Volume 16 - Machining Part 9 ppsx

Speed and Feed Optimum speed and feed for drilling depend on workpiece material, tool material, depth of hole, design of drill, rigidity of setup, tolerance, and cutting fluid.. Apart f

Drill Design. Many holes can be drilled with more than one type of drill, but discriminating selection may allow closer tolerances For example, if a screw-machine drill can be used, somewhat closer tolerances can be met than with a longer drill, because of the decrease in drill overhang In some applications, a screw-machine drill is used instead of a standard-length drill, thus eliminating the need for bushings Modification of the drill point often results in greater accuracy

Predrilling with a center drill often improves accuracy because the final drill has no chance to walk at the start Walking causes out-of-line drilling, which results in holes that are less accurate in size and roundness

Drilling in steps can improve accuracy, although with some types of equipment it can also reduce productivity For example, to drill a 75 mm (3 in.) deep, 19 mm ( in.) diam hole, it may be advantageous first to drill a 25 mm (1 in.) deep hole with an 18.6 mm ( in.) diam twist drill and then switch to an 18.2 mm ( in.) diam twist drill to drill a 50

mm (2 in.) deep hole This is followed by a 17.9 mm ( in.) diam twist drill to reach the final 75 mm (3 in.) depth desired Finally, a 19 mm ( in.) diam four-flute core drill is used as a roughing reamer to obtain an accurate hole

Drill bushings in fixtures ensure uniformly centered starts, which result in better alignment, and thus improve hole roundness, straightness, and accuracy of location

Rotating the workpiece (provided its size and shape permit) will invariably result in more accurate drilling than rotating the drill while the work remains stationary Machines in which the work is rotated include lathes, turret lathes, and horizontal multiple-station drilling machines A turret lathe also makes it possible to drill a hole in steps, completing

it with a core drill, and then to ream it for still greater accuracy

Speed and Feed

Optimum speed and feed for drilling depend on workpiece material, tool material, depth of hole, design of drill, rigidity of setup, tolerance, and cutting fluid Consequently, it is impossible to recommend speeds and feeds that are applicable under all conditions The nominal speeds and feeds given in Tables 6(a) and 6(b) are useful as starting points in selecting

an optimum combination for a specific job

Table 6(a) Recommended operating parameters for producing holes in a variety of materials with twist drills

Material drilled

Cutting tool material(a)

degrees

Point

style Aluminum and

(15-Light hand 0 Spear

Cast (soft)

(90-165)

Y 14-25 90-118 160-220 80-97

4.6-7.6

25)

Table 6(b) Feed rates for materials listed in Table 6(a)

Feed rate, mm/rev (in./rev), at a drill diameter of:

0.089 (0.0035)

0.114 (0.0045)

0.13 (0.005)

X 0.05

(0.002)

0.089 (0.0035)

0.15 (0.006)

0.216 (0.0085)

0.267 (0.0105)

Y 0.08

(0.003)

0.13 (0.005)

0.20 (0.008)

0.267 (0.0105)

0.317 (0.0125)

Z 0.08

(0.003)

0.15 (0.006)

0.25 (0.010)

0.394 (0.0155)

0.483 (0.0190)

When unfavorable conditions prevail, such as less-than-normal rigidity or restrictions on the use of cutting fluid, slower speeds and lower feed rates than those given in Tables 6(a) and 6(b) may be required Oversize drilling (Fig 28 and 29), increases as speed increases Therefore, drill grinding practice, drilling rigidity, and drill design are closely related to the accuracy that can be obtained at a given speed Feed rate must not exceed that at which chips can be flushed away Clogging of chips decreases accuracy and eventually leads to drill breakage

Gun drills ordinarily have carbide tips, which can withstand greater speeds The usual practice is to operate them at considerably higher speeds but lower feeds than those for high-speed tool steel twist drills This allows gun drills to form thin chips that are more readily flushed away by cutting fluid under pressure Nominal speeds and feeds for the gun drilling of ferrous materials with carbide-tip drills are given in Table 7

Table 7 Recommended starting conditions for gun drilling

Alloy steel to 240 HB Tool steel to 200 HB

mm in kPa psi rev/

6

diameter

Coolant

pressure

Aluminum Gray cast iron to 180 HB Ductile cast iron to 200 HB

mm in kPa psi rev/

8

(a) Maximum allowable unsupported gun drill shank length Length in excess of these values will cause shank

• Aluminum and aluminum alloys: Kerosene, kerosene and lard oil, and soluble oil

• Brass and bronze: Dry Deep holes: kerosene and mineral oil, lard oil, and soluble oil

• Magnesium and magnesium alloys: Mineral lard oil, kerosene, or dry

• Copper: Mineral lard oil and kerosene, soluble oil, or dry

• Monel metal: Mineral lard oil

• Low-carbon steels: Mineral lard oil

• Tough alloy steels: Sulfurized oil

• Steel forgings: Sulfurized oil

• Cast steel: Soluble oil

• Wrought iron: Soluble oil

• High-tensile steels: Soluble oil

• Manganese steel: Dry

• Cast iron: Mineral oil

• Malleable iron: Soluble oil or dry

• Stainless steel: Soluble oil

• Titanium alloys: Soluble oil

• Tool steel: Mineral lard oil

• Abrasives, plastics: Dry

• Fiber, asbestos, wood: Dry

• Hard rubber: Dry

Detailed information on types of cutting fluids and principles of selection and use are described in the article "Metal Cutting and Grinding Fluids" in this Volume

Cutting fluids are used in most drilling applications, except on such materials as cast iron (for which an air jet may be used instead) or when the use of fluids is incompatible with subsequent operations or the end use of the part The use of sulfurized cutting oils is almost mandatory for gun drilling operations because of the more accurate dimensions and smoother finishes that are usually required

Variables Affecting Drill Life

Apart from drill design and material, and rigidity of the setup, the variables that most affect the service lives of drills are speed and feed and the hardness and composition of the work metal

Speed and Feed. Figure 30 relates speed and feed to drill life The curves shown in Fig 30(a) for drilling 4130 and

4340 steels at 341 HB (37 HRC) indicate that:

• Drill life decreased rapidly as cutting speed was increased

• At a given cutting speed, drill life was shortened when the feed rate was increased from 0.05 to 0.13 mm/rev (0.002 to 0.005 in./rev)

Fig 30 Effect of speed and feed on drill life Holes for both series of tests were drilled through 13 mm ( in.)

thick specimens 75 to 102 mm (3 to 4 in.) in diameter, with 6.4 mm ( in.) diam drills (118° included point angle, 29° helix angle, 7° clearance), using a 1:1 mixture of thread-cutting oil and light machine oil as cutting fluid Drills for data in (a) were 102 mm (4 in.) long and made of M2 high-speed tool steel Drills for data in (b), (c), and (d) were 70 mm (2 in.) long, of T15 high-speed tool steel, and ground with crankshaft points The end point of drill life was the breakdown of the drill or a 0.38 mm (0.015 in.) wear land on the drill margin, whichever occurred first

