Drilling Accurate Holes In Acetal

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UncleD

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I have a project where I'm drilling 1/2" holes in Acetal (delrin) and press fitting a sintered bronze bushing. Doing this in a lathe.

Having a bit of trouble getting an accurate hole drilled. About 20-25% of the pieces are oversized just enough that the bushing fits loose enough that I don't think they will stay.

Are there any tricks to this that anyone knows of? Should I slow my speed down? Speed up? Right now I'm running about 800 rpm.

I have tried just drilling using a 1/2" bit, drilling with a 15/32" bit and reaming. Both have the same outcome.

I did check my tailstock alignment to be certain it was ok.

Any ideas?

Thanks!
 
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Knurl the bushing mabey, let drill cool,
 
When I need a really accurate hole in Delrin, I bore it. The problem with Delrin and other plastics is thermal expansion so precision drilling to size can be difficult. If the hole has to be a specific size, get close and let the part cool so you can measure it accurately. Then bore it to size - its quick.
 
The drill operation should be only for roughing. And not for final size. Using a drill bit does not guarantee concentricity and or size. As said, boring is one option for final operation. Delrin is very easy to machine. You can sometimes just use the boring bar to bore out the hole and not even use a drill bit for roughing…Dave
 
I have found that machining some plastics requires a spindle speed much slower than you would expect. The problem is that the plastic melts rather than cuts. Slowing down the spindle and keeping the feed up seems to help. I machine a lot of UHMW and generally have had good luck with slower speeds. Really sharp tools is mandatory.
 
Drill then ream the hole small then press the metallic bearing into the plastic material, it will move.

The word plastic was originally an adjective meaning "malleable or easily formed", it became a noun during the last century with the advent of man made organic materials commonly called plastic. Please note that some materials known as such have little plasticity.
 
if you can drill it to a 64th under and ream to final dimension you'll get within .001", in my experience
the more material you try to ream out the, more error you will have through thermal expansion.

optimally, you'd make the bore to hold the bushing .0005" -.001" smaller than the od of the bushing you are trying to press into it.
you may need to ream the bushing ID if the interference is too great with the shaft
 
Ulma Doctor said:
if you can drill it to a 64th under and ream to final dimension you'll get within .001", in my experience
the more material you try to ream out the, more error you will have through thermal expansion.

optimally, you'd make the bore to hold the bushing .0005" -.001" smaller than the od of the bushing you are trying to press into it.
you may need to ream the bushing ID if the interference is too great with the shaft
Click to expand...
It is unlikely that a sintered bronze bearing pressed into a POM (Delrin) bore of this size, even .005" small will result in reduced diameter of the bearing, depending on wall thickness of course, sintered bronze is far less plastic then POM. Chasing tenths would be a complete waste of time and effort.
 
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