Ground to order

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END USER: General Broach Co., (517) 458-7555, www.generalbroach.com. CHALLENGE: Increase capacity and maintain quality while grinding broaches for aerospace applications. SOLUTION: CNC surface and creep-feed grinder with auxiliary grinding head and large work volume. SOLUTION PROVIDERS: United Grinding Technologies Inc., (937) 847-1214, www.grinding.com; and Tooling Systems Sales Inc., (248) 656-2035

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General Broach Co., Morenci, Mich., designs and manufactures broaching tools and machines and provides broaching services. GBC puts special emphasis on tools and machines to broach aerospace parts. 

In broaching, a tool with a series of progressively larger teeth is drawn through or across a workpiece to create a feature, usually an irregular shape such as a keyway or spline. In succession, the teeth rough, semifinish and finish the desired form. Broaching generally is performed on a specialized broaching machine in a single pass. “The broach kind of whittles away at the part,” said Larry Stover, general manager of GBC’s broach tool division. 

Courtesy of General Broach

General Broach employs a Blohm Planomat CNC surface and profile grinder to produce broach tool segments with complex “fir-tree” profiles. 

Typical of the aerospace part features cut with GBC broaches are slots in titanium or nickel-base alloy turbine engine compressor discs. The slots hold turbine blades around the disc periphery and are generally broached to an irregular V-shape that resembles a fir tree. A single turbine disc may require 90 to 100 slots. 

View this brief video to see an animated demonstration of slot broaching.

Depending on the disc being machined, a full set of broaches may consist of eight to 10 segments of tools that together measure 100 " to 300 " in length. Individual segments range from 4 " to 39 " long, 0.6250 " to 2.5 " wide and 1.0 " to 2.5 " high. In the first segments in a set, the teeth usually are simple shapes. In succeeding segments, the teeth become larger, change form and eventually reach the final fir-tree configuration. The tools usually are ground from PMT-15 P/M alloy, Stover said, adding that they are rarely coated because most are titanium-based coatings, which chemically react with titanium workpieces. 

A few years ago, GBC had been grinding turbine broach segments on a manual 6 "×18 " Gardner surface grinder fitted with a CNC wheel dresser. As demand for the broaches grew, production speed and capacity became an issue. Stover, then production manager at GBC, set out to find a way to increase capacity. 

To find the equipment needed to accomplish this, Stover took a lead employee from each GBC department involved with the tools to IMTS. At the trade show, GBC staff researched different grinders, means of inspection and software. 

The consensus choice for a grinding machine was a Blohm Planomat CNC surface and profile creep-feed grinder from United Grinding Technologies Inc., Miamisburg, Ohio. The Planomat machines are assembled from base modules, including the machine bed, column, wheelhead and machine table, chosen to meet specific end-user requirements.

Brad Contrucci, president of Tooling Systems Sales Inc., Rochester, Mich., which represents United Grinding in that area, met with Stover after the show. After discussing part requirements, Contrucci suggested machine options suited to GBC’s needs. Contrucci said GBC’s deep understanding of tooling geometries and the broaching process facilitated the company’s selection process. 

GBC purchased a Planomat 612 grinder with a 24 "×48 " work envelope, a main spindle with a 400mm wheel and a small, high-frequency spindle on a separate axis. GBC also specified that a rotary disc dresser be mounted on the machine table. 

The grinder’s CNC capability facilitated fast changeover between parts. That’s important because there typically are a lot of changeovers in the aerospace parts industry, Stover noted. 

In addition to speed, a CNC grinder assures accuracy and repeatability. Those qualities are critical, according to Stover, because the aerospace manufacturers applying the tools must hold tolerances within 0.0003 " to 0.0005 ". In light of the flight-critical nature of aerospace products and the expensive exotic alloy workpiece materials being broached, trial-and-error is not acceptable. “When you manufacture a tool it has to be right, because if you scrap one of these aerospace parts, you are looking at a cost of several thousand dollars,” he said.

The machine’s high-speed auxiliary grinding head significantly contributed to productivity in grinding backoff, or relief, behind the cutting edge of each broach tooth. The relief is critical for providing clearance between the workpiece and the cutting teeth. 

Previously, backing off the individual teeth was time-consuming. Tooth forms typically are relieved from 1° to 3°, with some requiring as much as 5° of relief. “We start at the back of the tooth and work towards the cutting edge, and we have to be able to get the grinding tool off that tooth without hitting the cutting face of the next tooth,” Stover said. 

The auxiliary head on the Planomat machine enabled the desired tooth form to be dressed onto a wheel as small as 50mm in diameter. The small wheel can grind the relief while not interfering with the next tooth’s cutting face. Use of the small-diameter wheel on the auxiliary head “probably cut at least 60 percent of the form grinding time,” Stover said. 

Stover added that the new machine’s generous work envelope was “a good fit for us” considering the range of broaching segment sizes the shop produces. 

To meet growing demand for its broaches, GBC has since acquired another Planomat machine.