Tests on 4340 steel at 514 HB (52 HRC), as shown in the bar graphs of Fig 30(b), 30(c), and 30(d), indicate trends somewhat different from those observed for the softer steels Maximum drill life was found at the intermediate feed rate

of 0.025 mm/rev (0.001 in./rev), with shorter drill life observed at either a higher or a lower feed rate Only at 0.025 mm/rev (0.001 in./rev) feed did drill life decrease progressively as speed was increased; at both the higher and the lower feed rates, maximum drill life was obtained at a cutting speed of 9.1 m/min (30 sfm)

The use of a light feed reduces cutting temperature and cutting force However, if the feed rate is reduced by half, the area

of chip passing over the cutting edge is doubled As a result, tool wear is likely to increase Therefore, whether or not reducing the feed rate is advantageous depends on whether the lower cutting temperature and lighter cutting force offset the increased area of chip passing over the cutting edge At high cutting speed, for which cutting temperature is a critical factor in drill life, reducing the feed is advantageous At low cutting speed, for which cutting temperature is less critical, the longer chip and the increased rubbing over the cutting edge are likely to offset the advantages of lower feed, and drill life is likely to decrease

Hardness and Composition of Work Metal. The effect of workpiece material on tool life in drilling carbon and low-alloy steels similar in composition and microstructure can be interpreted in terms of hardness The results of one comparison of this type are given in Fig 31 Although these results were affected to some degree by the use of a different tool material for drilling the softest steel, drill life (the amount of metal removed before the development of a predetermined wear land on the drill) was progressively shorter for increasing hardness of the steel being drilled

Fig 31 Effect of workpiece hardness on drill life for 6.4 mm ( in.) (top row) and 13 mm ( in.) (bottom row)

diam holes Factors considered were cutting speed (a), feed rate (b), removal rate (c), and drill life (d) Comparative drilling tests were conducted on three similar steels, each at a different hardness On each steel, the tools and machining conditions used had been previously determined to provide maximum economy in drilling that material Drill life was based on the development of a predetermined amount of wear on the edge

of the drill

The composition of carbon and low-alloy steels is usually of only secondary importance in its direct effects on drill life Effects of practical significance include those of free-cutting additives, differences of 0.10% or more in carbon content, and substantial differences in the content of alloying elements Some of these differences in composition also produce changes in hardness Composition is of primary importance when it is necessary to improve drill life by heat treatment or cold reduction of the work material, or by selection of a more suitable work material

Effect of Workpiece Hardness on Cost

The hardness of the workpiece material exerts a major influence on drilling cost Meaningful comparisons of the effect of hardness on drilling cost can be made for steels that are similar in composition and microstructure

Drilling Cost. Although drilling cost almost always increases with increasing hardness in the range above 35 HRC (330

HB), the reverse effect is observed for low-carbon steel at lower hardness For low-carbon steel, there is usually an optimum hardness range for lowest drilling cost; this range varies considerably according to composition For some low-carbon steels, a heat-treating operation is warranted to increase hardness before drilling

Overall Cost. In considering overall economy of manufacture, cost-per-pound differences among mechanically equivalent steels should be evaluated, along with the differences in cost of machining the steels Even when machining costs for a grade of steel that is difficult to drill are much greater than for a similar grade that is not, the machining costs may be outweighed by the difference in cost per pound of the two steels The heavier the part and the less machining done

on it, the more important material cost is as a factor in total cost

Determination of Optimum Speed and Feed

A wide range of speeds and feeds can be employed to yield acceptable results in the drilling of most materials, particularly those that present no unusual difficulty in machining For materials that are unusually hard, soft and gummy,

abrasive, or otherwise difficult to machine, the range of economical operating conditions is narrow For these materials,

an optimum speed and feed can be determined by making drilling tests to establish the conditions for minimum cost per cubic inch of metal removed

Optimum drilling conditions are determined by changing one variable (speed or feed) while holding the other constant The first step is to select a near-optimum feed Speed is then adjusted in increments to find the most economical speed Selection of the near-optimum feed is based on previous experience After this first series of operations (the speed search), speed is held constant, and feed is varied in increments to verify or correct the initially selected feed rate (the feed search)

The development of a predetermined wear land on the edge of the drill is the criterion for tool life Different speeds and feeds cause wear at different rates Attainment of a predetermined amount of wear, which is ascertained by the use of a hand microscope, marks the point at which each test is stopped

Cost Factors. In such a study, three factors are included in the determination of machining costs: use cost, change cost, and operating cost Operating cost is based on a standard cost per minute for all operations Tool-change cost

tool-is obtained by the use of a specific rate of tool changing assigned to the operation Tool-use cost includes the cost of new tools and the cost of regrinding

Total cost per hole drilled for each test condition is the sum of the following:

• Operating cost times the number of minutes required to drill one hole

• Tool-change cost divided by tool life in holes per tool

• Tool-use cost divided by tool life in holes per tool

Dividing this total by the volume of metal removed per hole gives the drilling cost in dollars per cubic inch of metal removed for each test condition

Testing to Evaluate Drilling Conditions

Laboratory tests are usually conducted to develop and evaluate drilling techniques and drill designs and materials, without concern for machines and fixtures This requires close control over test conditions (especially rigidity) and the provision

of adequate, smooth power to the drill Production tests are used to evaluate drill performance in combination with drilling machines and fixtures Accelerated tests may lead to erroneous conclusions because the wear or failure may be different in practice

Variation Among Drills. There is considerable variation in performance among drills of the same type and even on resharpening a single drill Therefore, enough drills should be tested to allow for this variation, and drills should be sharpened often enough to represent shop practice Sharpening should reproduce the original point of the drill and should include removal of the part of the drill that has metal pickup on the margin or has reverse back taper The web should be thinned to its original dimensions, and the resharpened drill should be free from burns and checks

Criteria for Drill Life. Preliminary testing is often necessary to determine realistic criteria for drill life Workpiece material, cutting fluid, and operating conditions influence the type of failure Useful criteria include drill noise, drill wear, inability to cut, total failure of drill, inaccurate hole size, poor hole finish, burrs, drill breakage, and increase in the amount of torque required for a given drilling operation

Interpretation of Results. Because of variations in drills, test conditions, and work material, a substantial number of tests usually must be made As an illustration, differences of 30% or more between groups of identical drills can be detected reliably with about six drills of each type run through several sharpenings Statistical analysis can indicate whether observed differences are significant

