High Precision Fixed Abrasive Finishing

A variety of high-precision processes have been in use, and new processes are constantly being developed to achieve extremely close geometric tolerances or to improve surface finish. The objective of all high-precision processes is to achieve geometrically precise components or surfaces of controlled texture or surface finish (Fig. 11). Table 5 compares the processes described in this section. A parallel set of processes using loose abrasives, called lapping, buffing, and polishing, are reviewed in later sections of this article.

Table 5 Comparison of high-precision finishing processes

Process features

Process category

Honing

Superfinishing

Flat honing

Microgrinding

Bonded abrasive tool used

Yes

Yes

Yes

Yes

Cutting speeds

<500 sfm

<500 sfm

<500 sfm

500 to 12,000 sfm

Oscillation of abrasive tool

Yes

Yes

No

No

Amplitude of oscillation of abrasive tool

Large

Small (about 1 to 6 mm)

Abrasive product size relative to the work size

Small to large

About equal

Large

Varies widely

Frequency of oscillation

Small (100-200 strokes/min)

Large (300-4000 strokes/min)

Abrasive tool motion

Linear or reciprocation

Angular, linear, or reciprocation

Rotation

Rotation

Work material motion during the abrading process

Fixed

Circumferential rotations

Epicyclic

Large range of motions and velocities are involved

Work surface

Generally straight cylindrical internal

External or internal surfaces, not limited to

Generally flat and parallel surfaces. Occasionally

Large range of surfaces and

configuration

surfaces

straight surfaces

cylindrical or sphere

contours

Amount of stock removed

Rough 0.010 to 0.015 in.

0.0001 to 0.0005 in.

Rough 0.010 to 0.015 in.

Similar to honing and superfinishing

Finish 0.002 to 0.006 in.

Finish 0.002 to 0.006 in.

Surface texture controlled

Yes

Yes (to a lesser degree than honing)

Maybe

Maybe

Abrasive tool bond type

Vitrified (also metal bond for superabrasives)

Vitrified

Vitrified (also metal bond for superabrasives)

Resin, metal or vitrified using conventional abrasives or superabrasives

Fig. 11 Applications of high-precision processes using bonded abrasives. (a) Honing is most commonly used to correct the internal geometry of a bore hole. (b) Superfinishing is most commonly used to improve external

surface finish. (c) Flat honing is most commonly used to improve flatness and the parallelism between surfaces.

Honing is a low-velocity abrading process that uses bonded abrasive sticks to remove stock from metallic and nonmetallic surfaces. As one of the last operations performed on the surface of a part, honing generates functional characteristics specified for a surface, such as geometric accuracy, dimensional accuracy, and surface features (roughness, lay pattern, and integrity). It also reduces or corrects geometric errors resulting from previous operations.

The most common application of honing is on internal cylindrical surfaces (Fig. 12a). However, honing is also used to generate functional characteristics on external cylindrical surfaces, flat surfaces, truncated spherical surfaces, and toroidal surfaces (both internal and external). A characteristic common to all these shapes is that they can be generated by a simple combination of motions.

Fig. 12 (a) Schematic representation of honing. (b) Schematic representation of superfinishing

Superfinishing is a low-velocity abrading process very similar to honing. However, unlike honing, superfinishing processes focus primarily on the improvement of surface finish and much less on correction of geometric errors (Fig. 12b). As a result, the pressures and amplitude of oscillation applied during superfinishing are extremely small. This process is also referred to as microhoning, microsurfacing, and microstoning.

Flat honing is a low-velocity abrading process, similar to honing except that a large flat honing surface is used to simultaneously finish a large number of flat parts (Fig. 13). The predecessor to the flat honing process was hyper lap, in which the lapping plate was simply replaced by an abrasive product such as a grinding wheel. Modern flat honing machines "float" the abrasive product such that it can be applied under controlled pressure against the work surface. It is also critical to ensure that the flat honing tool wears uniformly and accepts uneven work surfaces that require correction during the honing process. These processes also true and dress the flat hone in a manner very similar to a grinding process. With suitable adaptation, flat honing equipment can be used for finishing cylindrical surfaces such as pins or rollers, as well as spherical surfaces such as balls.

Fig. 13 Schematic representation of flat honing

Flat honing requires abrasive wheel(s) that will produce the required surface finish and accuracy of flatness and parallelism. To be efficient, the wheel(s) must continue to cut, load after load, without requiring frequent truing. The accuracy obtained depends on the flatness of the wheel surfaces. If the faces wear, then they must wear away evenly in order to keep the wheel surfaces flat.

At the time of this writing, the use of diamond or CBN wheels for flat honing is relatively new. Research is presently being done to flat hone hard ceramic materials with metal bond, fine-grit diamond wheels. The anticipated benefits of diamond wheel honing over conventional grinding are cost effectiveness, less surface damage, better accuracy and finishes, less chipping, and easier fixturing.

The coolant normally used in the flat honing process wets the small particles generated during the machining process and carries them away. In addition, it cools the workpiece and provides lubrication. Mineral oil or mineral seal oil are conventional fluids that meet these requirements and still have low enough odor and high enough flash point to be feasible and safe. Water-based coolants have specific applications, and some workpieces composed of carbon and other ceramics are often processed using water.

Microgrinding is akin to the precision grinding processes described above, except that extremely fine abrasives are used (50 ^m and finer). The cutting velocities in microgrinding range from very low (500 sfm) to as large as those used for grinding (6000 to 12,000 sfm). This process is also called fine grinding or microfinishing grinding.

The Future. Each of these high-precision processes is an emerging technology, and many advancements can be expected, driven by functional or performance improvements of industrial components and their systems. For example:

• Honing and microgrinding are used to achieve closer-fitting cylinders and pistons that reduce leakage past the piston and improve performance efficiency in hydraulic cylinders and automotive engines.

• Flat honing and microgrinding improve the quality of flat and parallel surfaces that are used to align, join, or seal other surfaces.

• Microgrinding can be used to modify surfaces ground to a certain feature to improve the bearing area, which in turn improves load-bearing capacity (in the case of bearings) or signal processing capability (in the case of the magnetic heads used in computers).

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