Abrasive Belt Grinding and Polishing

Endless cloth belts precoated with abrasive are widely used in grinding and polishing operations. In comparison to grinding with bonded wheels, they offer the following advantages:

• Ability to finish large areas simultaneously

• Elimination of the need for costly wheel setup operations such as balancing, truing, and dressing

• Less heat generation in polishing because of the larger surface area of belts

• Ease of setup and ease of changing the abrasive belts

• Flexible or compliant abrasive tooling, not easily prone to chatter

• Smaller inventory of polishing accessories

Typical configurations of abrasive belt applications are shown in Fig. 14, 15, 16, and 17. In general, the abrasive belt is held between a drive roll and a contact roller. The belt tension determines its torque-carrying ability and hence the power delivered to the grinding zone. The work is usually applied against the belt under controlled force or pressure. Occasionally the work may be fed at a constant rate against the abrasive belt. The belt is replaced when it loses or sheds the abrasives or when the rate of material removal is decreased.

Line Diagram Abrasive Belt Grinder
Fig. 14 Schematics illustrating the primary components of coated abrasive precision grinding machines. (a)

Rotary-table type. (b) Reciprocating (bed) table type. (c) Vertical-platen type (with hydraulic feed table)

Fig. 15 Schematics illustrating the primary components of coated abrasive precision grinding operations. (a) Sheet dimensioning. (b) Conveyor grinding. (c) Vertical-platen sanding. (d) Coil

Contact wheel

Contact wheel

Fig. 16 Schematic of backstand grinder having coated abrasive belts for use in off-hand rough grinding operations
Abrasive Belt Grinding Diagram
Fig. 17 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

As in grinding wheel applications, the abrasive belt applications can be manual or make use of mechanical drives. In manual applications, the operator can vary the contact area during use to maximize the material removal rate and minimize the force or effort required. Mechanical means are adapted for large-volume production processes, usually involving components of constant geometry.

Coated abrasives are a composite of three components: backing, adhesive, and abrasive. In general, the backing for a belt is either paper or cloth. Different weights of paper and types of cloth are used, depending on requirements for strength, flexibility, and water resistance. The adhesive is usually resin or glue and is used to bond the abrasive to the backing. It is applied in two coats, with the first layer ("make coat") anchoring the abrasive in place on the backing. The second layer ("size coat") is applied over the abrasive to further strengthen its bond to the backing.

The abrasives most commonly used on coated abrasive belts are aluminum oxide and silicon carbide. Coated products containing CBN or diamond abrasives are beginning to be developed, with unique application for finishing steel components and glass or ceramics, respectively. The abrasive is applied to the backing by electrostatic deposition, then passed through an electric field where an electrostatic force propels it into the resin, while the orientation of each abrasive grain on the backing is controlled. This results in a sharp coated abrasive product containing many exposed, well-oriented cutting points.

An efficient belt grinding operation uses the coarsest grade that produces an acceptable finish. Depending on the objective of an operation, more than one abrasive grade may be needed. When both stock removal and finish are required, a sequence of abrasive grades is used. The coarsest grade is determined by the amount of stock removed and by the type of material. It should be the finest possible grade that still removes stock at an acceptable rate so that no excessively coarse scratches are produced. The finishing grade is determined by final finish requirements. If a number of grit sizes of intermediate grades are skipped, some deep scratches will remain that decrease buffability. In general, more grades can be skipped in the coarser grades and when finishing a softer material. Parts with small contact areas allow more grades to be skipped and require fewer finishing grades because of increased buffability. Table 6 shows the abrasive grades used for a given operation and the approximate number of grades that can be skipped.

Table 6 Abrasion c

irades for various operations

Operation

Grade range

Condition

Stripping

12-20

Removal of old finishes, rust, and other materials that tend to load belt

Heavy removal

stock

24-30-36-40

Used for rapid stock removal and large depths of cut. Skip 1 to 3 grades.

Medium removal

stock

50-60-80

Average stock removal and progression from rougher finishes. Skip 2 grades.

Light stock removal

100-120-150

Used in operations requiring minor dimensional changes. Skip 1 or 2 grades.

Finishing

180-220-240

Negligible stock removal. Produces the desired appearance or surface for plating. Skip 0 or 1 grade.

Polishing

(280-320)-(360-400)-(500-600)

Preparation for mirror or near-mirror finishes

In many operations, the surface finish specifications are determined by scratch depth, which is affected by factors such as abrasive grade and type, lubricant, material to be finished, and finishing conditions (e.g., contact wheel and abrasive speed). Figure 18 shows the typical range of surface finishes that can be obtained with a given abrasive grade. Finishes outside this range are used for some applications, so the graph should be used only as a general guideline.

Fig. 18 Surface finishes obtained with varying grades of abrasive belts

Contact wheels over which the abrasive belt rides provide pressure of the belt against the workpiece. Depending on its hardness, the contact wheel can provide either high unit pressure, hard wheel, or low unit pressure, soft wheel. Selection of contact wheel directly affects the rate of stock removal, the ability to blend in polishing, the surface finish obtained, and the cost of the polishing operations.

Although it is possible to operate a contact wheel satisfactorily at speeds from 10 to 50 m/s (2000 to 10,000 sfm), normal operation speeds usually range from 18 to 38 m/s (3500 to 7500 sfm). Table 7 illustrates and describes the principal types of contact wheels and indicates their applicability to various grinding or polishing operations.