Caution must be exercised in interpreting test results because of the wide discrepancies shown by machining tests in general For example, consider the data shown in Fig 30(a) Results were essentially the same in drilling 4130 and 4340 steel at 0.13 mm/rev (0.005 in./rev) feed, but speed for a given drill life was 40 to 50% higher for 4130 at 0.051 mm/rev (0.002 in./rev) feed It could validly be concluded from these results that this lot of 4130 could be drilled at higher speeds

than 4340 at a feed of about 0.051 mm/rev (0.002 in./rev) However, no conclusion could be drawn about 4130 and 4340 steels in general without additional testing

Drilling Steel Having 48 to 55 HRC Hardness

Successful drilling of steel that has been heat treated to a hardness of 48 to 55 HRC depends mainly on the design of the drill, the rigidity of the machine setup, and the choice of tool material As a rule, improvements in drill design and in machine and workpiece rigidity are of more direct benefit to drilling performance than is a change in tool material A number of other conditions may also affect drill performance, and in certain cases, they may become critical Among these conditions are feed and speed, the efficiency and adaptability of power equipment (portable or stationary), and the accuracy of working surfaces of the drill The following three examples illustrate the effects of operating variables in drilling hardened steel

Example 5: Effect of Nitriding on the Drill Life of a High-Speed Tool Steel Drill

Blind holes, 6.91 mm (0.272 in.) in diameter and 15.75 mm (0.620 in.) deep, were drilled in 4335 steel hardened to 48 to

50 HRC The drills had a life of only one to four holes per sharpening, and a breakage rate of 1 drill for every 16 to 20 holes When the drills were nitrided, drill life was increased to 18 to 20 holes per sharpening, and drill breakage was reduced to 1 drill for every 50 to 60 holes

All drills were standard heavy-duty drills ground from solid heat-treated high-speed steel Each had a split point ground with a positive axial rake of 3 to 4° on the split portion, an included point angle of 135° and a lip relief angle of 5 to 7°, ground flat 0.8 mm ( in.)

Drilling was done in a radial-arm drill press with multiple speeds and automatic feed Speed was 147 rev/min (3.20 m/min, or 10.5 sfm), and feed was 0.05 mm/rev (0.002 in./rev)

Example 6: High-Speed Tool Steel Versus Carbide Drills

For H11 steel at 50 HRC or below, M33, M34, and M36 high-speed tool steel drills of heavy-duty construction gave good results Positive-drive equipment was used, with drill speeds of 7.6 to 9.1 m/min (25 to 30 sfm) and feeds of 0.013 to 0.018 mm/rev (0.0005 to 0.0007 in./rev)

Material harder than 50 HRC required solid-carbide drills with short flutes Speeds of 12 to 15 m/min (40 to 50 sfm) and feeds of 0.013 to 0.025 mm/rev (0.0005 to 0.001 in./ rev) produced satisfactory results The design of the solid-carbide twist drills used for holes of less than 6.4 mm ( in.) diameter in this material is illustrated in Fig 32(a) and 32(b); the design of straight-flute solid-carbide drills, for holes 6.4 mm ( in.) in diameter or larger, is shown in Fig 32(c)

Fig 32 Details of solid-carbide drills used successfully on H11 steel harder than 50 HRC (a) and (b) Twist

drills, for holes less than 6.4 mm ( in.) in diameter (c) Straight-flute drills, for holes 6.4 mm ( in.) in diameter or larger Dimensions given in inches

Example 7: Selection of Carbide Grade

Three grades of carbide (C-1, C-2, and C-3) were tested to determine which was most suitable for drilling H11 steel sheet (5% Cr, 1.5% Mo) heat treated to 54 HRC Two sheets of this material, each 2.10 mm (0.083 in.) thick, were drilled simultaneously with 6.35 mm (0.250 in.) diam carbide-tip drills, without a cutting fluid

The softer 6% Co grade (C-1) failed because of abrasion, excessive burning, and chipping all of which ultimately resulted in breakage The hardest (and therefore most brittle) grade, C-3, which contained only 3% Co, failed by excessive chipping and breaking during drilling The C-3 also chipped and broke in handling

The most satisfactory carbide was the harder of the two 6% Co grades, C-2, which showed only slight wear after testing; this grade was selected for machining on a production basis (Although the C-1 and C-2 grades tested both contained 6%

Co, the C-2 was harder because of its finer grain size, which decreased the size of the softer cobalt lakes between grains.)

The design of these drills is shown in the upper portion of Fig 33; the tool life for each grade of carbide is plotted in the lower part Composition, hardness, and grain size of the three grades of carbide were as follows:

C-2 88.25 5.75 6.0 91.8 1-2 (40-80)

C-3 87.00 4.0 6.00 3.0 92.2 1-3 (40-120)

Fig 33 Top: design of carbide-tip drills used to drill H11 steel sheet at 54 HRC Bottom: comparison of tool life

for the three grades of carbide tested on drills of the above design for performance in drilling 6.35 mm (0.250 in.) diam holes through two 2.11 mm (0.083 in.) thick sheets of this steel Dimensions in figure given in millimeters

Indexable-Insert Drills (Ref 2)

The most important recent advance in drilling technology was the development in the early 1970s of drills with indexable carbide inserts These tools can produce relatively shallow holes from the solid at faster rates and lower cost than high-speed steel twist drills in many applications

Studies have shown that about 60% of all drilling applications in industry are considered to be short holes having depths

up to about three diameters Many of these holes, as well as others up to five times the drill diameter, can be drilled with indexable-insert drills Others are not practical to produce with these tools, because the holes are too small in diameter or because inadequate machines (with respect to speed, power, and rigidity) are employed

The hole diameters that can be produced with indexable-insert drills vary with the tools available from different manufacturers A common range of diameters offered by some manufacturers is 19 to 76 mm ( to 3 in.) The smallest tool commercially available drills holes 16 mm ( in.) and the largest, 127 mm (5 in.) in diameter Some of these tools contain up to six inserts Figure 34 shows two- and four-insert versions

Fig 34 Two types of indexable carbide insert drills (a) Two-insert drill (b) Four-insert drill

Advantages

The major advantages of indexable-insert drills are increased productivity, reduced costs, and better versatility

Increased Productivity. The use of carbide inserts brings drilling close to the machining rates possible with turning and milling The higher cutting speeds possible permit holes to be drilled substantially faster than with high-speed tool steel twist drills and even faster than with carbide spade drills The cycle time on transfer machines and other high-production applications is often determined by the capabilities of the high-speed tool steel drills used With the faster penetration rates of indexable-insert drills often matching the rates of other operations, cycle times can generally be reduced Potential productivity is also increased because the almost flat lead angle of indexable-insert drills results in a shorter feed stroke before cutting, compared to twist or spade drills that have point angles (Fig 35) For example, the feed stroke for a twist drill may be 13 mm ( in.) compared to 2 mm ( in.) for an indexable-insert carbide drill in the case

of a 41.4 mm (1.63 in.) diam hole being drilled to a 6 mm ( in.) depth in high-carbon steel; at a 260 m/ min (860 sfm) speed and a 500 mm/min (19.7 in./min) feed rate, the carbide drill speed is eleven times faster than the high-speed tool steel twist drill (45 mm/min, or 1.8 in./min)

