Grinding machines

A surface finishing process, that is removal of irregularities from machined surfaces, may be performed either by employing hardened steel tools in such operations as filing, scraping and burnishing, or by employing abrasives for surface refinement in grinding, lapping, honing, superfinishing and polishing operations.

Grinding is the process of removing metal very accurately and economically by means of solid or sectional abrasive wheels rotating at a comparatively high speed. Originally employed for sharpening tools, grinding has become a useful and accurate finishing process for both hardened and unhardened metal parts, especially in the mass production of precision parts. By means of grinding, articles made of hard material, can be brought to a true finish with very close tolerances. Surface finishing operations involving the application of abrasive of smaller size than those used in grinding are known as lapping, honing, superfinishing and polishing. In all these operations only a small amount of metal is removed in surface finishing.

Lapping is used for accurate fitting many machine tools and mating parts. The process consists of rubbing their surfaces together under load, a very fine abrasive material being placed between them. The surfaces are rubbed until all their irregularities are removed and the surfaces fit closely.

Honing is employed for producing a very fine finish by using a tool referred to as a hone. Hones of special shapes used for finishing internal surfaces of cylindrical work may contain several abrasive blocks, which can be moved out from the axis thus giving the tool any desired diameter.

Superfinishing is a finishing process which brings metal to an extremely smooth surface by using a bonded abrasive stone held in contact with the work under low pressure. The work rotates against the stone at low speed, at the same time the stone being given a random motion. Grinding is performed on machine tools known as grinding machines, which use rotating grinding or abrasive wheels for producing cylindrical, conical, or flat surfaces.

Many different types of grinding machines have been developed for grinding various kinds of work. The grinding machines are generally classified into several groups: bench grinders used for tool sharpening and general off-hand grinding usually consist of a motor with a two-wheel spindle replacing the motor shaft; cylindrical grinders are used for grinding cylindrical and conical work. They are essentially similar to engine lathes, but the carriage and tool post are replaced by a wheel stand carrying a grinding wheel. Internal grinders are grinding machines used for finishing internal surfaces of cylindrical or conical shape.

Centreless grinding machines used for external cylindrical grinding have no centres to support the work, and the part to be ground is held on a work rest between the grinding wheel and a suitable regulating or feed wheel which imparts axial motion to the part. Internal centreless grinders are employed for finishing roller bearing races and bushings. Disk grinders used for plane surfacing operations are similar to bench grinders, but are equipped with metal disks to which abrasive disks are connected.

Universal grinders can perform both internal and external grinding of cylindrical and conical surfaces.

Surface grinders are designed for grinding flat surfaces. Machines of this type are divided into two groups: planer-type surface grinders whose table is rectangular in shape and traverses under the wheel, and rotary type surface grinders whose table is circular in shape and rotates under the wheel.

The surface grinder has a 18 by 11ј-in. work table with a maximum travel of 19ј -in. on vee and flat guideways. Steplessly-variable traversing speeds from 3.3 ft per min are provided hydraulically. There is no handwheel for longitudinal movement, but the table can be traversed slowly under the control of a separate lever or handwheel, for example, when wheel dressing is to be carried out. A cross travel of 12ѕ-in. is provided for the saddle, which is guided by central and outer bed-ways, and feed can be applied continuously at rates ranging from 8 to 96 in. per min, or intermittently at the ends of the table movement. Lubricant is delivered to the central bed-way by a built-in pump, and to the other ways by means of rollers. Drive to the wheel spindle, which runs at 2950 rpm, is taken from a 3-h.p. motor through a flexible coupling, and the bearings are lubricated by a built-in pump. The wheel-head can be swiveled on the saddle by a pinion and segment gear, and may be set in the horizontal or vertical position, or at any intermediate angle. It can be secured to the saddle in required position, by means of a clamp, and is provided with a sine bar attachment to facilitate accurate setting for angle. Disk-shaped grinding wheels up to 8-in. diameter can be mounted on the spindle. Alternatively, a 6-in. diameter segment-type wheel may be employed. Work up to 11ј-in. diameter high may be ground with a disk-shaped wheel, and up to 9ј-in. high with a segment wheel. The handwheel for traversing the wheel-head saddle on the column ways can be set in two positions on its shaft to give coarse and fine adjustment, and provision can be made for down feed to be applied automatically, in increments ranging from 0.0002 to 0.0008 in. per table stroke. Alternatively, hydraulic equipment can be provided, which enables the wheel-head saddle to be continuously traversed on the column ways for a pre-set distance under the control of upper and lower stops.

Grinding wheels consist of abrasive grains held together by some bond such as clay, shellac, or rubber. The hardness of the abrasive, the shape and form of the grain structure and the tenacity of the bond are each important in grinding operations. The grade of a grinding wheel denotes its hardness, which cannot be accurately determined by bond mixture or the method of manufacture. Wheel grade is often indicated by letters, running from E, soft, to Z, extremely hard. Grinding wheels are dressed or trued by metal "star" wheels or by mounted diamonds to remove metal particles or dull grains of abrasive, and to restore their original shape and accuracy.

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