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.

[mongocanfly]

Cutting oil both cools and lubricates...its either 1 of 2 or both things...the bit is dull or the bolt could be surface hardened...meaning you drilled out the softer center and have hit the hard surface...
I've had issues drilling some bolts...and case hardened is usually the reason...

[e015475]

For the most part, using cutting oil when you are hand drilling just serves to help you clear chips out the flute of the bit. It is very unlikely you put enough heat into the fastener to 'heat treat' it

A lot of larger bits have a chisel point in the middle and only work well with a pilot hole. Did you run out of pilot hole? I start mine with a sharp 1/8" bit all the way to the bottom of the hole.

While it'd be almost impossible to change the hardness of the fastener, you can dull a drill bit pretty fast by overheating it at too high of RPMs and putting a drop or two of oil on it isn't going to make much difference. Different drill bit brands will have different coatings and materials and one might be more robust to higher speed than the other. Look at the cutting edge of the bit and see if there's chunks missing out of the cutting edge or it is a straw color.

Learn how to sharpen a bit on your grinder or buy yourself a DrillDoctor.

[Terrysch]

Cutting oil keeps the temperature down, not increases it.
But a drop will not do that. When you look at machining, the cutting fluid is pouring on the tool and cutting piece to wash away the chips and cool.

There is some new high tech cutting fluids, that do only use a mist. Keeps the machining chips cleaner so worth more as scrap, plus reduces amount of cutting oil purchased and used, which is expensive - oil!!!!!.
But they are only for aluminum - aerospace especially.

I am retired mechanical engineer who worked most of my career at John Deere and Mercury Marine with background in machining steel and aluminum.

[Terrysch]

Forgot one comment. Sorry.
Mongocanfly gave the correct answer.

[e015475]

You might get a kick out of this.........AvE on drilling holes-

https://www.youtube.com/watch?v=QhpOg186fks