Turning plastic on a metal lathe

[Mr Ron]

Over on a woodworking forum, I frequent, a question was raised about turning wood on a metal lathe. The answers were obvious; sawdust and oil don't mix, metal lathes are too slow for wood turning, etc. I want to turn some UHMW on my lathe, but now I ask you; are plastic cuttings harmful for a metal lathe? I have turned nylon and UHMW and the shavings come off in one continuous long strand, not dust, so I think I'm OK.

[BadDog]

Plastics are very often turned on metal lathes with no adverse effects. I would guess there are probably some plastics that may off-gas (room temp or turning heat?) corrosive fumes, but these won't be the common things like UHMWPE and such. I've turned quite a bit of it, most often for making plane bearings (bushings) for a variety of things that don't need bronze, and it works very nicely with easy cleanup. The biggest problem is managing the string that results as letting it get tangled in the cut can really trash your finish, thought that's about the worst as long as you don't let it snag a finger or something.

One thing you may want to do is some reading on tool forms ground from HSS for plastics. Lots to discuss, but the main point is absolutely silly sharp, and most tools ground for aluminum will work well enough, though plastic allows for a lots of more unusual bits as well. With the exception of some very sharp micrograin carbide (and similar), my suggestion is you should probably just ignore all that and use HSS ground to suit.

[Glenn Brooks]

I just had an occasion to turn some HDPC on my lathe. Nothing to it. Although it needs to be held good and tight with a threaded arbor and nut, or maybe a four jaw chuck. Milling is another thing altogether. The pieces I was working with tried to go over the top of end mill and then squirted right out of my vise with only mild tool pressure. It can’t hardly be held in place except by nailing down all four quadrants. This stuff is self lubricating, and flexible as a noodle when put under pressure. If I were to attempt to mill any hi def plastic again I think I would crew or bolt to an underlying metal plate, then hold the plate in the vise and mill the plastic.

BTW, actually saw dust and way oil mix quite well, as it turns out. South Bend originally marketed their 1920’s 9” lathe as dual capacity -wood and metal - machines. At the Hawaii Railway Society back shops last winter I turned down 20’ or 30’ of 1” oak and fir plugs for a 1915 passenger cAr restoration - using a SB 9 and the metal tool bit. Worked like a charm. Turns out the saw dust doesn’t hurt anything on the lathe. mixed with way oil, it actually helps preserve the surfaces, and prevents rusting.

Glenn

[Steggy]

The portholes in my F7 are made of cast acrylic and were turned in a lathe. The cut was so smooth the surface was almost glassy in appearance. There were no ill effects on the lathe.

[Harold_V]

I am unaware of any of the common plastics that might be detrimental when being machined. Glass filled materials are an exception. They do offer considerable abrasion of precision surfaces, to say nothing of the uncomfortable itching of working with glass fibers.

One of the worst parts I ever machined were some made from ½" thick brown colored fiber glass board. There was no end to the itching, and no shop vac at my disposal to limit dispersal. A miserable and nasty job, to be sure.

Not having much in the way of wood working equipment (I don't like wood working), I've used both my lathe and mill for multiple projects where wood was required. I've repaired drawer slides, made replacement pieces for antique furniture, even made patterns for metal casting. I do NOT subscribe to the unproven notion that such use is bad for the machine. If would be far less damaging than cutting hot rolled steel or any scaly (or rusty) material. I do suggest one avoids wood that isn't reasonably dry, however, as it does lend the possibility of rusting, and it doesn't machine well. Keep the ways well oiled and wipe down the surfaces after machining. No big deal.

They're your machines. Use them as you see fit. You don't want to turn wood on them? Don't. No law says you must. Plastics? They can be a pleasure to machine, especially Delrin.

H

[NP317]

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

[Inspector]

I think the reasoning for not turning wood on a metal lathe is because most woodwork is sanded to make it smooth. So the sanding dust would have been the reason for not doing it. Put very powerful dust capture at the point of sanding and that would be minimized if not eliminated. Problem is virtually nobody puts a 6" or 8" bell mouth hood ducted to a 5hp collector on their metal lathes. Hard enough to find guys doing it for a wood lathe.

Pete

[toddalin]

I turn acrylic on my lathe all the time. Real easy to make pullies.

[Harold_V]

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

Interesting observation. I've machined it numerous times (both milling and turning) and have not noticed any problems.
If you've had a personal experience, I'd be interested in reading what transpired.

H

[John Hasler]

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

Machining isn't going to get PVC hot enough to release chlorine.

[NP317]

To answer your interest:
I restored and Managed the Student Machine+ Shops at the University of Washington Mechanical Engineering College in Seattle, WA for 11 years.
After getting the shops restored to sort-of modern usage, it had 1 excellent Bridgeport mill, four new 2-axis CNC BP clone mills, and six 14-40 lathes, 1 of them CNC. Plus MANY more machines, welding shop, casting foundry and wood shop. 76 machines in all.
Due to the heavy usage of these shops, especially toward the ends of each academic quarter, I developed a student "Shop Master" program allowing qualified students to open the shop after hours. The ShopMasters were not allowed to run machines while on duty, because their job was to manage the other students, It was an earned position that carried significant respect and weight, even when job hunting post-graduation.

All shop users were cautioned about the chlorine outgassing of PVC materials. And given the CNC machining capabilities, high material removal rates using carbide cutters were not unusual. Occasionally, I would arrive to work in the mornings and find one of the mills covered in a fine patine of light rust! By checking the machine sign-out records, I would contact the user and always learn that Yes, they had machined PVC, and NO, they had not pre-oiled the machine surfaces or wiped it down after finishing. The ShopMasters were also on the hook to clean the machines along with the student. Fine Scotchbrite, oil, and shop clothes...

So that experience provided documented PVC machining and the associate results. We all learned from those experiences.
I had a reputation for running a very careful and tight set of Shops. 'Took much effort on my part,
but we produced many ME Graduates with exceptional Real World skills, along with all the math and theory.
UW ME College still has that good reputation.

So that's my personal experience with PVC machining and the resulting machine surface rust produced.
~RN

[Harold_V]

Always happy to hear of real life experiences. Yours was a very interesting report. Your comments are Information I will file in my head for future recommendations.

Thanks!

H