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Turning Acrylic on metal lathe--your suggestions please

stuball48

Stainless
Joined
Sep 10, 2006
Location
Dickson, TN
Have friend who ask me to turn a 2" bar of acrylic? Can that be done on a metal lathe well enough to get a smooth finish? What cutting fluid----speeds and feeds? Or do I need to "trial and error" it with a piece of scrap acrylic? Any suggestions would be appreciated. Thank you
 
High clearance (15 deg ish ) zero rake HSS tools, honed really sharp - as in 10x glass sharp.

Keep the speed down (300ft/min max) and the feed up (0.008" / rev plus) and use coolant.

Drilling - needs more clearance on the drills, and keep the speed downn to 100' / MAX at a high feed with coolant,. and do NOT let the flutes block.

Polish with wet & dry paper with coolant, then peanut oil with the paper, etc etc etc. according to requirements.
 
Limy Sami said:
High clearance (15 deg ish ) zero rake HSS tools, honed really sharp - as in 10x glass sharp.

Keep the speed down (300ft/min max) and the feed up (0.008" / rev plus) and use coolant.

Drilling - needs more clearance on the drills, and keep the speed downn to 100' / MAX at a high feed with coolant,. and do NOT let the flutes block.

Polish with wet & dry paper with coolant, then peanut oil with the paper, etc etc etc. according to requirements.

All the above, followed by chemical or flame polishing - chemical polishing on acrylic can be done with either a chloroform or methylene chloride vapour bath (outside or with fume extraction!) - methylene chloride's the main constituent of Nitromors paint stripper, but in the USA may be EPA outlawed as it's pretty damn toxic...

Dave H. (the other one)
 
I do a lot of acrylic for a puny company that makes most all the airbrake stuff in the world. These are test fixtures for valving and poppets and they specify: "optically clear" so they can see the action of the part. I've tried heating with a torch which will clear it, but it distorts the image. Maybe I shouldn't use a rosebud! I would go with what Limy said, plus I use rubbing, then polishing compound after wet sanding. Then if you wanna get crazy...toothpaste. Makes a difference.
 
All the above, followed by chemical or flame polishing)
I found flame polishing to be problematic. It certainly does leave a smooth surface, but it leaves a very slight yellowish color, indicating that it the temperature has caused a chemical change. However, the material is extremely easy to polish. I started with 400 grit paper, and by the time I got to 1000 is was optically quite nice. I continued to 2000 grit paper (~10 microns), leaving it quite transparent despite the grit size being ~20x the wavelegth of light. Although the surface was good enough for my purposes, I have 5 micron and 2 micron grit available, so I embedded those in wet cloth. I couldn't see any improvement, and didn't bother checking with a microscope to see if there actually was any improvement in surface finish, even though I'm sure there was.
 
First lets hope it's cast and not extruded. If cast: use a ground and polished carbide insert with a .008 nose radius. Rough at .005" in/rev. 500 sfm and .050" doc. Finish at .001-.002 in/rev. 800 sfm and .010" doc. I prefer mist coolant but hey what ever works for ya. A good buffing wheel and a blue bar would take care of your optically clear finish in no time flat. my .02
 
Heat is the killer on acrylic. Flood coolant, and a good feed have helped me get an acceptable finish, but nothing I'd call smooth.

Compressed air also works quite well, but as above does not leave a real smooth shiny finish.
 
All the good advice about tooling and coollant.
To get a beautiful finish polish with BRASSO and soft cloth, We made see through visual go/nogo guages for production, cheap easy to use for operators. They could see if the product fell between limit lines. Often they lasted 200,000 parts with a 100% inspection two ends.
 
I spent a number of years as a plastic fabricator/machinist type, and found that turning acrylic is best done with a large nose radius on the lathe tool, something on the order of .06-.12" radius or thereabout, ground onto a HSS tool, and use the tool then with plenty of relief on the side and front and under the cutting lip, 10-15 degrees or so. With a broad tool nose radius, you can achieve an almost polished finish on a finish pass on the lathe. It's also important to be working with zero rake angle (scraping the material, not digging in to create a chip like metals). This can also apply to twist drills if you have holes to drill.

A strong air blast also is very effective at cooling the cut and getting the chip away. As noted by others, heat is your enemy. I always work dry on machining operations as long as chip control is good, but if you need to do fine finish polishing, wet sanding with 220->400->600 grit SiC wet-dry paper will get you to the point where some light buffing with white compound and a loose cotton buff will be all that's required for cosmetics.

If you need the part to be long-lasting and as durable as it can be, do not flame-polish, but go with sanding and buffing where polishing is necessary. The flame polishing will create a surface layer of highly-stressed material, just waiting for an opportunity to relieve itself by crazing. A wipe with any sort of solvent-based cleaner will often be the trigger for almost instantaneous reaction. Don't ask me how I know. The vapor-polish process is also a possibility, but I have never actually performed it. I have always been curious about the likelihood of crazing with the solvent attack on the surface, but not curious enough to get set up to do it, with all the vapor control in place.

