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Grinding, or abrasive machining, refers to processes for removing material in the form of small chips by the mechanical action of irregularly shaped abrasive grains that are held in place by a bonding material on a moving wheel or abrasive belt . In surfacefinishing operations (e.g., lapping and honing) these grains are suspended in a slurry and then are embedded in a roll-on or reference surface to form the cutting tool. Although the methods of abrasionmay vary, grinding and surface-finishing processes are used in manufacturing when the accuracy of workpiece dimensions and surface requirements are stringent and the material is too hard for conventional machining.

Grinding is also used in cutoff work and cleaning of rough surfaces, and some methods offer high material-removal rates suitable for shaping, an area in which milling traditionally has been used.

Grinding is applied mainly in metalworking because abrasive grains are harder than any metal and can shape the toughest of alloys. In addition, grinding wheels are available for machining plastics, glass, ceramics, and stone. Conventional precision metal and ceramic components and ultraprecision electronic and optical components are produced using grinding.

Mechanics of Grinding. Three types of energy are involved in grinding . Rubbing energy is expended when the grains (cutting edges) of the grinding wheel wear down. As they wear, they cut less and produce increasing friction, which consumes power but removes less material. Plowing energy is used when the abrasive does not remove all of the material but rather plows some of it aside plastically, leaving a groove behind. Chip-formation energy is consumed in removing material from the workpiece as the sharp abrasive grain cuts away the material (or chip) and pushes it ahead until the chip leaves the wheel.

The grinding wheel experiences attritious wear as the abrasive grains develop wear flats from rubbing on the workpiece, or when grains break free from the bond material. Attritious wear gives rise to rubbing energy resulting from friction, and thus can lead to thermal damage as power consumption increases without an increase in material removal rate. The wheel can wear through fracture, predominating at relatively high in-feed rates. In this case, pieces of the abrasive grain break free and expose a new, sharp surface.

Materials can be classified as either easy to grind or difficult to grind. For easy-to-grind materials, most of the power consumption becomes invested in chip formation; thus, rubbing and plowing energy are minimal. Difficult-to-grind materials involve considerable rubbing and plowing energy since theforce required to remove chips is comparatively high.

Types of Grinding. In surface grinding, the grinding wheel traverses back and forth across the workpiece. Grinding can take place by using either the periphery or side face of the wheel. The table holding the part may also reciprocate. Surface grinding is done most commonly on flat surfaces and surfaces with shapes formed of parallel lines, e.g., slots.

Creep-feed grinding is a form of surface grinding in which the wheel feeds into the workpiece at a low rate (0.4 to 40 in./min, 10 to 1000 mm/min) while grinding at a large depth of cut (0.04 to 0.4 in., 1 to 10 mm, or deeper). A large amount of material can be removed with one pass of the wheel, comparedwith conventional surface grinding in which the wheel makes many quick passes over the workpiece at slight depths of cut. This process is limited by the large amount of heat generated at the grinding arc which can result in thermal damage (grinding “burn”). Application of coolant is critical in creep-feed operations. CBN wheels, with their good heat transfer property, can also reduce the severity of burn .

Cylindrical grinding produces round workpieces, such as bearing rings, although some machines can also grind tapered parts. The workpiece is mounted to a spindle and rotates as the wheel grinds it. The workpiece spindle has it own drive motor so that the speed of rotation can be selected. Both inner surfaces (internal cylindrical grinding) and outer (external cylindrical grinding) can be worked, although usually the same machine cannot do both.

• Plain grinding — the wheel carriage is brought to the workpiece on its spindle and in-feeds until the desired dimensions are reached
• Traverse grinding — the rotating workpiece is mounted on a table that reciprocates under the wheel; the grinding wheel is stationary except for its downward feed into the workpiece
• Plunge grinding — the table with the rotating workpiece is locked while the wheel moves into the workpiece until the desired dimensions are attained

Centerless grinding is a form of cylindrical grinding. In this method workpieces are not held in a centering chuck but instead rotate freely between a support, regulating wheel, and the grinding wheel.

