Volume 16 - Machining Part 17 pptx

Manual-stroke external honing has replaced lapping in some applications, because: • Honing is usually faster • Soft metals can be honed without being impregnated with abrasive • The us

Prepared proprietary oils that contain buffers are widely used Before use, these oils (which may also contain rust inhibitors and deodorants) are diluted up to 95% with kerosine

The use of too much buffer may detract from its beneficial effects An excessive amount:

• Reduces the cutting action of the abrasive

• Produces smoother finishes

• Requires higher pressures or lower rotational speeds

• Lowers the ability of the fluid to dissipate heat

• Impairs fluid distribution

• Increases requirements for refrigeration and filtering

Regardless of the type of fluid used, it should be delivered to the honing stones in a constant and generous supply The fluid also should be filtered through a system that removes particles coarser than 15 m (600 in.) The system should be kept free of water and stray oil (such as from the hydraulic system), which adversely affect the properties of honing fluids

In many plants, 17 to 20 °C (62 to 68 °F) is the preferred temperature range for honing fluids Controlling the temperature becomes more important as tolerances become closer If temperature is allowed to rise, dimensions may become inaccurate and the fluid may break down, causing excessive stone wear and changes in cutting characteristics In production installations, heat exchangers are often used to maintain close control of honing fluid temperature

Example 1: Variations in Dimensions and Finish for 900 Cylinder Blocks

The data plotted in Fig 19 represent results of a quality control check made on 99.31 mm (3.910 in.) diam cylinder bores

in gray iron blocks for V-8 engines Bores 2 and 7 were measured in 11 blocks from a production run of 900 The conditions employed in honing these bores are presented in Table 10

Table 10 Processing details for honing cylinder bores in V-8 engine blocks

Processing details(a)

Machine production rate 70 blocks/h

Spindle speed 204 rev/min

Spindle reciprocation 78 strokes/min

Stock removal:

Amount 0.051-0.102 mm (0.002-0.004 in.)

Honing fluid Mineral seal oil(b)

Stone life per set (c) 450 blocks

Size control Spindle-mounted plug gage

Dimensional tolerance Max out-of-roundness and taper, 0.025 mm (0.001 in.)

Finish 0.50-0.89 m (20-35 in.)

Crosshatch angle

22 °

(a) Bores are classified in five sizes differing 0.013 mm (0.0005 in.) in

diameter, for selective fitting of pistons

(b) At 20 °C (68 °F), heat exchanger is required for maintaining this

temperature, and honing fluid must be free of water and tramp hydraulic oil

(c) Silicon carbide stones

Fig 19 (a) Surface finish, (b) taper, and (c) out-of-roundness variations obtained in honing Data represent

measurements on cylinder bores 2 and 7 in 11 gray iron blocks for V-8 engines, selected from a run of 900 Measurements were made on blocks 1, 50, 100, 200, 300, 400, 500, 600, 700, 800, and 900

The honing fluid was maintained at 20 °C (68 °F) by the use of a heat exchanger, and was constantly filtered Less than 2% of the 7200 bores honed required a repair operation because either taper or out-of-roundness exceeded the specified 0.025 mm (0.001 in.)

Surface Finish

Surface finish of 0.25 to 0.38 m (10 to 15 in.) can be obtained easily in production honing, and finish of less than 0.050 m (2 in.) can be achieved and reproduced A range of roughness is sometimes specified In other applications, a maximum surface roughness is specified Under carefully controlled conditions, surface roughness can be maintained within a close range, as indicated in Fig 19

Size of grit in the honing stones is the main factor controlling surface finish When grit is fine, the finish will be fine (other factors being equal); but as grit size is decreased, rate of stock removal is also decreased, as described in the following example

Example 2: Honing Gray Iron to a Finish of 0.25 to 0.38 m (10 to 15 in.)

In honing gray iron (hardness, 170 to 195 HB), a finish of 0.25 to 0.38 m (10 to 15 in.) was desired Silicon carbide stones with a grit size of 180 produced a roughness of 0.63 to 0.75 m (25 to 30 in.) The required finish could be obtained with 320-grit stones, but the time required for honing made the use of this grit size impractical The problem was solved by first rough honing with 180-grit stones and then finish honing, in another setup, with 320-grit stones

Rough finishes are sometimes improved by using a dwell time at the end of the honing cycle that is, by continuing the rotation and reciprocation action for a few strokes after feed-out ceases and pressure drops off In manual honing of a particular bore, use of this technique reduced surface roughness from the normal 0.50 to 0.25 m (20 to 10 in.)

Honing Practice for Internal Diameters

Honing is widely used for finishing bores in engine cylinders, cylinder liners, and bearing bores Procedures for honing similar parts may vary from one plant to another, depending on quantity, available equipment, and established plant practice

As a rule, honing stones and techniques used for honing cast iron are different from those used for aluminum alloys However, there are exceptions as in the case of an assembly of cast iron and aluminum in which the two metals were honed simultaneously, with the same abrasive, because it was the simplest way to achieve a proper fit

Small Bores. Conventional manual-stroking honing tools (Fig 9) are available for use in bores as small as 1.6 mm (in.) in diameter in parts such as fuel nozzles, miniature bearings, and heading dies, as in the following example

Example 3: Honing Very Small Bores

Dies for cold heading tiny rivets and screw blanks had bores as small as 1.6 mm ( in.) in diameter Bore length varied, but was usually 25 to 50 mm (1 to 2 in.) Figure 20 shows one of the heading dies, which was made of tool steel, and a typical product of the die Holes were drilled and reamed about 0.075 to 0.13 mm (0.003 to 0.005 in.) undersize before heat treatment After hardening, they were honed, using manual stroking, to an accuracy of 0.0025 mm (0.0001 in.) for both roundness and straightness

Fig 20 Bore of die for cold heading the rivet shown at the left is typical of small bores finished by

manual-stroke honing Dimensions given in inches

Large Bores. The maximum diameter and length of bore that can be honed is limited mainly by the size of the equipment required for the workpiece and by the power required for the tools Equipment with drive motors of up to 37

kW (50 hp) is available for honing steel shells of 1040 mm (41 in.) inside diameter (ID) and 19 m (63 ft) length

Cylinder shells for hydraulic hoists on regulating gates for dams are examples of large bores that are honed In one honing operation, 0.75 mm (0.030 in.) of stock is removed from a 6.4 Mg (14,000 lb) shell of 760 mm (30 in.) ID and 7.9 m (26 ft) length to obtain a total envelope tolerance of 0.050 mm (0.002 in.) Before honing, the average out-of-roundness is 0.41 mm (0.016 in.)

Short Bores. Several different techniques are used for honing short bores These are particularly applicable when bore diameter exceeds length In the simplest method, several pieces are stacked with bores aligned, clamped tightly by any suitable means, and honed as a unit For example, 13 mm ( in.) long rings 38 mm (1 in.) in inside diameter can be honed in stacks of eight In effect, this would be the same as honing a single piece 100 mm (4 in.) long Stacked parts may

be either manually or power stroked However, for successful results from this technique, the parts must have parallel sides to permit building a straight stack that can be clamped tightly and provide a straight bore Another technique that has proved successful for honing short bores is shown in the following example involving automotive-engine connecting rods (compare with newer method shown in Fig 15)

Example 4: Short-Bore Honing Technique for Connecting Rods

A power-stroking horizontal machine was used in high production for honing 61.54 mm (2.423 in.) ID crankpin bores simultaneously in eight connecting rods Figure 21 shows the fixture and the honing tool Eight rods were stacked between 4.8 mm ( in.) wide parallel separator plates, resulting in an effective bore length of 260 mm (10 in.) The tool had three banks of four honing stones Each stone was 9.5 mm ( in.) square and 57 mm (2 in.) long A two-station, rotary index table allowed the operator to unload eight completed rods and to load eight unfinished rods while eight other rods were being honed A precheck plug probed the rods in the loading station to determine whether they had been bored to proper rough size The operation completed one bank of rods in 45 s, floor-to-floor time, and a production rate of about 600 rods/h was obtained In honing, 0.075 mm (0.003 in.) of stock was removed, a finish of 0.75 to 1.14 m (30 to 45 in.) was produced, and inside diameter was controlled within 0.13 mm (0.0005 in.)

