But using shapers was far from an ideal solution, because operators could not produce quality splines consistently using the tools. As a result, the gear maker was making too many scrap parts. “We’ve been making this gear for a number of years, and we knew we had to come up with a way to make them more economically.” says Loran Lantz, the Fairfield tooling engineer who supervised the project.
Finally, Fairfield’s engineers asked the tool designers at General Broach Co., Morenci, MI, if they could come up with a tool that would do the job. The tool General Broach delivered four months later is a unique, modular broach made up of 46 separate, replaceable inserts, shave rings, and wafers. The 600-lb. tool offers the speed and precision of broaching without the burdensome maintenance costs. Using the tool, Fairfield’s operators have been able to produce the helical gears 10 to 15 times faster, with less scrap, and with greater accuracy than they could with traditional shaper tools.
“With shaper cutting, you’re constantly relying on the operator to hold size,” says Lantz. “The biggest advantage of this broach is size control. Each part is consistent within a few tenths, so we’re producing a more precise spline with fewer rejects.”
The broach assembly consists of a large, precision-ground, HSS shaft that acts as an arbor. The shaft has a standard automatic 2 1/2″ puller and retriever at each end. Nine holder segments are mounted onto the arbor, including five wire-EDM-produced wafers that locate the broach tooling in relation to the arbor centerline. To eliminate unpredictable machining and wear factors when producing the wafers, General Broach machined them from prehardened 5162 high-chromium mold steel. The other four holder segments are simple spacers fitted between the wafers to lock them in place.
A pilot on the broach guides the gear blank onto the tool assembly. It presses the holder wafers and spacers against a flange built into the arbor. A back-up plate positioned behind the rings is threaded into the arbor and also serves as a rear pilot to guide the part as it comes off the broach.
Bolted around the diameter of the broach behind the pilot are 24 replaceable inserts measuring 21 1/4″ long. These hardened, ground HSS tools rough out the form and finish the root diameter of the spline. General Broach treats the inserts with a ferritic nitrocarburization process to improve lubricity and reduce galling during the broaching operation. The inserts are designed to enter the bore at various locations to increase tooth engagement and reduce the harmonics normally produced by extreme cutting-force variations. As a result, the entire broaching process is smoother and quieter.
Eight shave rings, cut on wire-EDM equipment, produce the finish involute spline profile. These rings feature full form relief to reduce friction during the broach cycle. They are coated with titanium nitride (TiN) to improve lubricity and tool hardness and to increase abrasion resistance. The wire-EDM process holds the shave rings to 0.0002″ profile and 0.0005″ cumulative tooth-spacing tolerances. These tolerances are much tighter than what could be achieved with conventional broach tools.
Although it cost Fairfield a substantial amount to have a broach custom designed and built, the company believes it made a wise investment. “One concern we had to face up to was the initial expense of the broach,” says Lantz. “It was greater than the shaper we’ve been using all along. But we’re convinced that we’ll save money with this tool. We’re already saving a lot of manufacturing time.”
Using the custom broach, Fairfield should see significant savings in repair and replacement costs, according to General Broach. Fairfield’s initial costs covered the materials, heat treatment, and coatings used in the original tools’ construction. If the gear maker were using a conventional broach, it would have to incur these costs every time the tool became worn or broken, because the entire tool would have to be replaced.
With this broach, however, Fairfield will only have to replace the perishable inserts and shave rings. The broach’s arbor, wafers, spacers, and pilots are made of durable materials and will rarely need to be replaced. Even routine maintenance should be easier, because the rings can be sharpened without removing them from the assembly.
To gain further economies and flexibility, the permanent portions of the assembly are engineered to accommodate variations from the original design. By adding and subtracting rings, for example, Fairfield operators can compensate for deviations caused by heat-treatment distortions in their parts without needing to purchase a separate tool with the necessary geometry.
“We’re very pleased with the results,” Lantz says “From the initial designs, through prove-out, and now in production, our experience has been very positive. This is really a different kind of broach for us. We were a little uncertain in the beginning, but it’s solved an important problem for us. We get more consistent quality on a precision part, faster production times, and reduced operating and maintenance costs.”
