milling with vacuum table

[Bob W.]

I currently use venturi pumps and they work great. The problem is the o-rings leak a little, at least the ones I use. It is cord stock bought in 5' lengths and you simply form a loop with it. If a plate is surfaced flat, and o-ring groove cut, and a hold drilled in the center for the vacuum to be pulled there will be problems. The work piece will form a seal to the plate and restrict air flow. my air flow grooves are susally .020" deep so it doesn't take much.

 

The best lab grade pump on the planet will not hold stock down better than 14.7 psi. It is not the vacuum holding the work piece. It is atmospheric pressure. The ultimate holding difference between a scientific pump and a venturi pump is probably less than 1.5 psi.

[TREV]

WOW,

 

dont check back in for a day and all kinds

of talk.

for tose of you allresdy doing this thanks for your input.

we will see how i make out.

 

trevor

[Rob @ Target Machine]

Bullet Bob - Venturi pumps are only capable of .5 torr(approximately 30 Hg) our Scroll Pumps can pull 9 x 10-2 torr. Are you telling me that the "holding power" difference would be negligible between the two vacuum differentials?

 

I kind of understand the theory you are pointing out, but a [9 x 10-2] or .9 torr vs. .5 torr difference in vacuum makes no notable difference in holding power???

 

I never really "put it to the test" with high cutting forces, but one would think that kind of vacuum should make some kind of difference.

[35K chipper]

i have used molecular drag pumps that will go to 1 x 10> -12 millitorr like deep space vacuum and not any measurable difference. you would do more if you blow on top of the part with an air gun or pressurized your shop or moved some place below sea level.

[Bob W.]

Atmospoheric pressure (at sea level) is 14.7 psi. If every molecule of air is removed from the bottom of the plate via a vacuum pump, that is the amount of force available to force the plate down, ~14.7 psi. If a vacuum pump can only remove 90% of the air the plate will be held down with a force of ~13.2 psi. Even at 10 torr a venturi pump is removing 99% of the air for a holding force of 14.5 psi so the difference between a lab grade pump and a venturi pump is .2 psi in holding force.

[Bob W.]

+1000 35K Chipper

[Rob @ Target Machine]

Thanks Bullet Bob.

 

I know that principle, but wasn't sure what the relationship was.

 

eg. torr vs. % of vacuum