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Kurt Haller RV-9A
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Date: 1-7-2012
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Number of Hours: 8.00
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Manual Reference: 8
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Brief Description: Stnlss drilling: 6 broken bits, cheap bit best!??
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Bought 8 bits from OSH Aircraft Supply: 2x 1/16", 2x 5/64", 2x 3/32". These were cobalt steel split-point bits...at least the package claimed they were. Also got 2x #40 bits, split point but "generic" tool steel w/black oxide finish. OSH had no Rapidtap cutting fluid. Neither did Workbench H/W...but they did have plumbers pipe-thread cutting oil, so I got some of that.
Hoo boy, what a mess. The 3/32" bits wouldn't drill the stnlss for anything. I tried the smaller diameter bits and would eventually break thru, but chip generation was minimal during the so called cut. Just couldn't get enough pressure on the bit for it to dig in. Once thru, the 3/32" bit would enlarge the hole to final size well enough. But eventually, the small dia bits started breaking when my tired arms would place a unintended side load on the bit.
Eventually, with a little more than half the holes drilled, I managed to break all 6 of the cobalt bits! So, expecting total defeat, I tried the black oxide #40 bits, with the cutting fluid, and full trigger on the low-rpm setting of the cordless drill. Crick...crick...hey, look pigtails...broke thru in a few seconds! The rest of the holes were easy as pie.
Finished by copying the plans to layout the elliptical holes on the bottom of F-972 where the landing gear mains come thru. Drilled in the center of the cutout, roughed out the shape with nibblers. Will complete the fine finishing off the plane.
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