Machining Delrin to high Flatness and Parallelism

[RooneyJC]

Hello All

I need to machine a plastic piece that is rather thin with high flatness, parallelism and surface finish callouts. I was thinking of lightly thinning some stock high-maintainability Delrin. See the drawing below:

http://imgur.com/igV3fsB

The piece is 190mm X 100mm. Now these tolerances may be unreasonable, but what could be realistically be hit by a CNC milling machine without getting too expensive or complex? Is there a plastic material more suitable to this application than Delrin?

Thanks!
Justin

 

[MikeHalloran]

"high-maintainability Delrin" ? Never heard of it.

Ordinary Delrin can be machined as flat as you specify.

Don't expect to measure it half an hour later and get the same numbers.

If you assemble the part between two other parts, and e.g. rely on the flatness for sealing, do the assembly immediately after finishing.

Remember that Delrin is a condensation polymer, and has a thin layer of 'foam' in the middle of its cast form. You probably need to try shaving both sides of 1/4" cast material, or bandsawing the outer faces off 1" cast material.
I don't know which will warp more.


Mike Halloran
Pembroke Pines, FL, USA

 

[RooneyJC]

Thanks Mike. Yeah I'm just talking about Delrin Acetal Resin. I assume the tolerance changes will occur due to the part cooling? Do you have any idea by how much the variation could be? The part will not be used in a sealing application, but as a spacer in an automated machine that will need to draw vacuum through it, thus the need for high flatness and parallelism. I don't want to use a metallic material as I need the part to be plastic for inductive sensing purposes.

I believe 3/8" sheet stock will be purchased and then about 2.86mm will have to be removed total, from either side.

 

[Demon3]

How about achieving the same effect by getting some metal sheet and then adhering some plastic tape on either side? Contact C S Hyde for PTFE or any other type of tape. Will be way easier to get the right thickness and tolerances and way cheaper too.

Dr. Chris DeArmitt

Plastics consultant to the Fortune 500:

http://www.phantomplastics.com

Webinars on plastics, fillers & impact modification:

http://www.plastictraining.com

 

[btrueblood]

We have fairly good luck in getting 3/8" - 1/2" PTFE double-disk ground to a precision (+/-.001") flatness/parallelism, but we specify same with the material restrained between metal plates (as that's how it gets used). Oh, and surface finish is probably closer to 32 micro-inch rms, not 16.