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djtrickdog
06-21-2007, 02:25 PM
i attached a 4 horse shop vac to my table i rescently just made, it has holes on the top every inch on a 2'x2' surface. i figured if you turned the vac on, you hand or shirt would get a little stuck but ntohing happens like that, its just like i feel flowing air, is it because if i put my hand on one spot and cover it, it will just suck from the other side of the surface? i tried a towel and covered the entire surface and it tuggged some bit but not hard, but then again, towels have holes so the air will juss seep through. any suggestions?

drcrash
06-21-2007, 06:19 PM
i attached a 4 horse shop vac to my table i rescently just made, it has holes on the top every inch on a 2'x2' surface. i figured if you turned the vac on, you hand or shirt would get a little stuck but ntohing happens like that, its just like i feel flowing air, is it because if i put my hand on one spot and cover it, it will just suck from the other side of the surface? i tried a towel and covered the entire surface and it tuggged some bit but not hard, but then again, towels have holes so the air will juss seep through. any suggestions?

The air is flowing through the towel.

You normally can't feel the flow of air through the platen holes. (But if you cover it with a sheet of plastic, it'll stick.)

djtrickdog
06-21-2007, 06:24 PM
The air is flowing through the towel.

You normally can't feel the flow of air through the platen holes. (But if you cover it with a sheet of plastic, it'll stick.)

ok thats what i thought. so it should be all good?

Geof
06-21-2007, 07:13 PM
ok thats what i thought. so it should be all good?

Maybe not; a Shop Vac does not pull much vacuum.

A really good vacuum cleaner will pull negative pressure equal to about 120 inches of water, most of the Shop Vac type will probably poop out at half this. Twelve inches of water is equal to almost 0.5psi so 60" is not quite 2.5psi. That is not very much; over an area of one square foot, 144 sq inches this gives a hold down force of 360 lbs. You could probably lift the corner of a piece of material held down with only this force and that lets air in and you lose any hold down.

A vacuum cleaner may be okay for large areas when you are taking light cuts but you should test whether you can slide the material on the table when the vacuum is on. Lifting is not the biggest problem, sliding sideways under the force of the cutter is.

drcrash
06-21-2007, 07:53 PM
Geof,

djtrickdog is using the shop vac for vacuum forming, not a hold-down for machining. (This is the vacuum forming forum.) The issues are a bit different.

(Of course, shop vacs are overrated for vacuum forming, too. They don't pull harder than a similarly-powerful canister vacuum from a thrift store. My 1000-watt Shark hand vac pulls as hard as my 5.5 HP shop vac.)

Geof
06-21-2007, 08:00 PM
Geof,

djtrickdog is using the shop vac for vacuum forming, not a hold-down for machining. (This is the vacuum forming forum.) The issues are a bit different...

Sorry, saw mention of 'table' and developed tunnel vision.

I would have thought Shop vac type devices where totally inadequate for vacuum forming unless you are dealing with very simple forms and very soft and thin material.