We seem to be doing a lot of parts out of A/C ply for various contractors in town these days. The obvious problem of getting ply of decent quality so that it's flat enough to stick down to the spoilboard has proved challenging.
When we do it get it to stay, we have problems keeping it there for all the parts. I've been using an onsrud 60-051 Low helix upshear rougher for it. That 3 flute bit is meant to move fast and, combined with the upshear motion, the little adhesion the ply has is lost almost immediately. I've slown it down to 12k RPM and 615 in/min and it still has problems.
Can anyone recommend a different bit? Maybe just the downshear version of what we've got?

Is it 3/4 material? Average size of parts? and is the whole sheet lifting or parts below a certain size?

You are right, a down shear version of your bit would be better for hold down, just remember to put the finished side on top. As you have probably already experienced, this may not be best as the sheet may hold down better one way and not so much on the opposite side.

You might first like to try leaving an onion skin or tabs, which may help depending on how warped the sheet is.

I have started offering my clients the option of tabbing all the parts and sending them back the sheet with the parts attached.

Most appreciate the cost savings because they have some kid in the back whose pay rate is a lot less than my shop rate.

Even if they have to bill their shop rate, it's usually 1/2 my cnc rate, so they come out ahead.

I have less scrap to deal with and less parts handling.

Thanks for those.
Yes, it's 3/4" material. We have to press it down for the vacuum to hold, so once a part comes loose, the whole thing lets go. Frequently, this results in the sheet being dragged around the table stuck to the bit. I think that's a function of the upshear.
The parts aren't that small.
The stuff I cut out of A/C doesn't really matter if it has 1 sloppy face, so I can just use the downshear and whatever face goes down will get a little chipping.
I should have mentioned that I have been onion skinning the whole thing. The problem is that even if I onion skin as much as 1/16", when the sheet pops up a little from the spoilboard, it cuts all the way through.

If you tab it and leave it as a sheet you can put screws in the corners.

Even you have to get your guy to cut the tabs it might be less stressful.

The way i do it is:
I strap the board down and seal the edges of the processed board and the spoil board with masking tape .
I cut the parts not all throw the depth(i leave 0.3 mm)
then i cut the parts with a knife.
I don't need to use a fancy bit for that.
GOOD LUCK

This is a bit of an art.

I tried a 3/8 downshear version of the upshear that you use. It worked great.

Ply usually has one side that holds down better than the other, so choose carefully.

I cut full depth less 1/4 inch on the first pass. It's amazing how often a sheet that won't sit flat will do so once a bunch of grooves 1/2 deep are cut in it.

And tabs. I use 1/4 thick tabs meaning that a section of the sheet can be 1/8 off the table and still not move.