[4]

dull bit or really hard aluminum?

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ARC-170

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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.
 

Tom1948

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6061 Should not be that difficult to drill. Of course you need a good bit going in the right direction.
 

Cadillac

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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.
 

yendor

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It should should have spun out a nice spril of swarf.

Dull bit
 

cbellanca

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Very dull bit, or running in reverse. lets see a pic of the bit.
 

ARC-170

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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.
 

coherent

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Almost looks like there is a sleeve in the hole in the photo... Incorrect speed, dull bits or incorrect bit angle will cause excessive friction and heat. Aluminum is "gummy" when cutting and drilling and chip evacuation is important. A sharp bit and peck drilling and lifting the bit out completely each peck while using a coolant or cutting oil will keep the bit cool and avoid the issues it looks like you're having.
 

pontiac428

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Dull bit, but one other consideration- is that 6061 heat treated (T6)? Depending on the needs of its original source, it may or may not be.

Also, I am not a fan of 3-in-1. Alumicut or any other engineered cutting fluid would be preferable to a general purpose household oil. A squeaky cabinet hinge does not have the same heat and stability requirements as a cutting tool.
 

markba633csi

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Many speed charts are intended for maximum material removal in the least amount of time, for production shop needs. Not as important for home/hobby situations. 1000 rpm or so would be fine for 1/4" drill in aluminum, with a sharp bit.
M
 

tq60

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We always twist the bit by hand against a finger tip to feel if it is sharp.

Aluminum can be nasty stuff

Extruded aluminum is real gummy and will make a wad over the cutting edge and give similar results.

A drop of oil in the hole helps as well as tuning up cutting edges.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

MarkM

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I say it s a bushing. Heat expanded the hole and drill pressure pushed the bushing down.
 
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