Disc Grinders and Double-Disc Grinding
Disc grinding is similar to surface grinding, but with a larger contact area between disc and work-piece. Disc grinders are available in both vertical and horizontal spindle types. Double disc grinders work both sides of a work-piece simultaneously. Disc grinders are capable of achieving especially fine tolerances.
Surface grinding is the most common of the grinding operations. It is a finishing process that uses a rotating abrasive wheel to smooth the flat surface of metallic or nonmetallic materials to give them a more refined look by removing the oxide layer and impurities on work piece surfaces. This will also attain a desired surface for a functional purpose.
The surface grinder is composed of an abrasive wheel, a work-holding device known as a chuck, and a reciprocating or rotary table. The chuck holds the material in place while it is being worked on. It can do this one of two ways: ferromagnetic pieces are held in place by a magnetic chuck, while non-ferromagnetic and nonmetallic pieces are held in place by vacuum or mechanical means. A machine vise (made from ferromagnetic steel or cast iron) placed on the magnetic chuck can be used to hold non-ferromagnetic work-pieces if only a magnetic chuck is available.
Factors to consider in surface grinding are the material of the grinding wheel and the material of the piece being worked on.
Typical work-piece materials include cast iron and mild steel. These two materials don’t tend to clog the grinding wheel while being processed. Other materials are aluminum, stainless steel, brass and some plastics. When grinding at high temperatures, the material tends to become weakened and is more inclined to corrode. This can also result in a loss of magnetism in materials where this is applicable.
The grinding wheel is not limited to a cylindrical shape and can have a myriad of options that are useful in transferring different geometries to the object being worked on. Straight wheels can be dressed by the operator to produce custom geometries. When surface grinding an object, one must keep in mind that the shape of the wheel will be transferred to the material of the object like a reverse image.
Spark out is a term used when precision values are sought and literally means “until the sparks are out (no more)”. It involves passing the work-piece under the wheel, without resetting the depth of cut, more than once and generally multiple times. This ensures that any inconsistencies in the machine or work-piece are eliminated.
A surface grinder is a machine tool used to provide precision ground surfaces, either to a critical size or for the surface finish.
The typical precision of a surface grinder depends on the type and usage, however ±0.002 mm (±0.0001 in) should be achievable on most surface grinders.
The machine consists of a table that traverses both longitudinally and across the face of the wheel. The longitudinal feed is usually powered by hydraulics, as may the cross feed, however any mixture of hand, electrical or hydraulic may be used depending on the ultimate usage of the machine (i.e., production, workshop, cost). The grinding wheel rotates in the spindle head and is also adjustable for height, by any of the methods described previously. Modern surface grinders are semi-automated, depth of cut and spark-out may be preset as to the number of passes and, once set up, the machining process requires very little operator intervention.
Depending on the work-piece material, the work is generally held by the use of a magnetic chuck. This may be either an electromagnetic chuck, or a manually operated, permanent magnet type chuck; both types are shown in the first image.
The machine has provision for the application of coolant as well as the extraction of metal dust (metal and grinding particles).
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