NOV 2018

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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Page 6 of 23 | The Machine Tool Search Engine 5 a milling machine can produce a surface finish of around 32 microinch Ra and a lathe can produce a surface finish of around 16 microinch Ra. Grinding is required for a surface finish of 16 micro inch Ra and below. In fact, grinding can produce a super finish of 8 microinch Ra and below, and in some cases achieve a 2-microinch Ra, considered to be a micro finish. Super finishes are accomplished using two different, fine-grit abrasive wheels, as well as a polishing wheel when necessary. When grinding for accuracy or surface finish, the amount of material left to remove after machining is usually somewhere around 0.010 inch. The finer the surface finish required, the finer the wheel grit or polishing wheel needed. The cycle time to achieve the finished part size also becomes longer. Ideally, the least amount of material should be left after machining to provide just enough stock for the grinding operation to clean up to finish size. This approach will provide the optimum cycle time for the grinding operation. An Abrasive Process Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool. A grinding wheel consist of hard, sharp-edged particles, so that as the wheel spins, each particle acts like a single-point cutting tool. Grinding wheels are available in a multitude of sizes, diameters, thicknesses, grit sizes and bonds. Abrasives are measured in grit or particle size, and range from 8-24 grit (coarse), 30-60 (medium), 70-180 (fine) and 220-1,200 (very fine). Coarser grades are used where relatively high volumes of material must be removed. Finer grades are generally used after a coarser grade to produce a higher surface finish. Grinding wheels are made from a variety of abrasive materials including silicon carbide (generally used for non-ferrous metals); aluminum oxide (used for ferrous high- tensile-strength alloys and wood; diamond (used for ceramic grinding or final polishing); and cubic boron nitride (generally used for steel alloys). Abrasives can be further classified as bonded, coated or metal-bonded. Bonded abrasives consist of abrasive grits that have been mixed with binders and then pressed into the shape of a wheel. They are fired at a high temperature to form a glassy matrix, commonly known as vitrified abrasives. Coated abrasives are made of abrasive grits bonded with resin and/or glue to flexible substrates such as paper or fiber. This method is most often used for belts, sheets and flap disks. Metal-bonded abrasives, most notably diamond, are held together in a metal matrix in the form of a precision wheel. The metal matrix is designed to wear away to expose the abrasive media. Abrasive Bonds A bonding material or medium holds the abrasive grit within the grinding wheel and provides bulk strength. Open space or porosity is intentionally left within the wheel to enhance coolant delivery and release chips. Other fillers may be included, depending on the wheel's application and type of abrasive. Bonds are generally classified as organic, vitrified or metal. Each type offers application- specific benefits. Organic or resin bonds can withstand harsh grinding conditions such as vibration and high side forces. Organic bonds are particularly suited for high stock removal in rough applications such as steel conditioning or abrasive cutoff operations. These bonds are also beneficial for precision grinding of ultra- hard materials such as diamond or ceramics. Vitrified bonds provide excellent dressability and free-cutting behavior when precision grinding ferrous materials such as hardened steel or nickel-based alloys. Vitrified bonds

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