Original Date: 09/20/2004
Revision Date: 01/18/2007
Best Practice : Gear Grinding/Hob Sharpening/Worm and Worm Gear
As an adjunct to the core business of hobbing precision gears, Forest City Gear sharpens its own hobs, precision grinds gears, and manufactures high quality worm and worm gear sets for precision alignment applications. These functions support and complement Forest City Gear’s precision hobbing and shaping capabilities.
Forest City Gear’s (FCG’s) fundamental business is hobbing and shaping (cutting) gears. The company developed high precision hobbing capabilities through the acquisition of state-of-the-art hobbing equipment and the pursuit of continuously improving quality. Many of the gears FCG cuts are carburized and hardened after hobbing. This heat treatment process can introduce enough distortion to reduce a precision hobbed American Gear Manufacturers Association (AGMA) quality 11 gear to one with several of the metrics falling below AGMA quality 8. FCG re-cuts some of these distorted heat-treated gears with solid carbide hobs to restore the pre-heat treatment quality; however, to achieve true high precision, FCG finish-grinds the gears after heat treatment by using its four, high quality gear grinding machines.
Three of FCG’s gear grinding machines are form grinders (Figure 2-1). The shape of the space between an adjacent pair of teeth is dressed into the grinding wheel, and gears are ground one tooth at a time. The form can be adjusted to provide sophisticated profile modifications with designated shapes in four regions. Grinding wheels use state-of- the-art SG-3 and SG-4 abrasives. These machines can grind either internal or external, spur or helical gears. FCG also has a threaded wheel grinder with a rack form dressed into the outside diameter of a large grinding wheel. This grinder develops the involute shape on gear teeth through a generative process and can grind external gears at a high production rate. With its gear grinding capabilities, FCG can set-up one of the grinders and produce gears with complex shapes in a matter of hours versus days often required in other shops with older and less sophisticated equipment.
As part of its dedication to high quality gear hobbing, FCG pays a premium to buy precision hobs and sharpens them to ensure that they retain their high precision. Rigid quality control is exercised through the sharpening process to ensure that the hobs are correctly mounted for sharpening, the sharpened faces are properly oriented to the hob, and there is no index error between the freshly sharpened faces. Most of the hobs feature ceramic coatings to enhance their cutting performance. FCG ensures that these coatings are properly stripped and the hobs are re-coated to return them to their original precision after sharpening.
FCG also uses ultra precision hobbing machines, which allow hobbing gears with up to AGMA quality 15 index and runout errors. This capability produces worm and worm gear sets for precision alignment applications. End products include equipment such as devices to aim telescopes, medical positioning equipment, precision rotary tables, and military sighting mechanisms. This small but profitable adjunct to FCG’s core business is a result of the company’s dedication to buying the best equipment and making the highest quality product.
Figure 2-1. Form Grinder
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