Fig 21 High-production honing of automotive parts Fixture designed to hone crankpin bores on eight

automobile connecting rods simultaneously, using a single honing tool Rotating fixture permitted loading and unloading on one side while parts on the opposite side were honed

Blind holes are bores that have a bottom, shoulder, or other obstruction that prevents a tool from passing completely through The three most common types of blind holes are shown in Fig 22 Most unrelieved blind holes can be honed satisfactorily, but there will always be some unfinished area at the bottom The amount depends on length of bore, type of material, tolerance required, and amount of stock removed Under the best conditions, dead-blind holes can be honed to within about 0.38 mm (0.015 in.) of the end Any relief will improve results; as much relief as possible is preferred Sometimes an unrelieved blind hole is in effect provided with a relief because specified tolerance and finish need not be met at the bottom of the hole

Fig 22 Three types of blind holes

Special tools may be required, depending on whether or not relief (or on how much relief) is provided For example, the unrelieved 13 mm ( in.) diam bore shown in Fig 23(a) was manual-stroke honed to within about 0.38 mm (0.015 in.) of the end with a special tool having a hard-tipped honing stone (Fig 11c) If adequate relief is provided, conventional tools are satisfactory For example, cylinder heads in lawn mower engines (Fig 23b) can be manual-stroke honed in high production with conventional tools, because of the generous relief (about 6.4 mm, or in., wide) at the blind end Although both parts shown in Fig 23 were manual-stroke honed, similar parts are frequently honed by power stroking

Fig 23 Blind holes honed by different methods (a) Unrelieved blind hole that required a special tool (see Fig

11c) for honing (b) Relief that permitted use of a conventional honing tool in the bore of a cylinder head for a lawn mower engine Dimensions given in inches

Delivering enough honing fluid to the work area is often a problem in honing blind holes When a hole has a bottom opening (Fig 23b), fluid can be pumped through a plastic tube inserted in the opening When a hole has no bottom opening, the flow of fluid should be directed parallel to the mandrel, into the mouth of the bore

In manual honing, blind holes are more difficult to keep straight than open holes A truing sleeve (dummy workpiece) is frequently used to keep the shoes and stones straight and parallel; also, the stone and shoe are made shorter than the blind hole Experienced operators have found that using a series of short strokes with an occasional stroke all the way out of the mouth is the best practice, until the hole is close to final diameter This keeps the bottom slightly larger than the mouth Straight strokes are then used for finish honing

Tapered Bores. Part size, angle of taper, and length-to-diameter ratio determine the method used in taper honing Short tapers are honed using a machine and tool such as that shown in Fig 24 The machine has a head that can be positioned for any desired degree of taper, and the reciprocating tool holds a single stone The workpiece is rigidly clamped in a fixture that rotates This method is most commonly used for producing tapers on parts for which the length of honed area

is less than the diameter As the length of the taper increases in proportion to the diameter, however, the practicality of the method decreases, because the longer and more slender tools lack adequate rigidity

Fig 24 Machine and tooling for honing short, tapered bores

Applications of this method of taper honing include special bearing rings and parts that use end tapers for sealing, and bores in gears that must fit tapered shafts For example, drum-to-barrel seals in a 20 mm gun must have a taper of 0.050 mm/mm (0.050 in./in.) of length at each end and roughness less than 0.25 m (10 in.) To meet these requirements, 0.01

to 0.05 mm (0.0005 to 0.002 in.) of stock must be removed from the critical surfaces

Taper honing long bores in large parts is far more complex than honing short tapers A major portion of the stock is removed by step honing In this operation, a straight stroke is used, its length being progressively reduced to form a rough taper consisting of a series of small steps The taper is then finished in a second operation in which a sine bar regulates the increase and decrease of the diameter on the return and forward stroke of the honing cone

Special Shapes. Machines and tools have been developed for honing various special shapes For female splines, honing stones must be narrower than the spline width (preferably no wider than half the spline width) to allow for oscillation Machines and tools for honing splines are designed to produce simultaneous reciprocation and oscillation, rather than reciprocation and rotation Relief bores are commonly honed by contour boring

Special Applications of Honing

A few special uses of honing should be enumerated as a means of indicating the potential of the honing method beyond the field of its basic and most extensively accepted applications These are related to the honing of internal cylindrical surfaces by using regular abrasives for obtaining specific dimensional conditions of the work surface Among these related processes are:

• Hone forming

External Honing. Honing has been used to only a limited extent for finishing outside diameters, largely because required dimensions and finish can be produced at less expense by other processes, such as centerless grinding However, advances in metrology and improved honing techniques have resulted in an increase in the number and scope of applications of external honing

Special machines and special adaptations of conventional machines (such as lathes) have been tooled to hone outside surfaces of metal parts With these machines, either power or manual stroking may be employed

Fixtured external honing (power stroking) is widely used for pieces that are not adaptable to competitive methods A notable example is the finishing of grooves in bearing races Special machines that simultaneously rotate the workpiece and oscillate the stones (Fig 25) produce the crosshatch lay pattern characteristic of a honed surface

Fig 25 Fixtured honing of grooves on external surface of bearing rings with simultaneous oscillation of honing

stone and rotation of workpiece

Manual external honing is applicable to the removal of small amounts of stock from external diameters of a wide variety of sizes and shapes The honing of lengths up to 3 m (10 ft) is common practice Conventional honing machines are generally used for rotating workpieces up to 610 mm (24 in.) long Lathes or drill presses are preferred for longer workpieces

Tools such as that illustrated in Fig 26 are available for honing parts ranging in outside diameter from about 3.05 to 69.85

mm (0.120 to 2.750 in.) With this setup, the sides of the tool are gripped and stroked over the rotating workpiece out and cutting rate are controlled by applying pressure to the honing-control lever, which will move through a preset distance Size is controlled automatically by setting the micrometer stone feed-out so that the honing-control lever will be against the stop pin when the correct size is attained The only adjustment needed during the honing operation, even in production runs, is a slight additional stone feed-out to compensate for stone wear A turn of the honing-control lever will instantly disengage the stone from the work for quick gaging or unloading, but will not change the setting on the micrometer stone feed-out

Feed-Fig 26 Assembly used for manual-stroke honing of outside diameters See text for discussion

With the setup shown in Fig 26, a line of stones with opposing guide shoes, or opposing stones, can be used For honing long parts (up to 610 mm, or 24 in.), multiple holders that contain as many as three stones (or shoes) in line may be used for correcting waviness The torque arm can be used to offset the tendency of the tool to turn A guide bar mounted on the machine acts as a stop for the torque arm This type of tool can produce dimensional accuracy to 0.0025 mm (0.0001 in.)

or better and surface roughness as low as 0.050 m (2 in.)

Manual-stroke external honing has replaced lapping in some applications, because:

• Honing is usually faster

• Soft metals can be honed without being impregnated with abrasive

• The use, in honing, of multiple-length stones and shoes allows better control of bow and waviness

Long anodized aluminum tubes for in-flight refueling are honed externally in a lathe, the honing tool being moved by hand, and the nozzle for the honing fluid moving with the tool Crankpins of some crankshaft are honed the same way at overhaul

Gear-tooth honing is an abrasive process designed to improve geometric accuracy and surface conditions of a hardened gear The teeth of hardened gears are honed to remove nicks and burrs, to improve finish, and to make minor corrections in tooth shape Gear teeth are honed on high-speed machines specially designed for the process (Fig 27) The honing tool is like a gear driving the workpiece at high speed (up to 30 m/min, or 100 sfm) while oscillating so that the teeth slide axially against the workpiece

Fig 27 Honing teeth of helical gears

Spur gears and internal or external helical gears ranging in diametral pitch from 24 to 2.5, in outside diameter from 19 to

673 mm ( to 26 in.), and up to 75 mm (3 in.) in face width have been honed on these machines Finishes of 0.75 m (30 in.) are easily achieved, and finishes of 0.075 to 0.10 m (3 to 4 in.) are possible Both taper and crown honing can be done

Tools used in honing gear teeth are of two types, a helical gear shape tool made of abrasive impregnated plastic, and a metal helical gear with a bonded abrasive coating that is renewable The plastic tool, which is discarded at the end of its useful life, is widely used The metal tool is used mainly for applications in which plastic tools would be likely to break; also, it is used primarily for fine-pitch gears

Plastic tools are supplied with abrasives of 60-grit to 500-grit size Size of abrasive, gear pitch, and desired finish are usually related as:

Finish Grit size Gear pitch

Applicability. The use of honing for removing nicks and burrs from hardened gears can result in a considerable cost

saving in comparison to the usual method In the usual method, the gears are tested against master specimens on sound test machines Nicks indicated are searched for and removed using a hand grinder The gear is then retested to make certain the nick has been removed When honing is used, all of these various tests and procedures can be eliminated

Some shape correction can be achieved in the removal of 0.050 mm (0.002 in.) of stock by honing A helical gear 127

mm (5 in.) in diameter may show lead correction of 0.010 mm (0.0004 in.), involute profile correction of 0.0075 mm (0.0003 in.), and eccentricity correction of 0.010 mm (0.0004 in.)

