How do you hold sheet material?

[cnczoner]

I regularly cut acrylic with my mill, and the best way I've found so far to support the material (so it does not lift/flap during cutting) is to screw it down as much as possible. But that gets tedious and time-consuming, even with a powered screwdriver.

For larger pieces, I also slide a strip of aluminum pressed onto the acrylic, and close to the cutting bit, but that's not exactly safe.

I know of the existence of vacuum tables, but I've never seen one in action, and I wonder how I would not cut into the vacuum table, when I need to cut all the way through the acrylic. So what other techniques are there?

Pic beow is with a cell-phone, but you can see the #6 screws holding the sheet at regular intervals.

Thanks,
-Neil.

[Jason3]

When I've had to cut acrylic, I've used double sided carpet tape on an MDF spoil board. Keep it away from the cut line though - the cutter can smear it along the sides of the cut and then you have another operation to clean it off

No doubt there's a better way - I can't wait to find out what it is!

Regards, Jason

[makinoman]

Not sure if it would help in your situation but when we cut thin material here, we use a vacumn fixture. For your situation if you could space the pockets farther apart it would work better. You can mill down into the fixture where the pocket will be that you have to cut through the material, but you will have to have a rubber seal go around the pocket to keep air from leaking through which would result in the loss of the vacumn.

Sorry if I am confusing you, hard to explain without pictures.

[cam1]

Mitee-Bite makes a film that is easily applied and removed.
It's called Mitee-Grip:
http://www.miteebite.com/

regards

[ImanCarrot]

Thanks for that. I do a lot of thin stuff and will order some of that Mitee-Grip stuff (I note they have a UK distributor). Previously I was heating the parts and waxing them onto a spoiler. This looks much simpler and less messy.

Thanks again!

[Edit]Have ordered the stuff. Went for someof the paper stuff, some of the mesh and some of the stick sticky stuff... will let you know how I get on! gotta be better than what I'm using at the moment... I getthe occasional component grabbing the tool and launching itself across the shop![/Edit]

[cam1]

Hey: It's Friday time for 2 pints and some crisps ImanCarrot.(I'm on the same plan)

cheers

[volthause]

For vacuum plates there is card-like material that's about 0.025" thick, and is air permeable that goes on top of the vacuum plate, then your part on top of that. Milling through the part by 0.010" you are only milling into the card-stock, and not into your vacuum plate.

[PlasticWorker]

Cnczoner,

That Mitee-Bite stuff looks like too much work, applying, heating, reheating. Wheeew I'm tired already.

I'd use double face tape before that. I have DC tape that is almost impossible to remove. Try TESA vinyl 4970. Alcohol will remove the residue.

The reason they call the vacuum hold down boards "spoilboards" or " sacrificial" boards is they get cut. The idea is not to cut into them any more than necessary.( one or two thousanths) When they get too chopped up to hold, you reface them. You can reface 1/2" board quite a few times before it needs to be replaced.

If you really cant live with that, here is another technique. If you are using paper masked acrylic, just cut down to the paper but not through. If you have bare material, all the sign suppliers carry transfer paper that you can use to mask the bottom of the material.

The type of fixture that Makinoman was mentioning is a dedicated fixture as follows:

We usually use acrylic as the surface of a vacuum box. Surface it so it is flat. Cut a groove into the acrylic an eighth or so inside the part outline of each piece. We use 1/8" diameter soft rubber tube gasket so we cut the groove .125 X .100 deep. This leaves the gasket slightly higher than the acrylic surface. Drill a hole through the acrylic into the box below. These are inside the gasket line for each piece. When vacuum is applied the material will pull down, squishing the gasket until the material hits the acrylic surface. You want a little gasket squish to help hold the seal. You may have to adjust the gasket groove depending on your gaskets durometer. When you cut the gasket, leave it slightly longer than you think so the ends stay tight together. With this type of fixture can use a small Gast vacuum pump because the air flow is easier and there is no air leakage. I always put a vac gauge in the system to be sure the material is down tight and there are no leaks. If there are any internal cutouts or holes they need to be gasketed too.

Just as an observation, it looks to me as if there are way too many screws in your picture. You really shouldn't need a screw on every corner of each piece. I hope you are using straight flute router bits and not an end mill. If we are cutting small parts that have a tendency to move, we cut the profile and leave five thou., then as a last pass we cut all the way through. The last pass doesn't put much stress on the part to move.

Sorry if this was too verbose.
There was a vacuum hold down discussion on alt.machines.cnc a while back. (Thats a newsgroup for those who might not know.)

Just my two cents

[ImanCarrot]

Hey: It's Friday time for 2 pints and some crisps ImanCarrot.(I'm on the same plan)

lol and it's now Sunday Evening. Work tomorrow. Poo.

PlasticWorker: Lots of good tips there! but I can't use really sticky tape stuff, when I try to get the stuff off the table I have to lever it off with a screwdriver and it wrecks it. No good. I cannot have any burr.

I'm gonna hold the substrate on with mind controll

[FPV_GTp]

Mitee-Grip looks awesome

cheers

[cam1]

Many roads lead to Rome.

cheers

[PlasticWorker]

ImanCarrot,

We make lots of very delicate items with the DC tape.
As long as your "prybar" is softer than the material you cutting it won't chip or scratch it.
Also, if you give yourself a place to pry from underneath you can't break the edge. For example, if you drill a shallow 1/2" diameter hole in the sacrifical board at the edge of the part. So the hole is half exposed and half under the finished part, it gives you a place to pry up from under the part without being at the edge.
The only problem with DC tape is that it is fairly slow, so we don't use it for production runs.

Another way to skin the cat.

It looks as if the bottom edge ( lower right in the photo) is straight. If it is you could cut your blank into a strip than makes five pieces ( one machined row, instead of three rows ). Attach a stop block to the table ( a piece of material the same thickness as your work and the same length x 2" wide). Mount five destaco toggle clamps to the stop block so that the pad of the destaco is near the center of your finished piece and the bar of the clamp crosses the bottom of the part where you are not going to machine it. This allows you to clamp each piece. Now you only have to machine the three sides of each piece. The toggle clamps are really quick. If you wanted to be away from the machine for longer than it takes to machine five pieces, you can make multiple set ups on the table.

OK that's it , I quit.
Fixturing is a whole art unto its self. There are books that cover just that one subject.

I hope in some small way I have helped,
PW