# Machining plastics



## yclo (Jan 16, 2008)

Made myself a 123 carrier out of ertalyte, o-ring sealed to keep out moisture.



 





  



One problem I had is the long string-like "chips" keep wrapping around the part, are there ways to avoid this? Sometimes the chips from the previous operations get caught into the new chips that are wrapped around on the part and makes a big mess.

-YC


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## will (Jan 16, 2008)

There might be some way dealing with the angles on the cutting tool. 

You could hand feed the carriage and stop every 1/4 turn or so, this will break the chips. Do the final finish cut with the power feed to give a smooth finish. 

Someone who works with plastics might have a better answer.


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## 65535 (Jan 16, 2008)

I'm going to say it's the plastic more than the machine, that plastic is made up of semi-crystalline fibers, which probably don't chip like metals do when machined and some plastics, maybe try delrin? Just make sure you can get the strings away.


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## scott.cr (Jan 16, 2008)

Sweet carrier!!!

Parts wrappage has been a headache with me too when machining Delrin. I don't know how different it is than the PE you're working with, but I'll usually break the chip with a box-cutting razor and then make a light finishing cut to clean up the workpiece.


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## modamag (Jan 16, 2008)

This is what I do when I turn plastic on my lathe.

1) Low speed to minimize spinning chips
2) Deep cut makes the chip break easier
3) HSS instead of my typical favorite carbide with high + rake & clearance

4) Autofeed and contantly remove long chip as a last option cause your hand shouldn't be near any moving part my safety instructor would say.

I have very little experience with milling PE so I would not be able to comment there.


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## modamag (Jan 16, 2008)

BTW: very classy holder. You don't really need the thread do you? :nana:


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## Torque1st (Jan 17, 2008)

Experimentation with any plastic is required. Use HSS to experiment if standard metal cutting shapes do not work. Try to grind a small step in the top surface so the chip coming off the edge hits the step. Sometimes an angle to the step to twist the chip will help. Faster feed rates will also produce a heavier chip that is more prone to breaking. More or less top rake can also help when cutting plastic. Keep plenty of front clearance. Some plastics work well with standard metal cutting tools but not all.


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## Anglepoise (Jan 17, 2008)

Nice container yclo.
I have never yet found a good solution. A variety of different chip breakers make little difference.

It just something we live with.
Aluminum gets its 'False edge'
Cast iron is gritty,dirty and abrasive.
SS and Ti work harden
And plastic coils......


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## Rothrandir (Jan 17, 2008)

Anglepoise said:


> Nice container yclo.
> I have never yet found a good solution. A variety of different chip breakers make little difference.
> 
> It just something we live with.
> ...



And what about brass? 

Cloey, as you've probably figured out by now, that's par for the course with plastics. All the good tips have been mentioned here, and are all spot on. HSS tooling is better than carbide, sharp, positive cutting edges, lots of clearance, hard feedrates, interrupt the cut (by grooving the outside before turning, or by feeding forward then back a little)
Turning from spindle outward can also help push the chip off the part, depending on the part geometry.

Really though, you ain't got it that bad. Those strings can be a much bigger hassle in a production environment.


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## greenLED (Jan 17, 2008)

Have you guys machined "friendly plastic" by any chance?


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## Data (Jan 17, 2008)

Carbide insert lathe tooling would be OK if you sharpened it yourself. It is not the carbide that is the problem it is that it is usually not razor sharp on purpose. Uncoated HSS is often razer sharp and that is good for plastic. Having said that, for a mill, a new uncoated carbide endmill is razor sharp and will cut plastic perfectly.


Cheers


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## sortafast (Jan 17, 2008)

gotta love the mess. I have the same issue on my lathe with delrin, nylon, UHMW whatever. I always seem to for get to get rid of all the Al swarf laying around and it all gets mixed together. But nice holder BTW.


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## sawlight (Jan 17, 2008)

That stuff looks just like UHMW, no fun for breaking the chips, it's just a stringy mess!! Don't use indexble cutters, the finish looks like crap!! HSS with a fine point works well, use auto feed and "bump" the handle as it feeds someimes helps, this depends on how slack you're machine is.If it's a loose machine it will break easy, if it's tight you will have to bump it a bit harder. Bump the oposite way it's feeding.
I always enjoyed cutting it myself, make a pass, through the shavings under the tailstock, and pass it again!! Don't even think twice about taking .100" a pass, and you can single point threads in a single pass, if you dare. If you don't leave at least .020" on you're final pass other wise it will get "fluffy" and you will have to file it out.
Vegetable or Canola oil works great for a cutting fluid when machining this and Lexan (it wont stain the Lexan BTW!! Trust me, good info!!) and is easy to clean up.
I've never had the rake make a difference, the sharp angled point helps point the chips at you, and away from the spindle.
Hope that helps some.


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## yclo (Jan 17, 2008)

Thanks for all the comments and tips guys. :thumbsup:

Now I'm curious, how does a CNC machine deal with stringy chips?

-YC


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## Rothrandir (Jan 18, 2008)

Carbide can of course be used, and with the right inserts works well, but HSS will take a sharper edge than carbide will, and is certainly easier to modify if needed.
I've got some real high positive carbide inserts that work great, but in truly finicky applications (the likes of which most will never see) simply using HSS over carbide can make a difference.