Fig 35 Feed strokes (dimension x) of three drills The indexable-insert carbide drill (a), shown here with a

trigon insert, has a much shorter feed distance than a standard twist drill (b) or a spade drill (c)

Reduced Costs. The use of low-cost inserts with multiple cutting edges eliminates regrinding costs The multiple cutting edges available provide savings from not having to replace the entire tool Indexing the inserts does not change their positions and the tool length, thus any tool resetting costs are eliminated

Versatility. Indexable-insert drills can be used as nonrotating tools for applications on lathes or other machines or as rotating tools on drilling machines, machining centers, and other machine tools The machines used, however, must be rigid, must be in good condition, and must have ample speed and power capabilities Some of the tools have the capability, when used on suitable machines, to perform boring as well as drilling operations For example, mounted on the cross slide of an NC lathe, some tools can be moved radially outward to drill holes larger than the tool diameter or make a boring pass, thus improving the accuracy and finish of the hole Other tools can perform turning, contouring, and facing operations, as well as multiple operations such as drilling, chamfering, and spotfacing

Limitations

The smallest-diameter hole that can be produced with the indexable-insert drills that are commercially available is 16 mm ( in.) The maximum drilling depth is generally two to three times the hole diameter Some tools, however, can drill to four times the diameter, and a few specials can drill to depths over five times the diameter Indexable-insert drills are not precision hole-producing tools, and subsequent operations may be required for improved accuracies and smoother finishes Small pilot holes are not useful and can be detrimental, and the tools cannot be used to enlarge existing holes

Indexable-insert drills require less thrust than twist drills because they have no webs or chisel edges, but they do require more power because of increased metal removal rates Rigid machines in good condition, with adequate speed and power capabilities, and cutting fluid under pressure are necessary to take full advantage of the productive capabilities of these tools Horsepower requirements increase proportionately with the drill diameter (Fig 36) Bench, upright, and radial drilling machines are generally not suitable for use with these tools, because they lack sufficient speed, power, or rigidity Safety guards are required on any machine used for drilling through holes with the workpiece rotating and the drill stationary because the slugs produced can be thrown outward at high velocity

Fig 36 Spindle power requirements for various diameters of indexable-insert carbide drills in three materials

(4140 steel, hard cast iron, and Type 316 stainless) under varying conditions of speed and feed A, 4140 steel,

105 m/min (350 sfm), and 0.18 mm/rev (0.007 in./rev); B, 4140 steel, 90 m/min (300 sfm), and 0.18 mm/rev (0.007 in./rev); C, hard cast iron, 105 m/min (350 sfm), and 0.30 mm/rev (0.012 in./rev); D, type 316 stainless steel, 75 m/min (250 sfm), and 0.13 mm/rev (0.005 in./rev); E, 4140 steel, 60 m/min (200 sfm), and 0.18 mm/rev (0.007 in./rev); F, type 316 stainless steel, 55 m/min (175 sfm), and 0.13 mm/rev (0.005 in./rev)

Indexable-insert drills can be used to drill many materials, but most are not suitable for laminated or stacked materials This is because the disks or slugs produced would be pressed into or welded to the next layer of material and because increased pressures can damage the inserts and possibly the drill body The surfaces of workpieces to be drilled should preferably be flat When using negative-rake inserts, convex surfaces can present problems, and concave surfaces are not generally recommended, because they might throw the drill out of balance Angular surfaces rising more than 1 mm (0.040 in.) in a 50 mm (2 in.) distance and interrupted cuts are also generally not recommended for use with most of these tools There are, however, successful applications with angular starting surfaces and interrupted cuts when positive-rake inserts are used

Tool Design Considerations

Indexable-insert drills of slightly different designs are available from various tool manufacturers Most consist of a hardened alloy steel body (with a straight or a taper shank) held by an adapter attached to the machine spindle Some tools are of one-piece construction and others two-piece, with one of the two pieces being an interchangeable cutting head or nosepiece

The drills have straight or helical flutes or grooves, and internal coolant holes Flute or groove design is critical; sufficient space must be provided for the rapid removal of a large volume of chips, but an adequate body cross section must be maintained for strength and rigidity Coolant enters the tools, usually through tapped openings, and reaches the cutting zones from orifices near the inserts Chips and coolant exit through the external flutes or grooves on the tools In applications in which the tool rotates, an inducer or coolant collar (as with other coolant-fed tools) is required unless coolant is supplied through a hollow spindle on the machine

At the cutting end of each flute or groove, recessed pockets are provided to locate the indexable inserts Depending on the drill diameter and design of the tool, one to four inserts are generally used A few drills, using toolholding cartridges, hold

as many as six inserts Some have a centrally located insert, positioned slightly ahead of the others, for self-centering purposes The inserts are mounted in positions and at attitudes to counteract each other's lateral cutting forces, thus minimizing side loads This is necessary because the tools are not guided by the holes being drilled and because guide bushings are not used

Insert Configurations

Round, square, triangular, diamond, trigon, trochoid, parallelogram, hexagonal, and octagonal carbide inserts are used by different tool manufacturers (Fig 37) The inserts may or may not be the same size, depending on the drill design and diameter Some inserts have dimples, grooves, or special geometries for chip-breaking purposes

Fig 37 Typical insert configurations used in indexable-insert carbide drills (a) Square (b) Trochoid (c) Trigon

The geometry and positioning of the inserts are important to the performance and efficiency of the drills Continuing improvements are being made in insert geometries to ensure constant chip control Depending on the insert shape and application, some drills are equipped with negative-rake inserts, requiring negative-rake placement Others are designed for positive-rake inserts that are placed to provide either a neutral or a positive axial rake angle Positive geometries generally develop less cutting and axial thrust forces and require less horsepower than negative geometries Effective rake

in a negative insert, however, can be higher and can develop lower forces than a positive-rake insert Replaceable anvils under the inserts can help prevent damage to the tools in case of insert breakage

The grade of carbide used for the inserts varies with the application Classification C-5 is extensively used for drilling many steels; C-2 is used for cast irons and nonferrous metals Coated inserts (especially titanium nitride) are also widely employed because they permit higher cutting speeds (to 305 m/min, or 1000 sfm, or more) and are less prone to the formation of built-up edges Inserts made from different grades of carbide are sometimes used in the same tool, depending

on insert position and cutting speed

Inserts are held in the drill pockets by screws or cam-locking pins, eliminating the need for clamps, which would obstruct chip flow It is important that the screws or pins be securely tightened, but not overtightened Loose inserts will cause chatter and possible tool breakage Some indexable-insert drills, especially larger-diameter tools, are equipped with cartridges that reduce costs in case of insert failures because they provide protection for the main body of the tool

