Techspex

NOV 2018

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

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T O D A Y ' S G R I N D I N G T E C H N O L O G Y 6 If you want to achieve a tight part tolerance and a fine surface finish while consuming less production time and lower operator involvement, now is the time to look at the latest in grinding technology. are specifically designed to provide extremely strong adhesion to CBN grains through a chemical reaction, thus enabling an excellent ratio of stock removal to wheel wear. Metal bonds offer excellent wear resistance and form-holding ability. They can range from single-layer plated products to multi-layered wheels which can be made extremely strong and dense. Metal-bonded wheels can be too tough to dress effectively. However, newer wheels with a brittle metal bond can be dressed in a manner similar to vitrified wheels and have the same beneficial free-cutting grinding behavior. Wheel Dressing During the grinding process, the abrasive wheel can wear, become dull, lose its profile form or "load up" as swarf or chips stick to the abrasive. Then, rather than cutting, the abrasive wheel begins rubbing the workpiece. This condition creates heat and reduces the effectiveness of the wheel. When the wheel loads up, chattering will occur, and the workpiece surface finish will be affected. Cycle times will increase. At this point, the wheel must be "dressed" to sharpen the wheel, thereby removing any material lodged on its surface and returning the wheel to its proper form, as well as bringing fresh abrasive grit to the surface. Many types of wheel dressers are utilized in grinding. Most common is a single-point, static, on-board diamond dresser that sits in a block, usually positioned on the machine's headstock or tailstock. The face of the grinding wheel is passed over this single-point diamond and a small quantity of the abrasive wheel is removed to sharpen it. Two or three diamond blocks can be used to dress the face, sides and form of the wheel. Rotary dressing is now becoming a popular method. A rotary wheel dresser is coated with hundreds of diamonds. It is often used in creep-feed grinding applications. Many manufacturers have found rotary dressing to be superior to single-point or cluster dressing for processes that require high part production and/or close part tolerance. With the introduction of vitrified superabrasive grinding wheels, rotary dressing has become a necessity. A swing dresser is yet another type of dresser that is used for large form wheels which require deeper and longer dressing travel. Off-line dressers are used primarily to sharpen the wheel away from the machine while using an optical comparator to verify form profiles. Some grinding machines use wire EDM to dress metal-bonded grinding wheels still mounted in the grinder. Machine Construction On a surface grinder, workpieces are most often held with a magnetic chuck, vacuum chuck or special fixtures bolted directly to the table. For cylindrical grinding, the workpiece is normally held between centers, in a collet, with a three- or four-jaw chuck or on special fixtures. Tool and cutter grinders most often use precision collets. To consistently produce part accuracy of 0.0002 inch and below, with super finishes under 16 microinches, grinding machines must be designed to control vibration and thermal growth. Machine bases are often

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