centerless-ground_parts.pdf

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41. CENTERLESS-GROUND PARTS

41.1. THE PROCESS

Centerless grinding is an abrasive method for finish-machining cylindrical

surfaces. Workpieces are unmounted but pass between two opposed

grinding wheels. There are several variations of the process:

In through-feed grinding the part passes axially between the main grinding

wheel and the regulating wheel; a metal blade supports the workpiece at the

correct height. This method is limited to cylindrical parts without heads,

shoulders, or other projections that would prevent through movement of the

part between the wheels. It is illustrated schematically in Fig. 4.14.1.

Infeed grinding differs from through-feed grinding in that the workpiece

does not move axially during grinding and in that the grinding wheel may be

dressed to the shape to which the part is to be ground. The process is

comparable with plunge or form grinding on a cylindrical grinder. The

grinding wheel is fed into the work and retracts for workpiece removal. The

infeed method is not quite as rapid as through-feed grinding. (See Fig.

4.14.2.)

CENTERLESS-GROUND PARTS
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Figure 4.14.1. Through-feed centerless grinding.

Figure 4.14.2. Infeed centerless grinding. (Courtesy Cincinnati

Milacron.)

Figure 4.14.3. End-feed centerless grinding. (Courtesy Cincinnati

Milacron.)

End-feed grinding is used for tapered work. Either the grinding wheel or the

regulating wheel or both are dressed to the proper taper. The workpiece is

fed axially to a fixed end stop. Figure 4.14.3 illustrates the process.

Internal centerless grinding is used for ring- and sleeve-shaped parts. The

workpiece is supported between three rolls, which establish its location and

cause it to rotate. An internal grinding wheel removes stock. Since the part is

located from its outside surface, the ground inner surface is nearly perfectly

concentric with the outside.

41.2. TYPICAL APPLICATIONS
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The centerless-grinding method can be applied to solid parts with diameters

as small as 0.1 mm (0.004 in) and as large as 175 mm (7 in). Rings and tubing

somewhat larger250 mm (10 in)can be centerless-ground with equipment

currently available. Parts as short as 10 mm (0.4 in) and as long as 5 m (16 ft)

are centerless-ground.

Centerless grinding produces accurate cylindrical surfaces. The method is

ideally suited for pins, shafts, and rings when close-tolerance outside

diameters, precise roundness, and smooth surfaces or all three are required.

Long, slender parts that would be subject to deflection in conventional

cylindrical grinding can be ground accurately by the centerless method.

Screw threads also can be ground with this process.

For through-feed grinding, the surface to be ground must be a straight

cylinder. If the part has two or more diameters, only the largest can be

ground with the through-feed method. Figure 4.14.4 shows a variety of parts

ground with the through-feed centerless method.

The infeed method is slightly slower but possesses the advantage of having

the capability of producing multidiameter or formed surfaces. Tapered parts

or parts with steps are ground with this method. Valve tappets, arbors, yoke

pins, shackle bolts, and distributor shafts are typical examples.


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Figure 4.14.4. A variety of parts produced with through-feed

centerless grinding. (Courtesy Cincinnati Milacron.)

41.3. ECONOMIC PRODUCTION QUANTITIES

Centerless grinding is primarily a mass-production process. It combines high

output levels with the precision of finished dimensions.

Short parts processed by the through-feed method and using an automatic

magazine or hopper feed can be centerless-ground at rates as high as 6000

pieces per hour. Times may be considerably longer for end-feed or infeed

processed parts, particularly the latter when larger diameters, greater stock

removals, or multiple diameters are involved. For infeed grinding, a rate of 30

to 240 pieces per hour is more typical. For through-feed grinding, production

rates are more easily stated in terms of length processed per minute. Rates

range from 1 to 4 m/min (3 to 12 ft/min) per pass to as much as 9 m/min (30


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ft/min) per pass. From one to six passes may be involved.

Centerless grinding is more rapid than conventional cylindrical grinding

because loading and unloading time is much shorter. There is also no need

for drilling center holes on the ends of the workpiece.

Setup times for centerless grinding range from a few minutes to as much as 4h, depending on the changes required and the machine used. Sometimes a

change in the material being ground may require more setup time than a

workpiece change because it may entail changes in wheels and coolant.

