Volume 16 - Machining Part 15 doc

Superabrasive Wheel Bonds Although conventional abrasive grinding wheels are limited to vitrified and organic bonds, superabrasive grinding wheels can utilize metal and electroplated bo

Figure 18(a) shows a turning operation on a lathe The work rotates at a certain speed, VC, and the tool has a depth of cut (DOC) and axial feed per revolution along the part Figure 18(b) shows how these are related The three forces acting on the tool are shown in Fig 18(a):

• Cutting force, FC

• The force radial or perpendicular to the work surface, FR

• Axially, the feed force, FF

Figure 18(c) shows how these three forces are resolved into FN, a normal force Drilling, boring, and milling operations can be similarly modeled

Fig 18 Nomenclature for turning operations on a lathe (a) Schematic of workpiece and tool setup (b) Relation

of depth of cut to axial feed per revolution and workpiece speed, VC (c) Vector diagrams of the cutting (FC),

radial (FR), and feed-force (FF) components that form the resultant normal force (FN)

Figure 19 shows an external grinding system There are two forces: FN in the normal direction and FT in the tangential direction As the slide is fed into the part at the rate F, it induces FN, which causes the workpiece to be ground at a radial rate W and the wheel to wear at a radial rate S

Fig 19 Schematic of an external grinding system The terms in the schematic are used to calculate the metal

removal rate (ZW), the wheel wear rate (ZS ), and the power consumption

In Fig 18 and 19, Z is a volumetric removal rate In cutting, Z is the metal removal rate calculated as shown in Fig 18(b) For grinding, ZW is the metal removal rate, and ZS is the wheel wear rate In both cases, the tangential force, FC or FT,

multiplied by the work speed in cutting, VC, or wheel speed in grinding, VS, is the power used

System Graphs

Turning. Figure 20 shows turning data on 4130 steel tubing with four tools having different rake angles Figure 20(a) plots metal removal rate against the normal force, and four linear relationships describe the slope The tool with the 45° rake had a small threshold force and the steepest slope, while the 25° rake tool had the largest threshold force and the

shallowest metal removal parameter (MRP) slope (metal removal rate per unit force) This means that for any Z value

(any horizontal line from the vertical axis) the 45° tool would always use the least force and that the force would increase

as the rake angle is reduced to 40, 35, and 25°, which would always use the highest force

Fig 20 Turning data for 4130 steel tubing using four tool bits having different rake angles at VC of 0.45 m/s (90 sfm) (a) Metal removal rate plotted against normal force yields MRP slope (mm 3 /s, kgf): A, 36.3; B, 22.0;

C, 13.7; D, 5.73 (b) Power plotted against metal removal rate yields SP slope (W, s/mm 3 ): A, 0.846; B, 0.98;

C, 1.037; D, 1.47

Figure 20(b) plots power against metal removal rate Again, there are four linear relationships, with the slopes designated specific power, which is the power required to cut at some removal rate The worst tool for cutting efficiency, the 25°

rake, used the most power at any Z value; the best cutting tool, the 45° rake, used the least power

This method provides two values, the metal removal parameter and the specific power, which describe the cutting and power requirements of each system Usually, for cutting, the normal forces are ignored (but sometimes measured); each power is divided by its own metal removal rate, and different values of power/metal removal rate are obtained from five individual tests These values are usually plotted against metal removal rate, showing that specific energy decreases with higher removal rates In fact, Fig 20(b) would simply show that a threshold power exists If there were no threshold power, all five power/metal removal rate values would be nearly identical

Figure 21 shows graphs of the hard turning of 58 HRC high-speed tool steel with a cubic boron nitride tool Again, there are two numbers describing this system: the MRP slope (Fig 21a) and the SP slope (Fig 21b)

Fig 21 Turning data for 58 HRC high-speed tool steel cut with a CBN tool having a -6° rake Depth of cut was

2 mm (0.08 in.), and VC was 1.33 m/s (260 sfm) (a) Metal removal rate plotted against normal force yields the metal removal parameter of 7.0 mm 3 /s, kgf (b) Power consumption plotted against metal removal rate yields the specific power of 2.7 W, s/mm 3

External Grinding. Figure 22 shows external grinding results on soft steel using CBN-electroplated wheels (one layer

of abrasive on a steel hub) The 36-grit abrasive wheel cut freer (higher MRP slope) and used less power (shallower SP slope) than the 80-grit abrasive wheel

Fig 22 External cylindrical grinding of 4150 steel at 23 HRC using CBN-electroplated wheels An oil coolant was

used with VS of 57 m/s (11,200 sfm) Wheel grit sizes: A, 36 grit; B, 80 grit (a) Workpiece metal removal rate plotted against normal force to obtain metal removal parameter (b) Power consumption plotted against

workpiece metal removal rate to obtain specific power

These and other tests, including turning with negative-rake tools to simulate grinding, are shown in Fig 23, which is a plot of the SP slope versus the MRP slope The best direction on Fig 23 is down (low power) and to the right (low normal force) All cutting results are in the best direction in the lower-right section, but the negative-rake cutting (simulating grinding) is in the upper-left or worst position The upper-right section, which offers higher power but low normal force, contains the grinding results, although there was a 6:1 difference between the worst and the best grinding results This means that ordinary cutting uses power much more efficiently than grinding, except where negative-rake tools are used Therefore, the low-power aspects of cutting influence the ability to send a chip sliding up an inclined tool plane (positive-rake tools) However, cutting uses normal forces as high as those in grinding because both sets of data are generally in the same area on the right of the graph This means that the normal or deflection forces in cutting can be as large as those in grinding There was an 8:1 change in normal force in both the cutting and grinding data

Fig 23 Specific power plotted against metal removal parameter to forecast optimum operating conditions for

turning and grinding CCC, ceramic-coated carbide

References

1 R.P Lindsay, The Effects of Grinding Fluids on the Performance of CBN and a New High Alumina

Abrasive, Japan Society of Precision Engineers, 1987

2 R.S Hahn and R.P Lindsay, Principles of Grinding, Mach Mag., July-Nov 1971

3 R Snoeys and J Peters, The Significance of Chip Thickness in Grinding, Ann CIRP, Vol 23 (No 2), 1974, p

227-237

4 G Pahlitzsch and R Schmitt, Abrichten Von Schleifscheiben Mit Diamantstucken Rollen, Ann CIRP, Vol

17 (No 2), 1969

5 R.S Hahn and R.P Lindsay, "The Production of Fine Surface Finishes While Maintaining Good Surface Integrity by Grinding," Paper presented at the International Grinding Conference, Carnegie-Mellon University, 1972