The advisability of using honing for salvaging hardened gears hinges on cost considerations As the error in tooth shape increases, honing time increases and tool life decreases On the other hand, if the gears represent a large investment in production time and material, honing may be the most economical method

Because honing is not designed for heavy stock removal or tooth correction, it cannot be substituted for grinding or shaving of gears Rotary shaving usually leaves gear teeth smooth within 0.25 to 1.00 m (10 to 40 in.)

Plateau honing produces a special plateau finish, which removes the surface peaks but retains the deep valleys Such a finish has been found desirable in engine performance because the valleys act as oil reservoirs for improved lubrication, especially during engine break-in

A plateau finish is produced by first rough honing to final size Then the surface is finished with a finer-grit stone for about 45 s, depending upon the amount of plateauing desired The plateauing operation, with a 600-grit stone, removes so little stock that the bore diameter is not measurably increased

Flat honing is a term designating a method and the equipment by which the flat surfaces of component parts produced

by other methods are improved with regard to both flatness and parallelism of opposite surfaces One of these surfaces may be that on which the part is located during the honing of the opposite face, or both faces may be honed simultaneously on machines operating with two honing disks

The equipment used is similar in appearance to rotary face-grinding machines, but it is adapted to honing, a method which differs from grinding particularly in the low cutting speed of the abrasive disk, the applied speed being comparable to that used in conventional honing The bonded abrasive disks used in flat honing are generally not intended for substantial rates

of stock removal and thus can have very fine grains, promoting the development of a high-grade finish, even of the order

of 0.025 m (1.0 in.) Ra when needed The spindle of the honing disk used on flat honing machines can be raised and lowered by an air or hydraulic cylinder

Single- or double-surface flat honing machines are designed for high-production uses, finishing typically 1200 to 1800 parts per hour in a fully automated operation controlled by a timer

On two-wheel machines (see Fig 28), the top wheel, lower wheel, and workholder each have separate drives and controls Machines are available with automatic controls to gradually increase pressure on the top wheel during the honing cycle Automatic size control is also available The workpiece carrier is part of an epicyclic sprocket holder

Fig 28 Two-wheel flat honing machine

Electrochemical Honing. In this process, metal is removed by introducing an electrolyte into a gap between a cathodic honing tool body and an anodic workpiece Direct current from the power source is conducted to the tool through a brush assembly that acts as the cathode, while the workpiece becomes the anode

The tool mandrel is made of metal and has a series of small holes which provide channels for the electrolyte circulating under controlled pressure The electrolyte has the additional role of being a coolant and flushes away the chips that have been sheared off by the honing stones

Abrasive honing stones are nonconductive and are limited to removing the electrochemically loosened metal particles, thereby controlling the geometric form and the size and texture of the produced surface Thus, the electrochemically honed surface has the same characteristic crosshatch pattern and is essentially stress free, as is the surface produced in a regular honing process by purely mechanical action

The abrasive stones continuously remove from the work surface the oxides developed by the electrochemical action, leaving the surface clean This makes use of an electrolyte much less corrosive than that needed in the electrochemical

machining process Nor is it necessary for the electrolyte to be dispensed at high pressure, a process characteristic that calls for great structural strength of the equipment in the mechanically unassisted electrochemical machining

The electrochemical honing process is carried out in a manner quite similar to regular honing, combining the rotation and reciprocation of the tool at controlled speeds However, the tool mandrel must have good conductivity and all elements of the equipment that are exposed to the corrosive environment must be made of corrosion-resistant materials

Hone forming (HF) is a recent development that constitutes a marriage of two different processes, honing and electrodeposition The process is used to simultaneously abrade the work surface and deposit metal It produces work surfaces that combine the benefits of both processes, that is, a geometrically and dimensionally controlled surface with a functionally favorable texture developed on a cladding that is concurrently electrodeposited on the base metal In some of its basic principles, the method is a reversal of electrochemical honing

The gap between the anodic honing tool body and cathodic workpiece is kept small Current densities used in HF operations are many times those employed in other electrodepositing methods Rate of deposition varies from 0.018 to 0.05 mm/min (0.0007 to 0.002 in./min) and up to 0.64 mm (0.025 in.) of metal has been deposited effectively in the laboratory

In the following description of the equipment and process, a cylindrical internal surface or bore is considered; however, the method is applicable to flat or external round surfaces as well

Hone forming is carried out on special equipment, which includes the machine for supporting the fixture and for actuating the rotating and reciprocating tool with its controlled feed motion Additional elements of the equipment are the rectifier

to supply the direct current, the solution tank, and the circulating system The insoluble anode is part of the honing tool and is connected to the positive side of the rectifier The workpiece functions as the cathode and is connected to the negative side of the rectifier The gap between the anode (tool) and the cathode (workpiece) is kept small, while through that gap at controlled velocity passes the solution, which, together with the workpiece, is contained in a sealed circuit Figure 29 shows the components of a typical hone forming machine

Fig 29 Principal components of a typical hone forming machine

After the workpiece has been cleaned of oil and other potential contaminants, it is mounted in a special honing machine and, as the first phase of the process, the bore is honed in a conventional manner This part of the operation produces a clean base metal surface with accurate cylindrical form

Subsequently, the metal-depositing process is started, keeping the anode at a controlled distance from the work surface, which acts as the cathode The electric power supply is turned on, and the electrolyte is circulated between the anode and the cathode

During this process the honing action continues, but with reduced stone pressure, thus ensuring that the deposited metallic layer will develop into a work surface of the desired form (roundness), size (diameter), and surface texture (crosshatched) The process is terminated when the desired size of the bore has been reached, as determined either indirectly, by the elapsed time, or directly, by means of a gage

The entire process is carried out within a very short time, of the order of about one minute, because the cladding of the surface is many times faster than in conventional bath plating and has the added benefit of obtaining a precisely honed work surface with controlled characteristics Other advantages of the new process are the elimination of most of the time-consuming preparatory and postprocess operations, such as masking, washing, neutralizing, and so forth, which are necessary for attaining the same results with the application of conventional procedures

In hone forming applications, the workpiece must be conductive To date, most materials used in hone forming are copper, bronze, tin, nickel, cobalt, or chromium plated and are hone formed on workpiece materials such as iron, steel, stainless steel, and bronze Theoretically, any surface that can be honed can be processed with hone forming

Although the immediate uses of this new method are mostly in the area of work salvaging out-of-tolerance parts and reconditioning worn surfaces, its potential is much wider and includes production line operations

Microhoning

The term microhoning is considered a descriptive designation for a method that uses hones (bonded abrasives) for tools and produces an accurately controlled surface whose parameters are measured in micro (very small) units Essentially, the method is applied to round surfaces, both external and internal, of cylindrical, tapered, or spherical general form, and less frequently, to flat surfaces with limited areas All of these contain surface elements that are either straight lines or circular arcs

The method operates with tools made of bonded abrasives, with the active surfaces having forms complying with the contacted area of the work surface The abrasive tools, while being held against the surface of the rotating work with a controlled, light force, generally effect a rapid, short-stroke, reciprocating (oscillating) motion in a direction parallel with the surface elements of the processed work surface

Microhoning can be done on centers or with centerless through-feed machines With centerless microhoning, 60 to 70° of the total workpiece circumference is in contact with one or more fine-grit stones

Comparison of Microhoning with Regular Honing

Characteristics common to regular honing and microhoning methods are:

• The use of bonded abrasives for tools

• Operation with low cutting speeds and producing light cutting force

• The combination of a rotational and a linear motion between the tool and the work; both methods produce a cross-hatch pattern, but in microhoning it is less distinct

• The objectives of the process, that is, improving the form, size, and surface texture of the work surface without causing metallurgical damage