Indexable-Insert Drill Applications

Indexable-insert drills are primarily used for producing holes in steels and irons More ductile materials such as aluminum and copper are also drilled with these tools, but chip ejection may be a problem for some applications When soft, ductile, and gummy materials are being drilled, chip control can be a problem The thicker chips produced with neutral or negative-rake inserts tend to pack in the flutes or grooves The tools are not suitable for drilling soft materials such as rubber or plastics

These tools are especially advantageous for medium- and high-production applications on NC and transfer machines They are also used for many low-production applications on manual machines, lathes, and other machine tools if the machines have the required power, speed, and rigidity Table 8 lists suggested cutting for uncoated and titanium nitride coated inserts

Table 8 Recommended cutting parameters for indexable drills with uncoated and titanium-coated carbide inserts

Speed Drill size

Standard length Stub length

Feed Material

mm in m/min sfm m/min sfm mm/rev in./rev

Insert style(a)

0.08- 0.005

0.003-SPGM C-5(b)

36.5 1 -

0.08- 0.006

0.003-SNMG C-5(b)

47.5 1 -

0.10- 0.008

0.08- 0.005

0.003-SPGM C-5(b)

36.5 1 -

0.10- 0.006

0.004-SNMG C-5(b)

47.5 1 -

0.08- 0.008

0.08- 0.005

0.003-SPGM C-5(b)

36.5 1 -

0.10- 0.006

0.004-SNMG C-5(b)

47.5 1 -

0.13- 0.008

0.15- 0.010

0.08- 0.005

0.003-SPGM C-5(b)

Tool steel

36.5 1 -

0.10- 0.006

0.004-SNMG C-5(b)

47.5 1 -

0.13- 0.008

0.15- 0.010

0.006-

SNMM-ND

C-5(b) 19-30

0.10- 0.008

0.004-SPGM C-2(c)

36.5 1 -

0.13- 0.010

0.005-SNMG C-2(c)

47.5 1 -

0.15- 0.012

0.20- 0.015

0.008-

SNMM-ND

C-2(c) 19-30

0.08- 0.004

0.003-SPGM C-2(c)

36.5 1 -

0.10- 0.005

0.004-SNMG C-5(b)

47.5 1 -

0.10- 0.006

0.13- 0.007

0.005-SPGM C-5(b)

36.5 1 -

0.13- 0.007

0.005-SNMG C-5(b)

47.5 1 -

0.13- 0.007

0.003-SPGM C-2(c)

36.5 1 -

32-1

25-35 75-110 25-35 75-110

0.08-0.10

0.004

0.003-SNMG C-2(c)

47.5 1 -

38-1

25-35 75-110 25-35 75-110

0.08-0.10

0.004

0.20- 0.010

0.008-SPGM C-2(c)

36.5 1 -

0.20- 0.012

0.25- 0.015

0.010-SNMP C-2(c)

1 49-75

0.30- 0.020

0.012-SNMP C-2(c)

Source: Metal Cutting Tools, Inc

(a) Inserts are designated by a letter code indicating such specifications as shape, clearance angle, tolerances,

and hole size

Small-Hole Drilling (Microdrilling)

The drilling of small holes (diameters of 0.025 to 3.2 mm, or 0.001 to in.) requires machines, drills, techniques, and operator skills different from those used in conventional drilling

Machines used for drilling small holes are usually bench-mounted and resemble a jeweler's drill press (Fig 2) Alignment of spindle and table at any position should be within 0.013 mm in 152 mm (0.0005 in in 6 in.), or at a radius equal to half the length of the table Runout of the spindle should not exceed 0.0025 mm (0.0001 in.) in any position Spindle speeds are high, ranging from 3000 rev/min, for drilling holes near 1.6 mm ( in.) in diameter to 20,000 rev/min or higher for drilling holes 0.25 mm (0.010 in.) in diameter or smaller

To minimize vibration, the machine is belt-driven by a balanced, vibration-damped motor mounted on a separate stand The required sensitive feed is provided by a balanced crossarm or by a rack and pinion controlled by a knoblike wheel

Larger machines for microdrilling have precision chucks or collets to hold the drill However, for drills smaller than 0.38

mm (0.015 in.) in diameter, a one-piece spindle and drill that revolves in a jeweled V-block is used

Drills larger than 0.20 to 0.25 mm (0.008 to 0.010 in.) in diameter may be of either the twist or the spade type (the latter are also known as pivot-type drills) Drills smaller than this are usually of the spade type, although twist drills have been made in diameters as small as 0.099 mm (0.0039 in.)

Drills smaller than 0.38 mm (0.015 in.) in diameter have their own mandrels When drills are broken, mandrels are returned to the factory for the insertion of new drills Incorporating the drill in a mandrel makes it possible to grind the drill point concentric with the mandrel Drills are available in three classes of tolerance:

Carbon-tungsten special-purpose tool steels such as F2 or F3 are extensively used as materials for small-diameter drills High-speed tool steels are used less often for these drills than for larger drills

Techniques. To produce small holes that are accurate in size and finish, extreme care must be taken in grinding the drill and in the drilling technique (centering and feeding) because:

• Space for chip removal is limited by the greater ratio of web thickness to diameter of small-hole drills

• Pressure on the end of the drill is greater

• Longitudinal and torsional deflections are greater because of the high ratio of length to diameter

In small-hole drilling, the formation of metal powder rather than chips may result This condition may lead to packing, causing the drills to break Packing can be alleviated by frequent clearing of the hole by complete removal of the drill and application of a lubricant Frequent tool withdrawal to clear chips is known as peck drilling One successful technique is the following:

• The drill is withdrawn for chip clearance after the initial penetration has reached a depth no greater than three times the drill diameter (a lesser depth for initial penetration may be required for some materials)

• Another withdrawal is made after penetration has progressed for 1 times the drill diameter beyond the first cut

• The drill is again withdrawn following each succeeding cut to a depth of three-fourths the drill diameter

Speed and Feed. The spindle speeds used for drilling small holes are high compared to those used for ordinary drilling, but surface speeds, for holes down to about 0.20 to 0.25 mm (0.008 to 0.010 in.) in diameter, are not necessarily different The objective is a speed-and-feed combination that produces a true chip If excessive speed is used, it becomes impossible to obtain a feed great enough to form a chip

For drilling holes smaller than about 0.20 mm (0.008 in.) in diameter, the smaller the drill, the lower the speed that is permitted Applications have been reported in which speeds as low as 50 rev/min (0.003 to 0.006 m/min, or 0.01 to 0.02 sfm) were used for drills approaching 0.025 mm (0.001 in.) in diameter

