Need Help!- holding/milling UHMW

[theatrewizard]

Greetings. I'm trying to figure out the best way to clamp and mill what will be a slide bushing of uhmw.
Rough dimensions are 1.625w X 1.75h X 18.0 long.
I need to cut a 1.0" wide X 1.5" deep slot the full 18" length, with out having the remaining sides buckle, or bend inwards in any way.
Does anyone have suggestions on type of endmill, as well as how to hold something that long?
Any advice would be great.
Thanks ahead of time.

Chris

[One of Many]

You might use 2 vises and cut it long to have something on the ends to grip it.

Leave it maybe 1/2" wider and thicker while the slot is cut net on center. Then flip it over with a support bar in the slot so the width and thickness can be cut net.

I'd cut the width with a left hand spiral, right hand cutting EM forcing the cutting direction down, then deck the top with a low or no helix EM. Remove it from the vises and cut to 18" on a bandsaw.

Some of the problems with UHMW is that it is dimensionally unstable due to thermal expansion while being cut. Besides the fact it is resilient to being cut, so the best plan is to use very sharp cutters that push or have no helix that can pull it and minimize friction heating with coolant and optimum feed and speeds. Keep the chips clear so they don't try recutting also.

DC

[Geof]

Make a wooden trough the same width, really close, as the UHMW. The best way to do this is to screw down one side of the trough onto a piece of plywood clamped to the table then hold the other side firmly against the UHMW and screw it down.

Have the sides very slightly below the top of the UHMW and screw on lips that just overlap the UHMW and hold it down along its entire length.

Have a stop at each end so it cannot slide and away you go.

The trick with holding UHMW is to spread the clamping over the whole length and not rely on friction to hold; hence the wrap-around trough with lip idea.

[theatrewizard]

Thanks guys. I should be able to make a decent go of it.

Chris

[cut more]

If you need to do a bunch of these, there is an easier way in my opinion,
I machine UHMW and can hold +- .005" easily.

Make a vacuum hold down plate. All you would need would be a 3/4 or 1" thick peice of aluminum, Clamp it in your vise. Mill the top flat, Mill a slot into the face ~ .23" deep with a 1/4 ballmill. Use 1/4" rubber gasket material in the slot. I use an automotive vacuum pump as the vacum source.

As for cutters, you can't beat the Onsrud type, O-flute cutters for UHMW, I use a 1/2" d. cutter @ 6000 rpm. 1/2" doc. 60-75"/min and you have a glossy finish.

Hope that helps,
Cutmore

[big_mak]

Garr 242M endmills work fine for UHMW, just as Geof says, try to hold as much of the part as possible, and don't overtighten it or you could have an issue with it pulling up.

Geof's solution will keep this from happening.

[-Chris-]

You might try making some soft jaws spanning 2 vices then cutting dove tail cuts in them and have your first offset cut dove tails in UHMW to clamp on to and cut to height @ end of Operation -Chris-