Particular differences between regular honing and microhoning are described below

The Stroking Length. Honing is essentially a long stroking method, capable of covering work surface lengths several times that of the stones During the process the stones travel a substantial distance (usually expressed as a fraction of the stone length) over the ends of the processed work surface

In microhoning, the stone reciprocation is over a very short length, typically of the order of about 1.0 to 4.1 mm (0.040 to 0.160 in.) For that reason the stone motion is often referred to as an oscillation

Frequency of the Reciprocation. In honing, the number of strokes per minute varies from very few, in the case of very long strokes, to about 100 or 200 in general operations The stroking, although usually hydraulically actuated, in the case of low-production or medium-production machines may also be manual

Microhoning operates with a very rapid reciprocating motion, its frequency varying, according to the applied system and stroke length, in the range of 300 to 2500 cycles per minute The reciprocating motion has mechanical or pneumatic drive

Rotating Member. In honing, it is the tool that rotates (with the rare exception of the external honing of very long parts); in microhoning, it is always the workpiece that rotates in front of the tool whose reciprocation is along a fixed path

Contact Area. In honing, usually several stones, at essentially uniform circumferential distances, make simultaneous contact with the work surface, each stone taking part in the stock removal

In microhoning, a single stone, except for the rare multistone applications on large diameters, covers only one segment of the work surface, although the width of the stone produces a wraparound effect

Work Surface Configuration. Stroking in honing is always along a straight-line path, which limits the general form

of the surface adapted to honing The long stroking of honing also excludes surfaces that do not permit free stroking, although the honing of surfaces with one free end, for example, blind holes, is possible Honing is used nearly exclusively for bores

Microhoning, which operates with very short strokes, can be applied also to surfaces with circular cross-sectional contours, such as spherical or toroidal shapes Equally adaptable to external and internal surfaces, the short stroking also permits the microhoning of surfaces bounded by shoulders on both ends, requiring a very narrow undercut, comparable to that provided in cylindrical or internal grinding

Amount of Stock Removal. Honing can be used for efficiently removing substantial amounts of stock In rough honing, of the order of 0.25 to 0.38 mm (0.010 to 0.015 in.) can be removed, occasionally, considerably more In finish honing, about 0.05 to 0.15 mm (0.002 to 0.006 in.) can be removed from the part diameter This stock-removing capacity

of the honing method permits the correction of various types of bore form irregularities

Stock removal in microhoning is usually of the order of 0.0025 to 0.0075 mm (0.0001 to 0.0003 in.), consequently, the general form irregularities, for example, ovality, taper, and so on, can be corrected only within a very limited range

Characteristics of the Produced Work Surface. Honing is generally used to produce surface finishes in the range

of 0.8 to 0.2 m (32 to 8 in.) Ra Such surface texture values are adequate, or even desirable, for many types of bores

finished by honing Often even rougher surfaces are required, or, in rare cases, a finish of the order of 0.10 m (4 in.) Ra

is specified It is feasible to produce even finer finish values by honing, but generally at the price of rapidly decreasing productivity

Microhoning can produce, in efficient operation, surface finishes of the order of 0.10 to 0.05 m (4 to 2 in.) Ra, and, upon occasion, even finer finishes This single parameter, the average surface roughness, does not convey, however, all the functionally important surface characteristics that can be developed in regular production by microhoning These will

be discussed in greater detail under a separate heading

Basics of the Microhoning Process

The operating principles of the microhoning process were illustrated earlier in Fig 3 The diagram illustrates that an abrasive stone, whose operating face complies with the general form of the work surface, is forced against the rotating workpiece to exert a specific pressure while carrying out a short-stroke reciprocating motion

In operation, the abrasive stone will first make contact and act upon the protruding elements of the work surface Protuberances can result from surface form irregularities such as lobbing, chatter marks, waviness, and so on, and can also constitute the peaks and crests of a rough surface Generally, irregularities from both sources are present, with the roughness superimposed on the form irregularities

By abrading the protruding elements of the surface, the action of the microhoning stone will gradually equalize the work surface, correcting some of the form irregularities and reducing the roughness The major protuberances of the surface will be abraded first by the concentrated action of the total stone pressure; consequently, the rate of surface improvement will be rapid in the starting phase of the operation As the contacted work surface area continuously extends, the rate of stone penetration will become slower, partly because the specific pressure on the contacted areas decreases, and partly because the stone surface starts to glaze, due to the reduced dressing action of the work surface The latter condition, although detrimental to the abrading ability of the stone, is beneficial to the development of a better finish on the microhoned surface, which is the prime objective of the process The optimum balance between the effective abrading and the dependable smoothening ability of the stones can be controlled by the proper selection of the process variables, such as reciprocating and rotating speeds; applied pressure; and form, dimensions, and composition of the stones

The described progress from fast abrading to the slower smoothening action could be extended to produce very substantial improvements of the work surface by microhoning However, it would take an excessive amount of time for a stone that is capable of rapid abrading action to lose its cutting ability to the degree that it could produce a very smooth surface For that as well as for other reasons, that is, to retain the unimpeded abrading capacity of the stone, the microhoning process is usually operated in the range of its highest efficiency The approximate extent of that range can be expressed conveniently by the ratio of surface roughness improvement, which usually varies from 4:1 to 8:1, although these values are not absolute limits The lower ratios commonly apply to the effective range for coarse surfaces, such as

improving the finish from 6.3 to 1.5 m (250 to 60 in.) Ra, while the roughness of a fine-ground surface can be reduced

efficiently from 0.40 to 0.050 m (16 to 2 in.) Ra

Surface texture is seldom measured, although it can affect certain functional characteristics, such as the sealing capacity and lubricant retainment of the surface At a relatively higher roughness level, a cross-hatch pattern can be observed on the microhoned surface; that pattern tends to change, as the process progresses, into a nondirectional pattern,

or to disappear entirely, resulting in a reflective surface

These changes of the surface texture, brought about by microhoning, when applied to the distinctly directional pattern produced by grinding are generally beneficial to the functional properties and service life of the work surface

A rough surface will make contact with a mating part through its protruding crests, and the extensive lower-lying areas will have no functional role Reducing the roughness of the surface without changing its alternating crests and valleys will only partially remedy the unfavorable conditions of high wear and friction in the contact area

The optimum contact condition is created by cutting down the crests and transforming them into wide plateaus, a process brought about by microhoning The effect of microhoning on an originally rough surface is visualized by the schematic cross-sectional diagrams in Fig 30, which illustrates in five steps (corresponding to the indicated processing times) the manner in which the rugged surface resulting from a preceding machining is improved by changing it into a surface containing wide plateaus

Fig 30 A rough surface, (a), being gradually improved by microhoning, that is, by cutting down the protruding

crests and finally developing a surface consisting of wide plateaus The elapsed time and corresponding surface roughness obtained at each interval for the five cross sections are: (a) 0 s, 1.25 m (50 in.), (b) 10 s, 0.38

m (15 in.), (c) 20 s, 0.28 m (11 in.), (d) 30 s, 0.20 m (8 in.), (e) 0.050 m (2 in.) The diagrams exaggerate the cross-sectional contours of the work surface for visualizing the effect of microhoning on the work surface texture

Process Parameters in Microhoning

Type and Grit of Abrasive Stones. Both types of abrasive materials are used for microhoning stones:

• Aluminum oxide abrasives (which fracture less easily), for carbon and alloyed steel

• The more friable silicon carbide, for very soft or very tough types of steels, as well as for cast iron and most types of nonferrous metals

The hardness of the stones, controlled by the percentage of the bond, varies from J (very soft), to P (very hard) The former is used for extremely hard alloys, chromium plates, and so on, while the hardest bonds are needed for cast iron and nonferrous metals

The dimensions of the active stone face are determined by the size of the work surface For external cylindrical surfaces, the width of the stone is about 60 to 80% of the part diameter, but generally not more than about 25 mm (1 in.) For work diameters larger than about 150 mm (6 in.), microhoning heads, with several stones arranged along an arc in compliance with the work surface, are frequently used The length of the stone is usually somewhat less than the length of the work surface, but not more than about three times the width of the stone For the microhoning of longer work surfaces, an additional traverse movement is needed The thickness of the stones is controlled by the mounting dimensions, the access length (which is particularly essential for honing inside shoulders), and an adequate wear depth; this latter component is substantially more, in some applications, than the stone width

