Questions Machining Delrin

Recently, I experimented with cutting ⅛" Delrin sheet to fabricate a turntable mat for a vintage Sony audio turntable. This job requires high precision both for fit and for weight balance. I ran into a few problems and would appreciate any guidance.

I have a S4 XXL machine with the new CC VFD spindle.The machine is running great.

  1. I used an Amana 51411-K zero single flute mill designed for plastic with the default Carbide Create settings. I found that all my circle contours (inner and outer) were slightly smaller than intended and did not fit the turntable in critical areas. I was able to correct by adding 0.25 mm to the measurements. Is this the “right” way to do this?
  2. Delrin sheet is subject to warping and really needs to be surfaced. On YouTube I saw Delrin surfacing using a Shear-Hog SH150 with a ½" shank and significantly more powerful machine. It looked pretty straightforward. Is there a recommended mill / approach for surfacing Delrin on the S04?
  3. Is it possible to do round-overs on Delrin? My earlier attempt to use a small two-flute 3/16" round-over bit on acrylic generated heat and not a smooth rounded-over surface.

I hope not to mangle too much Delrin on failed tests. Any guidance would be much appreciated!



It’s definitely the most efficient/straightforward way. You may still want to understand the sources for dimensional inaccuracy (my take on this topic here), but at the end of the day, after applying the usual best practices to calibrate the machine and reduce tool deflection and runout, we all end up using an offset determined experimentally, to get a perfect fit.

You can use any square endmill you have on hand, or possible a surfacing bit (like the McFly), the only trick with plastics is to feed fast (and then some) to avoid melting. Fast and shallow does the job.

When in doubt, feed (much) faster when cutting plastics. Scary as it is, it’s the answer 99% of the time to avoid melting. Bump up the feedrate until you get it snows chips (rather than a stringy mess, or worse, no strings or chips at all which means you are only melting material and not cutting it). Using single-flute endmills help too, but for a roundover I doubt a single flute ball endmill exists (I never checked…)


I think he meant a roundover/corner rounding bit. I’ve never seen a single flute in either.
However, if you’re handy with a grinder you could easily make a 2-flute into a 1-flute by grinding ~0.005" off of one of the cutting edges. You want the rest of the material there for balance, but the 2nd flute will now only serve to clear chips. :wink:


Thank you for helpful suggestions. I’m going to experiment with ramped up speeds on my Delrin scrap pieces and see how things go. Great to get helpful tips.


And I’m sure many folks know but there’s bound to be some lucky folks out there that don’t:
Burning delrin creates formaldehyde fumes so while I know we all practice the utmost safety at all times… maybe try to be just a bit extra safe when dialing in your delrin recipe :slightly_smiling_face:

I liked working with it quite a bit all things considered. Be sure to show off whatever you come up, it sounds like a super cool project.
(And I’m personally invested in how you accomplish the round over for one of my own projects)

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Thanks, Tyler, for the formaldehyde tip! I was aware that Delrin can give off fumes but didn’t know the details. I use a powerful ventilation fan and also am focused on not burning any plastics! I’ll let you know how things turn out.


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