Grinding

Grinding is used for preliminary surface conditioning before polishing, and as a means of removing excess metal from weld beads and flash from forgings and castings. It also is used for dressing gas-cut welding edges. These operations usually require coarse or rough grinding. However, as the refinement of the surface increases with the use of finer abrasives, grinding begins to approach polishing; the dividing line between fine grinding and polishing is seldom clear cut.

Solid wheels are used for coarse grinding and include the vitreous and rubber-bonded or plastic-bonded types. They should be free-cutting to avoid loading and glazing. Abrasives frequently used are aluminum oxide (Al2Ü3) and silicon carbide (SiC), in grit sizes ranging from 20 to 36 (for initial or coarse cutting) to 60 (for subsequent finishing work). Surface speeds for solid wheels usually range between 25.5 and 30 m/s (5000 and 6000 sfm). For safety, wheels should never be operated above their maximum permissible speed. To realize the maximum cutting efficiency of which they are capable, they should not be run at less than recommended speeds.

Wheel Operation. Grinding wheels should never be forced; forcing causes excessive wheel breakdown and localized high temperature in the workpiece. In addition, the wheel should not be allowed to ride on the workpiece with insufficient pressure, because this causes rapid glazing of the wheel.

The elimination of heat buildup in localized zones is of major importance with stainless steel. The low thermal conductivity of stainless steel contributes to increased thermal distortion. This applies particularly to the chromium-nickel grades, of which the coefficients of thermal expansion are relatively high. Holding the metal at low temperature avoids heat tinting, which becomes evident at 230 to 260 °C (445 to 500 °F) and above. Marked increases in metal temperature can reduce the hardness of heat-treated grades and precipitate carbides in the unstabilized chromium-nickel grades, which are susceptible to intergranular corrosion.

Rough surfaces on weldments, castings, and forgings are cut down to the general finish contour with a solid wheel of either Al2O3 or SiC of grit size from 20 to 40, and with a bond loose enough to allow the wheel to remain open and free-cutting without excessive wheel wear. Again, it is important to prevent local overheating, which can cause either mechanical or metallurgical damage, and to avoid excessive metal removal, allowing sufficient material for further finishing. Surface contamination is not important at this stage of finishing.

The next roughing operation, which in many applications may replace the solid-wheel operation, requires the use of a portable disk grinder, powered by air or electricity, using abrasive disks coated with Al2O3 ranging in grit size from 80 to 120. Localized overheating may be prevented by applying a small stream of water or water-soaked rags to the side not being ground.

Weld Beads. Excess metal in weld beads ordinarily is removed by grinding, although an initial cut may be made with a cold chisel when the size of the bead warrants. Grinding procedures and precautions conform to those previously described, except that the width of weld beads precludes right-angle cutting with successive grit sizes. Usually, a raised bead will take a fairly coarse grit at the outset. Canting of wheels must be avoided; otherwise, grooves may be cut parallel to the bead and undesirable thinness will result. Limiting stops attached to portable grinders may be installed to prevent canting and excessive metal removal.

When grinding is to be followed by polishing, as for weld joints on polished sheet, the grinding operation must terminate sufficiently above the level of the base metal to allow enough metal for final polishing to finish flush, without a ridge or groove. Limiting stops on grinding machines are helpful.

For economic reasons, grinding of weld joints in cold-rolled or polished sheet should be held to a minimum by using welding procedures that avoid high beads of excess metal. The grinding step can often be eliminated, allowing the workpiece to go directly into the polishing operations where finer abrasives are used.

Metal adjacent to beads that are being ground should be protected from flying bits of metal cuttings by shields of material such as paper. Wet rags may be laid on the workpiece to absorb heat and reduce thermal distortion, particularly on thin-gage material.

Progressive Grinding. To remedy an existing surface condition, such as removing scale patterns or indentations from hoisting clamps on an annealed plate, a series of wheels of decreasing grit size is often needed. The initial grit size is selected on the basis of which coarseness is needed to remove the major portion of the unwanted condition. After the workpiece has been partly dressed down, the operation is completed by using a graduated series of successively finer wheels until the desired final finish is attained. The use of a relatively soft plastic wheel impregnated with fine, sharp grit makes it possible to reduce the number of finishing operations, because of the combination of free cutting and wheel resilience for ease in blending.

The direction of wheel traverse across the work is changed by 90° with each change in grit size to remove residual grinding lines. As each change is made, workpiece surfaces should be brushed thoroughly to remove any particles of the preceding abrasive or of metal cuttings that mar the performance of the finer grit to follow. The progression from coarse to fine grit size may be made in steps of 20 to 40 mesh.

When using a flat disk grinder, with which cutting is performed against the face (instead of the rim) of the wheel, a rotary or circular traversing motion is most frequently used. This eliminates the need for reversing the direction of grinding with each change of grit size.

Belt Grinding. Belts carrying abrasives of various grit sizes are widely used for grinding and polishing stainless steel surfaces. Although many complex shapes can be belt ground, a simple projection may make belt grinding impossible; for example, the studs welded to a cookware pot or pan to which the handle is affixed. In this application, the finishing operations, from grinding to color buffing, must be completed before the studs are welded in place. Grit sizes and belt speeds for abrasive grit roughing and polishing are given in Table 9.

