Collet

Tool holder runout directly affects hole quality in deep hole drilling. I have measured the runout difference between collet holders and side-lock holders on dozens of gun drill setups, and the gap is significant enough to dictate which holder I use for each job. A difference of 0.01mm in runout at the holder can become 0.05mm or more at the drill tip. Problem Description Runout at the cutting edge causes the drill to cut unevenly. One side of the drill takes more material than the other, producing an oversized hole and accelerated tool wear. On gun drills, which rely on a single cutting edge and two guide pads, runout pushes one guide pad against the bore wall harder than the other, causing rapid pad wear and poor surface finish. ...

I had been running gun drills for about three years when I thought I had the tool holding figured out. Tighten the collet until it feels solid, then give it another quarter turn for good measure. That was my system. It had worked fine on every machine I had run, so I did not question it. Then we got a new job running 8mm gun drills in 4340 steel, about 500mm deep. The first few holes went fine. Good chips, consistent surface finish, everything looked normal. Around the fifteenth hole, I heard a noise I had not heard before. It was not the usual cutting sound. It was a sharp crack, like a twig snapping, followed by the spindle stopping hard on a torque alarm. ...