The grit size of the stones is generally selected from a wide range to suit the condition of the machined surface and the objectives of the process The type of microhoning work required can vary from roughing (applied either as a preparatory operation to a subsequent finishing or as a single process for general-purpose work) to fine, or even very fine finishing For coarse microhoning applied to improve the finish and the functional properties of rough-machined surface, grit in the range of 60 to 220 is used For general-purpose microhoning work to produce a finish of the order of 0.40 to 0.20 m (16

to 8 in.) Ra, grit sizes 320 to 500 are needed Fine finishing is carried out with grit sizes in the 600 to 800 range, while for very fine finishing, stones with 1000 to 1200 grit may be selected

Stone Pressure. One of the characteristics of the microhoning method is the use of a relatively low stone pressure to avoid a deep penetration into the work surface, which could leave furrows and generate heat The shearing effect of the grains is limited to the protruding elements of the surface, and that action can be achieved by causing the stones to bear against the workpiece with a light force

For average work, stone pressure in the range of 140 to 275 kPa (20 to 40 psi) is generally used, raising the pressure for roughing to about double these values For very fine finishing, particularly for soft material, stone pressure as low as 14 to

34 kPa (2 to 5 psi) may be applied

Work Speed. Most commonly, microhoning is applied to the surface of rotating workpieces that are of cylindrical and, less frequently, of tapered shape, referred to as OD However, flat surfaces, either uninterrupted, such as the end faces of bearing rollers, or of annular shape, such as flange areas, are also adapted to microhoning Longitudinal flat surfaces, which require linear traverse movement, can also be microhoned, although that method is seldom applied because flat surfaces, unless they are located in a recessed position, are generally adaptable to lapping (see the article "Lapping" in this Volume)

The work speed, therefore, is generally specified as the surface speed of the rotating workpiece, with its most commonly applied values falling within the following ranges:

• For roughing, 12 to 15 m/min (40 to 50 sfm)

• For finishing, 30 to 60 m/min (100 to 200 sfm)

To produce a very fine finish and also to transverse workpieces at a high feed rate, substantially higher surface speeds, up

to about 120 m/min (400 sfm) are also applied

At the lower work speed, the microhoning process generally develops a very fine but still distinguishable crosshatch pattern, which may be the desirable surface condition in many applications, although it reduces the reflectivity or shine of the work surface At higher work speeds, that pattern disappears, and a brighter surface is developed

Stroke Length and Speed of the Stone Reciprocation. The fast reciprocation of the stones in a short stroke is one of the essential characteristics that sets microhoning apart from regular honing The length of the stroke differs somewhat in various systems, some of which employ a single stroke length, for example, 4.76 mm (0.1875 in.), while others are designed to provide variable stroke length, adjustable over a range of about 2.00 to 5.10 mm (0.080 to 0.200 in.)

The actual linear speed of the motion of the stone is the function of the stroke length and the rate of reciprocation These two factors are also referred to as the amplitude and the frequency of stone reciprocation Stone speed can be adjusted to the requirements of the operation by varying either of these factors

The resulting linear stone speeds differ over a rather wide range according to the system used and the condition of adjustment Typical extreme values are 3.18 to 20.3 m/min (125 and 800 in./min)

Microhoning Applications

Microhoning lends itself to cylindrical (internal and external), flat, tapered, toroidal, spherical, and barrel-shaped surfaces

on automotive products such as ball-and-roller bearings, transmission shafts, crankshafts, shock absorber piston rods, valve shaft diameters, and universal-joint spiders Figure 31 shows two typical setups used to machine two different types

of bearing races Table 11 lists typical production rates and finishes obtained with microhoning on automotive components

Table 11 Typical microhoning production rates for automotive components

Finish after

grinding

Finish after

microhoning Automotive component

parts/h Tappet head 0.76-

(a) Starting finishes for pressure plates and brake drums are for turned surfaces rather than ground

In some cases, pressure plates are ground to a finish of 0.50 to 0.63 m (20 to 25 in.)

Fig 31 Setups for microhoning (a) ball bearing races and (b) roller bearing races

Microhoning is very effective for finishing ball-shape components to a unique degree of sphericity Such shapes, executed

to an excellent finish, are required in engineering production for valve balls, pump pistons, and so on An uncommon, but important application of spherical microhoning is in the manufacture of prostheses, specifically, artificial hip joints The accurate sphericity and very high finish of the ball and socket surfaces, which is accomplished by microhoning these

surgically installed elements, ensures their dependable functioning and capability to withstand load of 1.18 kN (265 lbf)

or more

Lapping

Revised by Pel Lynah, P.R Hoffman Machine Products

Introduction

LAPPING is a low-speed low-pressure abrading operation that accomplishes one or more of the following results:

• Extreme dimensional accuracy of lapped surface (flat or spherical)

• Close parallelism of double-lapped faces

• Refinement of surface finish

• Extremely close fit between mating surfaces

• Removal of damaged surface and subsurface layers that degrade the electrical or optical properties

In general, the quality that can be obtained by lapping is not easily or economically obtained by other processes

Loose abrasive, carried in an appropriate vehicle, is used on cast iron laps in 99% of the lapping applications, but there are some isolated fixed-abrasive applications that are classified as lapping It is difficult to make a clear distinction between lapping and honing Lapping is the lower-pressure, lower-speed, and lower-power application of the use of fixed abrasives Furthermore, the fixed abrasives of honing are usually limited to the conventional resinoid or vitrified face wheels, while the fixed-abrasive lapping operation often uses unconventional media, such as urethane-impregnated pads, polyvinyl alcohol and abrasive mixed, foamed, and cured to a hard, cellular block, plated or surface-bonded diamond, or thin abrasive-filled vinyl films The usual definition of lapping is the random rubbing of a part against a lap, usually of cast iron composition, using an abrasive mixture in order to improve fit and finish Lapping operations usually fall into one of two categories: individual-piece lapping and matched-piece lapping

In individual-piece lapping, abrasive is rubbed against the workpiece with a special tool called a lap (usually of material softer than the workpiece), rather than with a mating workpiece surface When loose abrasive is used, the lap is usually made of soft cast iron (typically close-grain cast iron or meehanite metal) or a soft nonferrous metal Laps made

of bonded abrasive also can be used, as discussed above

Individual-piece lapping is most effective on hard metals or other hard materials It is used to produce optically flat surfaces, to produce accurate planes from which other planes can be located (as for gage blocks), and to finish parallel faces

The machine lapping of individual pieces, either one per cycle or in multiple-piece loads, represents the bulk of the production lapping currently done in industry Single-side flat lapping machines, double-side planetary machines, cup lapping machines for spherical surfaces, and specialized single- or double-plate machines (such as ball, roller, or pin laps) constitute the vast majority of lapping installations

In matched-piece lapping, sometimes called equalizing, two workpiece surfaces separated only by a layer of abrasive mixed with a vehicle are rubbed against each other Each workpiece drives the abrasive so that the grit particles act on the opposing surfaces Irregularities that prevent the surfaces from fitting together precisely are thus eliminated, and the surfaces are mated In many cases, a part is first lapped individually and is then mated with another part by this method, before the two are stocked as a pair of lapped-together parts

Matched-piece lapping enables mating parts (such as the heads and blocks of internal combustion engines) to form tight or gas-tight seals without the need for gaskets It also eliminates the need for piston rings in fitting some plungers to

liquid-cylinders Other common uses of matched-piece lapping include fitting tapered valve components (Fig 1) and mating two

or more gears in a set

Fig 1 Tapered valve components finished by matched-piece lapping for precise fit of mating surfaces

Process Capabilities

Parts that are processed by lapping are constructed of a variety of materials, ranging from metal parts for tooling, gaging,

or sealing to electronic crystals such as quartz piezoelectric frequency devices and silicon semiconductor material for integrated circuit manufacture Tungsten carbide, ceramic, and glass components; aluminum computer disks; tool steel slitter blades; saw blanks; and jade decorative tiles are among the applications that demonstrate the diversity of the lapping process

The size or weight of the workpieces that can be lapped is limited only by the available equipment Parts finished

by lapping range in weight from a fraction of an ounce to hundreds of pounds

Workpiece Shape. Tools and methods have been devised for lapping virtually every shape of workpiece on which a

lapped surface is desired Lapping is most widely used for finishing flat surfaces or outside and inside cylindrical surfaces The process can also be applied to balls, rollers, cones, double-curved surfaces, assembled bearings, and shapes such as gear teeth