Feeds as great as 0.025 mm/rev (0.001 in./rev) are often used However, hand feeding is normal for drilling extremely small holes, and the rate of feed will vary widely among operators Successful drilling of small holes requires considerable operator skill and feel for feeding the drill In drilling extremely small holes, even an experienced operator will often break one or two drills when starting a work period before this feel is regained

Cutting fluid requirements for drilling small holes differ somewhat from those for large holes, mainly because intermittent withdrawal of the drill disposes of chips and also cools the drill Therefore, in small-hole drilling, the main requirement of a cutting fluid is lubrication The usual practice is to coat the drill with lard oil (or a similar lubricant) at each withdrawal, either by hand brushing or with a lubricant dispenser

Example 8: Microdrilling of a Copper-Nickel-Tellurium Alloy

The 0.635 mm (0.025 in.) hole in the copper-nickel-tellurium alloy part shown in Fig 38 was made with a standard twist drill 0.635 mm (0.025 in.) in diameter, with flute length of 7.9 mm ( in.) The part was drilled in a multiple-operation machine The drill was withdrawn six times to clear the chips Drilling was more difficult than normal for this type of operation because the hole depth and flute length were the same This drilled hole represents about the maximum depth that a standard twist drill of 0.635 mm (0.025 in.) diameter can produce within the 0.076 mm (0.003 in.) total tolerance specified Processing details are given with Fig 38

Speed, at 7875 rev/min, m/min (sfm) 15 (52)

Feed, mm/rev (in./rev) 0.25 (0.001)

Cutting fluid Sulfochlorinated oil (plus fat)

Drilling time per hole, s 18

Drill life per grind, pieces 1200

Fig 38 The ratio (12.5:1) of depth to diameter of hole in this part was the maximum capability of the standard

twist drill used to drill a copper-nickel-tellurium alloy Dimensions in figure given in inches

References

1 L.E Doyle, C.A Keyser, J.L Leach, G.F Schrader, and M.B Singer, Manufacturing Processes and

Materials for Engineers, Prentice-Hall, 1985, p 600

2 Machining, Vol 1, Tool and Manufacturing Engineers Handbook, Society of Manufacturing Engineers, 1983

Reaming

Introduction

REAMING is a machining operation in which a rotary tool takes a light cut to improve the accuracy of a round hole and

to reduce the roughness of the hole surface Most reamers have two or more flutes, either parallel to the tool axis or in a helix, which provide teeth for cutting and grooves for chips

Reaming and boring are related processes, and sometimes their applications overlap (There are even tools identified as reamers by some and as boring tools by others.) Hole diameter and length, or required straightness or tolerance, usually indicate whether reaming or boring is to be used

Process Capabilities

Although steels that range in hardness from about 15 to 30 HRC are the metals most often reamed, the process is widely used for finishing holes in cast iron Reaming is also used for the softest nonferrous metals as well as for steels with a hardness of 52 HRC or higher

Hole Diameter. Most holes reamed are 3.2 to 32 mm ( to 1 in.) in diameter Reamers are commercially available for holes as small as 0.35 mm (0.0135 in.) in diameter, and specially designed reamers are available for 0.1 mm (0.005 in.) diam holes Solid reamers are available for holes up to 50 mm (2 in.) in diameter and are specially made for holes up

to 75 mm (3 in.) in diameter Reamers of other types are available for holes up to 150 mm (6 in.) in diameter

The hole length that can be successfully reamed depends on reamer diameter, method of holding and driving the reamer, and required dimensional accuracy Reamer diameter determines the maximum length of the reamer cutting edge, which affects the length of hole that can be reamed accurately For example, the cutting edge of a 0.35 mm (0.0135 in.) diam reamer may be as long as 19 mm ( in.) (a length-to-diameter ratio of 55:1), but that of a 150 mm (6 in.) diam shell reamer is usually no longer than 150 mm (6 in.) In most applications with standard reamers, the length of the hole being reamed ranges from only slightly longer to considerably shorter than the cutting edge of the reamer However, the length

of the cutting edge does not necessarily limit the length of hole that can be reamed Shank length can be extended to permit the reaming of holes several times longer than the cutting edge of the reamer, but this makes it difficult to guide the reamer and to hold dimensional tolerances

In the horizontal reaming of holes several times longer than the cutting edge of the reamer, the difficulty of maintaining finish and dimensions is sometimes increased by misalignment in the machine This can be minimized by the use of reamers with shorter cutting edges With special tools, such as gun reamers, accuracy can be attained in reaming holes many times longer than the cutting edge

Stock Removal. Most reaming operations are not intended for the removal of large amounts of stock This can usually

be done more economically by other processes, such as drilling, boring, or core drilling When more than 0.5 mm (0.020 in.) on diameter must be removed from a hole less than 50 mm (2 in.) in diameter, special reaming methods or boring is usually considered Special reaming methods may include the use of gun reamers or rough and finish reaming with different types of tools

The practical minimum stock for reaming is greatly influenced by workpiece composition and hardness Because reaming

is a cutting operation, chip formation is required for efficient operation If too little stock is being removed, the reamer will burnish the work rather than cut it; this will result in damage to the reamer and the work surface For soft metals, the removal of 0.2 mm (0.008 in.) on diameter per pass is near the minimum, depending on hole length and tool rigidity For harder metals, because of the difference in chip formation, this amount can be reduced to 0.13 mm (0.005 in.) For the removal of less than 0 13 mm (0.005 in.) of stock, another machining process, such as honing, is usually preferable

Tolerances. Reamers are ground to size to eliminate tool adjustments during production runs Tolerances of 0.025 to 0.075 mm (0.001 to 0.003 in.) on diameter are practical in production reaming Tolerances of less than 0.025 mm (0.001

in.) can be maintained, but this requires closer-than-normal control of reamer dimensions, reaming feed and speed, and all other operating variables For reaming to extremely close tolerances, it is sometimes helpful to reduce the back taper of the reamer slightly and to match the guide bushing with the reamer so that minimum clearance may be obtained

The finish of a reamed hole depends on workpiece hardness, condition of cutting edges, feed, and speed Under optimum conditions, it is possible to obtain finishes of 1.00 m (40 in.) or less However, in the production reaming of annealed steel, 2.50 to 3.20 m (100 to 125 in.) is more common When extremely smooth surfaces are required, methods such as honing or burnishing should be considered

Workpiece Material and Hardness

The hardness of carbon and low-alloy steels has a greater effect than composition on reamability The results of tests to determine the effect of workpiece hardness in reaming are shown in Fig 1 Speed, feed, metal removal rate, and reamer life were compared for reaming holes of 6.4 and 13 mm ( and in.) diameter in three similar low-alloy steels (4130,

4330, and 4340) at 15, 47, and 52 HRC, respectively With increasing hardness of the steel workpiece, the metal removal rate was progressively lower because of the reduced cutting speed and feed rate Life of the M10 high-speed steel reamers used also decreased progressively despite the reduction in speed and feed

Reaming conditions (holes of both diameters)

Type of machine Vertical four spindle

Depth of through holes, mm (in.)

