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I am in need of some help with turning Polyethylene, white. I have got the finish pretty much perfect but I have to stop the machine after every tool to cut the stringer (birds nest) off of the chuck, turret etc. Does anyone no if it is possible to break a chip with this stuff. I am Turning with polished CNMG 432 inserts.
Thanks for your help
Quote:
Originally Posted by CNCMAN 83
Good luck! You're going to get a lot of stringy chips just because its Polyethylene. CNMG 432, CNGP 432, are good choices. Your best chances of keeping the strings down is to minimize how much the material extends beyond your collet. That will force the chips to the front of the workpiece as much as possible and hopefully fall off when your tool retracts. I've sometimes used a 'chip puller' to help the chip off the part. Its nothing more than a wire that drags across the part as you're positioning to the front of the workpiece.
You may be able to minimize the birds' nest during roughing by taking a very aggressive cut and using G74 or G75 with a very short peck and retract distance. There is not much you can do about finishing cuts but if you have really nice sharp tooling and leave a very small amount to take off for finishing the strings break fairly easily.
You can also take a 35 degree tool, small radius for aluminum (very sharp edge prep, high rake angle) and back turn away from the spindle Z+.
Otherwise like someone already said G74 or G75, tiny retract moves, and again the sharper the tooling the better.
You're right in leaving a small amount for the finish. Back turn the finish pass as well if possible.
Best way to eliminate bird-nesting when turning plastic is to cut from the chuck out. Works EVERY time. Where is it going to get wrapped up?
Another option is to do an X move about 1/2 to 2/3 DOC and drag the tool in Z without the spindle turning, acts as a chip breaker when the material is being turned. Of course this works better with a HSS insert than a carbide insert.
I've always wanted to try a live milling pass in the Z direction to interrupt the chips into C shaped rings. With a small cutter, you might not sacrifice too much stiffness and you the interrupted cut might be ok in such a soft mat'l.
Using live-tooling REALLY ups the cycle time, where as a quick zip in Z at 2/3 X DOC takes 2 seconds max. Trust me, the interrupted cut is the least of your worries with any kind of plastic. I used to produce HDPE fittings for the oil and gas industry, 6" up to 18", on an old manual lathe. With a good sharp HSS bit, I could gouge a good strip of material off as fast as I could spin the handwheel, before engaging the clutch and taking another pass.
Quote:
Originally Posted by MadMax
Birds nesting on the part is fairly easy to clear with your proposed method, but it doesn't solve the issue of chip wrapup on the tools or the chuck which is a cumulative issue. I'm not disagreeing that live tool passes aren't slow. I've just found that I've had to take a lot of time tradeoffs with unsupervised machining of parts.
I wonder if it might be possible for CNCMAN to aim his coolant jets to blow the chip away from the tools and chuck. I have sometimes used an airgun to hose the chips in a particular way during tryouts to keep the cutting area clear while working acetal. It works pretty nice, but it eats up a lot of air. Perhaps there's a decent fluid flow trick that could help.
When I've done some HDPE turning, I've used Iscar cut-grip groove/turn tools to good effect. Machining from the chuck outwards is also a good idea.
The groove turn tool permits a very aggressive feedrate without producing a shabby surface. You could probably turn at .03" to .04"/rev. The broad flat nose of the tool with proper chip breaker geometry, helps keep the chip on top of the tool instead of sneaking around the corner like happens with pointy tools. Even CNMG is too pointy.
This tooling also gives you the option of plunge roughing, which may work really well if it produces good wound up coils, and this automatically breaks the chip when you move to the next position.
Another problem with birds nest is coming up to the shoulder, tool armed with a knot of material. Sometimes it helps to use a cutoff blade to machine a groove at the shoulder, leaving room for the chip ball. I've also had some luck with making a series of grooves to give the chip ball the opportunity to get blasted off with coolant or cold air.
Doing the finish cut in Z+ is about the only way I know to ensure no chip problem.
On my side, when I turned Nylatron (I know it's not the same density of the HDPE but...) I use a round HSS drill rod which I shaped a very sharp tool that I mount on a tool holder. I stoned the tool front and side face to polish it to make very clean cuts. And using a round drill rod will allow me to rotate the tool if my chips doesn't go where I want. Also I make a ONE pass turning at full speed with a very aggressive feed to make a thick chip. This way, the chip goes 15 feets away of the chuck with a great loop. Also, I make a pause of 1 second when I'm at the end of the pass to seperate the chip from the blank.
The Nylatron part is a 5" bushing. The worst part is to turn the inside of the tube. I use a small boring bar with a TCGX AL insert. Again, I make a ONE pass turn with an agressive feed at full speed. The chips came out of the blank and fall again at 15 feet of my TL-2 :D
Can you rig up an air blast?
I found nothing helps get rid of the problem like a shot of air right at the cutting point.