When a new wheel is installed on the grinder, it must be trued before use. The cutting surface of a new wheel will run out slightly due to the clearance between the wheel bore and machine spindle (Figure L-51). Truing a wheel will bring every point on its cutting surface concentric with the machine spindle. This concentricity is important for achieving smooth and accurate grinding conditions.
In most grinding operations, small chips of workpiece material can become lodged in the cutting surface of the grinding wheel. In addition, if the wheel bonding hardness is excessive, dulled abrasive grains can remain in the grinding wheel. Both of these conditions will impair the cutting efficiency, and these particles must be removed as needed to maintain proper cutting action. This process, termed dressing, is important in obtaining good results in grinding.
Dressing is the process of sharpening a grinding wheel. grinding wheel must run true with every point on its cutting surface concentric with the machine spindle. As the wheel becomes loaded with workpiece material, it must be dressed to restore sharpness. It must also run in balance because of its great speed. These procedures of truing, dressing, and balancing are important parts of grinding operations,
and the purpose of this unit is to familiarize you with them.
Describe truing, dressing, and balancing of grinding wheels.
Distinguish the difference between the objectives of truing and dressing a grinding wheel.
Correctly position a single-point diamond dresser in relation to the grinding wheel.
Both truing and dressing remove a certain amount of material from the grinding wheel. Wheels should be trued and dressed only enough to establish concentricity or to expose new sharp abrasive grains to the workpiece. Some estimates are that more abrasive material is sacrificed by dressing than is actually used to remove material from workpieces.
Optimal matching of grinding wheels to the grinding operation results in an essentially self-dressing condition, where the force of the grinding action is sufficient to release dull abrasive grains from the bond and keep the grinding wheel sharp.
The need to retrue a grinding wheel typically relates to the losing of form control, such as when the edges of the wheel wear away an unacceptable amount. The need to dress a grinding wheel is typically related to maintaining surface finish and metallurgical requirements.
The cluster dresser (Figure L-53) may be wide enough to reach across the entire cutting surface of the wheel, but traversing the dresser is the most common way to dress the grinding wheel.
Dressers are always solidly mounted. The diamond and its holder are mounted on the grinder chuck so that they can be traversed across the cutting surface of the wheel. The dresser must be positioned off center on the wheel on the outgoing rotation side (Figure L-54) to prevent the dresser from getting caught and being pulled under the wheel.
Dressers are sometimes marked with arrows to be pointed in the same direction as wheel rotation.
When truing, lower the wheel head while traversing the diamond with the table travel until you find the highest point on the wheel. Continue to traverse the dresser across the wheel and feed down .001 in. after each pass. Each time, a little more abrasive will be removed from the wheel. When the dresser is cutting all around, the wheel has been fully trued. Do not remove any more abrasive than is necessary to
achieve concentric running of the cutting surface. Truing may be done dry if the diamond is given a few seconds to cool after each pass has been made. If the truing is done with grinding fluid, it is important to flood the diamond continuously; otherwise, the diamond could be fractured by becoming hot and then having fluid splash against it. After truing and dressing, it is desirable to “break” the sharp corners
of the wheel with a dressing stick, leaving a small radius.
This will prevent the sharp corners from leaving undesirable Figure L-54 This is one of several ways of mounting a dresser on a surface grinder. The dresser with its diamond is placed and magnetically secured on a clean magnetic chuck. Note that the diamond is slanted at a 15-degree angle and slightly past the vertical centerline of the wheel (Courtesy of DoALL Company). feed-line scratches on the workpiece. (The term truing also applies to correcting axial runout, particularly in operations such as tool and cutter grinding, and cylindrical grinding
into shoulder features.)
After the wheel has been in use, the diamond will again be used to remove embedded workpiece particles and remove dulled grains to expose new sharp abrasive grains. This is the process of dressing and is done in the same way as truing. The speed of traverse can influence the surface finish that can be achieved on the workpiece. A slow dressing traverse will cause the diamond to machine the abrasive grit smoother, which in turn will result in a smoother workpiece finish (by the mechanism of burnishing) but with less efficient grinding. A rapid
dressing traverse will leave a sharper wheel, but the surface finish will be rougher. In many cases the same wheel can be used for both roughing and finishing; a coarse dress for roughing multiple parts, followed by a fine dress for finishing the parts. It is important not to try to rough grind the next batch of parts without redressing the wheel. The effect of different dressing procedures can be substantial;for example, a
slowly dressed 60-grit wheel can be made to behave like a rapidly dressed 120-grit wheel for finishing, but the reverse is never true. Grinding machines may have a built-in dresser with micrometer feed (Figure L-55).
Because cubic boron nitride and diamond abrasives are expensive, it is not desirable to true or dress these wheels excessively. The bore of a diamond wheel is machined so that it will closely fit the grinder spindle (Figure L-56). When a resinoid-bonded diamond or CBN wheel is mounted, it may be adjusted to run true by using a dial indicator. The wheel is tapped lightly using a block of wood (Figure L-57). If the
diamond wheel cannot be adjusted within tolerance, it must be trued. A single-point diamond or cluster dresser must not Figure L-57 Runout on a diamond wheel is checked with a dial indicator and must be within .0005 in. for resinoid wheels or .00025 in. (half as much) for metal-bonded wheels. (Some metalbonded wheels are constructed with a concentric groove for indicating purposes.) Tapping a wooden block held against the wheel to shift the wheel on the spindle is often enough to bring it within limits.
Otherwise, it will have to be trued. be used for this purpose. A better alternative is to use a brake-type truing device, with a 46- to 60-grit silicon carbide grinding wheel (Figure L-58). Metal-bonded rotary diamond dressing wheels of from 80 to 150 grit are also used effectively on CBN wheels for this purpose. Truing glazes resinoid-bonded superabrasive grinding wheels, so they may need to be conditioned afterward to work well. On wheels less than 12 in. in diameter, this can be done by hand using a fine-grit aluminum oxide dressing stick and a small amount of grinding fluid. The dressing stick is pushed into the wheel by hand, and gradually, as the bonding material is
eroded away, the wheel begins to consume the stick more rapidly. When the rate increases sharply, the dressing is done.
This process may use up 2 or 3 cubic inches of a dressing stick in the process. Conditioning with larger wheels is generally done by initial grinding of a few pieces at a reduced rate until the wheel “opens up,” then gradually bringing the grinding rate up to what is termed a steady-state force level.
Figure L-58 This is a brake-type dresser. The abrasive wheel in the dresser is being used to dress a metal-bonded diamond
wheel on the grinding machine.
NEW TECHNOLOGY
As more vitrified bond CBN superabrasive wheels are being utilized, more research into the effective truing and dressing of these grinding wheels is taking place.
Many brake-type dressers are being replaced with motor-driven dresser rolls, which can be adjusted accurately for setting the speed of the dressing roll.
In one study it was found that a motor-driven diamond dressing roll being driven at a surface speed of .8 that of the grinding wheel,
with an infeed of .0005 in. and a single traverse at .003 in. per revolution, after concentricity was achieved, resulted in nearly complete elimination of the need to condition or break in the grinding wheel.
Stick dressing after such a truing/dressing actually resulted in somewhat poorer grinding action.
Electroplated superabrasive wheels should not be either trued or dressed, as the abrasive is only a single layer deep.
The newer vitrified bond CBN wheels typically do not need to be “conditioned” after they are dressed.
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