32 (1 )

Stock reamed on diameter, mm (in.)

0.4 ( )

Cutting fluid Sulfurized oil:mineral oil (1:1)

Criterion for tool life Surface finish of 1.0-1.2 m (40-46 in.)

Reamer details 6.4 mm

( in.) holes

( in.) holes Number of flutes 4 8

Land width, mm (in.) 0.175 (0.007) 0.23 (0.009)

Radial rake angle 3° 2°

Radial relief angle 17° 12°

Chamfer angle 42° 42°

Chamfer relief angle 6° 5-6°

Fig 1 Effect of the hardness of a low-alloy steel workpiece on reaming conditions and reamer life (M10

high-speed steel reamers) (a) 6.4 mm ( in.) diam holes (b) 13 mm ( in.) diam holes

Soft metals such as aluminum and brass can be reamed at speeds five to ten times greater than those for annealed steel However, free-cutting low-carbon steel, such as 1113, can be reamed to a smooth finish at nearly the highest speeds On the other hand, low-carbon steel that does not contain additives for free cutting, such as 1015, produces stringier chips, thus yielding a rougher finish and requiring slower speeds As carbon content is increased, even in free-cutting steel, abrasiveness is increased, and this shortens tool life

Machines

Probably every type of machine capable of rotating a tool or a workpiece has been used for reaming Reamers are sometimes driven by hand-held air or electric motors, especially when only a few parts require reaming or when the equipment must be taken to the workpiece Relatively large workpieces are rotated in an engine lathe, and the reamers are fed from the compound rest or the tailstock of the lathe Most production reaming, however, is done in drilling machines

or as one machining step in a turret lathe or other multiple-operation machine Machines having automatic feed can hold close tolerances more consistently than handfed machines The various machines used for production reaming are described in the article "Drilling" in this Volume

Selection. In many applications, reaming is supplementary to, and performed in the same sequence with, other operations, such as drilling Under these conditions, the machine is selected mainly for the primary operation

For the production reaming of holes less than 32 mm (1 in.) in diameter, machines that rotate the tool and hold the workpiece stationary (drill presses, for example) are usually the most practical and economical Guide bushings can be used in these machines to maintain tolerances of 0.075 mm (0.003 in.) or less for holes that are several diameters long For maximum accuracy, however, it is preferable, if size and shape of the workpiece permit, to rotate the work and hold the tool stationary This also applies to the reaming of holes that are considerably larger than 32 mm (1 in.) in diameter

or that have a length-to-diameter ratio of more than about 8:1

Size and shape of the workpiece are often major factors in the choice of a machine for reaming and for drilling or other operations that precede reaming In many cases, although only relatively small holes are to be reamed, the workpiece is too large or heavy to be rotated in machines such as turret lathes In other cases, workpiece size or weight may allow rotation, but asymmetric shape makes rotation impractical For either condition, the reamer must be rotated in machines such as drill presses or boring mills Relatively small holes in extremely large workpieces are often reamed by portable, hand-operated machines

Reamer Materials

Hand reamers are usually made of a carbon or low-alloy tool steel, such as W1 or O1, hardened to 62 HRC or higher Reamers for machine operation are made of either high-speed steel or a lower-alloy tool steel for the shank, with carbide inserts for the cutting edges The high-speed steels most used for reamers include T1, M2, M7, and M10 For reaming especially hard or abrasive metals, high-speed steels with a higher vanadium content, such as T3, T15, M3, or M4, are often used because they give longer life than the lower-alloy steels

Because the load imposed on the tool in reaming is usually far less than in drilling, reamers require less toughness than drills Instead, reamers should be of maximum hardness (65 HRC or higher) to obtain optimum surface finish and tool life

Many standard and special reamers are made of solid carbide or contain carbide inserts Although more expensive than high-speed steel, carbide will often outlast it ten times or more when reaming steel at near-optimum hardness ( 20 HRC) Because of the longer life of carbide reamers, they are preferred for use on steel harder than 40 HRC

For the efficient use of carbide, maximum rigidity is essential in the machine, reamer, and workpiece Even with a machine in first-class condition, if the unguided or unsupported length of the reamer is more than six times its diameter, the use of carbide becomes questionable If chatter develops, the life of carbide tools will be markedly shortened If chatter is likely to occur, high-speed steel reamers should be used

Reamer Design

The design of a typical straight-flute solid reamer is shown in Fig 2 The cutting angles and other tool details given in the accompanying table usually are not appreciably modified for reaming different work metals, except that the chamfer relief angle is generally increased to 12 to 15° for soft metals such as aluminum

Margin, mm (in.)

1.6 ( ) Chamfer length, mm (in.)

1.6 ( )

Chamfer angle 45°

Chamfer relief angle 6-9°

Radial rake angle 7°

Fig 2 Typical design of a straight-flute solid reamer Details in the table are for reamers 13 to 50 mm ( to 2

in.) in diameter "Actual size" in illustration refers to the actual measured diameter of a reamer, which is usually slightly greater than the nominal size to allow for wear

Sometimes, however, minor changes from the typical values shown in Fig 2 can improve results For example, the normal 45° chamfer angle at the lead end of a reamer is sometimes modified to obtain a better finish on the reamed surface

The eight flutes shown in Fig 2 are typical of a 25 mm (1 in.) diam reamer Fewer flutes are used in smaller reamers, and more are used for larger ones (50 mm, or 2 in., diam reamers usually have 12 or more flutes)

If a reamer has too many flutes, it will not provide enough space for chips If it has too few, it is likely to chatter, especially if it is a straight-flute reamer Other possible considerations in choosing the number of flutes are discussed in the following two examples

Example 1: Six Versus Ten Flutes

Changing from a six-flute chucking reamer to one with ten flutes increased tool life by nearly 35% Comparative results from 12 production runs with each type of reamer were:

Pieces per grind (average) 1,052 1,417

Pieces per tool 12,624 17,004

Both types of reamers were operated at a speed of 30 m/min (100 sfm) and a feed of 0.74 mm/rev (0.029 in./rev)

Example 2: Revised Reamer Design to Meet Close Tolerances

Close tolerance (±0.01 mm, or ±0.0005 in.) was specified on the diameter of the flat-bottom blind hole reamed to a depth

of 2.00/1.97 mm (0.786/0.776 in.) in the alloy 377 forging shown at the top of Fig 3 Parts were rejected because the eight-flute reamer originally used (lower left, Fig 3) was forced off center in bottoming out, causing out-of-roundness at the base of the hole

