Machining UHMW polyethylene question

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dml66

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I'm working with UHMW polyethylene "bar stock" that's 3/4" tall, 1" wide, in this case 12" long. My understanding is black UHMW is reclaimed (recycled?) product, whereas white UHMW is virgin. This material has a very low coefficient of friction which is a property I need for my project; it's slippery as all get out.

These particular black bars from McMaster-Carr are manufactured using an extrusion process, the white stuff is skived. The problem with the black bars is they are very flat and smooth on the top and bottom which is exactly what I want but, the bars as-sold are really made by sawing a sheet into bars; the sides have obvious saw marks and, while still slippery, are uneven, and not desirable. What I need in this application is for at least two adjacent sides to be extruded-smooth. The white stuff that's skived is very smooth but far, far from flat.

uhmw bar stock_rev.jpg

Here's what I've tried so far with mixed success: Since dimensionally, the bars are slightly oversized (1.05" or so), I fixed a 1" aluminum straight edge to the smooth top, centered, and ran the pieces past a ball-bearing Freud router bit, 1" tall, 1/4" shank, installed in a finish router that's table mounted. The aluminum edge acts as a bearing guide. The result is an improvement over the saw-marked finish, but I'd like it to be smoother. The main problem I see with this method is no matter what I do, there's chatter. A secondary issue is the removed material must be electrically charged; it clings to everything, including the surface being milled.

At this point, I'm thinking I may try breaking out my larger router, Hitachi with a 1/2" collet, to see if a stiffer 1/2" shank bit may do better in the chatter department. I'd have to whip up a table mount for it.

It seems to me a plane of some sort may also do a good job, the last one of those I touched was in high school carpentry class. If a plane does give a nice, smooth finish, it can't be at the expense of squareness.

So, figured I'd ask here is anyone has worked with this, or similar material, or maybe has an idea for another approach?

Thanks.
 
Probably your best results would be with a milling machine and a new, sharp endmill
but a router setup could work well also provided you can arrange a stable fence
Static electricity is very much a problem with this stuff, especially in cool, dry weather
Some anti-static fabric spray might help
 
I have worked with it.
If you have a mill, it fly cuts well. Or face mills.

If you have a jointer, I would try that. if not, a router table with the outfeed fence shimmed with two playing cards from a deck of cards. Set the outfeed level with the router bit and it will act as a jointer. The chips are stringy... If you have a spiral bit, use that in the router.

To get a real straight cut, you may want to back the opposite side with a piece of aluminum or wood that's straight. Use doublestick tape to attach .. the idea is to provide backing so it loses its floppiness/flexibility.
 
I have good results with plastics like that drawing a sharp blade across the surface in a scraping action. A jointer blade makes a good scraper. If you vary the angle to the direction of the draw, you can get a reasonably flat surface. For a final surface finish flame polishing should give you a finish similar to the extruded surface.
 
This is the router bearing jig I made up, and couple pieces that were router milled. The finish is actually not too far from good enough, I don't have a mill, think I'll keep going with the router to see if I can get it over the finish line.

uhmw jig.jpg
 
I have cut it in the lathe only, but that was enough to get a feel for how it cuts and it seems to me that the best way to plane it flat would be to literally plane it flat. With a plane. I feel like the single continuous cut from a sharp hand plane would yield the best possible result. If the more informed suggestions above didn't pan out, my next attempt would involve sharpening the old hand plane I haven't touched in years.
 
Since you have the aluminum jig and can screw it to the blocks i'd try a solid carbide helical pattern bit. 1/2" shank in a router table would give the best results, and be safer with a small part. up or down cut bit will not matter, but the compression ones can leave a 'center' line. up cut in a router table would probably be easier making part from slick stock, since the bit will want to pull the part to the table. I have never had much luck with the router-table-as-jointer trick. The AL jig looks good, just needs fussing with the details.
 
Some people have had success running it through a thickness planer. I have NOT done this myself although I have worked with UHMW quite a bit. Google “thickness planing UHMW” and you’ll get quite a few hits to get a better idea of whether you want to try it or not.
 
One thing I didn't mention is cutting it on a tablesaw. My Forrest WWII blades leave a smooth as silk finish, even on the plastic. But you need to push it faster, so it doesn't melt. again it pays to attach the one side to a stable piece so it cuts straight and averages out the highs and lows of the original rough cut against a fence. I have cut blocks for jigs on the tablesaw.
 
woodchucker said:
One thing I didn't mention is cutting it on a tablesaw. My Forrest WWII blades leave a smooth as silk finish, even on the plastic. But you need to push it faster, so it doesn't melt. again it pays to attach the one side to a stable piece so it cuts straight and averages out the highs and lows of the original rough cut against a fence. I have cut blocks for jigs on the tablesaw.
Click to expand...
Forrest also makes beautiful no melt plastic blades!
 
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