dull bit or really hard aluminum?

ARC-170

Jeff L.
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Oct 17, 2018
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I was drilling a 1/4" hole into what I think is 6061 aluminum plate (1/8: thick) with a standard drill bit using a little 3 in 1 oil for lube/coolant and got an entry hole that looked like this:
290798
and an exit hole that looked like this:
290799
I had a piece of wood underneath and the wood was burned. It was very hard to push down on the drill press.

Anybody ever seen this before? The bit is now dull, but I'm not sure if it was before I started. Is there aluminum sheet that is hard enough? This was scrap, so it wasn't labelled.
 
6061 Should not be that difficult to drill. Of course you need a good bit going in the right direction.
 
Whatever you were doing it was pushing the material not cutting it or barely. Like said not enough relief clearance on bit or flat out dull. Picture of the drill bit would tell a story.
 
It should should have spun out a nice spril of swarf.

Dull bit
 
Very dull bit, or running in reverse. lets see a pic of the bit.
 
It was a right-hand drill going forward.

According to my chart, the RPM is supposed to be 6,112. Here's the math: cutting speed x 3.82/dia of bit=RPM. So 400 (cutting speed of aluminum with HSS) x 3.82/.25=6,112. I set it for less, about 4,000 RP, which seemed high to me. I have chart on my drill press that indicated a speed of about 4,000 so that's what I used.

I'm pretty sure the bit was dull. I use a 1/4" drill quite a bit, so that's shouldn't be a surprise. I just didn't think to look at it before I chucked it up.

The bit had aluminum on it that I picked off with a blade. I'll try and remember to post a picture.
 
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