Table 9 Grit sizes and belt speeds for abrasive belt roughing and polishing of stainless steels(a)

Product

Roughing

Rough polishing

Polishing

Obstruction or roughness removed

Grit size, mesh

Belt speed

Grit size, mesh

Belt speed

Grit size for for successive stages, mesh

Belt speed

Polishing aid(b)

m/s

sfm

m/s

sfm

m/s

sfm

Billets, alloy or stainless steel

50

17.9

3500

G

Cutlery, stainless steel

Blades--tapering

50

28

5500

80

28

5500

180

28

5500

G

Forks or knives

80

26.5

5200

1500

26.5

5200

G

Spoons

150, 220

26.5

5200

G

Hypodermic needles, stainless steel

320(c), 400(c)

23.5

4600

WSO

Jet blades, stainless steel

Airfoil

50

28

5500

120

28

5500

O

Longitudinal

80, 100, 120, 150, 180

(d)

(d)

O

Stainless steel

Coil (series 300)

Pits

60

17.3

3400

80

17.3

3400

120, 150

17.3

3400

O

Pots and pans

Wrinkles

80

25.5

5000

220, 320(c)

25.5

5000

G

Press plates

Scratches

80

20.4

4000

100

20.4

4000

120, 150, 180, 240, 320

20.4

4000

G

Sheets, No. 3 finish

80

20.4

4000

100

20.4

4000

G

Sheets, No. 4 finish

Inclusions

100

20.4

4000

120

20.4

4000

150(c)

20.4

4000

G

Sheets, No. 7 finish

Inclusions

100

20.4

4000

150

20.4

4000

180(c), 240(c), 280(c)

20.4

4000

G

Tubes

150

17.9

3500

220, 280, 320

23

4500

O

Turbine nozzles and buckets

80

25.5

5000

120

23

4500

(a) Abrasive belts coated with Al2O3, unless grit size is footnoted to indicate otherwise.

(c) Silicon carbide.

(d) Fixture abrasive, reciprocating.

Mechanized Belt Grinding. In mechanized belt grinding, longer belts provide a longer belt life and dissipate heat more effectively. Thus, longer belts frequently are operated without a coolant, eliminating a post-grinding cleaning operation.

In belt grinding of rectangular stainless steel sinks, for example, it was necessary to use short belts to reach all internal surfaces. Each sink was ground in a machine with four grinding heads indexing about the main column of the machine while the sink was held in a cradle that rotated in a horizontal plane. A narrow belt ground the radii between the bottom and the side walls of the sink. Wider belts were used to grind the side walls; these belts were comparatively short and required the use of a coolant. Belt abrasives varied in grit size from 80 to 220 mesh, depending on the desired finish.

On a typical grinding machine using a 2200 mm (86 in.) wide abrasive belt and powered by a 250-hp ac motor, a positive hydraulic reciprocating drive permits instantaneous variations in table speed; belt speed is fixed at 25.5 m/s (5000 sfm). The 2200 mm (86 in.) belt travels over a conventional vertical-head assembly consisting of a dynamically balanced upper idler roll of steel and a rubber-covered serrated lower contact roll. A pneumatic belt-centering device ensures positive tracking of the abrasive belt and is adjustable to compensate for belts of different widths. Incorporated into the entire worktable is a 2200 mm (86 in.) wide vacuum chuck 4.9 m (16 ft) long. To produce single or compound tapers, a worktable can be tilted to any angle, for either right-hand or left-hand tapered sheets.

Belt life is influenced primarily by belt speed, type of material being ground and its hardness, pressure of the belt against the work, type of contact roll, type of lubricant (if a lubricant is required), and uniformity of the finish desired. In belt grinding of stainless steel, the recommended belt speed is approximately 20 m/s (4000 sfm).

With the exception of the precipitation-hardening stainless steels, the hardness of the steel has a greater effect on the life of a grinding belt than its composition. Hardness also affects surface finish, and a high-quality finish is easier to obtain on a harder stainless than on one that is softer. With precipitation-hardening alloys, best grinding results are obtained with a waterproof cloth abrasive belt and a water-soluble oil lubricant.

Pressure of the workpiece against the abrasive belt is probably the most important factor affecting belt life. Excessive pressure on a new belt causes glazing of the abrasive and greatly reduces cutting action. Therefore, new belts should be subjected to very light pressures during the break-in period and until the belt is capable of maintaining a uniform cutting action. A light belt pressure is preferred, and increased stock removal should be obtained by changing to a coarser belt.

The contact roll that backs up the abrasive belt is another important factor; a properly serrated contact roll may increase belt life by as much as 60%. The angle of the serration affects both belt life and finish. To obtain fine finishes, contact rolls serrated at an angle of 75° to the axis of the spindle should be used. Rough finishing requires a 30° angle. For

general work, contact rolls may be made with a 45° serration, using a 9.5 mm (- in.) groove and 9.5 mm (- in.) land.

Hardness of the contact roll should range between 50 to 65 on the Shore scleroscope A-scale.

When lubricants are used, they should maintain free-cutting (nonloading) edges, add color to the finished product, maintain a cool cutting surface, and be easy to apply. Lubricants may be in the form of grease sticks, waxes, or cutting oils. Cutting oils generally are more effective when they are diluted as much as 4 to 1 with kerosene.

Safety. Metal fines collected in a container near machines during the belt grinding of stainless steel should be removed regularly, because the fines, together with polishing compounds or oils that are collected with them, constitute a potential fire hazard. Fires in the duct system can be extremely serious because of the high air flow in the ducts. Fire-extinguishing equipment should be close to any machines using abrasive belts.

When wide abrasive belts are used, equipment with automatic tracking to center the belt in relation to the work is advantageous. All abrasive belt machinery should be equipped with motors that are totally enclosed.

Safety training for operators should begin with thorough instruction in the proper use of equipment, because most severe injuries result from improper use. Most common injuries are burns, cuts, and eye injuries. Serious accidents may arise from the snagging of parts because of improper loading, improper use of lubricating devices, and careless placement of hands and arms while the machine is in operation.

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