The force of the rotating grinding wheel holds the workpiece against the support. The supports are usually stationary and so a flow of lubricant is required to reduce friction between workpiece and support.

Abrasive belt machines use a flexible fabric coated with an abrasive stretched between two rollers, one of which is driven by a motor. Usually, the abrasive coating is aluminum oxide for steels and bronzes and silicon carbide for hard or brittle materials. In the metal industries, common use of such machines is for dry grinding of metal burrs and flash and polishing of surfaces. However, some fabrics permit use of grinding fluids to enhance chip removal and provide cooling and lubrication, which results in better cutting action and longer belt life. Honing, lapping, and polishing use abrasives to improve the accuracy of the form of a workpiece or the surface finish beyond the capabilities of grinding.

• Honing is a low-surface-speed operation, usually performed on an internal, cylindrical surface but possible also on external ones. Stock is removed by the shearing action of abrasive grains: a simultaneous rotary and reciprocating motion of fixed abrasive in the form of a stone or stick. Finishes range from under 1 to 50 min. (0.025 to 1.3 mm). The development of CBN has revolutionized the honing process because this material easily outperforms conventional abrasives such as aluminum oxide, lasting up to 100 times longer.

• Superfinishing, like honing, uses fixed abrasives in the form of a stone. Unlike honing, which has a helical motion inside a bore, superfinishing uses high-speed, axial reciprocation combined with slow rotation of the outside diameter of the cylindrical component being processed. The geometry produced by a previous operation generally is not improved.

• Lapping is a fine-finishing, abrasive machining process used to obtain superior finish and dimensionalaccuracy. Lapping is unlike other finishing processes because some of the abrasive is loose rather than bonded. In general, lapping occurs when abrasive grains in a liquid vehicle (called a slurry) are guided across a workpiece by means of a rotating plate.

• Polishing uses free abrasive, as in lapping, but requires a soft support unlike the relatively hard support used in lapping. The total depth of cut during polishing can be as little as nanometers where chemical interactions will play a stronger role than mechanical or physical interactions. When the depth of cut is greater than 1 min. (0.025 mm), the interactions are usually of a mechanical nature. Many industrial components, especially electronic and optical, required highly polished surfaces.

A grinding wheel consists of thousands of small, hard grains of abrasive material held on the surface in a matrix of bond material. Grinding wheels must be resharpened on occasion. Dressing, not always required, sharpens the grains before grinding. Truing operations ensure the wheel conforms to the required cutting shape and will rotate concentrically to its spindle.

Because grinding wheels have relatively high mass and high operating speed, they must be precisely balanced. Imbalance causes vibrations that reduce the quality of the workpiece, hasten the wear of the spindle and bearings of the machine, affect other devices mounted on the grinder, and possibly transmit vibration from the grinder through the shop floor to other machines. Mounting of the wheel on the spindle and subsequent wheel wear can degrade the balance of the rotating system. Wheels are balanced by moving counterweights on balancing flanges. Some machines have an automatic balancer that shifts internal counterbalance masses.

Coolants are usually sprayed on the grinding zone to cool the wheel and workpiece, lubricate the surface to reduce grinding power, and flush away the chips. Excessive heat can damage both the wheel and workpiece by inducing undesirable physical changes in materials, such as metallurgical phase changes or residual stresses, or softening of the bond material in the grinding wheel. Coolant application is especially important in creep-feed grinding where the wheel-to-workpiece contact arc is long, heat generation is high, and the chips produced and abrasive lost from the wheel must be flushed away.

JobSmart™ Leather Hole Punch & Grommet Setting Tool Kit

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Punch clean, neat holes and set grommets in leather and other soft materials with the JobSmart™ Leather Hole Punch & Grommet Setting Tool Kit. Made of drop forged steel and are heat treated for durability Comfortable plastic coated handles for easy grip Hole punch tool has interchangeable punch heads Lifetime warranty [Read more]