6 R Snoeys, M Maris, and J Peters, Thermally Induced Damage in Grinding, Ann CIRP, Vol 27 (No 1),

1978, p 571-581; C.P Bhateja and R.P Lindsay, Ed., Grinding: Theory, Techniques and Troubleshooting,

Society of Manufacturing Engineers, 1982

Selected References

• J.R Besse, "Practical Creep Feed Grinding," Paper MR87-820 SME, Society of Manufacturing

Engineers, 1987

• Injury in Ground Surfaces, Norton Company, 1973

• R Komanduri, Some Aspects of Machining With Negative Rake Tools Simulating Grinding, Int J

Mach Tool Des Res., Vol 11, 1971, p 223-233

• R Komanduri, W Konig, and H Tonshoff, Machining of Hard Materials, Ann CIRP, 1984

• R.P Lindsay, "On the Metal Removal and Wheel Removal Parameters Surface Finish, Geometry and

Thermal Damage in Precision Grinding," PhD thesis, Worcester Polytechnic Institute, 1971

• R.P Lindsay, "On the Surface Finish Metal Removal Relationship in Precision Grinding," ASME

Paper 72-WA/Prod-13, American Society of Mechanical Engineers, 1972

• R.P Lindsay, "Sparkout Behavior in Precision Grinding," Paper 72-205, Society of Manufacturing

Engineers, 1972

• R.P Lindsay, The Effect of Wheelwear Rate on the Grinding Performance of Three Wheel Grades,

Ann CIRP, Vol 32 (No 1), 1983, p 247-249

• R.P Lindsay, "The Effect of Contact Time on Forces, Power and Metal Removal Rate in Precision

Grinding," Paper presented at the International Grinding Conference, Lake Geneva, WI, Society of Manufacturing Engineers, 1984

• R.P Lindsay, System Parameters for Cutting and Grinding, in Proceedings of the Ninth Annual

Conference on Composites and Advanced Ceramic Materials, American Ceramic Society, 1985

• S Malkin and N Joseph, Minimum Energy in Abrasive Processes, Wear Mag., Vol 32, 1975, p

15-23

• R.F Pugh and R.F Pohl, "High-Speed Metal Removal," Special Publication ARLOD-SP-82004,

U.S Army Armament Research and Development Command, 1982

Grinding Equipment and Processes

William N Ault, Norton Company

Introduction

METAL IS REMOVED from the workpiece by the mechanical action of irregularly shaped abrasive grains in all grinding operations This article will discuss grinding wheels and disks, coated abrasives, and grinding machines and processes These processes include:

Grinding Wheels and Disks

In their simplest form, grinding wheels can be thought of as multitooth cutters They consist of three primary components:

• Abrasive (the cutting tool)

• Bond (the toolholder)

• Porosity or air for chip clearance and/or the introduction of coolant

Each of these components has a profound effect on the grinding process

Standard Marking Systems for Grinding Wheels. Abrasives can be classified as conventional abrasives or superabrasives (see the article "Superabrasives" in this Volume) Figure 1 shows the standard marking system for conventional abrasive products (aluminum oxide or silicon carbide abrasives) Figure 2 shows the standard marking system for superabrasive products (diamond or cubic boron nitride, CBN, abrasives) Although standard marking systems are available, many parts of the markings have no standard of measurement

Fig 1 Standard marking system for conventional aluminum oxide and silicon carbide abrasive grinding wheels

Fig 2 Standard marking system for diamond (a) and cubic boron nitride (b) superabrasive grinding wheels

Abrasive Type and Grit Size. In both marking systems, the first two components of the marking deal with the abrasive type The second component defines the chemistry of the abrasive, while the first defines the specific type of that abrasive The second two components deal with the size of the abrasive particle The third position defines the sieve spacing to which the particle corresponds In other words, the grit size is roughly equal to the linear holes per inch of a sieve that the particle would just pass through A 60-grit particle, for example, would pass through a 56-mesh screen but would be caught on a 64-mesh screen Table 1 lists the mean particle sizes for various grit sizes The grit size varies indirectly with the particle size The fourth position further describes the particle size distribution by defining the combination of grit sizes that has been used to manufacture the grinding wheel There is no industry standard for grit size combination

Table 1 Mean particle sizes for grits used in conventional abrasive grinding wheels

Particle size

(mean) Grit size

Note: Grit size varies indirectly with particle size

Bond Designation and Grain Spacing. The latter part of the marking deals mainly with the bond and the spacing of the grain in the bond For conventional abrasive wheels, the fifth, sixth, and seventh positions are the bond hardness (or the amount of bond), the porosity or grain spacing, and the bond type For superabrasive wheels, those positions are again the bond hardness, the concentration or the amount of grain in the abrasive section (and actually, by inference, the grain spacing), and the bond type

bond and the letter Z being hard and holding the abrasive tightly into the grinding wheel The definition of what constitutes a given grade letter for hardness or harshness of the grinding action varies by bond and by manufacturer because there are no industry standards

in superabrasive wheels other than the relative scales In conventional abrasive wheels, low structure numbers are dense and have little porosity (consider that the number is the distance between the abrasive grains), while high numbers denote porous products In superabrasive wheels, higher concentrations have more superabrasive particles

conventional and superabrasive marking systems further defines the bond type and is vendor specific In superabrasive product markings, the ninth position denotes abrasive layer thickness Superabrasive wheels often contain thin rims of expensive abrasive in a bond matrix or a nonabrasive preform or holder

Abrasives

In addition to particle or grit size, abrasives have a number of properties that determine their efficacy in the grinding process These properties include chemistry, crystal structure, hardness, durability, friability, and sharpness

