machining ultra high molecular weight polyethylene

[shazam/TPP]

anyone have any experience with this type of material? we are having a heck of a time, using fullerton g3 aluminum end mills and parts are warping. our vacuum chuck won't hold it due to the material's natural slipperiness.

help

[The Cathedral]

I have a lot of experience with the plastic, but none with a vacuum chuck. I take it using a vise is out of the question?

[shazam/TPP]

we can use a vise, i was just trying to mill the top and outer profile in one shot. since the vacuum chuck wasn't holding it, we put it in a vise to

take a light cut off the top to remove any twist or warping. it seemed flat at first but then started to warp, strange material.

[Mic6]

Careful in the vise also.  It'll squish up and usually...OUT

[shazam/TPP]

very light pressure is what I'm gathering.

[CADCAM3D5AXIS]

How about using some double back tape  and watch your depth of cuts and feed rate. sensitive plastics do not like to  be forced into submission. 

HTH

[Myth Project]

I too have machined a lotta plastics using dbl sided tape.  Works pretty good.

[Machineguy]

DOUBLE STICK TAPE

[shazam/TPP]

do you think freezing it to zero degrees F and then machining it would help? the part is .240 x 1.75 x 3.75 inches

[shazam/TPP]

any recommendations on brand of double face tape?

[jeff]

UHMW sucks!

If you let the part sit after machining, and come in the next day, you will find that it has moved, so if you have tight tolerances I suggest roughing to within .05" or more and letting it sit overnight and then finish.

 

For double sided tape, I use 3M. 

[Machineguy]

3M

#410M YT 10278

that's 1.0" wide

 

Acetone will break the adhesive. Just make sure the plastic doesn't react to it. all plastics are different.

[Machineguy]

If its UHMW, acetone won't bother it.

It machines like gum. Burrs are a bitch.

HSS cutters work best. New ones that is.

[shazam/TPP]

thank you all !! i will give these a try  

[Rstewart]

Use a Small endmill to induce less stress into the part = less warping

[djstedman]

+1 to sharp HSS cutters, carbide doesn't have as fine of an edge as a brand new HSS cutter.

 

Also.. balancing the amounts of material removed from each side of the part will help to minimize movement.. although minimize is just that.. this crap moves when you look at it..

[CADCAM3D5AXIS]

Use WD 40 to break the bond on the tape and then clean the plate to get the oil off before the next tape session. This works pretty good and does not harm any plastic.

[civiceg]

Have you tried putting weights on the part to achieve proper vacuum, and then removing them. I do a lot of thin copper on vac fixtures, even with straightening, they will not suck down without assistance. Sometimes it takes a 200 lb man standing on them, but once they are sucked down with no leaks, its good to go.

[The Cathedral]

These are some of the general things we keep in mind whenever taking on any kind of plastic job. These may or may not pertain to your job or you might already do it all but maybe it will help someone else--

 

We try to order stock that is at least an 1/8" to 1/4" thicker than the overall height of the part. That way we can machine everything "top down" on five sides while still having a thick base to keep the part rigid. Then we usually machine a pair of soft jaws that has the contour of the part for the final side. This way, the soft jaws hold the part firmly all around instead of focusing it on two contact points. Often, we'll machine a plug or insert if the part has a cavity. This goes inside to stop crushing, bending, or flexing.

 

Using sharp tools is critical and uncoated tools are best. While you aren't going to get any galling with coated tools like you would with aluminum, the coating takes away the sharp edge just enough to make a difference. Tooling that's been used before, even on aluminum, should be avoided. If you're looking for dedicated tooling, I suggest Ultra Tool (www.ultra-tool.com)

 

If you're having problems with the part sucking up, try using a down-cutting one- or two-flute endmill. This forces the plastic down into the vise instead of trying to pull it out. Harvey Tool has some good offerings.

 

Shallow DOC's work best with heavy WOC. I usually default to .050" and adjust depending on how it holds up and the type of plastic. Again, working "top down" this keeps the stock as rigid as possible by only removing mass where you are working. The long DOC, 5% stepover dynamic paths don't work so well with plastic. The extra time it takes can be made up by going several hundred inches per minute.

 

Using coolant can be iffy, and can be a big no-no depending on the type of plastic, especially if your customer is medical. Some plastic types will absorb coolant, which will make them swell and shrink and will contaminate them. If you're cutting a plastic that has a low thermal coefficient of expansion, a good method is to buy a cooler full of dry ice. Let the stock get cold and cut it using a cold-air gun.

 

If you have problems with burrs, instead of banging your head against the wall, send them out to be cryogenic deburred. Sometimes it can be cheaper to just send it out than to spend the time chasing it around.

[shazam/TPP]

thanks again all. yes we tried placing alot of force on the vacuum chuck with no luck. Cathedral great points, unfortunately i only have .030 stock

on the .240 thickness, i too like to have at least 1/8 plus extra stock on thickness so i can machine all 5 sides complete. i have ordered all brand new HSS tooling and am waiting for the tape, no one had it locally. keeping my fingers crossed.

[Eric@HorsepowerInc.]

Go and get yourself a hot glue gun and machine away! Use this trick all the time on paper thin parts, also works like a charm on a surface grinder to remove a bow that ordinarily the magnet would pull flat.

[shazam/TPP]

Eric that will be plan D, since plan A and B failed, double sided tape will be plan C. gotta laugh to keep from crying.     cheers to all the help.

[shazam/TPP]

thank you to all, the double sided tape ( 3M #410M YT 10278 1/2" wide) and the high speed tools worked. 

[Machineguy]

Glad it worked for you.

Just remember, that tape is good for 1-2 years depending how you store it. Write the date on the inside of the roll.

 

I cut a lot of the f'**n s**t here.

Best way to deburr the bigger stuff, 1. recut the top of the layer at Z +.001. Offset so about half the tool is over any edges. This cuts off most of any burr.

2. If you do the bigger stuff, go online and buy a box of the double sided nail files (the ones used in a nail salon) . sounds funny but its the best deburr tool for this stuff.

They have a plyable backing in between, so it bends a little when sanding.

 

Machineguy

[shazam/TPP]

I worked with a guy a few years back and he would take a 5-7" section of a used band saw blade (1/2" wide x .035) belt sand the teeth

off and then tape half of it so he could hold it. he would then use the square edge and quickly and lightly scrape it against the plastic

parts to deburr it. the blade would flex but it did remove the burr. i tried this method on this UHMWP and it worked pretty good, i will

also try your method too. thank you Machineguy!