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Superabrasive process shapes crankshafts ... fast.

If you've ever mowed a lawn, the odds are pretty good you've fired up a small engine built by Briggs & Stratton. The company annually produces approximately 7 million four-cycle engines ranging from 3.5 to 15 hp for push mowers, garden tractors, and other equipment.

You might not guess that Briggs & Stratton was one of the first--and remains one of the biggest--users of cubic boron nitride (CBN) grinding wheels in the United States. The company uses a high-speed superabrasive process to grind the bearing surfaces on ductile cast-iron single-throw crankshafts for 12- and 15-hp lawn tractor engines. The story of how B & S implemented the advanced process illustrates the persistence that is often required to take advantage of the capabilities of a new technology.

Program manager/manufacturing engineer John Edwards says Briggs & Stratton first checked out high-speed grinding (HSG) in the mid-1980s, when company manufacturing engineers were rethinking the crankshaft manufacturing process. "We looked at the whole process. Before, we used manual dedicated lathes, multiple single-punch grinds, a straightening process, hardening, milling keyways, and finishing. We looked at using HSG to upgrade our tooling technology and machine uptime."

The company considered CNC turning and conventional rough grinding, but eventually settled on the HSG process to provide most of the stock removal on the main bearings. The HSG machines were supplied by Guehring Automation Inc, Sussex, WI. The desire to machine the crankshafts after surface hardening to avoid the workpiece distortion associated with heat treatment was one reason B & S chose grinding over an upgraded turning process. The HSG process is performed after the cranks are case-hardened to hardnesses up to 65 Rc. The CBN wheels remove 0.125" of stock per side.

Mr Edwards says Briggs & Stratton's experience with HSG met with limited success at first, mainly because the aluminum oxide grinding wheels being used simply wore out too quickly to make the process economical. The decision was made to try superabrasive technology. In 1987, Guehring, GE Superabrasives, and B & S teamed up to make the "single CBN layer electroplated process" work in production.

Now, roughing is done in a cell consisting of four Guehring RB 625 HSG machines with 75-hp spindles. The single-layer plated CBN wheels used require no dressing, eliminating the expense of diamond dressing rollers. The workpieces are held in a headstock and tailstock between two opposing wheels. One wheel grinds one bearing surface, and the other wheel simultaneously hits the other.

In another part of the cell, Briggs & Stratton developed the concept of using two more Guehring machines to rough and finish two eccentric bearings on the crankshafts. Removing 0.125" of stock per side, the HSG process produces the required surface finish and holds bearing size tolerance to |+ or -~0.0005".

Grind time for the crankshafts is 12 to 16 seconds. Floor-to-floor time for the automated process, which is operated by a single setup person, is about 34 seconds. Life on the 20" diam CBN wheels can extend up to 330,000 pieces at speeds of 24,000 to 32,000 sfm.

To take full advantage of the productivity of the HSG process, Briggs & Stratton also developed and implemented a process for using a small (1" diam) CBN wheel for high-speed grinding of a 3" long keyway into the power takeoff end of the crankshafts. Grind time for the process is 10 seconds; floor-to-floor time for the manually loaded/unloaded process is 18 seconds. Life on the keyway wheel has exceeded 14,000 pieces at speeds of 17,000 to 18,000 sfm.

Even after the switch to CBN, kinks remained in the production process that needed to be ironed out, says Mr Edwards. For example, B&S design engineers standardized the product line from three cranks down to one to minimize both inventories of the expensive wheels and the two-hour changeovers needed for the dedicated tooling.

Another problem associated with the HSG process was handling the large quantities of grinding swarf generated. "Filtration is extremely important," says Mr Edwards. "We couldn't do HSG without it. For each machine, we use 150 gpm of straight oil at 250 psi, and we use heat exchangers that maintain the temperature of the cutting oil between 90 and 95 F." A blow-off of the cranks after high-speed grinding prevents oil contamination of other parts of the process, which use water-soluble coolants.

Was the effort to develop the HSG technology worth it? "We probably have material removal rates ten times faster than conventional grinding processes," says Mr Edwards. And, being able to grind the cranks after hardening has streamlined the production process by eliminating straightening.

Mr Edwards also is proud of the fact that B & S has been able to work with US companies to supply the CBN wheels. "We worked awfully hard to develop US sources. One of the things that we've been successful at in the last three years is working with the American companies who are competing head-on with European and Japanese suppliers," he adds.
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Manufacturing Solutions
Publication:Tooling & Production
Date:Sep 1, 1993
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