Properties of Abrasives

The chemistry of an abrasive can affect its ability to cut at grinding interface temperatures Diamond and silicon carbide are harder than aluminum oxide, but when steel is ground under high pressures, a chemical reaction occurs that degrades these abrasives compared to the relatively chemically inert aluminum oxide In a different vein, the chemical purity of an abrasive is often an indicator of crystal structure

Crystal Structure. There are basically three types of abrasive particle: monocrystalline, multicrystalline, and microcrystalline

Multicrystalline grains, usually made up of two to ten crystals, vary greatly with regard to durability and sharpness These abrasives fracture along crystal boundaries (often catastrophically) and along crystal planes

grinding, and they fracture along crystal boundaries In some applications, microcrystalline abrasives may have more usable volume than mono- or multicrystalline grains before being shed by the bond

Hardness. Figure 3 shows the relative hardness of various abrasives on the Knoop hardness scale Hardness is an advantage; it is inefficient to abrade a material with an abrasive that is not significantly harder than the material The

ability of an abrasive wheel to grind a material is normally measured by the G ratio (see the article "Principles of

Grinding" in this Volume), usually defined as the volume of metal removed per volume of wheel used Under optimum

conditions, superabrasive wheels, primarily because of their hardness, will yield G ratios hundreds of times larger than

those of conventional abrasive products

Fig 3 Knoop hardness ratings of various abrasives

Durability. Durable grains tend to withstand heavy grinding pressures without catastrophic wear Under light grinding pressures, they tend to dull, drawing higher power and giving better surface finishes unless there is vibration due to lack

of material penetration

Friability. Friable grains fracture to expose new sharp cutting points and may do so a number of times before the bond sheds the grain (Fig 4) Friable grains tend to remain sharp and tend to draw lower power, often giving rougher finishes because they do not dull as readily Very friable grains may not be efficient at high power levels and heavy grinding pressures because of premature wear With regard to conventional abrasives, friable grains are normally used on heat-sensitive and hardened steels to ensure material penetration at relatively low power and low frictional heat levels in precision grinding

Fig 4 Abrasive wear in a grinding wheel due to attrition and fracture (a) Attrition deteriorates abrasive grains

by the loss of fine particles This flattens and dulls the edges (b) Fracture deteriorates abrasive grains by the breaking away of relatively large pieces of abrasive crystals from the wheel surface

Grain sharpness is a function of crystal structure and the inherent ability of a given particle to cut prior to wear Grain sharpness can affect abrasives selection, particularly in such extremely low pressure applications as honing or superfinishing, in which there may not be sufficient force to fracture the abrasive particle

Types of Conventional Abrasives

The three prevalent types of conventional abrasives are aluminum oxide, silicon carbide, and zirconia alumina

Aluminum Oxides. A number of aluminum oxides have been developed for grinding applications Aluminum oxides are divided into two initial subgroups: fused and unfused

monocrystalline, and specialty abrasives

Dark aluminum oxides tend to be less pure, more durable, less friable, and multicrystalline They can also be mulled to a dull but even more durable shape They are normally used in medium- to high-pressure, relatively heat insensitive operations on medium-to-soft materials

White aluminum oxides are the most pure of the fused aluminas They are multicrystalline, very friable, not durable, and relatively sharp They are used in heat-sensitive operations on hard, ferrous materials

Monocrystalline fused aluminum oxides tend to combine a level of durability and friability This makes them efficient in medium-pressure heat-sensitive operations on a variety of ferrous materials

Specialty abrasives are variations of the above types They can be blends of dark and white aluminum oxides, coated dark alumina (which tends to increase its impact resistance), or chemically treated white aluminas (which tend to

ceramic-be less friable and more durable)

green crushing Therefore, they can be harder than fused aluminas that have been crushed after furnacing

A significant type of unfused aluminum oxide abrasive is seeded gel alumina abrasive Seeded gel is the purest of the aluminum oxides and the hardest ( 2150 HK) It is also durable, friable, and inherently sharp

Seeded gel is made by a ceramic process in which submicron particles are sintered to form microcrystalline abrasive grit particles A 60-grit particle of seeded gel contains billions of individual crystals Seeded gel is purer, harder (because it is not crushed after sintering), and maintains its sharpness longer than fused aluminum oxide

Seeded gel vitrified grinding wheels were introduced in 1987 and were commercialized in 1988 They have demonstrated the greatest utility for difficult-to-grind materials in which tight tolerances and no metallurgical damage are specified Typical optimized results are 3 to 5 times the life and 1.5 to 2 times the cut rate of fused aluminum oxides For the

precision grinding of difficult-to-grind steels and alloys, seeded gel will grind at higher pressures and infeeds than fused

aluminum oxides, without metallurgical damage and at significantly higher G ratios

Silicon carbide is manufactured in two purities: black or green

harder and sharper than aluminum oxide, but are less impact resistant Black silicon carbide tends to be efficient for grinding soft nonferrous materials; in such applications, its inherent sharpness inhibits heat generation

and/or materials that tend to fracture rather than form chips in the grinding process, such as ferrous carbides and other relatively soft ceramics

Zirconia aluminas are distinguished by their extremely high impact resistance These alloyed abrasives are of two chemistries: a eutectic ( 44% Zr2O) alloy and a 25% Zr2O alloy The eutectic alloy has some friability and is the sharper

of the two zirconia aluminas The 25% Zr alloy is the more durable and has a slightly higher impact resistance The zirconia aluminas are normally used in high-pressure, high stock removal operations on ferrous materials and are normally not associated with precision grinding

Types of Superabrasives

As previously noted, superabrasives consist of diamond and CBN Diamond has a relatively low impact resistance and is therefore used for grinding such materials as carbides, glass, and ceramics that fracture rather than chip when ground Cubic boron nitride has a high impact resistance and exhibits optimum results in the grinding of hard ferrous materials (>50 HRC) Superabrasives and their applications are discussed in the article "Superabrasives" in this Section

Abrasive Wheel Bonds

The properties of bonds that are important to the grinding process include the way in which the bond holds the abrasive, the rigidity and/or flexibility of the bond, the wear mechanism of the bond, and the inherent bursting strength or rotational strength of the bond under stress (Grinding wheels should never be used at speeds higher than the rated speed shown on the grinding wheel.)