Fig 3 Change from eight-flute reamer (bottom left) to single-flute reamer with wear strips (bottom right) to

eliminate out-of-roundness in reaming the blind hole in the forging shown at top Dimensions given in inches Workpiece hardness: 45 HRB

Changing to a single-flute reamer (lower right, Fig 3) and incorporating two wear strips to prevent the tool from being forced off center when bottoming out resulted in parts meeting required tolerance Both reamers had carbide cutting edges and the same speed and feed

• Amount of stock removed

• Type of fixturing, when used

• Accuracy and finish requirements

Hole Diameter. For holes more than 50 mm (2 in.) in diameter, solid reamers made of high-speed steel are seldom used, mainly because they would be too expensive Shell reamers are usually a better choice for holes of this size

Hole Configuration. When holes to be reamed have keyways or other irregularities, spiral-flute reamers are preferred This is because straight flutes may fail to bridge these irregularities, thus causing chatter

Hole Length. When hole length is not more than two diameters, several types of reamers are suitable However, as the length-to-diameter ratio increases, so does the problem of maintaining accuracy Guide bushings and pilots often solve the problem, but when accuracy must be maintained in long holes, special reamers are required

Amount of Stock Removed. When large amounts of stock are to be removed, a solid or a special reamer is usually

preferred Shell reamers are not suited to the removal of large amounts of stock In some heavy-removal applications, reaming is done in two stages, using a shell reamer for the second stage

Type of fixturing (when used) determines whether or not a piloted reamer is the best choice (see the section "Bushings and Fixtures" in this article)

Accuracy and finish requirements are related to several other factors For greater accuracy, the workpiece should

be rotated, if possible Therefore, the reamer shank must fit the chosen machine Accuracy depends on the rigidity of the setup and on the method of guiding the reamer Self-guiding reamers are usually chosen for greater accuracy and finer finish in reaming long holes

Production Quantity. For reaming a few pieces that can be produced with a single sharpening of the reamer, the simplest type is the logical choice For long production runs, adjustable types should be considered to minimize sharpening and downtime

Cost. Reamer cost must be considered from three related standpoints: initial cost, maintenance cost, and salvage value High initial cost is often warranted because of low maintenance cost; for example, reamers with inserted carbide blades

cost more than solid steel reamers, but may finish ten times as many holes per grind In addition, large reamers made of solid high-speed steel may seem excessively expensive, but their salvage value is high

Salvage Value. The amount realized from the sale of a worn-out high-speed steel reamer is small compared to its original cost It is common practice, therefore, to rework a worn-out solid reamer to a smaller size rather than sell it for scrap The economy of reworking depends mainly on whether or not the necessary equipment is available; the advisability

of buying such equipment depends on the amount of tool rework to be done Worn-out reamers can sometimes be sold to tool shops where they will be reworked for resale If this is done, the salvage value will be greater than if the reamers are sold for scrap

The extent to which a reamer can be reworked is also governed by the reamer design A large reamer can be reworked progressively to smaller and smaller sizes, but this practice is limited by reamer design The limit of decrease in reamer diameter by reworking is usually governed by depth of flute When the diameter is so small that a further decrease would require deepening of the flutes, the reamer is usually scrapped

Types of Reamers

Standard reamers include these principal types:

• Straight-flute chucking reamers

• Spiral-flute chucking reamers

Applications of Straight-Flute Chucking Reamers

Straight-flute chucking reamers are general-purpose solid reamers designed for use in various machines, such as drill presses, turret lathes, and automatic bar or chucking machines They are available with taper or straight shanks (Fig 4) and therefore can be held in collets or split bushings, or by setscrews These reamers are most readily available in diameters of 1.2 to 38 mm ( to 1 in.), in steps of 0.4 mm ( in.); they are seldom made in other sizes

Fig 4 Straight-flute chucking reamers

Straight-flute chucking reamers are normally pointed with a 45° chamfer and are suited to reaming almost all metals However, keyways or other irregularities in holes, tolerance requirements, blind holes, amount of stock that must be removed, or workpiece hardness may demand the use of reamers other than this general-purpose type

A jobber's reamer is a modified straight-flute chucking reamer The primary difference between the two is that the flutes

of the jobber's reamer are about twice as long in proportion to overall length

Applications of Spiral-Flute Chucking Reamers

Spiral-flute chucking reamers differ from straight-flute reamers only in that their flutes are milled in a helix They are used in the same machines and are available (with straight or taper shanks) in the same sizes as straight-flute reamers Spiral-flute reamers cut with a free-reaming action and are used for the more difficult-to-ream materials A typical spiral-flute chucking reamer is shown in Fig 5(a)

Fig 5 Redesign of spiral-flute reamer for keywayed hole Surface roughness of 2.3 m (90 in.), obtained with

conventional spiral-flute reamer (a) in reaming keywayed hole (b), was reduced to 1.25 m (50 in.) by modifying lead angle as in (c) Dimensions given in inches

Spiral-flute reamers are better than straight-flute reamers for reaming holes that have irregularities such as keyways (Fig 5b) The spiral cutting edges bridge these irregularities, and this minimizes chatter, surface roughness, and size variation and prolongs reamer life

Applications of End-Cutting Reamers

An end-cutting reamer, which may have either straight or spiral flutes, has no chamfer on the end for use as a lead; instead, the end has cutting edges at right angles to the reamer axis (Fig 6a) In this respect, end-cutting reamers resemble end mills

Fig 6 End-cutting reamers (a) A common type of end-cutting reamer used for finishing blind holes (b) When

guided in a bushing, an end-cutting reamer can correct dimensional deviations in through holes Dimensions given in inches

End-cutting reamers are used for finishing blind holes that must have little or no radius at the bottom A more important application is the correction of deviations from parallelism in drilled through holes A reamer having a chamfered end for

a lead will usually follow the hole already formed An end-cutting reamer, when guided by a bushing (Fig 6b), can correct out-of-parallelism by several hundredths of a millimeter

The main disadvantage of end-cutting reamers is that they produce comparatively rough surfaces When these reamers must be used (as for correcting hole deviations), they are usually used as roughing reamers, and a conventional reamer is used for finishing

Applications of Adjustable Reamers

Although a number of different types of reamers are adjustable (including floating-blade reamers and expandable shell reamers), the term adjustable reamer is generally used to refer only to a limited number of types Two of the more common types are inserted-blade and expanding-pin adjustable reamers

Inserted-blade reamers, which are made with and without adjustment for size, are toolholders in which slots are milled to receive inserted flat blades In the adjustable type (Fig 7), the blades are slid in angled slots by an adjusting nut

to change cutting diameter The adjusted blades are secured by a locknut and setscrews