# Drilling Accurate Holes In Acetal



## UncleD (Sep 27, 2016)

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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## dlane (Sep 28, 2016)

Knurl the bushing mabey, let drill cool,


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## mikey (Sep 28, 2016)

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.


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## Cobra (Sep 28, 2016)

+1 to boring to finished size.


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## chips&more (Sep 28, 2016)

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


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## JimDawson (Sep 28, 2016)

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.


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## Fabrickator (Sep 28, 2016)

Drill under size and finish with a boring operation, or use a reamer for final sizing.


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## Wreck™Wreck (Sep 28, 2016)

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.


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## Ulma Doctor (Sep 28, 2016)

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


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## Wreck™Wreck (Sep 28, 2016)

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


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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## UncleD (Sep 28, 2016)

So maybe get an adjustable reamer? I bought a 1/2" ream and tried it without success. Of course, I am very new to this and could have been doing something wrong, it did seem to chatter quite a bit when I was reaming.

In case anyone is curious, I have attached a pic of what I'm making. I need to run 500 of these, so the fewer steps in the process the better, not sacrificing quality though.


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## mikey (Sep 28, 2016)

Could you not bore a length of Delrin, press the bushing in and then part off pieces? Seems like it would be faster.


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## UncleD (Sep 28, 2016)

That's worth trying. I will give that a shot.

Another idea I just came up with, the issue being the hole is only a little too big. I'm going to try a 12.5 mm drill bit, that measures .4921" maybe that will be enough of a difference to make the bearing hold better. The acetal should stretch that much I think.


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## Wreck™Wreck (Sep 28, 2016)

If you are making 500 parts, ream them small, press in the bearing, buy a pin gauge for measurement, if any come up small toss them in the scrap and carry on.

Go-No go gauges are available from many sources including McMaster-Carr, just specify the go and no go dimensions http://www.mcmaster.com/#go/no-go-gauges/=14ddo6s

If you are buying the bearings there is no reason to think that the bearing manufacturer is any better at controlling  diameters then you are, if the gauge will not pass through the free bearing before installation then toss it.


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## JimDawson (Sep 28, 2016)

I would buy a 0.498 reamer for that job.  If the reamer is chattering, slow the spindle speed down.  The chatter will cause the reamer to cut oversize.


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## 4GSR (Sep 28, 2016)

With the healthy wall thickness you have there on the Delrin. I would put about .015" interference between it and the sintered bushing. Ream it with a 31/64" reamer and call it good.  Resultant hole size will probably leave a .008/.010" interference.  Will not hurt a thing. The Delrin will give somewhat to the bushing.  If it shrinks the ID of the bushing a little, run a adjustable guide bearing reamer thru the bore to size it.  Ken


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## Pissompons (Oct 1, 2016)

Uncle D, looks like you got lots of advice, but these two quotes are spot on for your issue. Make sure your tool is seated corrected to minimize run-out and a Reamer a couple thou short of final size should do the trick.  As JimDawson quoted, a reamer likes to be feed and act as a "sliding" blade instead of boring type of operation. Helps hold your hole concentricity, if you are worried about that.  Anyways, good luck with the application.


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## Metal (Oct 5, 2016)

Just a quick addon:
I've done a handful of uhmw, acetel and delrin, holes like this tend to end up different sizes if the bit is warmed up, I've consigned to drilling holes small and then reaming or pressing the bushing/bearing in after freezing it


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## bfd (Oct 7, 2016)

a trick to make the drill bit cut on size is to touch the outside of the cutting edge with a stone to round it over and make the outside of the drill cut to size instead of the cutting lips this will let the of the drill sixe the hole. sometimes the lips are uneven and the drill will offset from center and drill an oversize hole. use lots of coolant to avoid the overheating of the plastic. the plastic I have worked with did change the size greatly. bill another thing that you can use is a hougan bit. these drill accurate holes usually less than .001" oversize. bill


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