Vacuum holding workpieces

[ihavenofish]

so, im on the way to purchasing a gantry router specifically made for guitar milling.

im currently designing jigs for the neck of the first model. it takes 3 separate jigs which need to be clear on all sides of the work piece. this basically mans either tape or vacuum for holding.

id like to try vacuum fixtures. i can make them up easily enough. my issue is how big a pump do i need?

the fixtures will be "sealed", as in not drawing through a spoilboard or the like where the vacuum is leaking more than holding. the problem child i think is the neck laminates in profile. they have only 19 square inches of holding area, so about 190lbs give or take of holding pressure.

is that going to work? it "sounds" like it might be adequate, but i have no experience at all with vac fixtures. the fixture will have to hold 4 of these down at once on the active side of the machine, and 4 one the prep side. is there a formula for calculating how much power the pump will need to maintain pressure?

so many questions!!!! this is all much more complicated then my chisel. haha

[dougtyler]

Have you ever thought about creating the fixtures to incorporate hot glue to hold the piece in place. This would eliminate any distortion in the clamping. Just a thought.

[ihavenofish]

Originally Posted by dougtyler Have you ever thought about creating the fixtures to incorporate hot glue to hold the piece in place. This would eliminate any distortion in the clamping. Just a thought.

hadnt thought of hot glue. i dont think it would work here though. i need to mill the full perimeter of the peice, so there can be nothing on the sides. also i dont know if the glue is a good idea on expensive woods, does it soak into the grain at all like normal glue?

it also seems slower to use to me. the vacuum seems to be much more streamlined an aproach. the idea of this project is efficiency and volume with minimal human labour.

[ger21]

If you've got a really good seal, you can get away with a small pump. However, sometimes you need more CFM to get the seal started. I don't know any formulas for determining what you need.

[ihavenofish]

Originally Posted by ger21 If you've got a really good seal, you can get away with a small pump. However, sometimes you need more CFM to get the seal started. I don't know any formulas for determining what you need.

thanks. i figured better sealing = less power. but getting it started - especially on a wood like mahogany thats porous - might be tricky. my biggest worry is not having alot of surface area. i see guitar necks on vac holders held by the fretboard, where theres about 35 square inches of hold down and they are fine. maybe im worrying over nothing. ill just have to get a pump and try it.

[bostosh]

I have experience in Vacuum fixtures.
On Space Shuttle parts we could and did cut 4" billet to .035 floor thickness.
I could design fixtures for a neck easily.
O-ring or lip seals no problem.
110 V pump OK too.
The trick is in the bleed pattern to the port.
And a perfect match surface to surface.
If you wish to do fixture for a contour,
more tricky but accomplished.

Better yet,
a 4th axis would enable you to just have a universal fixture.
A rotary head and live center on a router would do.

[stevespo]

I don't think your neck laminates will present a problem. I cut loads of belly and pyramid bridges and they have only have a footprint of ~6 sq-in. One thing that will help hugely is indexing your parts with pins which will prevent the parts from twisting on the fixture.

Then the vacuum is only working to resist upward forces from your tools. So depending on your tooling, and how aggressively you are cutting, it might be possible for small parts to lift up and away from your fixture. If you're using straight bits, or downshear, this won't be an issue. Even so, you can get fairly aggressive with multi-flute upcut bits and not run into trouble.

I generally use HDPE fixtures with 1/4" gasketing (set in a channel) and a small 1/5 HP Gast diaphram pump and it has no problems holding down wooden parts during heavy machining. Someday I'll recut these fixtures in aluminum, but any "air tight" material works reasonably well.

Steve

[ihavenofish]

Originally Posted by stevespo I don't think your neck laminates will present a problem. I cut loads of belly and pyramid bridges and they have only have a footprint of ~6 sq-in. One thing that will help hugely is indexing your parts with pins which will prevent the parts from twisting on the fixture. Then the vacuum is only working to resist upward forces from your tools. So depending on your tooling, and how aggressively you are cutting, it might be possible for small parts to lift up and away from your fixture. If you're using straight bits, or downshear, this won't be an issue. Even so, you can get fairly aggressive with multi-flute upcut bits and not run into trouble. I generally use HDPE fixtures with 1/4" gasketing (set in a channel) and a small 1/5 HP Gast diaphram pump and it has no problems holding down wooden parts during heavy machining. Someday I'll recut these fixtures in aluminum, but any "air tight" material works reasonably well. Steve

cool. thanks guys. i think im quite a bit more confident in this working now.

the neck laminates were my big worry because they are 1.25" deep and ideally would have a full depth finish pass which could introduce a fair bit of upward force. initially i think it will be cut with a down spiral bit, but probably best done with a compression bit if i keep this thing going in volume.

there will also have to be a curved fixture for an arched back guitar. i imagine this is no more complex than any other fixures except it needs a precisely matched mating curve to the work piece.

i was looking at some of rickenbackers fixtures and they seem to more or less use exactly what i want.

[Montabelli]

I have used a 1.5 hp rotary vane vacuum pump with forms made from mdf , uhmw and corrion, etc with excellent results. I use closed cell neoprene gasket cord and if the parts are small and try to move, I grind drywall screws to a pin point and screw them into the back of the form and seal them in i have also used sticky back sand paper on forms where no pin marks are allowed to stop sliding parts. it works well for 3d parts too, it is helpful to ad a vacuum gauge to know the performance level of your jig. since mdf is porous it is some times beneficial to seal the form after cutting to aviod degrading the performance of the jig.

[ihavenofish]

Originally Posted by Montabelli I have used a 1.5 hp rotary vane vacuum pump with forms made from mdf , uhmw and corrion, etc with excellent results. I use closed cell neoprene gasket cord and if the parts are small and try to move, I grind drywall screws to a pin point and screw them into the back of the form and seal them in i have also used sticky back sand paper on forms where no pin marks are allowed to stop sliding parts. it works well for 3d parts too, it is helpful to ad a vacuum gauge to know the performance level of your jig. since mdf is porous it is some times beneficial to seal the form after cutting to aviod degrading the performance of the jig.

thank a bunch. great info. the sticky sandpaper sounds brilliant.