I have 1/2" and 3/8" mild (low carbon) steel plates I'm joining with a pattern of 5/16-18 bolts, and am drilling 11/32" through holes for these bolts. The first three holes went OK, but halfway through the fourth one, the drill started overheating and smoking, and stopped progressing. It was now blunted. So I stepped down to 21/64" and tried again in the same hole, but that one overheated and blunted, too. It looks molten around the leading edges.
Since I had already drilled 11/64" pilot holes and had the assembly clamped together to preserve hole alignment, I could flip it over and drill from the far side, following the same pilot hole. I only had these 11/32" and 21/64" bits, now ruined, so I used 23/64". That worked fine, and my holes are done.
But I want to learn from the experience. So, what's the lesson? What'd I do wrong?
More info: I'm drilling at about 1000 rpm per a chart from a drill bit maker. I'm using a fairly big nice drill press. These bits all happened to look pretty new, especially the /64" ones. The work is in a drill press vise clamped to the table. The holes are all perpendicular to the plates. I'm using a light oil as cutting fluid.
Maybe the first bit was just worn out (even though it looked good) and that made it overheat. But then why did the second one overheat in the same hole? Can an overheating drill bit harden the work beneath it? If this is what happened, I guess drilling to that point from the other side, the cutting edges now at a different angle, got through it just fine. Nothing felt funny about completing that hole.
Thanks!