I'm running an ertalyte job right now, and it's a pain. 2inches long and 1inch diameter with some grooves on the outside and hole through the middle with a big internal thread in the front.

I had to put a program stop in the machine to clear off the nest before doing a finish pass, otherwise it wraps around and destroys the part. A huge pain, but given the part shape, there ain't nothin' else to be done about it


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## PEU (Jan 18, 2008)

Cooling the piece also helps. 
I recall having all of the sudden a shavings toupee for not cleaning often 

Nice Container! if you invert the threads, making the cap female it can be made shorter.


Pablo


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## Torque1st (Jan 18, 2008)

Approaching the final diameter with a series of ~equal cuts allows you to judge how much to dial in for that final cut to take account of cutting forces and material response. You can hit very tight tolerances that way.

-As to stringy chips in metal. I almost lost my right index finger to a SS stringer when it cut 2/3 of the way right thru the first joint. It still "works" but hurts like hell at times almost 33 years later.


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## will (Jan 18, 2008)

Torque1st said:


> -As to stringy chips in metal. I almost lost my right index finger to a SS stringer when it cut 2/3 of the way right thru the first joint. It still "works" but hurts like hell at times almost 33 years later.



Safety - I worked in a small shop years ago. When I started there, I was taught by the shop forman. Most of the other workers ( five full time guys ) had a digit or two missing. The forman had all his, he was the one I listened to. It is very easy to lose sight of the fact that these machines have a lot of power and can do damage very quickly. 

Cleaning off chips - turn off the machine.


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## lotsaluck (Jan 18, 2008)

One word of caution when turning UHMD or Teflon... The strings are very strong for their size so under NO circumstances let them lay on or around anything (think arm here) that you don't want to suck into the chuck in an instant. Just a few strands around your wrist and you won't get the lathe shut down in time to save your arm. I know from experience turning UHMD on my 14 x 40. You will only do it ONCE.


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## Torque1st (Jan 18, 2008)

I was using a 3' long steel hook to collect a wad of stringers when it came loose from the work area. When I had a wad I would take the hook and all over to the chip dumpster which was 4 feet away. With a heavy welding glove on I would remove the wad from the hook and drop it in the dumpster. Unfortunately a long stringer from the machine picked that instant to lash out and entangle with a stringer from the wad, wrap itself around the chuck, and proceed with pulling the wad from my hand. The cut was a long one so stopping the machine to remove stringers in the middle of the pass was not possible. I actually finished the pass with my finger hanging loose b4 shutting down the machine. That was the nastiest batch of stainless I ever had to deal with. I don't even remember what alloy it was or which supplier it came from.

Modern CNC machines are nice in that they can be programmed to "bump" the feed and break chips.


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## gadget_lover (Jan 18, 2008)

Thanks for mentioning the strength of the stringers. I realized they are strong, but I never thought about a snagged stringer pulling my hands into contact with the chuck. 

Using Delrin, it's possible to find the right speed + feed to get a nice shower of chips, just like with aluminum.

Danile


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## yclo (Jan 19, 2008)

Ouch! Good tips guys, definitely safety first!

Container - Making the cap with female threads will definitely make it shorter, but what to do if you have a lathe that doesn't have power feed and you only have a tap/die set that is 18mm x 1mm. I'm stuck to making lights that use the same 18x1 thread for tailcap/body/bezel.

Have been trying to source a 13/16 x 20 tap/die set so I can make some E series parts, but haven't found anywhere that has decent shipping price. :help:

-YC


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## McGizmo (Jan 19, 2008)

It's great to have so much experience shared here! :thumbsup:

There is one additional device I found that has been an aid to cutting plastics for me on the occasion that I was trying to hold a tight tolerance and get a smooth surface. I came across a Vortec Cold Air Gun in searching for such a solution a few years back and especially when I was modding and needing to machine FR4 PCB's. Once the HDT of the resin is reached you stop cutting well. The Vortec gun is really handy but man does it suck the air!!!

I would think if you can hold tight enough tolerances on your parts and are using material with a high CTE, you could cool one part with the air gun while heating its mate with a heat gun and probably do some nice press fits that won't budge when the materials are at the same temp. If you could install a heat sink for instance in this manner, you would probably have a great thermal bond between sink and host with minimal resistance.

When my brother and I ran our Vespa shop, we had a customer who was an engineer in a cryogenics lab that did aerospace work. He impressed upon me that many materials can be made to shrink by cooling to a greater delta in dimension than they could be made to expand by heating. I only mention this because another thing I have found with machining plastic is that warm or hot plastic will measure different when cooled. IE. drilled holes often seem undersized to me after they cool if I let them get real hot while drilling them. :shrug:


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## Data (Jan 19, 2008)

Don, I got one of those cold air guns, it came with my Monarch lathe. It is fun to play with but I have not used it on any machining yet. It does not put out a super cold blast like a CO2 tank does but it may be enough for thin wall plastic. Thanks for the idea, I will keep it in mind if I aggressively cut plastic on the Monarch (on the Mori I run the coolant).


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## CNC Dan (Jan 20, 2008)

Chips not breaking is a big problem with plastics.
If you can interupt the cut to make smaller strings that can help.
Also clean away the chips often to prevent build up.

If you have the room, you can also try a shop vac to pull away the string as it forms.


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