How to hold perspex flat

Hi,

I am trying to engrave an A4 sheet of 2mm thick perspex with a 0.02mm depth of cut and then cut out the parts and my problem is getting the damn thing flat. I have secured a sacrificial piece of 12mm perspex to the table and skimmed it and have then secured the 2mm sheet to it with double sided tape. The problem is the tool is the correct depth at the start of the cut but there are areas where the tool is shy of the workpiece, where the cutter misses the perspex is different for every workpiece so I am assuming that the double sided tape is to blame (I cover 95% of the workpiece with the tape). I am wanting to engrave 12 of these onto the A4 sheet.

Attachment 27092

Superglue sprang to mind but thinking abuut getting the sheet off after engraving would be a ball ache - I suppose I could leave the protective backing on the perspex on the table but that would mean re-skimming the sacrificial mounting piece each time and I have about 6 A4 shees to cut.

Any ideas anyone?

With 0.02 DOC your always going to struggle with DS tape. But how good is your machine and how flat is the perspex to start with.? 0.02mm doesn't leave much room for error so it's more likely to be a mixture of few things.

A vacuum would be ideal way to hold it but could try low tack spray adhesive.?

I do these kind of jobs using normally a simple custom for the job vacuum fixture made from wood or plywood. Vacuum seals cut with scissors from 60x40cm EVA rubber sheet found from any asian shop for 60 cents / here in Spain/ .

Or use the thinnest double sided tape and make sure you have tape at the center so the piece will not bend. probably will have to take the protective sheet from the bottom side as this may cause the problems.

0.02mm depth of cut is too small. use spring loaded diamond drag for such jobs, as it gives exactly that depth/ width and is much cleaner from what i see on picture.

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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:

Attachment 27093

Quote:

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Originally Posted by 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...)

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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

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

Doddy - you have PM

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.

Still, ain't that a job for a diamond engraver?