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To the pro or semi pro guys out there that use cnc with vacuum fixtures....help!

I am wanting to move to vacuum fixturing and after seeing some of the fixtures pictured here on OLF, I know the parts I want to hold can be held. I have a decent (small) vacuum pump and am going to build a pvc tank or buy one of the Harbor Freight tanks to be able to have a little more vacuum.

My questions revolve around the gasket material, and how it is installed. What do you use, and how is it installed. Will round neoprene gasket material work, or is a flat gasket better. Any help here would be appreciated.

Mike

id be interested in this also as im tired of using double stick tape on the swing arm of my compound radiusing sanding machine. ive always wanted to go vacuum but was never familiar with it. any info would be apperciated of how to set it up to work that way........

The flat stuff generally seals better on one pair of faces since the cut faces aren't as smooth, whereas the round stuff is the same all the way around. No trick to using it, you just cut a channel and drop it in. I mostly use the round stuff now.

If you leave the gasket too proud of the surface then your parts might not lay flat, but if you leave too little of it proud then the gasket won't compress much and you run the risk of leaks or not being able to grab rougher workpieces. Soft rubber works, but it requires a very flat and smooth workpiece to get a seal.

It's all just closed cell neoprene foam cord, so you can probably get it at a local rubber/gaskets place as easily and possibly cheaper than any of the online suppliers.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

If I remember right, we used to setup the depth of the gasket channel to be about .010" or .020" less than the diameter of the gasket material (we were using round oring material). We'd match the width of the channel to the diameter (we were using 3/16 material from mcmaster & would splice the ends together using CA.

Trev

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http://www.PeakeGuitars.com

Bob could you further expound on your first statement about the flat stuff? Second, in using the round stuff, how much is too proud and how much is too little?




Trev are talking diam or radius. I can't imagine 10-20 thou giving you enough compression on the gasket, but then again, I a have never made these things!
Keep the info coming...please.

Mike

I use 3/16" od latex tubing with a.032" wall thickness. This very flexible tubing squishes easily and seals well as a result. I cut my groove 125% wider than the tubing and to a depth of 80%. So.. .234" wide and .15" deep in the case of 3/16" od tubing. I use several sizes and apply this formula to every size.

I also use .25" wide x .125" high, closed cell foam, adhesive backed weather stripping as a seal in situation where the geometry doesn't allow for the latex tubing. I usually cut it down the middle to create a .125" x .125 strip because the geometry is usually pretty tight to force the use of it. You can make 90 degree corners...a really tight radius. You can cut up weather stripping and join a dozen pieces together...and it will still seal. I've seen this stuff at both Ace Hardware and Home Depot. Foam stripping will squish down to almost nothing but will still be effective with only a tiny amount of squish. I design grooves for this (at the .125" x .125" size) at a .175" width and a .05" depth.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I've been getting cord, tape and sheet from Allstaradhesives.com Good guys to deal with. The sheet foam is pretty neat for complex parts. Here's a shot of a PG fixture.

Most parts we try to have a pair of index pins to locate the part and to hold it steady in the XY. If it's sheet stock we'll locate the pins on the edge.

It helps to have some PSA abrasive in the vacuum zone to keep the part from sealing off the vacuum.

Thanks Stuart and Sheldon.

I did find the Allstarabrasives site today through a link provided on another thread or forum but would like to find something I can buy locally.

Stuart do you glue the latex tubing together? I know the neoprene can be glued with ca but have no idea about the latex. I found a ton of latex tubing, but again it is net based so I will keep looking. I may try the weather strip but wasn't sure about the holding power. Thanks for the depth and width info on the grooves.

Now for anyone. Can these fixtures be built from mdf if they are painted or sealed with epoxy? I have quite a bit of a hdpe but it is not flat nor is it accurate and it is hard as all get out to machine with my high speed router. The slowest I can go with the router motor is 12k. I have not yet tried it at that speed, but at 22k and a feed rate of 100ipm I get the channel packed with chips that will remelt on the following passes. 100 ipm is a fast as I can run my machine without taxing it and getting some harmonics (vibration for those not used to a cheap cnc ). This hdpe is about 1.125 thick and I think it is glass filled.

Mike

Joining the latex tubing can be a challenge. I use CA and sometimes it works perfect and sometimes I struggle with it...and I can't figure out the variables. I've never failed to make it happen though, so technically, CA works. I think rubber cement might work better. I haven't researched it because I've always made CA work but there has to be some better out there.

Using foam is nice because it's pretty forgiving. You can butt two pieces together and stick them down to a surface and because the foam squishes so much, a seal easily forms.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Aluminum tool and jig plate makes the best jigs in my opinion. Phenolic is another common material. I have seen MDF vacuum jigs, but only for light cutting. The ones I saw were actually unsealed, but I would probably seal them. Also - MDF will warp when you cut it. You may want to laminate it to be at least a couple layers thick in order to try to gain some stability.

If I was going to make some vacuum jigs for my home shop, I would go with phenolic. Since my home machine is router based, I'd take light cuts and move quickly. Phenolic will smolder and ignite if you run at too slow of a feed rate...!

I believe the cord stock we were using was a type of neoprene...not sure of the exact stuff though..!

Trev

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http://www.PeakeGuitars.com

Thanks guys. Hey Trev, mdf will warp even if you don't cut it!

I want to make a couple of test fixtures to see if the vac pump will pull enough to hold the parts before investing in phenolic. I can tell you right now I can't afford aluminum. Machining that would cost me a small fortune since I can cut it smootly enough.

Thanks again. I am gonna try some of these things and will post back my personal findings so maybe this will help others as well.

Mike

The flat gasket strips are cut from sheet, so their sides are more porous than their 'tops' and 'bottoms' since the surface of the sheet has a nicer surface than the cut parts.

It depends on the fixture, but I try to get the round stuff down far enough that I'm not worried about it getting pinched over the lip of the groove it's set in. Try 3/4 diameter as a starting point for slot depth.

You can glue the neoprene end to end easily enough, but there's no real need to unless the gasket won't stay in the channel by itself. It makes just as good a seal if you butt it cleanly up against itself in the channel without adhesive and it can be hard to get a clean glue line unless you've done it quite a few times. If the glue line breaks, it'll make a leak that won't seal as the glued-up stuff is now hardened.

I used to order my stuff from AllStar before I found a local supplier.

If you can't do aluminum, I find acrylic is the next best thing. It machines well because it's hard, and if you have chip melting problems then you can just keep repeating the same pass until the cut is cleared out. Other plastics can get stringy and will never clear the chips to a sharp edge if the first cut isn't done perfectly (polyethylene is especially bad for this). Steel is better than aluminum, actually, but it's a real pain to work with and it's only better if you need it to be perfect dimensionally.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

A friend has been trying PVC 1" thickness for his vac plates. He makes signs using wood and foam and such. So far it has been working great. He's using surgical tubing for the gaskets. Not sure of the size though?

Just another thought on plate material?

Mike

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I'm outside looking in, just farther from the window than most.

Here is my first attempt at a picture in a post.
This is gasket from All-star, MDF sealed w/shellac ,channels cut with a 1/8" carbide bit in a plunge router, all but the last 1/16" buried. I use this fixture to hold plates to my surface plate for un obstructed planning.
It works great.