As also noted, if the material is extruded rather than cast, temp problems will be even more apparent. Difficult to get a really good finish on extruded material. Crazing possibilities are multiplied about 5X. A good lubricant is mineral oil if you are working so that you can use it effectively.
 
I've done a few jobs for a local lighting designer, taking acrylic and polycarbonate tubes of 2" and 6" diam, putting in a couple different types of grooves. Workholding is challenging, I ended up making a ring to go over the tube for ID jaws and "pucks" to support the inside for OD jaws. In both cases I made pucks to go in the tailstock end for a dead center. Cloth electrical tape on the jaws to protect the plastic.

+1 on the above for razor sharp honed HSS and watch for wear, the plastic seems surprisingly abrasive. If a tool digs, then kapowie- since they're so flexible I scrapped a couple tubes working the kinks out of the tooling. I tried a couple different cutting fluids, didn't seem to make much difference from cutting dry. Customer preferred the features be "frosted" so didn't mind the haze but didn't like any burrs or breakouts at corners.

I found I could plunge cut pretty well with a round nose tool, sharp vee too- but be fussy about the rake and honing the edge- needs to be as free-cutting as possible.
 
Aluminum geometry polished carbide inserts work nice for it, but if you want opticaly clear your probably are going to need a air spindle and PCD tooling to get it without polishing. Go carefull using chemicals around acrylic, some substances can cause the surface to craze at a later date (some alcohols can do it, but other solvents can too) Tooth paste does make a pretty good polish in a emergency but brasso has always been my goto for acrylic, will polish from about a 600-800 grit paper with ease, just keep the speed down + wet polish + wet sand if you can. Autosol will also polish it, but the extra dampness of brasso realy does help keep things cool. Don't forget cotton fibres if dry enough are nearly hard enough to scratch acrylic, so chose you polishing clothes carefully!
 
You have had good advice already but I would add some thoughts which will help a bit I hope, the material is an insulator, melts a low temperature, has high rate of thermal expansion and is brittle.
So use sharp tooling to minimise friction, take care with drilling as the chips will melt and then stick to the drill. This is helped by massive relief on the drill shank with just the tip on size, use coolant and be prepared to stop to let stuff cool down, I finish with metal polish after fine paper.
With care and patience you will get super finishes.
Peter
 
One more thing about vapor polishing, a good shop will anneal the parts in an oven to relieve stresses after vapor. On a side note, parts could probably be annealed also after flame polishing to relieve stresses too.
 
Specfab has it. Been years since I've done any work with it, almost 40 but it's just as I remember doing it. First job was at a small shop and the boss was Mr Hass (Rohm and Hass) right hand man in the machine shop before he retired from there. The factory where they made plexiglass was just 10 minutes from us. We did a lot of plastic work. I can remember using tools ground like specfab said and having streams of plastic shooting up to the rafters. Took almost nothing to polish. Used a broom to clean up all the hanging plastic streamers from the ceiling.
Bill
 
Huh. Interesting. We've been in the prototype phases of an internal product using many of these techniques. The requirements include having a bore through acrylic that is optically clear. We tried a bunch of different things: got quotes from people who chemical polishing, attempted flame polishing in house, etc.

After we nailed down the technique, I've left the project with my junior guy... so I think we ended up with a nice heavy boring bar (we found that rigidity matters a bunch), I believe a cutting edge for aluminum, and a relatively light cut with moderate (600 RPM) speeds and feeds (.005 in/rev?). I can check that when I get back in if it makes a difference. We tried a bunch of different coolants and ended up with a spray bottle filled with water and dishwashing soap. We polish with a rag and toothpaste. We ran a bunch of these through the CMM and found that our polishing wasn't changing gross geometry, and only removing about a tenth on the diameter. Our finish comes out very smooth, but hazy when we turn it, so the polish doesn't improve the roughness, just gets rid of the haze (microroughness?). We've been able to hold a 3.5" bore within .001 over 4" of length. That makes me quite happy.

The whole thing with getting the polishing right is what makes me nervous about sending it out. We put a decent amount of effort in to getting this thing right, but realistically we need to get it out to another shop so we can move on to other projects.
 
JaredM --
Finding a good plastic fab/machine shop will help a lot with worries about polishing, after some substantive discussions about methods and running a sample or two. You are also correct about dimensional control and finish control relating to rigidity, although I have noted that many general machine shops that don't do much with plastic over-estimate the difficulty of holding dimensions and geometric relationships (within the material's ability) on boring and drilling. You can have a lot more tool overhang with plastics then with aluminum or steel, and still have good control and finish. You may have better luck with finish with more relief under the cutting edge than with a standard aluminum (insert?) geometry.

Mr. Bridgeport --
That's what I'm talkin' about -- long acrylic confetti just sliding over the tool and shooting off into the air. You know the tool is working right when that's happening... The companies I worked for bought a lot of Rohm and Haas product back in the 70's and 80's.
 
I use HSS - high positive rake tools with a 1/32 to 1/16 nose radius and one whopper of a chip breaker ground in it. I use windex or even simple green as coolant. microfiber towel with red then white buffing compound has worked for me. I will have to check that flame finish process!
 








 
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