Table 7 Characteristics and uses of abrasive belt contact wheels

Wheel

Type

Material

Hardness

Purpose

Characteristics

1

Knurled or spiral grooved®

Steel

Rockwell C 52 to 55

Heavy grinding

Provides most aggressive action

2

Cog tooth(b)

Rubber

70 to 90 durometer

Grinding1-0"1

Fast-cutting, allows long belt life

3

Standard serrated(d)

Rubber

30 to 50 durometer(e)

Grinding®

Leaves rough-to-medium surface, excellent life

4

X-shaped serrations®

Rubber

30 to 60 durometer

Grinding, polishing®

Flexibility allows entry to contours

5

Plain face

Rubber

20 to ^ 40 durometer(i)

Grinding, polishing®

Allows controlled penetration of abrasive grain

6

Flexible

Compressed canvas

(k)

Grinding, polishing®

Tough and durable

7

Flexible

Rubber-coated canvas

Medium

Contour polishing

Contours well, yet gives substantial stock removal

8

Flexible

Solid section canvas

Soft, medium, hard

Polishing®"1

A low-cost wheel with uniform face density

9

Flexible

Buff section canvas1®

Soft

Contour polishing

For fine polishing and finishing, low-cost

10

Pneumatic drum

Inflated rubber

(o)

Grinding, polishing

Gives uniform finishes, adjusts to contours

11

Plastic foam

Polyurethane

Extremely soft

Fine polishing

Most flexible, for extreme contours

No. 14 standard face; 4-pitch 2 by 2 mm (— by — in.).

16 16

Land, 5 mm (— in.); groove, 14 mm( — in.); depth, 1.5 mm (— in.); cushion, 19 mm( — in.). 16 16 16 4

(c) For cutting down projections, such as weld beads, gates, risers, and sprues.

Land, 10 mm (— in.); groove, 10 mm( — in.); depth, 10 mm (— in.);cushion, 22 mm( — in.).

(e) Wheel also may be of dual density, with hard rubber; 60 durometer, at hub, softer rubber, 20 to 40 durometer, at working surface.

(f) For smoothing or blending cutdown projections or surface defects.

Land, 5 mm (— in.); groove, 14 mm( — in.); depth, 8 mm (— in.); slit, 13 mm (— in.) spaced; cushion, 22 mm( — in.). 16 16 16 2 8

(h) For light stock removal and medium polishing; preferred to standard serrated wheel for softer nonferrous materials.

(i) Softer wheels give better finishes. (j) For flat surfaces.

(k) Nine densities (very hard to very soft). Hard wheels can remove metal, but more slowly than wheel 2; softer wheels can polish to fine smoothness.

(l) Good for medium-range grinding and polishing. See footnote (k)

(m) Handles all types of polishing, giving uniform results without leaving abrasive pattern on work; adjusts to contours or can be preformed for contours.

(n) Can be widened or narrowed by addition or removal of sections.

(o) Hardness controlled by air pressure

Abrasive belt machines perform grinding, polishing, and deburring operations on bar, strip, coils, blanks, stampings, forgings, die castings, and sand castings. Parts made of metal, plastic, ceramic, wood, or rubber can be handled on this equipment. Any flat surface that requires finishing, sizing, deburring, and descaling can be processed. Parts can be fed against abrasive belt polishing heads by conveyor belts or feed rolls. Conveyor belts can be made from oil-resistant rubber, sponge rubber, or abrasive-coated cloth. Hold-down fingers and cleats are used to hold nonferrous materials for grinding or polishing operations and prevent slippage. Feed rolls are made of steel or rubber-covered steel.

Abrasive belts can be tensioned by mechanical or pneumatic means, and manual or automatic tracking devices are available. Automatic tracking devices are generally used for belts 300 mm (12 in.) or more in width. Manual or automatic adjustments for grinding pressure can be used to suit specific applications. Tandem arrangements of any of the machines mentioned can be made using as many polishing units as are necessary to produce the required successive grit finish in one pass through the machine. Conveyor belt machines can be furnished with an automatic turnover arrangement to polish or deburr both sides of parts. Abrasive belt heads or flap wheels can be mounted on the bottom side of the pass line to permit two-sided simultaneous polishing operations. Flat-surface polishing machines can be equipped for wet or dry finishing. A means for the application of oil spray, oil mist, or other wet polishing media may be provided.

A few of the applications for wide-sheet polishing mills include:

• Extrusion sizing, grinding, and deburring

• Prepolishing of stock before forming

• Polishing sheets to a high-quality satin finish

• Obtaining engineered finishes on stainless steel

• Upgrading finish of commercial cold-rolled stock

• Sizing operations on all types of sheet and coil stock

Sheet-polishing heads can be used as individual units or assembled in tandem arrays. Applications of these heads include prepolishing of carbon steel sheet for plating applications, conditioning of stainless steel coil stock, and polishing of sheet and plate to standard finishes. A broad (2 m, or 7 ft) wheel, angled to the sheet to be buffed, can achieve high degrees of microfinish and reflectivity by allowing the sheet to be transferred by the head or heads in a continuous mode or by having sheets placed individually on a reciprocating conveyor. The heads can be mounted at the top and bottom of the sheet in a pinch roll configuration. With some degree of oscillation, buffing streaks are avoided and uniform fine finish is achieved on both ferrous and nonferrous materials for finishing prior to the sheet's being blanked, stamped, or used full-dimensionally for other purposes.

Applications of Belts. Originally, the use of coated abrasive belts was limited to the finishing of flat surfaces, using straight-face contact wheels. Now that resin bonding of abrasives is practical, it is possible to produce belts with greater flexibility and improved joints. In addition, improvements in polishing machinery have made it possible use of coated abrasive belts for contour polishing operations. The selection of tough, flexible polyester backings also contributes to the ability of coated abrasives to contour polish. Extremely fine-grit abrasive belts are used for lapping and polishing applications. Use of superabrasive belts for ceramics finishing is also an emerging technology.

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