Using Metalworking Tools on Plastic

All depends on the actual plastic you're dealing with. Some can be worked like metal, some are gummy and soft and hard to handle while others are brittle and any odd stresses cause them to shatter.

With experience you can have good accuracy. I visited a one man shop that only machined plastics and as I remember he could hold tolerances of + - .oo1.

Sharp tools, lots of clearance , and expect to make some parts that are out of tolerance until you get things dialed in.

=20 Dan

Greetings Darren, I machine lots of plastics on machine tools made for metal. It's important to keep the plastic cool. Acetal plastics (Delrin and Acetron for example) will release formaldehyde gas when heated too much. This gas is corrisive as well as stinky and poisonous. Acetron can be knurled with standard bump knurls. It deforms remarkably well. Different plastics require different coolants. Water for acrylics, soluble oil for nylon, uhmw, acetal. The oil does stick to the plastics but comes off with the proper soaps. Oil must be kept away from acrylic. It will cause cracks. The cracks may not show for hours or days, or maybe not until the plastic is stressed. Sharp positive rake tools work best for most plastics. Nylon absorbs water so if it's going to be used as a water lubricated bearing it should be roughed and then soaked in water a couple days before finish machining. ERic

Thanks everyone.

I'm working on quite of few projects involving plastics and I guess my biggest concern was gumming up my metal working tools.

My immediate concern is a project that involves creating a series of

3/16" wide and 1/16" deep grooves in Nylon at 1/8" intervals, and I'm trying to figure out the best way to do this.

Darren Harris Staten Island, New York.

My experience with PTFE and similar plastics is that if you just want to whack things out and you don't need super high tolerances, you can just use your regular metalworking cutters.

I do mostly hobby stuff and rough prototypes, so I've never needed to attain high precision -- one of the nice things about PTFE and similar is that it's pretty compliant, which means that you can make pegs oversized and holes undersized, and when you push hard enough it all assembles just fine.

I suspect one of those air-powered Vortex cooling units would be real handy when machining plastics.

Since most plastics have VERY low heat conductivity, you have to work hard to keep it cool. Taking light cuts with very high feedrates is most of the secret.

Jon

I think I read somewhere that those can be made at home fairly easily. Not quite up to the efficiency of professionally made ones, but good enough to be useful.

The full name of the device is "Hilsch Vortex Tube". A Google search brings up some interesting hits, such as:

Robert Roland wrote in news: snipped-for-privacy@4ax.com:

The problem is that they take a lot more air than most home shops can supply. Lots of CFM at moderately high pressures.

Doug White

Doug White fired this volley in news:Xns9F9DD09D9B550gwhitealummitedu@69.16.186.7:

4cfm shouldn't be much past the ordinary HSM's compressor.

LLoyd

Yes.

Note that some materials will pinch down on a drilled hole, and otherwise make things more difficult. The main trick is to use *very* sharp tooling and tune the speed just right. Too fast and you will overheat the material and it will start to melt and string out. Too slow, and it will probably pinch more.

Delrin is my favorite of the plastics to machine. (And also rather expensive compared to some.)

You don't mention Teflon -- which is a nice material to act as a low-speed bearing, but again will be somewhat hygroscopic (as described later), so will change dimensions and get tighter or looser.

Define "decent accuracy/tolerances". Most plastics will retreat from the cutter somewhat, and spring back. This is why the sharpest tools are the best here.

Nylon (and some others) are hygroscopic -- they absorb water from the atmosphere, and change dimensions based on that. They also give up water to the atmosphere when the humidity is low -- changing dimensions in the other direction.

Look up the characteristics of the materials on web sites to have an idea what to expect.

Good Luck, DoN.

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Mostly not a problem. On a lathe, you will get big balls of strings from turning most plastics, but they can be pulled away by hand once the lathe is stopped.

Push a drill or some other tools too hard into plastic, and you will have the flutes fill with melted plastic, but once it cools, you can usually chip it off with no harm to the tool bit itself.

How precise do the grooves need to be. (Both the width and the spacing of the grooves.) That is the most hygroscopic of the materials which you have mentioned, so it will change dimensions somewhat. You may need to machine under maximum humidity or machine the grooves oversized to keep clearance.

I, personally, would use either a horizontal mill with a conventional milling cutter (not an end mill), or a shaper with a tool hand ground to the right width and to optimum sharpness.

You can use a two-flute end mill to do it -- but your speed will be more of a problem (risk of melting the Nylon and spoiling the workpiece). The depth is shallow enough compared to the diameter of the end mill so you should be able to do this in one pass. I doubt that you can set up with a liquid coolant (if you are still in your apartment), but you might be able to use compressed air to both remove the chips and to cool the end mill and the plastic. (There are things available which will turn compressed air into a flow of cold air out one end to cool the workpiece, and dump hot air out the other end which you will have to live with, because the hot air is produced by the process which also produces the cold air. But they take a *lot* of airflow, and you may not be able to run a compressor that powerful in your apartment. If you've gotten a shop space somewhere, your options are increased. Here are some examples on eBay:

# 110772235707 (two of them for $150.00 starting bid for the two)

# 290376851464 (single one for $144.00)

Good Luck, DoN.

That slot will be easy. You can probably get away with just an air blast to remove the chips and keep things cool. To insure a good finish and accurate size use an undersized endmill, 5/32 would be good, and climb cut both sides of the slot instead of just plunging in with a 3/16 cutter. But at such a shallow depth you might get away with 1 pass with a 3/16 cutter. Just make sure you use a sharp cutter. To help with getting a good finish on the bottom of the slot stone a small radius on the sharp corners of a new endmill. .005 radius is adequate. Can your slots have a small radius in the corners? If you use a brand new cutter with sharp corners the bottom of the slot may end up kind of furry. ERic

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O.K.

If you were working with high frequency RF, Teflon makes a wonderful insulator, but is probably overkill for the machines you are working-with/modifying/making.

You probably want the grooves slightly oversized to give the fingers room to spring.

O.K. Just two contacts per, then. Sort of like this as an end view?

+---------------+ | | | | | | | | | | +---------------+

I would suggest that would be preferred, since that will let you go a little too thick and make a nicer finish with the mill before you turn it to mill the grooves.

As for the mill to use -- use a *brand-new* end mill for this so it is at its sharpest, and later use those for metal working while you move newer ones into the plastic milling game.

Yes -- that makes a difference.

You might, if you clamp the plastic to the mill's table with the two sides to be grooved on front and back edges, try a 1/4" woodruff key cutter (if you can find one that big) which would be closer to the horizontal mill cutter which I said I would use by preference.

Understood. Compare it to Teflon. :-)

Useful -- though I did not notice the change in dimensions when exposed to water -- may have been there, I got tired of scrolling around the page. A printed copy of the right size would be easier to work with.

Good Luck, DoN.

"Lloyd E. Sponenburgh" wrote in news:Xns9F9DD6379E3CElloydspmindspringcom@216.168.3.70:

Maybe they've gotten more efficient, but the commercial ones I looked at years ago wanted 8+ CFM at 80 PSI or better.

Doug White

snipped-for-privacy@whidbey.com wrote in news:qhn3c7psi4auog48k4bhppb63rvdulbrj8@

4ax.com:

Rather than an air blast, use a vacuum cleaner with a small nozzle to get a good velocity. It will help keep the cutter a little cooler, and it also keeps the stringy chips from piling up.

Doug White

Its possible. But for best results, there are optimum cutter angles that are different for plastics than for metal. Drill bits, for example, have a somewhat sharper point than those for steel.

If you want to pick nits, there are optimum cutter angles for different kinds of metals as well as different plastics.