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I'm about to getting into vacuum clamping, but wanted to get some input because it is my understanding that if you cut through a work piece that is being clamped, the vacuum will suddenly release and thus, potentially ruin the piece.

So how do you all do it?
Are you making the clamping area within the inner-most part of the piece so that when you cut through you are outside of the clamping area?

How do you keep from marring/ruining the jigs that you cut through on (Is it recessed outside of the clamping area?)

Any photos and explanations would be greatly appreciated.

Best Regards,
Blain

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Thanks,
Blain

http://www.ullrichguitar.com

"89.67% of all statistics are made up on the spot."

Yes, that will indeed happen. Not much "potential" to it either. DAMHIKT



Yes to part one of your question. When I make a vacuum fixture, the first step is to face mill the top surface flat. When I use a vacuum fixture that will be cutting all the way through the part, I always zero at the top of fixture. When I'm cutting, I cut down to zero and that way I don't damage the fixture. It might leave some marks but any cutting is less than .001" deep so it's no big deal.

When I'm running 2.5d flat parts Iike plywood, I'll zero at the top of the material but I'll leave 1/8" skin at the bottom so as not break vacuum. When the part is done, I run it over to the flush trim bit in the router table.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Andy,
Thanks for the reply.
Makes perfect sense what you described about zeroing to the surface of the fixture and cut to zero.

So I'm guessing something like a bridge doesn't take a whole lot of clamping area.

Do you have room for those needle plates that you use also, or do you only do that for something larger like a finger board?

I guess I'm trying to visualize that plus the vacuum area all within the center of a bridge. Seems very tight. But then again, I have no experience with this sort of thing so I don't really know what the vacuum area should be anyway.

Best Regards,
Blain

_________________
Thanks,
Blain

http://www.ullrichguitar.com

"89.67% of all statistics are made up on the spot."

Bridges are too small to develop enough clamping force to keep them from squirming without additional fixation and unfortunately, the needle plates are too big for most bridge footprints. Although I've asked the manufacturer of the needle plates to make a smaller version, they said they were too busy to get something like that into production.

What I do is embed short needles into the fixture itself to keep it from squirming. With most bridges, I've been able to develop enough vacuum pressure to hold the thing down against the fixture and with the needles the bridges come out great. In the beginning I was using hand made needles and I made one too short and lost a couple bridges from one spot in my bridge fixture so be careful.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

A year or two ago I was contacted by a local Aviation Manufacturer to machine some parts for a new production run of a Classic Airplane...the de Havilland Beaver. They needed several hundred Compressor mounting blocks, which were made from Hardwood n the original plane Spec. They had a particular taper shape, with overall dimensions of 4 inched long x 1" wide and 3/4" thick. I had to produce several hundred of these. What I did was to place dozens of them inside of a 3 ft x 10" wide board of Eastern Maple. I then machined around the parts , leaving them attached by a 0.5mm thick skin. The board held down just fine on my vacumn table. Once cut, I flipped the nest over and ran the whole assemble thru my thickness sander. The sander removed the skin or web that was holding all the parts together , and because I had allowed for the thickness of the web in my dimensioning, I ended up with a batch of precisely thicknessed parts. Can't see why this wouldn't work for Bridges, brace stock etc.

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Jeff Scott

100 grit self adhesive abrasive in the vacuum zone works as well.

I use pins to prevent lateral movement (like Andy describes), and I groove out my fixtures so I can cut through the workpiece. Otherwise, I wouldn't be able to cut down to zero with a ballnose on things like a neck or a bridge. Just seal around any holes you're going to pierce in the part, or onion-skin it, to avoid it flying off.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

This all helps a lot everyone. I appreciate your input.

_________________
Thanks,
Blain

http://www.ullrichguitar.com

"89.67% of all statistics are made up on the spot."