Introduction

ABRASIVE FINISHING includes many commercial processes, which can be generally classified as rough grinding, precision grinding, or high-precision grinding. The distinctions among these categories are based on the desired material removal rates and the acceptable tolerance or finish (Table 1). In rough grinding processes, material removal occurs at a rapid rate, with less emphasis on surface roughness/tolerances. In contrast, high-precision methods emphasize the attainment of fine surface finish and close tolerances at the expense of material removal rates. In precision grinding processes, removal rates are balanced with requirements of surface characteristics such as form, surface integrity, tolerances, and surface finish (roughness, waviness, roundness, etc.).

Table 1 Characteristics of abrasive finishing processes

Finishing processes

Cutting speed

Material removal rate

Tolerance

Finish

m/s

sfm

mm3/mm/s

in.3/in./min

mm

in.

^m

^in.

Rough grinding

With grinding wheels

60-100

12,00020,000

300-1000

30-100

±6-25

±0.250-1.0

2.5-25

1001000

±2.5

±0.100

2.5-25

1001000

±0.13

±0.005

Belt grinding

15-25

3000-5000

6-3000

0.6-30

Precision grinding

With grinding wheels

Present

30-80

6000-16,000

0.1-50

0.01-5

±0.0025-0.125

±0.0001-0.005

0.751.25

30-50

Future

< 500

< 50

±0.25 ¡¡m

±10 ¡in.

0.0025

0.1

Belt grinding

15-37.5

3000-7500

High-precision abrasive finishing

Lapping

<0.25

<50

<0.5 ¡m

<20 ¡in.

0.0250.1

1-4

Honing

0.25-1

50-200

0.075

0.0075

0.5-1.25 ¡m

20-50 ¡in.

0.25-0.5

10-20

Polishing

Very slow

Very slow

(a)

(a)

(a) Surface finish altered; material removal rate is insignificant.

(a) Surface finish altered; material removal rate is insignificant.

The basic idea of abrasive finishing is to use a large number of multipoint or random cutting edges for effective removal of material at smaller chip sizes than in the finishing methods that use cutting tools with defined edges. Machining at small chip sizes allows improved finish, closer tolerances, more localized control, and generation of more intricate surface features. Cutting tools with defined cutting edges are less feasible and less practical if small chip sizes are desired. An effective way of delivering small cutting edges to the machining zone is to use abrasive grains incorporated into a bond or matrix material.

Recent Developments. Abrasive finishing processes have seen a spurt in new developments in the past 20 years or so, thanks to a number of technology drivers as well as user needs.

Tolerances. Improvements in product performance can often be related to improvements in abrasive finishing processes and their ability to achieve closer tolerances. For example, transmission efficiency is improved when gears roll against each other with less sliding. This is made possible by grinding the gear to closer tolerances. Efficiency of magnetic recording is improved when the magnetic recording heads are finished to closer tolerances and improved surface by improvements in grinding and lapping.

Consistency. When components are finished to greater consistency, inspection costs are reduced and the process becomes more amenable to control. As an example, in a centerless grinding process for a bearing component, the operation consisted of rough, semifinish, and finishing steps involving three operations, three machines, a large in-process inventory, and inspection between each operation. By consistent finishing with cubic boron nitride (CBN) grinding wheels, the process was reduced to two steps, rough and finish, with minimal in-process inspection.

Surface Finish. Constant demand for improvements in surface finish has resulted in new processes (e.g., superfinishing, flat honing, and microgrinding). These requirements span a wide range of materials, from metals to nonmetals and from ferrous alloys to ceramics.

Material Removal Rate. With the advent of new abrasives (CBN, seeded gel, etc.) and new processes (high-efficiency deep grinding, creep-feed grinding, microgrinding, etc.), the material removal rates achievable using abrasive finishing processes exceed the previously known capabilities.

Productivity. The technical benefits described above often translate into faster production rates, shorter cycle timing, improved yields, and so on. All of these gains lead to lower total cost per part or higher productivity. Advancements of this kind have been noted in aerospace, automotive, tool production, bearings, and computer component manufacturing operations that use abrasive finishing processes.

New Products. The range of new abrasive finishing products, introduced worldwide, includes novel shapes and configurations, a wider range of sizes, an expanded range of grit sizes, and new bond types. For instance, abrasives are now available in extremely fine particle sizes with uniform size and shape control, which were not commonly available only a few years ago.

Machine Tool Improvements. Developments such as multiaxis computer numerical control (CNC), higher horsepower, greater rigidity and accuracy, automatic wheel changes, and on-machine truing and balancing systems make it possible to achieve faster production, better quality, or both.

New Grinding Methods. Mirror-finish grindinguses extremely fine abrasive particles to achieve extremely fine reflective surfaces of precise geometry. These methods are used to finish precision molds and dies, such as those used for making contact lenses and other optical products. Grinding from solid combines the high-material-removal capability of modern abrasive finishing methods with the closer tolerances and forms that can be achieved using grinding processes. As an example, end mills of large sizes are now routinely ground from solid blank, eliminating premilling operations and expensive in-process inventory. Such methods are also advocated for grinding ceramics, which improves the yield and reliability of ceramic components while reducing total fabrication cost.

Processing of New Materials. Ceramics and other materials, such as high-silicon aluminum alloys, powder metals, and cements, are increasingly being finished using abrasive methods. As the abrasion resistance of the work material increases due to included second-phase material or hard particles, multipoint finishing methods are often preferable to single-point machining methods. With the advent of processes such as high-efficiency deep grinding, it is likely that some soft and easy-to-machine materials will be finished using abrasive finishing methods.

Abrasive finishing is used in industrial practice for a wide range of materials. Typical examples are shown in Table 2. This article provides a broad overview of the various categories of abrasive products and materials, abrasive finishing processes, and the mechanisms of delivering the abrasives to the grinding or machining zone.

Table 2 Relative usage of abrasive machining for various work material types

Work material

Relative use

Conventional abrasives

Superabrasives

Powders slurries, and compounds

Bonded

Coated/ impregnated

Plastics

Low

Low

Low

None

Composites

Low

High

High

Low

Metals

High

High

High

High

Steel

High

High

High

High

Glass

Low

High

High

High

Carbides

High

Low

High

High

Ceramics

Low

Low

High

High

Wood

Low

High

High

None

Stone

None

Low

High

Low

Minerals

None

None

High

None

0 0

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