Machinable Plastic

[cjs83]

I am looking for a cheap way to experiment with different ideas on my CNC machine, and was hoping that someone might be able to give me some information (websites etc.) on machining plastic. I want to use something soft, so its not hard on the cutters, but also hard enough that it will hold together and not flake badly as its being cut.

Anything would be helpful

[wms]

Try Acetal or delrin, (same thing). HDPE or UHMW are both good to machine and easy to get.

Do a google search and you will find info on all these.

[buscht]

Machinable wax is what is used
http://www.freemansupply.com/MachinableWax.htm

Or just use the type of stiff foam insulation board that is for sale at Lowe's or Home Depot. Its cheap and I've seen it up to 2" thick

[DEAN]

Delrin cuts really nice. Sharp tools, fast feeds

[cjs83]

Thanks for the information.

I have another question. I went to McMaster-Carr and looked at Delrin and the hardness rating is 89-94 Rockwell M. I'm not familar with the Rockwell M scale. Is this too hard to machine easily?

[wms]

CJ,

Delrin (trade name) or acetal cuts like butter. It is harder than some plastics but to an end mill or drill bit it is a piece of cake.

It is more brittle than say HDPE or UHMW and not as "tough" as Nylon, (which sucks to cut as it wraps around every thing and doesn't form a good chip).

Torlon is quite abrasive, but that's another story.

[Rekd]

Originally posted by wms CJ, It is more brittle than say HDPE or UHMW and not as "tough" as Nylon, (which sucks to cut as it wraps around every thing and doesn't form a good chip).

+1, the High Density PolyEthelyne and Ultra High Molecular Weight plastics are nice to machine. It's ok to hold onto also.

Delrin likes to move around and is slippery, but is real easy to machine.

Nylon truly takes a good touch to machine. A good fast touch.. I used to cut that as fast as the machine would move and it cut pretty good. You almost want to run it at 50% rapid when on an older machine that wont go too fast.. Once it starts to melt, your cutter's basically toast so you gotta really get the heat out with the chips. (It won't usually break right away, but it will load up solid and grow until it can't burn thru the plastic any more, then SNAP!)

'Rekd

[Rekd]

btw, on most plastics, your approach/exit points should be considered beforehand, as plastics tend to chip easily when milling out of a wall or corner. Always try to cut into the material.. reletively speaking

[Geof]

My shop machines Delrin on a regular basis both lathe and vertical cnc. It is one of the best engineering plastics for accurate machining but we have learnt a few things by experience. First; really sharp tools with maybe a small amount of positive rake but no rake works fine (Delrin machines somewhat like brass in lathe). Carbide insert tooling for lathe is just not sharp enough but solid carbide milling cutters are okay however because these do have an effective rake make sure your part is securely held. Hand ground High Speed Steel done on a very fine stone is good for lathe work the tool life is okay on natural (white) Delrin but is short on black Delrin because the black is actually finely divided carbon. It is possible to get by without coolant using very small cuts and low rpm and slow feeds but with flood coolant things can really hum; 5/8" two flute carbide, 10000rpm, 200ipm feed taking off 0.25 deep over a 1 inch length. Omit the coolant and you find Delrin burns with a clean blue flame and no smoke!

I have also machined UHMW (Ultra High Molecular Weight) Polyethylene and it is another nice plastic on both lathe and mill. Much softer than Delrin it distorts if clamped too tight so small cuts are required. UHMW also needs very sharp tools with large positive rake. Finely sharpened High Speed Steel on the lathe and high helix solid carbide on the mill. When machining UHMW you have a conflict because it cannot be held too firmly but the large positive rake tends to pull the part into the tool. In the lathe use full contact metal sleeves or collets rather than jaw chucks.

HDPE (High Density Polyethylene) Yuck!! I suppose it is higher density than poly bags but don't recommend even trying to machine it unless you want to move everything into a freezing chamber at -10 F.

Nylon like somebody mentioned can be machined but runs away from the tool and meltrs/burns badly without coolant. Lots of coolant, good sharp tools with a small positive rake can give good results. I have made hundreds of nylon bushings in years long gone by.

Acrylic, Perpsex, Plexiglas (depending on your country) machines nicely so long as you have the very expensive cast acrylic. Do not try machining extruded acrylic unless you like dodging shrapnel. Acrylic can also be polished to a perfectly clear gloss after machining. Start with very fine wet and dry paper moving down to 600 grit (wet of course) then change to cotton fabric with Brasso brass polish. Speed for lathe polishing should be around 100 rpm and care must be taken not to let the fabric get caught and take you for a ride. Or press too hard and let the brass polish dry or you will find friction causes heat. Milled facets on blocks of acrylic can be polished by laying the wet and dry paper or fabric on a sheet of glass much like lapping something.

Lexan, Tuffak, Polycarbonate ( the so called bullet proof or shatterproof plastic sheeting) can also be machined nicely with sharp high speed or high helix carbide mills. Polycarbonate must be very securely clamped because it will climb the cutter in a mill and chatter wildly. Speeds and feeds can be high like Delrin with flood coolant and the finish very nice but it cannot be polished easily.

Phenolic, the resin impregnated and compressed layers of paper or fabric, machines very nicely but smells terrible. Looks terrible too and is not cheap.

Realised I should add a comment about coolant. My shop uses Shell Dromus B at a dilution of 1 in 10 with water and we have found this does not cause problems with the plastics I mention. I have had experience with some straight cutting oils reacting with the plastics particularly acrylic and polycarbonate and causing crazing and stress cracking. Soapy water is also a good coolant for most plastics.

[keithorr]

How many posters here actually write the check for the delrin they machine? It is a nice material, but expensive. If you are looking for something to gnaw on while you learn, go to TAP Plastics or other retail plastics shops and see what they have in odd size scraps of whatever.