Acetal bore shrinkage

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hobgobbln
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Acetal bore shrinkage

Post by hobgobbln »

Prototyped a part for a guy last night from black acetal round. Small diameter, large bore, thin walls.
Last night the male and female halves fit great. This morning it seems the bore on the female half shrank. Not sure by how much but to much to use.

I'm going to be making a bunch of these little pieces. I remember reading about plastics moving so roughing first is a good idea. How long should I let the parts rest before I bore them to finish size? How much material should I leave when roughing?

Griz
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Harold_V
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Re: Acetal bore shrinkage

Post by Harold_V »

That type of movement is not limited to plastics, as metals, too, do the same thing. It's just a matter of degree.

I'm not sure there's a period of time that guarantees the end result you desire, but if you found change over night, consider giving the same amount of time before finishing.

For what it's worth, I found I had to rough, semi-finish, then finish the body of the extendable blowpipes (for bagpipes) I used to make. They were made of black Delrin, which I find to be very active when heavily machined.

In regards to how much to leave--my theory, one which has been proven in practice on numerous occasions, is to leave a bare minimum of material for finishing, as the less you remove, the less chance there will be further movement. That, of course, must reflect any movement from the roughing operation, otherwise the parts may not clean up. It is for that reason I gave my parts a semi-finish, as in roughing the movement was severe. Semi-finishing appeared to remove the vast majority of error, with final machining (only about .030" removal) yielding what could be considered stable parts.

You may have to experiment a little for your particular case.

I've done a great deal of Delrin work, often with thin cross sections. I've not ever had a problem I could not solve. It (Delrin) is wonderful material to machine, assuming you understand its characteristics.

Harold
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hobgobbln
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Re: Acetal bore shrinkage

Post by hobgobbln »

Ok. I'll make a couple test pieces and see how it goes. Wasn't sure if there was a standard rule.

Thanks Harold
Rich_Carlstedt
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Re: Acetal bore shrinkage

Post by Rich_Carlstedt »

Watch your temperature !
Plastic has a very high ( 4 times +) coefficient of expansion.
Never measure when machined and it is warm.....it will shrink

Rich
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Harold_V
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Re: Acetal bore shrinkage

Post by Harold_V »

Yep, what Rich said. Plastics, in general, do have a high coefficient of expansion. However, normal machining of Delrin has proven to be not much of an issue. Drilling can create considerable heat, but turning, using sharp tools and positive rake, not so much.

Harold
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Carm
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Re: Acetal bore shrinkage

Post by Carm »

I hesitated mentioning this...
Acetals are universally claimed to be non-hygroscopic but my experience is otherwise, particularly with close fitting tight tolerance work.
Large surface area e.g. thin walls especially prone.
gcarsen
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Re: Acetal bore shrinkage

Post by gcarsen »

Carm's response about the non-hygroscopic nature of Acetal brought back a funny memory!
when I was younger I worked in a Plastic Company that made tons of plastic sheets and moldings. I worked in the tooling dept and the Supervisor in Maintenance always seamed to have it in for me. I get to work one morning and he immediately calls me over to the maint dept with all the maintenance guys (about 12 guys) standing around a new Dryer machine and holding a book. He proceeds to ask me what a certain gauge on this new process machine does . its labeled something like Dehygroscopic something,, or other ? I don't remember exactly the wording as its been 20 years. I reply,
( and remember I am from the machinist side of the building where we only play with the lathes and mills),, it measures the amount of water left in the material being processed. he asked how did I know, well I explained about breaking the word down into the Latin components and what it meant. he stands there and loudly replies, "its not in the dictionary!" quickly I responded "nether is schmuck" . you talk about dropping a bomb, those maintenance guys went running trying to disappear as fast as they could!! the supervisor just stood there with a funny look on his face!! I went back to my area and to work.
he never messed with me again!!

But yes, Majority of plastics will absorb water. changing size with the humidity, let alone the temp. running coolant on the parts can cause size change when they dry out.
Some plastics take quite a time to set to there final size after manufacture. depending on the material. some don't reach there final size for several months. one thing that is needed to figure in injected molded tooling.

another thing, most plastic material you start from, being sheet, bar, or round stock is just like CRS steel. in that they are extruded and the outer shell is stressed different from the inside by just a bit from the process. so you need to ruff and then finish both sides or you will have the material moving all around!

then the Delrin/acetal thing ,, Delrin is Duponts trade name for Acetal,,

Grant
RSG
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Re: Acetal bore shrinkage

Post by RSG »

I machine acrylic acetate for fishing reel handles and one trick I have adapted is to do it with my coolant pump. It may seem odd but it keeps the tightest dims when run wet.
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hobgobbln
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Re: Acetal bore shrinkage

Post by hobgobbln »

Thanks for all the responses/info.

After pulling my hair out for two or three days and finally having to tell the customer I needed more time to get it right and explained why, he says "Well, can you make it out of stainless? That's what I was planning on using in the end anyway. I just figured plastic would be cheaper to prototype with." :evil:

After some swearing under my breath, a trip to the hardware store for a stainless steel bolt and an hour on the lathe he got his prototype and I learned a lesson on asking extra questions BEFORE I start.

Thanks again everybody,
Griz
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