Material Removal. Lapping is intended as a final finishing process that would be, in general, an impractical or uneconomical means of removing stock In most applications, less than 0.13 mm (0.005 in.) of material is removed from a surface by lapping However, occasionally (usually in flat lapping), 0.38 mm (0.015 in.) or even more may be removed

In a few cases, it has proved more economical to remove stock by lapping than to add a preliminary grinding operation

Selection of Abrasive

Silicon carbide and fused alumina are the abrasives most widely used for lapping Silicon carbide is extremely hard (2500 HV) Its grit is sharp and brittle, making it nearly ideal as an abrasive for many lapping applications because it continually breaks down to expose new cutting edges Silicon carbide is used for lapping hardened steel or cast iron, particularly when an appreciable amount of stock is to be removed

Fused alumina (2000 HV) is also sharp, but it is tougher than silicon carbide and breaks down less readily Fused alumina

is generally more suitable for lapping soft steels or nonferrous metals than silicon carbide

Boron carbide (2800 HV) is next to diamond in hardness and is an excellent abrasive for lapping However, because it costs 10 to 25 times as much as silicon carbide or fused alumina, boron carbide is usually used only for lapping dies and gages, which is often done by hand and in small quantities using little abrasive An example is synthetic sapphire for electronic applications The raw material cost is expensive, justifying a high abrasive-processing cost Relative costs for various quantities and grit sizes of silicon carbide, fused alumina, and boron carbide are compared in Table 1

Table 1 Relative cost (as of 1988) of abrasives used in lapping

Average size Cost $/lb Type of abrasive

m in

Grade

5 lb 10 lb 25 lb

5.0 200 1950 3.66 2.99 2.80 17.5 700 1600 3.09 2.44 2.27

Aluminum oxide

64.0 2520 1220 2.11 1.49 1.34 5.0 200 2950 11.89 10.99 10.57 22.5 900 2400 3.85 3.18 3.01

Table 2 Types and grit sizes of abrasives for various applications of lapping

Abrasive Relative hardness Grit size Typical applications

All-purpose compounds

Silicon carbide Hard and sharp 100, 220, 320, 400 Tool-room lapping

Corundum Medium soft 220, 240, 280 Tool-room lapping

Compounds for roughing, finishing, or polishing

400, 500, 600 Roughing softer steels

Corundum Medium soft

700, 800 Finishing softer steels

500, 600, 900 Roughing harder steels, stainless, chromium plate

Alumina Hard

2-10 m (80-400 in.) Finishing hard steels

900 Polishing hard steels

5, 10, 15 m (200, 400, 600 in.) Polishing hard steels

Alumina Medium hard

1-3 m (40-120 in.) Polishing stainless, chromium plate

Alumina Soft 1, 2 m (40-80 in.) Polishing

Silicon carbide Hard and sharp 600, 800, 1000 Roughing hardened steels; cast iron

600, 800 Finishing brass, bronze

Garnet Medium soft

10 m (400 in.) Polishing brass, bronze

Chromium oxide Medium soft 1 m (40 in.) Polishing stainless

Ferric oxide Soft 1 m (40 in.) Polishing soft metals

Cerium oxide Medium hard 1, 2 m (40, 80 in.) Polishing

The grit sizes most commonly employed in lapping range from 100 to 1000 (Table 2) However, abrasives are usually available in grit sizes from about 50 to 3800, and even finer

For lapping hardened steel to remove about 0.0051 mm (0.0002 in.) of stock and to produce a finish of less than 0.050

m (2 in.), a grit size of 280 is appropriate If finishing requirements are less stringent, 180-grit abrasive will be more economical because it removes metal faster than finer grit does

As the amount of stock to be removed increases, coarser grits are required For the removal of considerable amounts of stock, it is more economical to employ a roughing operation, followed by a finishing operation

When a substantial amount of stock is being removed, a fine finish can be produced without the use of a small grit size, because the originally coarse grit breaks down as lapping proceeds and progressively produces a finer finish If this technique is used, there must be enough stock allowance for lapping so that the deeper scratches formed initially by the coarse grit will be removed by the time final dimensions are reached

Grading. When an abrasive of a specified grit size is purchased, some of it will be finer and some coarser than the stated size The degree of grading is an important consideration in the selection of any abrasive Abrasives increase in cost as the grading becomes closer However, the use of a low-cost, loosely graded abrasive is not always economical, as demonstrated in the following example

Example 1: Closely Graded Abrasive for Greater Economy

A low-cost grade of silicon carbide that ranged in grit size from 100 to 800 was used for lapping piston rings A change to

an abrasive that was closely graded to a grit size of 600 reduced the overall cost of abrasive by 50%, even though the initial cost of the 600-grit abrasive was twice that of the low-cost grade The savings was made possible because the 600-grit abrasive contained more of the grit size that is most efficient for lapping; consequently, only one-fourth as much of it was required for removing the same amount of stock The 600-grit abrasive also gave a smoother finish with less smudging

Selection of Vehicle

Vehicles, or binders, for loose abrasives include a wide variety of compounds Some shops prepare their own formulations or modify standard compositions However, more consistent results can be obtained with standardized, commercial compounds

Two major factors in the selection of a vehicle are the material being lapped and the lapping method to be employed (inside or outside diameter, flat or spherical) Any vehicle should:

• Retain abrasives in uniform suspension and deagglomeration

• Serve as a cushion between surfaces being lapped (to minimize lap-to-part contact and yet avoid rolling action on abrasive particles)

• Adhere to laps and therefore minimize waste of compound

• Be noncorrosive to the material being lapped

• Be nontoxic to operators

• Be easily removable by cleaning

• Respond to temperature variations with the viscosity characteristics (stability or flexibility) desired in a given application Although rapid changes in viscosity are usually undesirable, in some applications it is important that the vehicle be able to change quickly from a grease to an oil when under slight heat and pressure and then revert quickly to greaselike consistency when pressure is released

Most vehicles have an oil or grease base, although some are made of water-soluble compounds The consistency of base vehicles varies from that of mineral seal oil (a water-white product having a viscosity slightly higher than kerosene)

oil-to that of heavy grease Common spindle oil is often used as a vehicle Commercial compounds contain mixtures of animal fat, vegetable oils, and mineral oils

Vehicles with an oil or grease base are usually used for lapping ferrous metals For specific applications in which grease

or oil would be objectionable (such as copper-base alloys and other nonferrous metals), water-soluble vehicles are available These vehicles, which are readily removed with water, are low-viscosity compositions of starches, bentonite, and soluble oils with rust inhibitors

Contamination is a potential problem in many nonmetal applications, especially in electronic components; therefore, clean-ability is the primary consideration Most commercial and proprietary vehicles for these materials are glycerine-base formulations; this provides a good-quality suspension, good film-forming properties (and therefore good lubricity), and a water-soluble mixture that is readily cleanable Clay or mica is occasionally mixed in to fill the voids between the abrasive particles, thus enhancing the suspension Ionic, charged, and submicron particles are also used as suspension

agents These particles affix themselves to the abrasive grain and produce electric repulsion forces to disperse and suspend the abrasive

Pure water is used in the lapping of glass and ceramic The abrasive slurry is generously applied and recirculated The resulting high fluid flow is used to keep the abrasive stirred and dispersed

Lapping Outer Cylindrical Surfaces

Outer cylindrical surfaces are usually lapped by one of the following methods:

• Ring lapping (a manual operation)

• Machine lapping between plates

• Centerless roll lapping with loose abrasive

• Centerless lapping with bonded abrasives

In addition to these techniques, special methods are used for specific applications, such as the lapping of piston rings and crankshafts Choice of method depends on part configuration, size of the production lot to be lapped, and cost Many outer cylindrical surfaces can be lapped with equal success by two or more methods

Ring Lapping

Ring lapping is the simplest method of lapping outer surfaces Designs of ring laps vary, but the assembly illustrated in Fig 2 is typical The lapping ring (or ring lap), usually made of cast iron, is manually stroked back and forth over the workpiece, which is chucked in a lathe or polishing head and rotated Lapping compound (usually of paste consistency) is often applied to the surface of the workpiece A manually adjusted screw is tightened, as required, to maintain a slight drag on the lap

Fig 2 Typical ring lapping assembly Drilled holes and slots permit uniform adjustment

