August 2007 Edition
Spotlight: Cutting Tools
No Room for Error
When each part costs more than $14,000, cutting thread holes has to be right the first time. This shop increased speed without sacrificing accuracy.
Gentz Industries, Warren, MI, makes turbine engine components for the commercial and military marketplace. It specializes in low-volume, high-precision parts from expensive and hard-to-machine materials, such as Inconel, titanium, and stainless. Gentz has no room for error or part-scrapping.
When Belinda Smith, a Seco Tools Inc., Warren, MI, application engineer, approached Gentz
Purchasing Manager Mark McWilliams and suggested the company switch to a 0.156" Seco threadmill for use on a combustor case component made of Inconel 718 heat-treated to 42 Rc, she was met with skepticism.
“We had already tested five competitive 0.156" size threadmills on this application, which requires the creation of 52 #10-32 thread holes,” McWilliams said, “all resulted in tool breakage. We weren’t real confident when Smith said Seco’s was better.”
Jet engine combustors are cleaned in preparation for threadmilling, much of which was done by handTo perform the threadmilling operation, Gentz used a 0.170" threadmill, which created undersized holes. Each of the 52 holes then had to be individually hand-tapped for finishing. The entire threading operation required two threadmills, making three passes, and about 15 taps to hand tap the part. Total time per component was 154 minutes of machining time plus another one to two hours for hand tapping.
The process, although inefficient, was workable. The Gentz team was not enthusiastic about changing a procedure that worked, especially when a forged combustor ring was $14,500 when it arrived at Gentz.
“Experience taught us that the 0.156" threadmill was just not suited for the job,” Jim Stevens, Gentz OEM manufacturing manager, said. “But, our long-standing relationship with Seco finally led us to try its solid carbide Threadmaster.”
The Threadmaster has a TiCN coating and micrograin structure that increases toughness and wear resistance compared to other solid threadmills.
Bumping Up the Numbers
Curt Hassan, Seco Regional Manager, recommended that Gentz more than double its feedrate from 78 ipm to 194 ipm. He also recommended increasing the machining center speed from 1,500 rpm to 5,000 rpm.
Each turbine engine combustor costs $14,500, so Gentz avoids scrap at all costs
“We were skittish about the speed increase,” Stevens said. “I couldn’t imagine the threadmill doing anything but falling apart on the Inconel.”
The Seco Threadmaster 0.156" completed the threadmilling operation in 53 minutes with only two passes, saving an hour in machining time and one complete pass. Hand-tapping time was reduced from a 90-minute average per component to 30 minutes.
To compensate for the increased feed and speed, some process changes were made: The programming parameters were modified with Seco’s Thread Milling Wizard software. It automatically generated the machine code based upon input parameters, reduced setup time, and created a perfect thread from the very first cut.
Gentz switched from a hydraulic chuck system to a shrink fit, thermal toolholding system offered by Seco.
“This helped us achieve less runoff, better tool life, and more even tool wear,” Stevens said. “With the machining center running at 5,000 rpm, the rigidity of the thermal toolholder is critical in achieving better tool balance.”
“Eliminating the hand-tap time was worth the change in this process,” Roger Bartolomei, Gentz COO, said. “Who would scoff at a savings of $62,000 per year?”
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