Conventional abrasive grinding wheels are held together by two types of bond: vitrified and organic Organic bonds are further divided into resin bond (and its subgroup plastic bond), rubber, and shellac In addition to vitrified and resin bonds, superabrasive wheels are available in metal and electroplated bonds

Conventional Abrasive Wheel Bonds

Vitrified bonds are chemical bonds that alloy with the abrasive at the abrasive/bond interface This rigid, glassy bond family is relatively inflexible and wears as a result of pressure fracturing the bond As the abrasive dulls and the power consumption needed to remove a chip increases, the bond fractures under the higher pressure Because of the rigidity of vitrified bonds, the abrasive tends to act sharper than in flexible bonds, causing easier chip formation but rougher surface finishes Vitrified bonds do not have high impact resistance and should not be used in high-impact, heavy-pressure operations such as foundry snagging or steel conditioning Vitrified wheels are typically used at 25 to 33 m/s (5000 to

6500 sfm) and can be run at more than 43 m/s (8500 sfm) only on specially designed machines and applications

Organic bonds are physical bonds that surround the grain to hold it in the wheel They tend to break down as the

abrasives dull and frictional heat increases

Resin bonds vary in rigidity from some of the more brittle phenolic or bakelite bonds to the more flexible epoxy or other plastic bonds Resin bonds can be reinforced with fiberglass layers to increase the rigidity of the wheels Some resin wheels can be operated at up to 81 m/s (16,000 sfm) Resin bonds are used in most rough grinding applications, including floorstand, portable snagging, weld grinding, and most cutoff applications In precision grinding, resin bonds are used in three situations: where system flexibility would break down a vitrified wheel (such as steel mill roll grinding and most centerless-bar grinding applications), where wheel geometry dictates some flexibility (such as the spiral fluting of drills), and for generating fine and cosmetic finishes (for example, hypodermic needle grinding)

Rubber bonds are the most flexible of the organic bonds The use of rubber bonds is decreasing because of the development of resin bond technology, particularly plastic bonds Rubber bonds have historically been used for polishing applications, wet cutoff operations, and centerless regulating or feed wheels

cutoff applications in which burn-free cuts were essential, for some heat-sensitive roll grinding applications, and for very fine grit polishing operations in which geometry is not critical and stock removal is minimal

Superabrasive Wheel Bonds

Although conventional abrasive grinding wheels are limited to vitrified and organic bonds, superabrasive grinding wheels can utilize metal and electroplated bonds in addition to vitrified and organic bonds

Metal bonds are used in superabrasive wheels primarily in ceramic and glass grinding operations, in which their tendency to wear by abrasion is an advantage Metal bonds range from relatively soft bronze bonds to carbide bonds In particular, they are used on materials that do not generate long chips in the grinding process

Electroplated superabrasive wheels consist of a single layer of abrasive plated to a metal preform These wheels are used for cutting exotic materials (such as fiberglass), for form grinding in which machinery limitations do not allow regeneration of the form in the wheel, and for various applications in which the inherent sharpness of electroplated wheels

is the key to rapid stock removal

Abrasive Wheel Porosity

Porosity or the grain spacing in grinding wheels serves two major functions: coolant transfer and chip clearance Heat transfer, chip removal, and lubricity are provided to varying degrees by coolants (including air) in a grinding operation (coolants are discussed in the section "Grinding Fluids" in this article) The grinding wheel is a balance between having enough cutting points for the work required and providing enough porosity to ensure freeness of cut

In conventional abrasive wheels, low structure products (that is, those with tight grain spacings) tend to be used in high-pressure applications such as rough grinding and cutoff Medium structure products (typically 5 to 9) are used for the precision grinding of small areas of contact, such as centerless and cylindrical grinding High structure numbers are used

in operations in which the large area of contact dictates the need for chip clearance High structure numbers also aid in carrying coolant into the cutting zone in such machining operations as vertical-spindle rotary-table surface grinding or creep-feed grinding

In superabrasive wheels, grain spacing is less influential in wheel performance because the abrasive, when properly exposed in the bond and when grinding the hard (or brittle in the case of glass) materials normally ground with superabrasives, tends to cut a very small chip The amount of superabrasive grain (related directly to the grain spacing) usually determines the product life of the wheel, and higher concentrations tend to give better surface finishes

Abrasive Wheel Configurations

Figure 5 shows the standard wheel configurations for conventional abrasive wheels Figure 6 shows some of the wheel configurations for superabrasive wheels

Fig 5 Standard wheel configurations for conventional abrasive grinding wheels

Fig 6 Typical wheel configurations for superabrasive grinding wheels

It is essential when using abrasive wheels to apply the grinding forces in the direction of support In other words, a type 1 conventional abrasive wheel should be used on the periphery, while a type 6 wheel would be used on the top of the rim Side grinding on straight wheels or grinding on the periphery of straight cup wheels is an unsafe practice and can lead to wheel breakage

Coated Abrasives

Coated abrasive products, historically thought of as sandpaper, consist of an abrasive whose grain spacing is tightly controlled, a bond, and a backing Although a grinding wheel can be thought of as regenerating itself to expose new cutting points during grinding, the coated abrasive product (like the electroplated superabrasive grinding wheel) has one layer of abrasive to do the work Figure 7 shows a typical coated abrasive product construction

Fig 7 Typical construction of a coated abrasive product

Coated Abrasive Composition

Abrasives. Conventional abrasives are almost exclusively used for coated abrasive applications With few exceptions, superabrasives are not used in coated abrasives In addition to aluminum oxide, silicon carbide, and zirconia alumina, coated abrasives are available in natural abrasives such as garnet, emery, flint, and crocus (iron oxide) However, natural abrasives are not normally used for metalworking and will not be discussed in this article

Bonds. As shown in Fig 7, the bond in a coated abrasive product consists of two coats: the making coat and the sizing coat The maker coat ensures the adherence of the grain to the backing in the proper orientation, while the size coat controls the exposure of the grain to the grinding operation A low size coat exposes more of the grain, thus making the product more aggressive, while a high size coat tends to inhibit the formation of large chips and generates better surface finishes on the ground part

