How to hold perspex flat

[Doddy]

Preston, Lancs?, I used to have (will have to have a hunt around) an A4 vacuum bed that I've simply never used. If I can find it you're welcome to it. I'm almost inclined to agree with Boyan, but thinking more a floating head. Or use a device attached robustly to the spindle body that presents a flat (non-scratch) foot at the material top level that sits around the cutter, so that as the cutter moves over the surface the foot presses the material around the cutter down and close to the machine bed. There must be a name for this (Quick!, to the patent office...)

EDIT:

Found it:

[Boyan Silyavski]

Preston, Lancs?, I used to have (will have to have a hunt around) an A4 vacuum bed that I've simply never used. If I can find it you're welcome to it. I'm almost inclined to agree with Boyan, but thinking more a floating head. Or use a device attached robustly to the spindle body that presents a flat (non-scratch) foot at the material top level that sits around the cutter, so that as the cutter moves over the surface the foot presses the material around the cutter down and close to the machine bed. There must be a name for this (Quick!, to the patent office...)

Yes there is such a device, buts its more for sheet cutting. Have seen it on you tube. But this is not the way for such small stuff. Its super easy to fabricate a simple A4 vacuum fixture

[iain1mm]

I agree with Jazz and wouldn't mind betting that it is variation in perspex thickness that is causing most of it. You can check this with a dial indicator once you have it held as well as possible in a custom vacuum fitting (I use PU model board mostly and a venturi vaccum generator on my compressor but a shop vac will do on non porous materials with light cuts)

Once you have determined how flat it isn't, you could probe a height map. There is a built in plugin with UCCNC called autoleveller which does this very simply and will compensate for tiny thickness variations if you have a good probe. It is one of many reasons why I use UCCNC. There may be plug-ins for Mach, Linux et al that do the same job.

Still a tricky one to engrave though. A bit of johnsons baby oil as a cutting fluid and running the job a couple of times may be in order. I have not tried a diamond drag knife so can't comment. I have just ordered one though! Cheers Boyan

[ngwagwa]

Well problem solved I think - have have engraved a smaller workpiece by using one coat of Bostik Spray Adhesive and it has cut 99% of the toolpath (I will write a touch up program just to cut the small area missed). I don;t know if the results are repeatable yet but the spray adhesive comes off easily with wite spirit so I think I have found a way forward.