Metal Grinding and Cutting

Cutting and Grinding

Grinding practice is a large and diverse area of manufacturing and tool making. It can produce very fine finishes and very accurate dimensions; yet in mass production contexts it can also rough out large volumes of metal quite rapidly. It is usually better suited to the machining of very hard materials than is "regular" machining (that is, cutting larger chips with cutting tools such as tool bits or milling cutters), and until recent decades it was the only practical way to machine such materials as hardened steels.

Industrial Abrasive Discs Linishing Belts Backing Pads
Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft's diameter by half a thousand of an inch (thou) or 12.7 um. Technically, grinding is a subset of cutting, as grinding is a true metal cutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is often understood to refer to the macroscopic cutting operations, and grinding is often mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though technically grinding is a subset of cutting. Similar abrasive cutting processes are lapping and sanding. Surface grinding uses a rotating abrasive wheel to smooth the flat surface of metallic or non metallic materials to give them a more refined look or to attain a desired surface for a functional purpose. The tolerances that are normally achieved with grinding are ± 2 × 10−4inches for a grinding a flat material, and ± 3 × 10−4inches for a parallel surface. (in metric units : 5 um for flat material and 8 um for parallel surface). The surface grinder is composed of an abrasive wheel, a work holding device known as a chuck, either electromagnetic or vacuum, and a reciprocating table. Typical work piece materials include cast iron and minor steel. These two materials don't tend to clog the grinding wheel while being processed. Other materials are aluminium, stainless steel, brass and some plastics. Cylindrical grinding (also called centre-type grinding) is used in the removing the cylindrical surfaces and shoulders of the work piece. The work piece is mounted and rotated by a work piece holder, also known as a grinding dog or centre driver. Both the tool and the work piece are rotated by separate motors and at different speeds. The axes of rotation tool can be adjusted to produce a variety of shapes. The five types of cylindrical grinding are: outside diameter (OD) grinding, inside diameter (ID) grinding, plunge grinding, creep feed grinding, and centre less grinding. A cylindrical grinder has a grinding (abrasive) wheel, two centres that hold the work piece, and a chuck, grinding dog, or other mechanism to drive the machine. Most cylindrical grinding machines include a swivel to allow for the forming of tapered pieces. The wheel and work piece move parallel to one another in both the radial and longitudinal directions. The abrasive wheel can have many shapes. Standard disk shaped wheels can be used to create a tapered or straight work piece geometry while formed wheels are used to create a shaped work piece. The process using a formed wheel creates less vibration than using a regular disk shaped wheel. Tolerances for cylindrical grinding are held within five ten-thousandths of an inch (+/- 0.0005) (metrical: +/- 13 um) for diameter and one ten-thousandth of an inch(+/- 0.0001) (metrical: 2.5 um) for roundness. Precision work can reach tolerances as high as five hundred-thousandths of an inch (+/- 0.00005) (metrical: 1.3 um) for diameter and one hundred-thousandth of an inch (+/- 0.00001) (metrical: 0.25 um) for roundness. Surface finishes can range from 2 to 125 micro inches (metrical: 50 nm to 3 um), with typical finishes ranging from 8-32 micro inches. (metrical: 0.2 um to 0.8 um) Creep-feed grinding (CFG) was invented in Germany in the late 1950s by Edmund and Gerhard Lang. Unlike normal grinding, which is used primarily to finish surfaces, (CFG) is used for high rates of material removal, competing with milling and turning as a manufacturing process choice. Depths of cut of up to 6 mm (0.25 inches) are used along with low work piece speed. Surfaces with a softer-grade resin bond are used to keep work piece temperature low and an improved surface finish up to 1.6 micrometres Rmax With (CFG) it takes 117 sec to remove 1 in.3 of material, whereas precision grinding would take more than 200 sec to do the same. (CFG) has the disadvantage of a wheel that is constantly degrading, and requires high spindle power, 51 hp (38 kW), and is limited in the length of part it can machine. To address the problem of wheel sharpness, continuous-dress creep-feed grinding (CDCF) was developed in the 1970s. It dresses the wheel constantly during machining, keeping it in a state of specified sharpness. It takes only 17 sec. to remove 1 in3 of material, a huge gain in productivity. 38 hp (28 kW) spindle power is required, and runs at low to conventional spindle speeds. The limit on part length was erased. High-efficiency deep grinding (HEDG) uses plated super abrasive wheels, which never need dressing and last longer than other wheels. This reduces capital equipment investment costs. (HEDG) can be used on long part lengths, and removes material at a rate of 1 in3 in 83 sec. It requires high spindle power and high spindle speeds. Peel grinding, patented under the name of Quick point in 1985 by Erwin Junker Maschinenfabrik, GmbH in Nordrach, Germany, uses a tool with a with super abrasive nose and can machine cylindrical parts. VIPER (Very Impressive Performance Extreme Removal), 1999, is a process patented by Rolls-Royce and is used in aerospace manufacturing to produce turbine blades. It uses a continuously dressed aluminium oxide grinding wheel running at high speed. CNC-controlled nozzles apply refrigerated grinding fluid during the cut. VIPER is performed on equipment similar to a CNC machining centre, and uses special wheels. Ultra-high speed grinding (UHSG) can run at speeds higher than 40,000 fpm (200 m/sec), taking 41 sec to remove 1 in.3 of material, but is still in the R&D stage. It also requires high spindle power and high spindle speeds. A grinding wheel is an expendable wheel used for various grinding and abrasive machining operations. It is generally made from a matrix of coarse abrasive particles pressed and bonded together to form a solid, circular shape, various profiles and cross sections are available depending on the intended usage for the wheel. Grinding wheels may also be made from a solid steel or aluminium disc with particles bonded to the surface.
Industrial Abrasives Metal Cutting Grinding Discs