Resin bonds predominate in metalworking applications, but some hand operations and light finishing are done with products having a resin maker coat and a glue size coat Glue bond is very heat sensitive and tends to give fine finishes while not generating heat in finishing Other bonds (such as vinyl acetate and varnish) are used on some specialty products, but these are atypical

Grain spacing in coated abrasive products tends to be designated by either open coat (abrasives uniformly distributed over 50 to 70% of the surface) or closed coat (abrasives completely covering the surface) An open coat product has significantly less abrasive and has widely spaced grains Open coat products are primarily used in woodworking applications either where heat generation while sanding relatively soft woods or where pine pitch, or other similar substances that tend to load the belts before the abrasive wears out are a problem Coated abrasive products for metalworking are almost exclusively closed coat products

The backing can be considered the toolholder in the coated abrasive product The major properties that determine the selection of the backing are as follows:

• Backing strength versus backing cost

• Backing rigidity or flexibility

• Aggressiveness of cut versus ability to polish

• Wet or dry grinding

• The friction characteristics of the backing

Backings are available in various weights, expressed as pounds per papermaker's ream In general, the heavier the weight

of the backing, the stronger and more expensive it is In addition, the heavier the weight of the backing, the more rigid and less flexible it is Heavy, rigid backings that can withstand high grinding pressures are used in coarse-grit rough grinding operations Light, flexible backings that may be able to conform to intricate shapes are used in finishing, polishing, and blending applications

The backing material and any surface treatment applied to the backing determine whether it can be used in water-soluble coolants The roughness of the back of a backing and its material will determine whether it has enough friction to grind under heavy pressures without slippage or whether it generates too much friction and causes heat problems during grinding There are four major types of backings: paper, cloth, fiber, and film

Paper backings are the lightest and have the least strength of all backing types Paper backings are available in five weights: A (the lightest), C, D, E, and F (the heaviest) Paper is normally used in light-pressure applications Unless specially treated, paper backings are not used with water-soluble coolants

Y weight, and the superheavy H weight Cloth backing materials range from cotton to synthetics (such as rayon) to polyesters Resin bond polyester belts can be used in water-soluble coolants, while cotton and rayon belts are normally not used in such applications

Cloth backings have two architectures: woven and stitched Woven cloth is manufactured by interlacing yarns at 90° angles to each other in the traditional weaving process Stitched cloth is made by a relatively modern process in which the yarns are not interwoven but laid on each other at 90° angles and then stitched together so that the yarns remain flat and uncrimped (and are therefore stronger) A unique variation of cloth backings is a mesh or screen cloth, which is used primarily in wet low-pressure finishing operations

used for dry, heavy-pressure applications

abrasive tends to cut freely without much heat Film backings are normally used for polishing and blending operations under light pressures

Flexing

Once produced, coated abrasive products are flexed before being converted into finished form Flexing is a controlled breaking of the adhesive bond holding the abrasive to the backing Light flexing is used to make a belt act aggressively but relatively unconformable Multiple flexing increases the flexibility and conformability of the product, usually at the expense of product life

Coated Abrasive Applications

Coated abrasive products are available in several forms: belts, disks, rolls, sheets, and specialty products Various abrasive products are shown in Fig 8

Fig 8 Various grinding wheels and coated abrasive products used in industrial applications

Abrasive belts can be made from cloth, film, or E- or F-weight paper (Fig 9) A length and a width of coated abrasive are joined together either by lapping the ends over and gluing them or by butting the ends and applying a patch to the nonabrasive side of the backing Lap joints can be skived or ground on the abrasive side to ensure that the joint is no thicker than the rest of the belt Skiving can weaken the belt at the joint and is normally done in finish-sensitive operations in which unskived joints leave marks on the ground part Butt joints can be straight (usually at a 55° angle with the side of the belt), or they can be scalloped to offer resistance to hinging or fatigue, particularly in form grinding applications Some applications demand special joint manufacture in order to eliminate marking of parts by the joints or

to inhibit joint failure caused by heat or the severity of the form to which the belt must conform or flex Belt sizes are specified by width and length

Fig 9 Coated abrasive belts with cloth, film, or paper backings

Abrasive disks can be made from any of the backings Fiber or heavy-cloth backed disks are usually mechanically held

to the grinder backup pad by either a flanging arrangement or a built-in metal twist-on locking device Cloth, paper, and film disks are normally glued to the backup pad, often by pressure-sensitive adhesive on the back of the disk Disks are specified by diameter and hole size (or blank if no hole) and for special applications may be slotted or may have a special contour (for example, an orthogonal shape)

Roll and sheet products for metalworking are often used in hand finishing and deburring operations and can be made from any backing Roll goods, specified as width (usually in inches or millimeters) by length (usually in yards or meters) are torn or cut to the desired length and are used by hand or in holders Sheet goods, specified by width and length, are used similarly

Specialty products, such as flap wheels and cartridge rolls, are used in a number of applications, most commonly on portable or bench-stand grinders They are made from cloth products

Grinding Fluids

Grinding fluids are introduced into grinding operations to:

• Reduce and transfer heat during grinding

• Lubricate during chip formation

• Wash the grinding wheel or belt of loose chips and swarf

• Chemically aid the grinding action or machine maintenance

Grinding fluids or coolants are used almost exclusively in precision grinding applications in which metallurgical damage

in ground parts must be minimized

Usage of Grinding Fluids

The heat transfer characteristics of water are well documented The lubricity provided by straight oil is practically dictated in high-conformity operations Water-soluble coolants provide varying degrees of compromise between these extremes

Lubricity is more important in high-conformity operations with materials that generate relatively large chips, such as Inconel and the bearing steels Lubricity is less important in low-conformity operations or with materials such as the carbides that tend almost to powder when ground

Chip Removal. Chip size also dictates the nature of the swarf transfer characteristics of a coolant Large chips may indicate that vigorous pressure is necessary to clean the wheel and that heavy coolant flow is necessary to carry the chips away Small chips may mean that the coolant may need to have the ability to agglomerate chips to ensure that they can be removed from the system by filtering

