Close-up of tapped hole and Carmex Mini-Mill thread tool
Rocket engine part machined from Inconel 718 at Protomatic
Protomatic Precision CNC Manufacturing, a high precision prototyping and manufacturing company, is no stranger to hard machining. A recent challenge involved machining and tapping an Inconel 718 rocket engine component. According to John Donajkowski, Manufacturing Manager, "Inconel represents a very difficult machining territory, especially when it comes to tapping. Material cost is extremely high and, if a tap breaks in the last hole, the part is scrapped. We never repair or rework critical parts due to broken thread mills. We have to drill and tap perfectly every time, and knowing our tool life is critical."
In addition to the preliminary machining operations, the rocket engine component required six 1/4-20 UNC 3B diameter threaded holes. In developing the tooling protocol, the initial trial involved a conventional tap that was acquired from a long-time source. "We had great support from their technical staff, but we were unable to tap more than one piece," said Donajkowski. "After the initial six holes, the tool would break in the second part. A second effort, from another supplier, involved a right-hand, single-point thread mill. Unfortunately, the performance was equally poor."
The solution involved the use of a Carmex 3-flute Mini-Mill thread mill operating at a 2.40 IPM feedrate at a spindle speed of 2,010 RPM. Under the test conditions, the Carmex thread mill increased the hole count from six threaded holes to 60 per thread mill, improving tool life by a factor of 10.
Doug Wetzel, Protomatic General Manager, said, "In testing any tooling, especially for super alloy parts, we not only determine tool life but analyze why one tool is superior to another. For instance, most thread milling operations involved a right-handed, bottom-to-top cut operation. In that process, the cutting edge of the tool broad faces the cutting surface, resulting in a large surface load that not only deflects the cutter but also requires significant coolant at the cutting edge. The incidental high temperatures shorten tool life and present a near impossible solution in blind hole tapping.
"The Carmex solution works because the rigid three-flute, three-thread mill is a left-hand tool that cuts from the top down. Due to what Carmex calls its 'helical advantage,' it provides a climb mill style of cut and substantially reduces the cutting load. This, compounded with a two-cut operation-first at 80% and finished with a 20% chip load, results in the 10-fold improvement in tool life, as well as predictable performance."
According to Jim White, Carmex Regional Manager, "One of the keys to Protomatic's success in dealing with the hard machining of complex parts has to do with their analytical approach. By understanding what makes one tool superior to another, they not only solve the problem at hand, but also add to a body of knowledge that is transferable to other situations. Their approach is an excellent example of the way in which our American manufacturers are emerging as superior competitors in the world market."
Founded by aerospace veteran Bill Wetzel in 1971, Protomatic serves industries that include automotive, aerospace, defense and optical. Its 30,500 sq. ft. facility employs specialty lighting and climate control.
For more information contact:
Protomatic Precision CNC Manufacturing
2125 Bishop Circle West
Dexter, MI 48130
734-426-3655
www.protomatic.com
Carmex Precision Tools
2075 State Hwy 175
Richfield, WI 53076
888-628-5030
sales@carmexusa.com
www.carmexusa.com