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Re: Turning plastic on a metal lathe

Posted: Mon Jun 17, 2019 8:53 am
by whateg0
I turn acetal, nylon, acrylic, and ABS on my lathe as the need arises. I also turn wood on it. And I sand the wood on it. Yes, it creates a lot of dust that has to be cleaned up. But, I just plan for it as part of the job. The only issue I have with wood is that if it's not clamped well, it walks out of the chuck. Plastic and wood are nice, though, in that you can run some impressive feeds. In fact, it took me awhile to convince myself running my CNC mill at it's max ipm that I needed to slow the spindle down, but doing so eliminated the problems I was having.

Re: Turning plastic on a metal lathe

Posted: Mon Jun 17, 2019 10:17 am
by Mr Ron
On a similar note, what is the best drill for plastic? I have twist drills, forstener bits and brad point bits. Which will give the cleanest hole and not distort the hole? The material I'm drilling through is UHMW, 0.416" thick.

Re: Turning plastic on a metal lathe

Posted: Mon Jun 17, 2019 10:41 am
by BadDog
I've turned PVC many times without issue, but that's at manual machine speeds. And my lathe ways stay all but dripping with oil, so that may contribute as well.

When I turn wood, cast iron, or rusty/scaly metal, I drape the ways and cross slide with a leftover length of thick/rubbery/soft tool box drawer liner. It works very well until you start getting into more ductile iron or cast steel (or below the rust/scale layer) which produces actual chips that carry more residual heat. Those tend to melt into it, so I don't use it for those materials. I have an old piece of thick cotton canvas that works better for those, or sometimes I just quickly wipe down between each pass. No more than I use it, I'm sure any additional wear won't be on the scale of anything I'll notice in my lifetime.

For drilling, depends on the plastic. Some like to grab and pull in the drill like brass. Others, like thicker UHMWPE, will expand slightly while drilling and then grab the bit with a death grip making extraction quite a chore. I've used "drubbed" bits like for brass in some cases with success, but heat generated can also be problematic. Others like acrylic want to crack. I don't remember which is what on material, but I've used brad point and step drills with success. I also tend to do incremental feed drilling, sort of pecking/clearing repeatedly to avoid grabbing and let the heat dissipate a bit, again depending on material, thickness, diameter, depth of cut, etc. Drilling some plastics can be a bit of a process in my limited experience. That's all by my memory (which isn't great), and it's been a while since I did much with plastics so YMMV, but that's my recollection anyway...

Re: Turning plastic on a metal lathe

Posted: Mon Jun 17, 2019 5:49 pm
by MrWhoopee
In "How to Run a Lathe" by the O'Brien brothers (South Bend Lathe Works), page 97 describes turning wood and plastics. They even made a wood lathe type tool rest and driving centers for their lathes. In my job shop we routinely turned plastics from UHMW to G10 glass reinforced epoxy and everything in between.

Re: Turning plastic on a metal lathe

Posted: Tue Jun 18, 2019 2:04 pm
by johnfreese
My biggest problem turning plastics is chip control. The softer plastics machine best with high positive rake. That pretty much guarantees one continuous chip for the entire cut. I had one job boring PVC pipe fittings. I set it up on the lathe with a high positive rake boring bar. I had the shop vac set up to capture the swarf.The swarf did not compact in the shop vac. I probably spent as much time emptying the shop vac and clearing the hose than I did machining. Later I set the job up on the mill. I made a 2 flute cutter having maybe 5* negative rake. The swarf from the negative rake tool was brittle enough to break up into shorter pieces, a foot or two long. I would occasionally shut off the mill and clear chips from the tool when the chip ball got too big. Maybe every 10 to 15 parts.

Negative rake will be my go-to for roughing PCV.

For milling the softer plastics like UHMW I favor wood working router bits or end mils designed for Aluminum. There are single flute router bits designed for thermoplastics but I have not yet tried one.

Re: Turning plastic on a metal lathe

Posted: Thu Jun 20, 2019 10:23 am
by Mr Ron
NP317 wrote:
Sun Jun 16, 2019 10:32 am
One plastic to be careful when machining:
PVC.
It can off-gas chlorine which will quickly rust any exposed metal surfaces.
I always prep the machines by applying a coating of oil for protection.
It wipes clean when the job in done.
~RN
Thanks for the heads up. Who would have thought.

Re: Turning plastic on a metal lathe

Posted: Thu Jun 20, 2019 10:31 am
by Mr Ron
I tried drilling a 3/4" hole in the UHMW using a 3/4", 2 flute, HSS end mill in scrap material and it cut like butter, so that is the route I'll be taking. I also drilled some 1/4" holes with a brad point drill and that cut cleanly. Tried a forstner bit and that didn't go so well; required a lot of pressure which in turn produced too much heat. Twist drills work OK, but need to be ground with a higher angle. I'm not going to buy a 3/4" MT2 drill (too expensive). The end mill seems to be the best way.