[FuriousGeorge]

I’ve been getting a very poor finish on a small delrin part (photos of part and fixture attached) and am wondering what is causing it. The whitish markings show up at the same height no matter what parameters I’ve changed. The also show up on 3 sides of the part but not on 1 side. There are pretty much in the same location on every part I cut.

I’m using a 2 flute 1/8th carbide EM (TiN coated). It’s a 0.01” finishing pass, 0.41” DOC, 5100 RPM and 60 IPM. I’ve varied the feeds between 40 and 90 and tried reducing the speed to 4800. I’ve varied the WOC between 5 and 10 thou, tried cutting it in 2 passes instead of one full depth pass and have tried two different carbide end mills. The only way I have been able to eliminate the marks is by moving up to a 0.25” EM going at 5100 RPM and 90 IPM. This produces a good finish, but I have to use an 1/8” EM to get a proper radius on the outside fillets.

Any thoughts on what could be causing this? I'm thinking that my surface speed is just way too low (167 SFM for an 1/8" EM at 5100 RPM), but how to I fix this when I can't increase my RPMs any more?

[Dr.Fiero]

Try redoing your tool path. Make the same part, but do one in climb, the other in conventional (ie, a CW and a CCW).

See if the affected area flips sides.

[FuriousGeorge]

I've actually tried that already (should have mentioned it in the first post). It made no change in the whitish markings, although climb milling does make the edges much cleaner than conventional. I've climb milled the part for pretty much every run except one conventional to see exactly what you suggested.

[SquibLoad]

Try cutting RPM way down, like by half or more. I've had synthetic materials do what you are seeing, and 'white stuff' appeared to be from the bit slightly melting the plastic. Cutting the RPM down solved it in my case.

[nitewatchman]

We machine a LOT of 3/4" MOS Nylon (Nylatron) and have seen the same problem. For us it is usually associated with chip rewelding to the part. Your chip load on the 1/8" endmill is quite high at nearly 0.006" per tooth. This is about double what we will run on a 3/8" or 1/2" end mill that has a much larger flute gullet. While the cutter can take the force it might not be rejecting the chips and smearing the chips across the cut surface as it leaves the cut welding the "fuxx" to the surface.

With that postulation, I don't have a suggestion to solve the problem.

nitewatchman

[FuriousGeorge]

Squibload,

You were right on the money. Slowed it down to 2500 RPM with a 20 IPM feed-rate and the white markings dissapeared. I still have to tweek the feeds and speeds a bit to get a better finish, but its pretty much there.

[SquibLoad]

Yea! Glad it worked.

First time I ran into that, it confused the heck out of me. Slowing RPMs down seemed counter-intuitive, but worked.

I've had to do the same thing with hardwoods for the classical guitar bridges I build in order to get a good finish, which is just the opposite of what woodworking practice would suggest.

Slower speeds and more flutes was the ticket.
India and Brazilian rosewood just don't machine the same as plywood or MDF.

[LeeWay]

Glad you got it working. I was going to mention that I use about 2200 RPM for black delrin on my lathe and get nice glass smooth cuts. When I went faster, I had similar issues as you when facing it. It has to be heat related I think.

[takewhatyoulike]

Things that helped me in machining Delrin and Turcite: 1) use of a sharp HSS cutter instead of Carbide 2) use of a Vortec model 610 air gun to significantly cool the part that helps prevent chip welding 3) in the lathe I use a pneumatic 5C closer such that collet clamping pressure can be easily and repeatability controlled using a differential air valve so as not to distort thin walled parts.

Don Clement

[tbaker2500]

Don,
I'll be embarking on some significant machining or delrin soon. I'm curious why a HSS cutter works better? Are you talking about surface finish or MRR? Is it the slower SFM keeps the part cooler? Thanks

[takewhatyoulike]

Not sure why. Just that a really sharp HSS tool works better than coated carbide for me in machining Delrin/Turcite for both surface finish and MRR. I run the highest speed/feeds possible on my machine. The Vortec cooler also helps, keeping plastic parts >30 degrees below ambient. YMMV

Don Clement

[SCzEngrgGroup]

I've been machining a lot of UHMW lately, which I assume is similar to Delrin, I run 5000 RPM, 50 IPM, using coolant, with a HSS 2-flute, and I have not see that kind of discoloration. Are you using coolant? Is there a reason NOT to use coolant with Delrin?

Regards,
Ray L.

[Peter Neill]

It's very simple, HSS is generally a *lot* sharper than Carbide and will actually cut the material more than it tears/rips it. Acetal has a melting range around 165-175 deg C and will easily melt from the friction of being rubbed by a cutter. I've never machined the stuff on a Tormach yet, but on the Bridgy I'm usually running around 500-1000rpm with 5mm to 1/2" HSS end mills, and I turn it on the lathe at around 5-600 rpm. You can generally mill & turn it without any coolant at all, but if you're deep drilling at all then you will very definitely need copious coolant - preferably water - as it will absolutely 100% heat up, grow in size, and then 'snatch' the part.

[tbaker2500]

Thanks for the info. I've been using water coolant when milling delrin with carbide, and LOOOVING how it machines. I'll try HSS next!