Chemical Grinding Aids. Some coolants are chemically defined to aid in the grinding process For example, specific coolants for grinding titanium combine chemically with the titanium chips to inhibit the exothermic oxidation of the chips during grinding, resulting in a minimum of metallurgical damage

Types of Grinding Fluids

Grinding fluids can be classified into five categories:

• Petroleum-base and mineral-base cutting oils

Water-soluble oils consist of a suspension of oil droplets in water The oil-base stock can also contain chlorine, sulfur, phosphorus, or fatty additives These coolants incorporate additives that emulsify or disperse the product when the concentrate is added to water These emulsions have very wide applications in all types of metal grinding

The addition of polar additives and/or EP additives can produce emulsions of greater lubricating value These solutions usually have a cloudy or hazy appearance They provide a good combination of the cooling and lubricating properties required for higher-speed, higher heat producing grinding situations Other advantages of water-soluble coolants over oils include:

• Higher heat removal and better cooling of the workpiece

• Improved operator acceptance due to improved cleanliness

• Reduced fire and health hazards

Water-soluble coolants can be used in most light-, moderate-, and heavy-duty grinding operations Water-soluble coolants with EP additives are replacing many oils in severe grinding applications

Synthetic fluids are water-dilutable coolants that contain no oil These products are also called chemical solutions or ionic coolants They usually contain synthetic lubricity and rust-preventive components and very effective preservative packages The older synthetic technology was based on nitrited components, but most of these products have been removed from the market because of their potential for forming the possibly carcinogenic nitrosamines Because they do not contain these materials, the newer synthetics offer a much safer alternative to oil-containing products Other advantages of synthetic fluids include:

• Easy mixing because all product components are water soluble

• Light or imperceptible residues that are easy to clean using the coolant itself or a mild alkaline cleaner

• Easy coolant maintenance and concentration control, with very low additive requirements

• Excellent rancidity resistance and long system life

Semisynthetic fluids are basically mixtures of synthetic and soluble-oil product components These products, sometimes referred to as microemulsions, include synthetic dispersions and some oil-accepting synthetics

Semisynthetic coolants usually contain a low percentage of oil (generally 5 to 30%) They are normally used in grinding applications in which high heat removal and moderate lubricity are needed Such operations range from general-purpose

to heavy-duty classifications Semisynthetic fluids are gaining acceptance because they incorporate the positive qualities

of synthetic and soluble-oil coolants The advantages of these products include:

• Better heat dissipation than the soluble oils

• Very good rust protection

• Very good rancidity resistance

• Very good acceptance by both the machine operators and maintenance personnel

Table 2 presents a generalized comparison of the characteristics of the grinding fluids and coolants

Table 2 Relative rating of the four types of grinding fluids on the basis of their properties

Grinding fluids Fluid property

Petroleum-base and mineral-base cutting oils

Water-soluble oils

Synthetic fluids

Semisynthetic fluids

Water plus additives

Note: A, excellent: B, very good; C, good; D, poor Source: Van Straaten Corporation

Fluid Cooling and Lubricity Properties of Oils and Synthetics. Petroleum-base and mineral-base cutting oils and water-soluble oils provide excellent boundary and fluid-film lubrication, and with added EP components, they can provide good chemical lubricity The chemical lubricant additives help to reduce the amount of heat generated in the grinding process In contrast to the oil products, synthetic and semisynthetic fluids provide low boundary and fluid-film lubrication as well as very good chemical lubricity

Because of the recent advances in machine technology and synthetic coolant technology, many grinding operations are being converted to accept water-base synthetic and semisynthetic coolants In most conversions from one type of fluid to another, it is recommended that the grinding wheels be reevaluated for the new lubricant Finer and/or softer grinding wheels may be optimal when switching from oil or water-soluble oil coolants to compensate for the more aggressive grinding action of synthetic and semisynthetic fluids These water-base fluids act more aggressively because they do not have a high degree of fluid-film and boundary lubrication therefore, they allow less slipping and plowing of the metal Other considerations for grinding with a water-base fluid include the fluid-handling system and filter design as well as possible waste treatment or disposal requirements Additional information on grinding fluids can be obtained in the article

"Metal Cutting and Grinding Fluids" in this Volume

Disadvantages of Grinding Fluids

The disadvantages of using grinding fluids include:

• Effect of the coolant on the surrounding environment

Loss of visibility has become less important as machinery has become more automated and is often justified by removing from the environment the dust particles prevalent in dry grinding Rust, foaming, and bacterial growth problems normally associated with water-soluble coolants can be controlled through additives and proper fluid maintenance

Flash point and the effect of the coolant on the surrounding environment problems normally associated with oil -have been addressed by manufacturers Synthetic coolants have been developed that approach the lubricity of oil with higher flash points and less tendency to pollute the local atmosphere Machine manufacturers have also further enclosed grinding machines to address these problems

coolants-The chemical disposal of used coolant has become a significant problem Coolant manufacturers have designed and are implementing coolant systems for which the need to dump coolant is minimized

Fluid maintenance is essential in any grinding fluid system The removal of grinding swarf by filtering and other methods

to ensure clean coolant at the grinding interface is necessary to reduce random scratching of parts and premature wheel wear or the need to dress the face of the wheel to clean it Controlling the pH of water-soluble coolants (normally between 7.5 and 9.0) helps control dermatological problems by inhibiting bacterial growth, lessens rust problems, and, in the case

of organic grinding wheels, may enhance wheel life because they are adversely affected by high-pH coolants Controlling the concentration of water-soluble coolants to ensure lubricity and proper viscosity is essential to process control

Optimum Grinding Fluid Parameters

It is also important that coolant supply be at the grinding interface and at the proper volume, pressure, and temperature Underwater grinding ensures maximum cooling, but is seldom feasible Coolant nozzle design and placement is important

to ensure maximum cooling and cleaning of the wheel Pressurized nozzles are used to ensure wheel cleaning and may be used to introduce the coolants to the wheel face at a velocity similar to the wheel surface speed to aid coolant transfer to the grinding interface (Fig 10) Refrigeration or other means of temperature control of the coolant as a process control ensures constant viscosity and heat transfer characteristics as well as temperature stability of the grinding machine and the accompanying assembly containing the fixture and workpiece

