Drilling Large Diameter Hole in Acetal Copolymer

bretthl

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I make this part frequently from 416. I thought I would try it using POM-C. My normal work path is to center drill followed by a 9/16" through hole followed by a 1-1/8" hole to a specified depth. The results shown below are not good using the 1-1/8" bit. I was running at 30 rpm and the bit was pulling itself through the material with such aggressiveness that it pulled the 3MT chuck shank from the drill press spindle.

How can I improve this?

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benmychree

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The pilot hole for a drill should be no bigger than the thickness of the web of the drill, this prevents wobbling of the drill that you are seeing. If hogging in is still a problem, you could grind a small flat on the cutting edge that is greater width than the feed on the drill, as one would do for a drill to cut brass. In other words, remove the back rake on the cutting edge, that is what is sucking the drill into the material.
 
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Karl_T

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That speed is WAY to low for acetel.

A huge drill might be problem no matter what you do. If you could find an HSS endmill that large, it would work better. You want super sharp HSS to cut it well.

A better option would be a smaller endmill on CNC or rotary table.

I used to cut a bunch of it. A vacuum to suck up the string really helps

read this
 
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RJSakowski

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For that big a hole in acetal, I would try a Forstner bit, most often used for holes in wood. There are no flutes to draw the bit in so you will have better control of the feed.
 

mksj

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Use an annular cutter if you need close tolerance, otherwise consider a fine tooth hole saw. One other option may be to sandwich the top with say an aluminum plate so the material is not pulled up or allowed to flex/chatter when drilling.
 

aliva

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Forstner bit is the way to go, just adjust your speed so you don't melt the material, it will also clog up fast keep cleaning.
 

plunger

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If you dont want to buy anything else maybe sharpen it sharper than 118 degrees and and flatten the cutting edge to a negative rake. In metal when you get chatter like that Ive found that throwing a piece of emery paper under the drill helps alot. Ive never tried it in acetal.I would think any annular type cutter would be far better.
 

Illinoyance

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You might want to "brass" your drill. Grind the face of the flutes to get zero rake. That should stop self feeding.
 

rzw0wr

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I have found that on plastics the flatter the point of the drill the better.

On small holes I use a brad point.

At work we would use a flat bottom drill at a slow speed.
 

bretthl

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"De-tuning" the bit worked perfect. The design of the part requires a 9/16" through hole followed a partial penetration with the 1-1/8" 118 deg bit bit so changing pilot hole size, Forstner or annular would not work. Kept the low speed and used autofeed. This stuff cuts amazing on the lathe with insert tooling at very high feed rates. Thanks all.
 
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