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.
I retracted the spindle and pulled the part off the machine. The drill had snapped about 30mm behind the carbide tip. The broken end was still in the hole. I spent the rest of the shift extracting it, cursing the tool, the material, and the machine in roughly equal measure.
I replaced the drill with a new one from the crib and went back to running. Same thing happened twelve holes later. Snap, torque alarm, broken drill stuck in the hole.
At this point I started looking for a pattern. Both breaks had happened at the same location on the shank, about 30mm behind the carbide tip. That is unusual. Normally gun drills break at the tip or at the drive pad. Breaking mid-shank suggests something else is going on.
I pulled a new drill out of the box and looked at it closely. The shank was ground smoothly, no visible defects. I rolled it on a flat surface to check for straightness. It was straight. I measured the shank diameter at several points. Consistent within a couple tenths.
Then I looked at the collet. It was a standard ER-32 collet, nothing fancy. I had tightened it the way I always did, with the collet wrench, giving it that extra quarter turn at the end. I decided to check the clamping force and borrowed a torque wrench from the maintenance crib.
I loosened the collet nut and retightened it with the torque wrench. The spec on the collet nut was 55 Nm maximum. When I torqued it to 55 Nm, I could feel how much less force I was using compared to my usual technique. I had been overtightening it by a wide margin.
Here is what I learned. An ER collet compresses evenly up to its rated torque. Beyond that, the collet starts to deform. The clamping force becomes uneven, concentrating at the front edges of the collet slots. That creates a stress concentration on the drill shank at exactly the point where the collet stops gripping. In my case, that stress concentration was high enough to initiate a crack on the drill shank after about ten to fifteen cycles of thermal expansion from drilling.
I replaced the collet with a new one, torqued it to 55 Nm, and ran the rest of the job. No more broken drills. The new collet also improved the runout at the tip, which I had not even realized was drifting. The holes got slightly straighter and the surface finish improved a hair.
I bought a torque wrench dedicated to collet nuts and hung it on the machine cabinet. Every operator on the floor got the same talk I had given myself. More clamping force is not better. The collet is designed to work at a specific torque range, and exceeding it damages the collet and the tool.
A cracked shank is a hard way to learn about torque specs. But I have not broken a drill from overtightening since, and that torque wrench has paid for itself many times over.