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Posted on : Friday 12th June 2020 03:54 PM

Parabolic Flute Drill

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I received an email from Lynn Fifer, a retired cutting tool professional and CTE reader since 1967, who shared his recollections about the development of the parabolic flute drill. Fifer said the development occurred in 1950s at the old Mohawk Tools Inc., Montpelier, Ohio, well before he started with the toolmaker in 1967. 

Fifer stated Frank Hofbauer, company founder and CEO, took a trip to his home country, Germany, to see some customers, as well as visit family and relatives. Volkswagen was one such customer, which was facing challenges with deep-hole drilling aluminum engine blocks. Hofbauer returned home with part prints and application information for his son Bill to work on. Mohawk also received aluminum engine blocks for test drilling.

“Like any tool engineer, Bill looked at the cross section of a drill and tried to come up with a design that would allow a lot of chips to flow up the flutes without any restrictions but still retain adequate longitudinal column strength,” Fifer noted. “He increased the web to about 38% of diameter for strength, which decreased the flute capacity. So to increase capacity, he rolled off the heal of the land to open up the flutes. With a heavy web at the point, and so as not to restrict chip flow, it would need to be a straight or parallel web.”

Usually, a drill with a thick web requires web thinning or a split point, Fifer continued. “A flatter point angle would direct the chips into the flute better, but the conventional split or crankshaft drill point didn’t work well in aluminum, so the split was modified with a radius and the web across the center was increased slightly for the aluminum.”

After successfully testing the drill, Fifer stated that Bill Hofbauer was not satisfied with having to take two passes during the flute grinding operation and brought in a man named Bill Hertlein to design a horizontal dresser to generate the parabolic flute in one pass.

Eventually, the head of Guhring at the time saw the unique drill at the VW factory, Fifer added. “A patent was never sought, and the GT 100 was born,” he noted about Guhring’s parabolic drill. “The rest is history.”


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