Fig 10 Typical air deflectors or chamber-type nozzle used to maximize fluid access to the grinding zone

Grinding Machines and Processes

The grinding of metals can be divided, with some overlap, into rough and precision grinding Precision grinding is defined as grinding to exacting tolerances and finishes Rough grinding is defined as dimensioning or removing excess metal

Some grinding operations are specific to the type of abrasive product used Two examples are grinding wheels for cutoff and wide coated abrasive belts for sheet polishing Other operations, such as centerless and weld grinding, may not be as clear cut as to what abrasive option should be chosen

In rough grinding, in which high stock removal is important and parts tend not to be as sensitive to metallurgical damage, higher wheel speeds in the range of 45 to 63 m/s (9000 to 12,500 sfm) are often used In precision grinding, the power required to run at higher speeds tends to make burn-free grinding more difficult, and minimizing metallurgical damage is more important; therefore, speeds of 25 to 43 m/s (5000 to 8500 sfm) are more prevalent

Most decisions on abrasive usage are to some extent a process of elimination For rough grinding, coated abrasive belts and disks and conventional abrasive organic wheels are used with few exceptions Coated abrasive products are used only

on materials softer than 50 HRC In heavy-pressure applications with high horsepower requirements, grinding wheels are generally used In light-pressure operations on soft materials, coated abrasive belts or disks are much more prevalent For precision grinding, coated abrasive products tend to be used on cosmetic rather than tight-tolerance applications Superabrasive grinding wheels are used almost exclusively on very hard ( 50 HRC) materials In rough grinding, dry grinding is the accepted practice, with abrasive product caused by the pressure and heat involved in the grinding process Power consumption is excessively high for removing workpiece material, with little regard for dimensional tolerances or slight metallurgical damage

Rough Grinding

Rough grinding can be classified into four categories: cutoff, portable, floorstand/swingframe, and steel conditioning Although cutoff is exclusively a grinding wheel operation, coated abrasives are often used in portable grinding, floorstand (typically referred to as offhand) and swingframe grinding, and steel conditioning (that is, the preparation of steel billets

by removing surface impurities prior to rolling or forming)

Cutoff. Most cutoff operations are done on ferrous materials using aluminum oxide or zirconia-alumina resin bond reinforced or nonreinforced type 1 wheels Reinforced wheels are used to withstand the incidental side pressures caused

by part movement during the grind Typical grinding wheel specifications would be 20 to 36 grit in grades P through U Figure 11 shows a typical cutoff application, while Fig 12 shows a portable cutoff machine

Fig 11 Dry cutoff of a pipe using a reinforced resin bond wheel

Fig 12 Use of a portable cutoff machine to dry cut a metal pipe

Portable grinding operations (listed from heavy to light stock removal) include snagging, weld grinding (Fig 13), shaping, blending (Fig 14), and polishing Although snagging tends to be done with zirconia-alumina or aluminum oxide resin bond grinding wheels and polishing with aluminum oxide or zirconia-alumina-coated abrasive disks and specialties, there is a good deal of overlap based on material configuration and composition, total stock removal, and the need for cosmetic finishes

Fig 13 Use of a type 27 reinforced resin bond abrasive wheel to grind welds

Fig 14 Use of a zirconia-alumina cloth abrasive disk to blend stainless steel seams with a portable grinding

machine

Floorstand (Fig 15) or offhand (Fig 16) and swingframe operations tend to split upon the subsequent use of the ground part If the part is subsequently polished (such as a hand tool, plumbing fixture, or turbine blade), it is usually offhand ground with a coated abrasive belt If the part is subsequently machined, ground, and so on, it is usually ground on a floorstand machine with a grinding wheel

Fig 15 Floorstand rough grinding of a casting using a zirconia-alumina resin bond wheel Note the pressure bar

used to increase the grinding rate

Fig 16 Front view (a) and side view (b) of a backstand grinder having coated abrasive belts for use in offhand

rough grinding operations

Steel conditioning is done on high-speed, high-horsepower grinders typically using zirconia-alumina reinforced resin bond wheels (8 to 12 grit, W to Z grade) or on specialized coated abrasive belt machines

Precision Grinding

Precision grinding can be divided between coated abrasive and grinding wheel operations, although there are areas of overlap Coated abrasive precision grinding is generally on softer (50 HRC and softer and more commonly 45 HRC and softer) materials at light grinding pressures and where part tolerances are measured only to the closest 0.05 mm (0.002 in.) or looser

Coated Abrasive Precision Grinding. A number of coated abrasive precision grinding operations are shown in Fig

17, 18, and 19 Abrasive machining, coil grinding, conveyor grinding, platen grinding, and sheet and plate dimensioning and polishing can be considered surface grinding operations (as discussed below), while centerless, cylindrical, and roll grinding have grinding wheel equivalents Coated abrasive belts are very sharp compared to grinding wheels; they maintain their cut rate through being replaced when dull, and as a result they work well in applications where light grinding pressures are dictated by part configuration (such as the centerless grinding of thin-wall tubing or sheet polishing) Coated abrasive products are also commonly used on soft nonferrous materials, which tend to load and clog grinding wheels

Fig 17 Schematics illustrating the primary components of rotary-table type (a), reciprocating (bed) table type

(b), and vertical-platen type (c) (with hydraulic feed table) coated abrasive precision grinding machines

Fig 18 Schematics illustrating the primary components of several types of coated abrasive precision grinding

operations (a) Sheet dimensioning (b) Conveyor grinding (c) Vertical-platen sanding (d) Coil grinding

Fig 19 Schematic of two different coated abrasive centerless grinding setups (a) Abrasive belt centerless

grinder with regulating wheel (b) Abrasive belt centerless grinder with regulating belt

Precision Grinding With Grinding Wheels. This precision grinding system consists of the grinding machine (and its inherent variables of rigidity, horsepower, ability to deliver and process coolant, throughput rates, ability to true and dress, and so on), the grinding wheel, the coolant, and the truing tool This section will discuss the machines; because grinding wheels and coolants have been covered previously in this article, a discussion of truing and dressing is appropriate

