Torque

Spindle power and torque are related but different specifications. Power determines how fast material can be removed. Torque determines how much cutting force the spindle can deliver at low RPM. For deep hole drilling, misunderstanding these two specs leads to undersized spindles that stall in the cut. I have seen a 35 kW spindle stall out on a 60 mm BTA drill because the machine peaked at high RPM and delivered only 80 Nm at the feed rate needed for BTA parameters. ...

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 estimate cutting forces to check whether the machine can handle the job before I start every deep hole drilling operation. Cutting forces in gun drilling have two components that I track separately: thrust force pushing the drill forward and torque resisting the rotation. Getting these wrong means either stalling the machine feed axis or snapping the drill shaft at depth. I have seen both outcomes on the shop floor, and neither is cheap to fix. ...