The ring lap should always be shorter than the workpiece, and if size permits, it should have adjustable slots Finishing the bore of a ring lap is critical It should be drilled, reamed, and honed (or lapped) to a size very close to the starting diameter of the workpiece; the screw adjustment should be used only to compensate for the slight decrease in workpiece diameter as lapping proceeds

Applicability. Ring lapping, when performed by a skilled operator, offers at least two advantages over machine methods:

• Parts can be produced to extremely close tolerances

• Out-of-roundness can be corrected to a degree not feasible by machine lapping

Aside from requiring operator skill, however, ring lapping is tedious and expensive, and it should be considered only when one or more of the following conditions prevail:

• Equipment for any other method is not available

• Workpiece is out-of-round

• Weight of workpiece is unbalanced

• Workpiece has two or more different diameters that must be lapped

• Workpiece has flats, keyways, or other interruptions on its cylindrical surface

• Only a few pieces are to be finished

The following example describes a specific application in which some of the above conditions existed and in which ring lapping was therefore the most suitable method

Example 2: Preproduction Versus Production Lapping

In many cases, ring lapping is used for parts being developed, and a more economical method is used when the parts are

in production This procedure was followed for the valve needle shown in Fig 3, which was ring lapped in small quantities during development but was machine lapped between plates (see Example 3) in production lots These needles,

in diameters of 6.4 to 9.5 mm ( to in.), were made of alloy tool steel and hardened to 60 to 65 HRC

Fig 3 Valve needle and ring lap for finishing small, preproduction quantities Dimensions given in inches

For ring lapping, each needle was chucked by its stem and rotated in a lathe at 650 rev/min The lap (Fig 3), which was made of cast iron, was stroked back and forth over the needle until grinding marks were eliminated The lapping medium with which the needle was coated consisted of chromium oxide mixed with spindle oil Lapping produced a finish of 0.050 m (2 in.) and maintained tolerances of 0.0013 mm (0.000050 in.) for straightness and 0.00064 mm (0.000025 in.) for roundness

To ensure straightness, the laps used had to be at least three-fourths as long as the area to be lapped The laps also had to

be inspected frequently and had to be reconditioned by being lapped with internal laps of similar material

Machine Lapping Between Plates

In the machine lapping of outer cylindrical surfaces between plates, the laps are two opposed cast iron or bonded-abrasive circular plates that are held on vertical spindles of the machine (Fig 4) The plates are usually 200 to 710 mm (8 to 28 in.)

in diameter, although larger sizes are available For the most part, plain-face laps are used, and for the greatest accuracy,

the width of the lap face should not exceed the length of the surface being lapped The workpieces are retained between these laps in slotted plates and are caused to rotate and slide They are given an eccentric, or in-and-out, motion to break the pattern of motion and to ensure that they move over the inside and outside edges of the lap This prevents grooving of the lap For short runs, an eccentric motion is unnecessary if the laps are kept flat by reconditioning

Fig 4 Typical vertical lapping machine for finishing cylindrical surfaces in production quantities Dimensions

given in inches

Cast Iron Laps. When cast iron laps are used, the lower lap is usually rotated and drives the workpieces The upper lap

is held stationary, but it is free floating so that it can adjust to the variations in workpiece size The lower lap regulates the speed of rotation because the workholder is not driven

The abrasive is used with a paste-type vehicle and is swabbed on the laps before the cycle is started Oil or kerosene is then added during the cycle to prevent drying of the vehicle, which may result in scratching

Because the upper lap floats, several parts must be lapped simultaneously A quantity of three parts will support the upper lap, but when only three parts are lapped, the machine will not produce straightness or a common size Therefore, it is advisable to lap a minimum of five parts; if this quantity is not available, the machine should be loaded with dummy parts The best practice is to put as many parts as possible in a load This reduces the pressure on each part and slows the operation Thus, the operator has more control and can secure desired tolerances more easily

Finishes as fine as 0.025 m (1 in.), with stock removal of 0.0025 to 0.010 mm (0.0001 to 0.0004 in.), are feasible when cast iron laps are used Diametral tolerances as low as 0.00050 mm (0.000020 in.), roundness within 0.00013 mm (0.000005 in.), and taper of less than 0.00025 mm (0.000010 in.) have been achieved However, such accuracy depends greatly on the accuracy achieved in prior machining operations

Bonded-Abrasive Laps. When bonded-abrasive laps are used, both laps are rotated, with kerosene or a similar

lubricant used as a coolant and to wash away chips or loose abrasive Because both laps are driven at higher speeds than those used for cast iron laps, the lapping action is more severe Consequently, the machine will not produce the extreme accuracy possible with machines using cast iron laps In addition, because bonded-abrasive laps must be dressed with diamond tools, it is not possible to make them as flat as cast iron laps, on which the machines regenerate flatness

The quantity of parts being lapped is less critical for machines using bonded-abrasive laps than for machines using cast iron laps, because both bonded-abrasive laps are rigidly supported on spindles and separately driven As few as three parts can be successfully processed in this type of machine

Applicability. Machine lapping between plates is an economical method of finishing outside cylindrical surfaces, provided its use is warranted by production quantities and is permitted by part configuration The process can be used for lapping parts a few hundredths of a millimeter to 75 or 100 mm (3 or 4 in.) in diameter and 6 to 230 mm ( to 9 in.) long Parts commonly lapped by this method include plug gages, piston pins, hypodermic plungers, ceramic pins, small valve pistons, cylindrical valves, small engine pistons, roller bearings, diesel injector valves, plungers, small rolls, and miscellaneous cylindrical pins

Either hard or soft materials can be lapped, provided they are rigid enough to accept the pressure of the laps Hard materials respond well to lapping and achieve luster Hard materials are also easier to control for tolerance because the hardness slows the operation Soft materials lap more rapidly and (especially when bonded-abrasive laps are used) often have a scratchy or dull appearance This can be prevented by using a polishing abrasive, such as levigated alumina, which reduces the cutting ability of the bonded abrasive

Limitations. A part with a diameter greater than its length is difficult or impossible to machine lap between plates For parts of this type, other methods of outer cylindrical-surface lapping are more practical

Parts with shoulders require special workholders that permit the shoulder section to be placed on the inside or outside of the lap face Parts with keyways, flats, or interrupted surfaces are difficult to lap by machine, because the variations in pressure that occur are likely to cause out-of-roundness If the relief extends over the entire length of the piece, this method of lapping cannot be used

Parts with raised hubs in the middle require special laps that are cut in such a way that they clear the hub Clearance is necessary between the hub and the work surface to allow for oscillation of the workpiece

Thin-wall tubing can be lapped, but if the walls are so thin that deflection is significant, it will be difficult to maintain roundness Parts that are hollow on one end but solid on the opposite end present problems in obtaining roundness and straightness, because the hollow end will deflect more under the weight of the upper lap Plugging the hollow end of the part will sometimes solve these problems

Because it is impractical to keep more than one working surface on the face of cast iron laps flat, workpieces with work surfaces that have different diameters require a separate operation for each surface It is usually impractical to machine lap workpieces with diameters that are greater than the diameter to be lapped

The outside edges of the plates lap at a faster rate than the inside edges; therefore, care must be taken to prevent the workpieces from becoming tapered One method of overcoming the problem consists of using short lapping cycles and, at the end of each cycle, turning the workpieces end for end in the slots in the workholder In addition, the workpieces should be removed from the slots after each short cycle, mixed, and then replaced at random in different slots This prevents the inadvertent placement of all larger pieces at one side of the workholder and smaller pieces at the opposite side Because the upper lap floats, this placement of the workpieces would make it difficult to produce accurate parts Taper can be minimized by positioning the workholder so that the parts in the slots are at a 15° angle to a radius, as illustrated in Fig 5

Fig 5 Setup for lapping production quantities of the valve needle shown in Fig 3

Because machine lapping between plates uses diametrically opposed laps, it cannot correct the out-of-roundness produced

by centerless grinding However, out-of-roundness of the type produced by grinding on centers can be corrected The following example describes procedures for machine lapping between plates

Example 3: Lapping Valve Needles to Close Tolerance

The valve needles described in Example 2 and illustrated in Fig 3, although ring lapped in preproduction, were machine lapped between cast iron plates in production Before being machine lapped, the parts were carefully ground for roundness and then (because the lap would ride on those parts that were largest in diameter) segregated into groups of 0.0025 to 0.005 mm (0.0001 to 0.0002 in.) diametral variation Both upper and lower laps were grooved to prevent the breakdown of sharp edges during lapping A laminated phenolic workholder designed to hold a maximum load of parts (Fig 5) was eccentric to the laps to provide an oscillating motion

In this operation, the cycle was stopped so that the parts could be measured with an electrolimit gage, a visual shadow gage, or an air gage If the desired size had not been attained, more finish lapping compound was added and lapping was continued Lapping produced a finish of 0.050 m (2 in.), roundness within 0.00064 mm (0.000025 in.), and straightness of 0.0013 mm (0.000050 in.)

