Drill bit Heating

When boring a hole using a drill bit and drill motor, why does the drill motor seem to have to labor harder (drill rpm's fall) just BEFORE the bit punches through? This becomes more obvious as the bit size increases. I have wondered if the increased heat from the friction as the metal thins contributes to this. Thanks Brian
- Brian (age 56)
Knoxville, TN.

That's a wonderful question. I don't know the answer but your idea makes a lot of sense. When there's a lot of metal behind the bit, it can carry the heat away better. When it gets thin, it will heat up more. There could be some other changes. For example, the metal could start to flex a bit when it gets thin. Another simple effect is that the farther you drill, the more friction occurs in the part already drilled. That would, however, give a gradually decrease in speed rather than a special effect near the punch-through point.

Does it happen more or less equally with metals with a variety of different thermal conductivities (steel is low, aluminum high)? 

Mike W.

(published on 08/17/2016)