Help Machining HDPE (I think..?!)

I have this plastic that I got scrap from a workshop but I am not sure it is HDPE or Delrin?!

I can’t seem to get my feeds and speeds right as I get a lot of swarf machining this stuff.

1200mm/min, 2mm DOC, 1.5mm WOC, 3.175mm 2 flute carbide end mill, 15000 RPM, I used climb milling as I first tried with conventional and the burr was much worse.

here is a photo of the end result, lots of burrs!

any advice on what it is and how to machine it would be greatly appreciated!


I think it’s HDMW (high density molecular weight). I cut some on the S03 last week and it was so fuzzy it looked like it had grown a cat around it. HDMW is extremely hard to machine and it is typically used in places where slick surfaces that resist abrasion are needed…like edges to fences and things like that.

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Things to try:

  • single flute endmill — ideally sharp, new, and w/ a flute shape optimized for plastics
  • leave a roughing clearance and take a finishing pass
  • see if freezing it will allow it to cut more crisply?

Please let us know what works and doesn’t, and esp. if you find a suitable endmill. Almost got sold some UHMW back when I was starting out and glad I said no.


I think you are came from a company that make aerosol production line machinery so this part was designed to have low friction etc.

I have tried a single flute and it left a horrible bottom surface finish.

I’m going to keep experimenting and we did I can get something to work, if not I might get some HDPE or Delrin instead hahaha

IF it is UHMW, climb cut, run fast, run sharp, and be fairly aggressive (fairly thick chips, which means 40 to 50% cutter engagement horizontal, some books I have recommend 0.25mm-- 0.010"-- or more chip thickness). Heat is the enemy, since the stuff grows a lot, is compliant (sanding it is impossible), and melts easily. Conventional cutting will rub on entry, whereas climb cutting gets in, and usually out, without rubbing. Too light a feed (low engagement) climb cutting will rub by pushing the material out of the wayl. Don’t bother with the 0.02mm finish pass. It will just make a mess. The finish pass will need to be fairly significant to prevent rubbing.

And what Will said: sharp cutter. One online source (top of the google search: ) suggests cutters for aluminum.

You might try an air jet for cooling. I have never tried liquid coolant with this stuff, but suspect that it would be more mess than help.


Wow thank you for that comprehensive insight! I think I’m gonna give up with this stuff, it’s a nightmare haha

This is why I hate working with plastics that I can’t identify positively. Actually, pretty much any material. Aluminum, steel, red metal, whatever. If I need to machine it, form it, or weld it, I want to know what it is. For things that don’t much matter, I go to the junk pile, and a lot of that is identified, as well. That said, I have some material that I have no clue what it is, but I know enough of the properties to be useful for many things, like the 32mmdia plastic rod (I think it is polyethylene) that gets used for a lot of non-critical small parts, and the 3/8 dia hard bronze brazing filler that is just handy for small knobs and sprung plungers for detents.

Take Will’s advice on the single flute upcut endmill…O flute I belive. I cut some UHMW recently with one and the cuts were sharp and crisp.

I have a bunch of this stuff lying around. Woodcraft used to sell it in the stores in bags of mixed colors and sizes. I bought a bag thinking I’d have the perfect use for it some day. That was about 15 years ago from the Woodcraft in Springfield, Va. The absolute best way to find the perfect application for it is for me to throw it all away. You know how that goes. It does turn pretty nicely on a lathe with very sharp tools


You need more Feed Rate 2000 or more .

Single flute, up cut. O flute. Aggressive. Hear, hear to @WillAdams!

I’m a big Onsrud booster. Expensive but they are works of art and last a long time. Highly recommended.

I do a fair amount of Delrin and HDPE. I use the Minimum Lubrication Quantity (MLQ) technique so I can cut at extremely high speed and yet achieve a near mirror like finish.

MLQ applies a small (tiny?) amount of lubricant/coolant (“a mist”) - there is little to no mess - via a special nozzle and compressed air. The air also assists with swarf (“chip”) removal (which is critical for fine finishes).

A bit of (constant) coolant/lubricant can dramatically improve cuts (and improve cutting speed) in plastics. Depending on what I have on hand, I use Kool Mist #77 or Durakut 7000B for cutting fluid.



While I use one of two commercial systems (FogBuster, Ekstrom Carlson Mistic Mist) it is quite easy to DIY a pretty darned good MLQ system for little to no money.