question about metal for machinists

[cadillac_al]

Today I had to drill out a stripped GM seat belt bolt. Actually started yesterday, I started in the hex hole with a 5/16 bit and it was drilling nicely. After I went down a 1/4" or so I figured I should put a drop of oil in the hole. Why? because when I am in machine shops I see them putting oil around the drill bit while drilling. I figure it's good for cutting or cools the bit or something. I start drilling again and it is drilling fine. I go down another 1/4" or so and I figure I need a 1/2" bit to take the head of the bolt off. I don't have one so I wait until the next day.

Today I figure I don't need a 1/2" bit, all I need is a 3/8" bit to drill out to the threads. I start drilling with a nice Irwin drill bit and it is going through like butter. I went down another 3/8" or so very easily. My gf was watching me and I told her that is the difference between good drill bits and cheap ones. I figure I should put a drop of oil in there so I don't ruin my nice bit. I put 1-2 drops of oil in the hole and start drilling again and it will not drill at all. I can't get one shaving to come up. WTF? This is blowing my mind.

My question for the machinists is, did I heat treat that bolt with the oil and make it impossible to drill? My theory is that the bolt was so hot from that easy yet aggressive drilling that putting oil on it heat treated it. OR, the oil just makes it too slippery to drill? I have used oil quite a few times before and never really knew if it helped or not but I have never seen a drill bit quit drilling immediately after a couple drops of oil is added. I don't mean flooding it with oil I actually count the drops 1, 2, that's enough. Anybody ever heard of something like this?

I suppose I can try that drill bit on something else tomorrow to determine if the bit just suddenly went dull or the bolt got hard.