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Faster grinding, better broaches.

Faster grinding, better broaches

One of our most recent improvements in broach manufacturing is grinding the tooth gullets of hardened M-2, M-3, M-4, and T-15 toolsteel broaches to full depth from the solid in a single creep-feed pass. This is quite an accomplishment, because typically these materials are hardened to between 62 and 66 Rc.

Formerly, gullets were cut by milling, with the broach in the unhardened condition. After hardening, the gullets were finish-ground with profiled aluminum-oxide ([Al.sub.2][O.sub.3]) wheels.

Although our milling operations were efficient, grinding was relatively slow and unproductive. Our [Al.sub.2][O.sub.3] wheels tended to grind hot, so we had to keep material-removal rates low to avoid burning ground surfaces.

Because [Al.sub.2][O.sub.3] wheels wore fast on hardened tool steel, frequent wheel dressing was necessary to hold the required profile. The resultant machine downtime reduced productivity substantially. And, even with frequent wheel dressing, it was difficult to hold close tolerances.

Electroplated CBN wheels

We solved our productivity and quality problems by investing in a CNC broach-grinding machine and electroplated cubic boron nitride (CBN) wheels. The CBN wheels are made by machining the desired profile into the periphery of a metal wheel core, then bonding a single layer of the superabrasive to the profile. Our wheels are made by Superabrasives Inc, Novi, MI.

CBN - a super-hard abrasive - wears hundreds of times slower than [Al.sub.2][O.sub.3] abrasive when grinding hardened tool steels.

Also, CBN - unlike aluminum oxide - is a thermal superconductor. Since CBN conducts grinding heat away from the material being ground, it helps to prevent burning, even when grinding hardened tool steels at high material-removal rates.

The broach grinder is designed to take full advantage of the inherent productive potential of electroplated CBN wheels. Our wheels are 6" dia, made with an extra-tough CBN crystal designed for high production rates on difficult-to-grind tool steels. Abrasive mesh size is 100/120. Wheel speed is 6000 rpm, and creep-feed rates are varied, depending on the width and depth of the gullet.

To reduce friction, we use a straight-oil coolant. Three nozzles direct large volumes of this fluid into the grinding zone. Relatively high coolant pressures are required to ensure good penetration into the grinding area and to keep chips from clogging the wheel.

Production results

For efficient use of machine time, we grind as many broaches as possible in the same setup. A different wheel serves each broach design, so setup consists of changing wheels, loading the magnetic chuck of the machine with broaches, calling up the right program from the CNC system's memory, and positioning the wheel for the first cut.

After the start button is pushed, the machine cycles automatically until the tooth gullets in all the broaches are machined. The electroplated CBN wheel cuts efficiently, holds the correct gullet form, including small radii, and requires no maintenance throughout its long useful life.

In the case of most broaches, gullet-spacing tolerances of [plus or minus] 0.002" are held routinely. However, the setup can hold tolerances as close as [plus or minus] 0.0005" (the limit of machine capability) when required.

Wheel life depends on width and depth of gullets and, to some extent, on the grindability of the tool steel. Typically, wheel life ranges from 1500 to 2000 inches of cut.

When the wheel no longer cuts efficiently, it's returned to Superabrasives Inc for recycling. The layer of spent CBN abrasive is stripped off and a fresh layer is electroplated to the grinding surface. Performance of recycled wheels is identical to that of new wheels, and recycling can reduce average wheel cost per broach substantially.

Good return

Our investment in CNC/CBN broach-grinding technology has proved to be a sound one. Eliminating the milling operation reduced broach-production time and cost substantially, and grinding from the solid with electroplated CBN wheels has helped improve performance of our broaches. Customers have reported broach-life increases of 10 to 20 percent and more. We think this is because grinding with CBN results in less thermal damage to the material being ground than grinding with [Al.sub.2][O.sub.3].

When overheated and cooled, broach materials may develop residual tensile stresses that promote fatigue cracking in service and thus shorten broach life. This overheating is less likely in the case of grinding with CBN wheels, due to their cooler-grinding characteristics and required lubrication. In fact, when broach materials are ground with CBN wheels, residual compressive stresses are set up that reduce fatigue failure in service, thus extending the life of the tool.

PHOTO : Electroplated CBN wheel on broach-grind-ing machine cuts full-depth tooth gullets from the solid. Strong flow of straight-oil coolant controls grinding heat and keeps the wheel clean.

PHOTO : Tooth gullets in this hardened-steel broach are ground from the solid by an electroplated CBN wheel. Width of cut is 0.187", and depth of cut is 0.110". Workpieces are ganged on a magnetic chuck.
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Title Annotation:electroplated superabrasive wheels
Author:Tussing, Herbert
Publication:Tooling & Production
Date:Dec 1, 1989
Words:831
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