Work Holding

[jbolt]

I have a smallish cnc router (18" x24" work area). I need to make some machine control panel face plates. My plan is to rotory engrave 1/16th engraving plastic. The router bed is 3/4 x 3 8020 t-slot with a 3/4" MDF on top.

What is the typical method of holding the thin plastic sheet? The face plates are about 12"x16". I also want to make machine button labels which are much smaller.

Thanks,
Jay

[Adrian]

I have done any for a few years but I used to screw a sheet of smooth tufnol down and then I stuck the thin sheet to it with double sided carpet tape.

[martin54]

If your going to be rotary engraving then you will need to ensure your machine bed is perfectly flat & your spindle is trammed properly, if you are going to be doing this sort of thing as part of a business on a regular basis then Adrians method is good but may leave you with quite a bit of cleaning up. If that's the case then you may want to look at buying or making a small vacuum hold down for doing this sort of work. More expensive option but could save a lot of time.

[CosmosK]

What I do is first surface the spoilboard so it's flat.

Then I use my pen marking tool to trace straight lines under where the text will go and the outline if need be.

Then double sided masking tape goes under the text (so they all have the same height adder). I also tape near the outline if I'm cutting it out. For cutouts, I tape inside or outside the line, but not on so the tape won't get cut, gumming up the end mill.

I did this cover plate with .030" ball end mill, one pass for the text, two passes to cut it out. I could probably cut it out in one pass, but the end mill pulls up, so I don't want to risk pulling the material up, which can happen.

edit: I sometimes run an indicator over the part when it's stuck down before I cut to make sure it's flat within a few thou. This method works great for thin plastic.

[jbolt]

Thanks all for the suggestions.

I have about 30 panels to make.

If I decide to build a vacuum fixture is there a preferred type for thin plastics? I have a 5 cfm 2-stage vacuum pump I use for vacuum bagging with a pre-charge tank. I've also seen systems using shop vacs, or commercial pumps drawing through a porous spoil board or a grid fixture using gaskets. With a vacuum fixture I'm assuming you do not fully cut through the material?

[martin54]

Look at something along the lines of these

http://support.vectric.com/tutorials/V8 ... g_GEN.html

If it's for thin material then you might not want to machine the channels so wide which will also aid your pump/tank in maintaining the vacuum. Some people leave an onion skin with vacuum hold down some machine all the way through.
I have made vacuum pucks from 2 pieces of 8mm acrylic bonded together (because that's what I had laying about) which seem to work very well, they wouldn't be any good for your application as each one is to small & using multiple pucks would leave gaps that were 2 large for such a thin material. I use a robinair 6cfm pump for drawing vacuum plus a degassing tank to act as a reservoir so similar set up to what you already have.

[Rcnewcomb]

Look at this for vacuum fixturing: ->The BradyVac II – A Production Vacuum Fixture on a Shoestring Budget

Towards the end of the article he talks about thin or flexible material.

[jbolt]

Hi Martin,

Was that the correct link?

[martin54]

Opps, sorry about that, it was this thread where there were a few but the channels in them may be a bit wide for thin material, will have to check Randalls link myself, not seen that before.

http://forum.vectric.com/viewtopic.php? ... ld#p174820

Basically the idea is something with small channels that can be fitted to an area of the machine when required rather than building something that covers the entire table & is a permanent fixture. Small channels means that you don't require a huge flow rate to maintain hold down.

[jbolt]

Okay thanks. I get the general idea.

Trick is finding a suitable material that does not cost a small fortune or will take time to track down. I guess I could try MDF and seal it.

My one concern is getting the gasket height correct to have a good seal but not cause the edges of the thin material over the gasket to be raised.

[Rcnewcomb]

My one concern is getting the gasket height correct to have a good seal but not cause the edges of the thin material over the gasket to be raised.

The people at All*Star Adhesive can help. They have a lot of good information on their site and were able to guide me when I was just starting to use their gasket material.

jbolt,
I see that you are just up the road from me in Mountain View. I'm in San Jose. We should get together for coffee and talk through some of this.

[martin54]

Okay thanks. I get the general idea.

Trick is finding a suitable material that does not cost a small fortune or will take time to track down. I guess I could try MDF and seal it.

My one concern is getting the gasket height correct to have a good seal but not cause the edges of the thin material over the gasket to be raised.

Small frame that ran round the very edge would ensure the thin material was held flat, would need to ensure your safe z was high enough to clear it but it wouldn't need to be very thick to start with, sure there must be other options, that's just one very quick thought.

I would definitely take Randall up on his offer if I were you.

[jbolt]

Randall, coffee sounds like a great idea. Would have to be on a weekend as I work in the east bay.

I will check out All Star thanks.

Jay

[jbolt]

Some success. I put this together with what I had on hand. Two layers of 3/4" MDF, 1" x 1" grid cut 0.240" wide x 0.230" deep for 1/4" cord. Six coats of varnish were applied as a sealer. I had some 1/4" burna rubber and 1/4" silicone tubing. The rubber works with thick material but I don't think it is compliant enough to get a good seal. It did not work with a 12" x 18" piece of 1/16" plastic. The 1/4" silicone tubing worked on the thin plastic but it only pulls about 7-8 inches of vacuum. I'm not sure how much vacuum I should expect. It does feel secure enough to cut. The only problem with the silicone tubing is where it rounds a corner it pinches and makes the tubing higher. I may try and miter the tubing at each corner. I think with a better gasket material this will work fine if I can get it to lay flat at the corners.

[Rick O.]

Another suggestion: Instead of trying to get a secure seal underneath the plastic, the plastic sheet could be dropped into a snug fitting recess (see example). Then just apply a strip of clear plastic (poly) packing tape on top, around the edges. This way, the tape will be pulled downwards into the tiny edge gap, making the seal tighter.

With this method, there is no need to seal the wood surface under the work piece. If the design has holes cut through the plastic sheet, place a piece of poly tape of rubber sheet over any holes as they are cut.

Instead of cutting a frame as shown in this example, strips of wood could be fastened to an existing vacuum table, as long as they fit firmly around the plastic sheet. For temporary vacuum fixtures, wood outside the immediate work area can be sealed with poly packing tape. It seals as well or better than varnish, and is fast & easy.