To recondition the laps, finish lapping compound was applied to the bottom lap The laps were then brought together and rotated until the edges of the grooves were sharp The laps required occasional regrinding to maintain a minimum groove depth of 1.6 mm ( in.) and width of 0.76 mm (0.030 in.)

Centerless Roll Lapping

In centerless roll lapping, only a single piece is processed at a time Therefore, this method is best suited to the lapping of small quantities of parts (usually, fewer than ten)

A typical machine (Fig 6) consists essentially of two 150 mm (6 in.) long cast iron rolls (one 150 mm, or 6 in., and one

75 mm, or 3 in., in diameter) and a reciprocating device for holding down the workpiece and controlling size The end of the fiber stick that holds down the work is provided with a 120° V-groove In operation, abrasive compound is applied to the rolls, and both rolls are rotated in the same direction-away from the operator and counter to the direction of workpiece rotation The larger roll rotates at about 180 rev/min, the smaller roll at about 90 rev/min The workpiece feeds across the rolls at about 50 mm/min (2 in./min) as the hold-down device is stroked back and forth to within 13 mm ( in.) of each end of the workpiece

Fig 6 Typical centerless roll lapping

The rate at which the workpiece feeds depends on the diameter of the piece For example, if a 13 mm ( in.) diam workpiece feeds at 50 mm/min (2 in./min), a 25 mm (1 in.) diam piece in the same setup will feed at approximately 25 mm/min (1 in./min) Slow stroking is necessary to obtain the best surface finish and control of size Stock removal in centerless roll lapping is usually 0.005 to 0.0075 mm (0.0002 to 0.0003 in.), depending on the finish obtained in the previous operation

The main advantage offered by centerless roll lapping is quick setup Therefore, the process is readily adaptable to frequent size changes in short production runs Limitations on the shape of parts for centerless roll lapping are similar to, but more stringent than, those that apply to machine lapping between plates When both processes are equally suitable for

a given application, the quantity of parts to be lapped determines which will be used

Centerless Lapping With Bonded Abrasives

Centerless lapping is a variation of centerless grinding (see the Section "Grinding, Honing, and Lapping" in this Volume) The machines for the two processes are similar in appearance, but the lapping machine is constructed to produce finishes

of 0.050 m (2 in.) or better, diametral accuracy of 0.0013 mm (0.000050 in.), and roundness within 0.00064 mm (0.000025 in.) The lapping and regulating wheels are 560 mm (22 in.) wide, which is much wider than those ordinarily used for centerless grinding Therefore, the work remains in contact with the lapping wheel longer and receives a finer finish

The regulating and the lapping wheels (both are bonded abrasive) can be angled so that their axes are not parallel Ordinarily, the regulating wheel is adjusted to a positive angle of 1 to 3° (depending on the production and finish requirements), and the lapping wheel is adjusted to a negative angle of about -4° When trued, both wheels assume a slight hourglass shape, which then allows them to wrap around the workpiece as it passes between them They also contact the workpiece at an angle to its axis, which is different from the axial-line contact of a grinding wheel This eliminates lapping marks

The finest finish obtainable in a centerless lapping machine requires at least three operations, each with a progressively finer lapping wheel, and a full flow of clean fluid (such as kerosene) as a coolant During the first operation, the workpiece is supported on a blade faced with hard steel or carbide For correcting out-of-roundness, the center of the workpiece should be slightly above the center of the wheels In the first operation, a maximum of 0.013 mm (0.0005 in.)

of stock is removed, and a finish of 0.10 to 0.15 m (4 to 6 in.) is obtained For the second and third operations, the workpiece is supported on a rubber blade and is centered on the wheels so that scratches are minimized During the second operation, a maximum of 0.0025 mm (0.0001 in.) of stock is removed, and a finish of 0.050 to 0.075 m (2 to 3 in.) is obtained During the third operation, practically no stock is removed, and a finish of about 0.050 m (2 in.) is obtained

Applicability. Centerless lapping is a high-production operation that is particularly suited to centerless ground parts that can be continuously fed, either manually or automatically Parts 6 to 150 mm ( to 6 in.) in diameter by 380 mm (15 in.)

long can be centerless lapped, and when a long bar feed is used, it is possible to lap parts 13 to 75 mm ( to 3 in.) in diameter and 4.6 m (15 ft) long Typical parts finished by centerless lapping are pistons, piston pins, shafts, and bearing races

Because little stock is removed in this process, only a small amount of correction can be made Therefore, parts must be previously ground to the required straightness and roundness

Parts with shapes that have no irregularities are ideally suited to centerless lapping, but irregularities such as those on the part shown in Fig 7 can be tolerated Such parts may, however, present problems in holding tolerances because of the undercut and the keyway Cross holes also add to the difficulty of holding extremely close dimensions in centerless lapping The production rates attained in centerless lapping and in two other processes for the part illustrated in Fig 7 are compared in the following example

Fig 7 Part lapped by three different methods Dimensions given in inches

Example 4: Production Rates for Centerless Versus Centerless Roll Versus Two-Plate Machine Lapping

The part shown in Fig 7 (52100 steel hardened to 61 to 63 HRC) was lapped by three methods for the removal of 0.005

mm (0.0002 in.) of stock to produce a finish of 0.025 m (1 in.) Productivity was as follows:

Method of lapping Pieces per hour

In machine lapping between plates, the two laps, which were of cast iron, were 400 mm (16 in.) in diameter and 75 mm (3 in.) thick The spider fixture was used Rotation speed was 100 rev/min, and an 800-grit abrasive was used in a water vehicle

Lapping of Outer Surfaces of Piston Rings

Special procedures are required for lapping the outer cylindrical surfaces of parts that are considerably greater in diameter than in axial length Piston rings are typical of such parts Lapping is especially necessary for a chromium-plated piston ring because the ring will quickly ruin a cylinder unless the minute chromium nodules are removed

Because of slight variations in machining, rings may have areas that exert low pressure During lapping, material will be removed faster from the high-pressure areas, thus causing a more even distribution of pressure around the ring, removal

of chromium nodules, and smoothing of the surface A specific procedure employed in the lapping of piston rings is described in the following example

Example 5: Lapping Eight Piston Rings Simultaneously

The stacking setup shown in Fig 8 was used for simultaneously lapping eight chromium-plated steel piston rings (hardness: 775 HV) The lap consisted of an outer cylinder that was a solid casting and an inner sleeve that could be replaced when worn out Replacing only the inner sleeve was more economical than replacing the entire cylinder The piston rings were reciprocated in the sleeve at 150 cycles/min by a special machine During each reciprocation, the stack

of piston rings was rotated 45° The abrasive, which was fed in through slots near the center of the sleeve, contained 10%

of 600-grit aluminum oxide mixed with 90% (by weight) of a commercial lapping oil

Fig 8 Setup and fixture for lapping eight piston rings simultaneously

This practice was used for rings 51 to 216 mm (2 to 8 in.) in diameter and 1.6 to 6.4 mm ( to in.) thick Productivity was 2 to 48 rings/min, depending on ring size Metal removal ranged from about 0.025 to 0.038 mm (0.0010

to 0.0015 in.) Size, which was controlled by the number of cycles, was checked by measuring the gap between the ends

of the ring when installed in a gage

Lapping of Crankshafts

Crankshaft journals and pins and a variety of similar cylindrical surfaces are often lapped when they require a finish better than that ordinarily produced by production grinding The pin and journal surfaces on crankshafts, for example, are ground to finishes of 0.63 to 1.40 m (25 to 55 in.), but a finish of 0.10 to 0.20 m (4 to 8 in.) is required for some applications This finer finish has been inexpensively achieved by the method described in the following example, in which all surfaces are lapped in one setup

Example 6: Machine Lapping of Crankshafts