In rough grinding, heat and pressure are used to maintain a somewhat steady-state grinding condition, but the tolerances and need for metallurgical integrity in precision grinding dictates a tighter control on the profile and sharpness of the wheel face A dull wheel face generates a smoother finish, but can introduce unacceptable vibration and heat into the system A sharp wheel face may remove material more quickly, but will not maintain tight finish and dimensional requirements in the finished part Precision grinding wheels are commonly trued and dressed to regenerate or correct grinding performance

Truing is defined as regenerating the dimensional integrity of the wheel, both in terms of roundness and profile Dressing

is defined as the regeneration of the desired cutting characteristics of the wheel Truing and dressing are commonly combined into one operation for conventional abrasive grinding wheels, but are often two separate operations for superabrasive wheels

geometries are maintained Superabrasive grinding wheels trued to within 0.0013 mm (0.000050 in.) concentricity when first mounted can outlast (by a factor of five) similar wheels not carefully trued

Dressing a wheel prepares the wheel face for the grind Vigorous dressing, which causes the abrasive particles to be exposed more in the bond matrix, tends to create sharp, free-cutting wheels that may wear more quickly Light, slow dressing that is relatively dull will give better surface finishes and longer wheel life

relatively easy with most truing tools; therefore, the wheels tend to be trued under conditions that generate the desired dressed face A sharp, dressed face is called an open face; a dulled face is called a closed face When truing superabrasive wheels (which is typically done in production grinding with diamond tools), the truing conditions tend to be set to minimize the damage to either the truing tool or the wheel because they are similar in hardness The resulting light-pressure/infeed conditions close up the faces of super-abrasive wheels, often to the point of optical reflectivity

Dressing is required for creating chip clearance and porosity in the trued wheel The dressing of superabrasive wheels is commonly done with soft conventional abrasive vitrified sticks, which relieve the bond without disturbing the superabrasive particles

There are four major types of truing tools:

• Steel cutters

• Conventional abrasive vitrified or boron carbide sticks or wheels

• Steel or carbide crush rolls

• Diamond tools

Steel cutters are used to roughly true coarse-grit conventional abrasive wheels to ensure freeness of cut after truing Vitrified and boron carbide sticks are used for the offhand truing of conventional abrasive wheels Vitrified truing wheels, such as those used on a brake-controlled truing device, are normally used for truing resin bond superabrasive wheels Crush rolls are used to crush true intricate forms into vitrified wheels, leaving a sharp, open-face wheel

Diamond tools are available as single-point tools, cluster or bar tools, multipoint nibs, dressing blocks, and rotary tools Superabrasive wheels for production grinding are usually trued with rotary diamond tools Diamond tools can be natural, synthetic, or polycrystalline diamond in a metal matrix

Single-point diamond tools consist of a single diamond in a holder These are the oldest of the diamond tools and have been replaced by more efficient tools in many applications Single-point tools are still used where imparting a truing pattern (such as a spiral on a lapping wheel) is beneficial or where constantly changing profiles make diamond rolls too costly

Cluster or bar tools are used on straight or mildly shaped wheel faces, particularly on wide wheels Multipoint nibs are used for truing straight faces into wheels Diamond dressing blocks are used almost exclusively for horizontal-spindle reciprocating-table surface grinders to generate specific forms

Rotary diamond tools are available as cups, thin rolls that traverse along the wheel face, and rolls that conform to the face

of the grinding wheel (Fig 20) There are three types of manufacturing methods for rotary diamond tools In the first method, sintered random set tools have the diamond set randomly on the preform and then held in by a sintered powder metal bond In the second method, the diamond pattern is meticulously set by hand and similarly affixed Lastly, reverse-plated tools are made by generating a negative of the preform, setting the diamonds on the negative, then plating the diamonds onto the positive preform Hand set rolls are used for mild forms, random set rolls for more intricate forms and where accuracy is of prime importance, and reverse-plated rolls for high-production, intricate form truing operations (for example, continuous-dress creep-feed grinding)

Fig 20 Rotary diamond tools Pictured are cups, thin rolls that traverse along the wheel face, and rolls that

conform to the grinding wheel face

With the exception of dressing blocks and most diamond rolls, diamond tools are traversed along the wheel face to generate the wheel profile (see the article "Thread Grinding" in this Volume) The distance the diamond tool travels per revolution of the grinding wheel is known as the dress lead The effect of dress lead is further discussed in the article

"Principles of Grinding" in this Volume

Surface Grinding

The subsegments of surface grinding are defined by wheel type and fixture or table type Common conventional abrasive wheel types are straight wheels, straight cup wheels, cylinder wheels, segments, and disk and lapping wheels The parts can be held on rotary tables, reciprocating tables, conveyors (Fig 21), or, in the case of disk and lapping wheels, between opposing wheels (Fig 22) Table 3 lists typical specifications for several types of surface grinders Surface grinding is by definition a high-conformity grinding operation requiring relatively free cutting, open wheels to remove material

Table 3 Specifications for several typical surface grinders

Size Main motor Weight Type (table-spindle)

mm in kW hp Mg lb Reciprocating horizontal 205 × 635 8 × 25 4.1 5.5 2.2 4,840

Reciprocating horizontal 610 × 3050 24 × 120 150 200 18.6 41,000

Fixed, horizontal 150 × 915 6 × 36 4.3 5.8

Rotary horizontal 610 24 15 20 5.5 12,200

Rotary vertical 915 36 26 35 6.4 14,000

Fig 21 Surface grinding operations that utilize either the periphery or face of the grinding wheel to grind flat

surfaces Horizontal-spindle surface grinders, which use the periphery of the abrasive wheel, can be either reciprocating (a) or rotary (b) Vertical-spindle surface grinders, which use the face (side) of the abrasive wheel, can also be either reciprocating (c) or rotary (d)

Fig 22 Use of the wheel face in double-disk grinders to machine the surface while the workpieces, sandwiched

between two abrasive disks (top and side view of each setup is shown), traverse along a straight line or in an arcuate path (a) Parts to be machined are pushed in and retracted by the drawer-like movement of a feeding slide (b) Parts to be ground move in an arcuate path while being transported in the nests of a rotating feed