Job shops can group lots of parts of like material and nearly the same

diameter and economically process lots of 100 pieces or less. Justification for

centerless grinding in comparison with center-type cylindrical grinding

depends on the availability of equipment and wheels and other tooling and

the complexity of the parts to be ground. Single-diameter parts can be

ground as easily with the centerless method as with center-type grinding if

quantities are 10 to 25 or more. Complex parts may require quantities of

1000 or more to justify the centerless approach.

41.4. SUITABLE MATERIALS

Centerless grinding has the same broad range capability for processing

various materials as the other grinding processes have. However, there is

one bonus with centerless grinding: brittle, fragile, and easily distorted parts

and materials are more suitable for centerless than for conventional center-

type cylindrical grinding. With the centerless method, parts are supported

along all or most of their length, and there is no end pressure or deflection

from wheel pressure. This makes the grinding of glass, porcelain, rubber,plastics, cork, and other brittle or easily distorted materials more practical on

centerless equipment.

For comments on the suitability of most materials to grinding methods in

general, see Chap. 4.15, Flat-Ground Surfaces.

41.5. DESIGN RECOMMENDATIONS

The following suggestions should be kept in mind by the designers wishing

to take maximum advantage of the centerless-grinding approach.


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Figure 4.14.7. Parts with irregular surfaces can-not be longer than

the width of the grinding wheel unless both infeed and through-feed

are used and the part is stepped in one direction as shown.

square, nearly square, or round ends. The included angle of a pointed end

should be 120 or less. (See Fig. 4.14.8.)

5. As with cylindrical grinding, it is best to avoid fillets and radii and instead

use undercut or relief surfaces. This eliminates difficult wheel dressing when

there is a fillet. (See Figs. 4.13.2aand 4.13.5.) When radii or fillets are used,

they should be as large as possible, and on any one part all of them should

be the same size to simplify wheel dressing.

6. When a part is designed for form centerless grinding (infeed method), the

form should be as simple as possible to reduce wheel dressing and other

costs.

7. If accuracy is critical, avoid keyways, flats, holes, and other interruptions

to the surface to be ground or make them as small as possible. (See Fig.

4.13.3.)
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Figure 4.14.8. Do not design the ends of centerless-ground parts to

have ground surfaces unless infeed grinding is used and the end

surface has an included angle of less than 120.

8. If a flat is necessary at the end of a shaft to provide a surface against

which a setscrew is to be tightened or for some other reason, and if

tolerances are tight, it is preferable to put flats on opposite sides of the part.

This prevents the tendency for a high spot to develop opposite the flat.

Another remedy is to retain a full cylindrical section at the end of the shaft.

(See Fig. 4.14.9.)

Figure 4.14.9. When interruptions are unavoidable, balance them if

possible or provide full cylindrical surfaces on both sides.

41.6. DIMENSIONAL FACTORS

Like other grinding methods, centerless grinding can be extremely accurateif all conditions are correct. These conditions include the condition of the

equipment, particularly wheel-spindle bearings, the use of the proper wheel

and coolant, and the evenness of the temperature of the workpiece, machine,

and coolant.

Roundness control in centerless grinding is remarkably good even though

the part does not rotate about a fixed center. The reason for this is the

geometry of the setup of grinding wheel, regulating wheel, and workpiece

support, which, if correct, systematically cause the grinding action to remove

the high spots of a part as it is rotated against the wheels.

Another favorable factor in centerless grinding is the fact that the setting of

the grinding wheel affects the diameter of the workpiece rather than the

radius from its center point, as in conventional cylindrical grinding. Thus a

factor of 2 is involved in the accuracy of wheel settings.

Table 4.14.1 presents recommendations for dimensional tolerances for

production centerless grinding.
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Table 4.14.1. Recommended Dimensional Tolerances for Centerless

Ground Parts

Recommended tolerance

Dimension Normal Tight

Diameter 0.0125 mm (0.0005 in) 0.0025 mm (0.0001

in)

Parallelism 0.0125 mm (0.0005 in) 0.0025 mm (0.0001

in)

Surface finish 0.20 m (8 in) 0.05 m (2 in)

Citation

James G.Bralla: Design for Manufacturability Handbook, Second Edition.

CENTERLESS-GROUND PARTS, Chapter (McGraw-Hill Professional, 1999, 1986),

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