View Full Version : The Sky's the Limit

05-02-2003, 04:47 PM
In any vacuum hold down system you are limited to 15 pounds per square inch, ie. the weight of the sky above. No matter how much grunt your pump has got you cannot go higher than this.
HOWEVER, a pressure chamber that uses the work as part of its seal could increase the effective pressure without limit. Even an old service station compressor can cough out 100 pounds or more.

05-02-2003, 07:57 PM
Well Simon,

One can also have a larger clamping area for the vacuum than the part itself. It is an interesting and can be an involved solution to a problem that may not exist. I have tried it and it works well. The fixture tends to be part specific and won't work for a lot of parts.

05-02-2003, 09:59 PM
Simon, 15 pounds of pressure per square inch does not sound like very much holding pressure but what you are talking about is called one atmosphere I only wish I could get a pump that would pull half an atmosphere, They measure vacuum in inch column feet of mercury. and if you can pull 7 inch of mercury I can tell you you will not be able to break the suction very easy. David in Wyoming

05-02-2003, 10:36 PM
Just an added thought, Back when I was a young and foolish pup I worked at a Uranium Mill (no I don't glow anymore [
] ) Anyway we used super large vacuum pumps to run the yellow cake fillters and the acid Filters to pull out the uranium from a mud slurry, it was capable of pulling a little over 12 inches of mercury and we use to have fun with the new guys by betting them that we could suspend an 80lb steel, ball mill, ball in mid air. Well we used one that was just larger than the intake tube for the pumps and if you got it in just the right spot it would hold the ball in mid air. But if you got it too close to the inlet, Ohboy! you had to hit it with a 60lb Sledge hammer with everthing you had.and at an angle almost 90 degrees to the tube. David in Wyoming

05-02-2003, 10:44 PM
Now Simon Tell me what part of the Story above is a Wyoming Cowpoke's Tell and what is not! David in Wyoming

05-02-2003, 10:58 PM
I know I know, That people in Wyoming even know about angles???? We are just getting these in Cnada Next month, They are said to be great.... Garsh LOL [

05-02-2003, 11:45 PM
Simon, 15 pounds per square inch may not sound like much, but have you ever tried to pull a wet rubber suction cup off a glass window? The qualifying words wet/rubber/glass give a hint to the secret - the physical conditions must be perfect.

A garage compressor is designed to produce positive pressure, not vacuum. The "pressure chamber" does not increase the pressure at all - it only prevents the compressor from having to run permanently. The chamber provides a reservoir and it smooths out big surges in demand. A chamber only has value for a table if you need a surge of vacuum to tighten your seals down when you have placed a new part.

Dave, 1 atmosphere pressure is about 30 inches of mercury, or 30 feet (360") of water (roughly speaking). That suction cup on the window has a full 30" mercury holding it because there is no sky (air) between it and the glass.

05-03-2003, 08:26 AM
Specific geavity Mercury is 13.6. 30" Mercury is equal to 408" water. But, for rough guessing an inch a foot is the number I use. And, the ShopBot doesn't seem to know the difference.

05-03-2003, 10:47 AM
I learnt my rough figures on the gold mines near Johannesburg. The mine buildings are 1600m above sea level, while the bottom of the mine is about 500m below sea level. Your ears can pop quite violently during the daily commute - 10% error either way was insignificant.

And the metric equivalent of 1 atmospere is roughly 1 kg per square centimeter (an area 10mm X 10mm)

05-03-2003, 12:07 PM
Gerald I fully agree and understand about atmospheric pressure I am a certified PADI and all you have to do is go down to 90 feet and feel the pressure and you FULLY understand. David in Wyoming

05-03-2003, 12:14 PM
PADI? I didn't know you were Irish!

(okay, some or other Diving Instructor)

05-03-2003, 12:31 PM
Ron, allow me to enlighten everyone on weight Measures of Pressure
1 pound per square inch = 144pounds per sq. ft.=0.068atmosphere=2.042inches of mercury at 62 degreesF.=27.7inches of water at 62 degrees F.=2.31feet of water at 62 degrees F.

1 atmosphere= 30inch of mercury at 62 degrees F.=14.7 pounds per sq. inch = 2116.3 pounds per sq foot=33.93 feet of water at 62 degrees F.

1 foot of water at 62 degrees F.= 62.355 pounds per sq. foot=0.433 pounds per sq. inch.

1 inch of mercury at 62 degrees F.= 1.132 foot of water =13.58 inches of water= 0.491 pounds per sq. inch

everyone got that, there will be a test on this tomorrow. David in Wyoming

05-03-2003, 12:36 PM
Gerald, ya a diving instructor and I'm half Irish too [
] David in Wyoming

05-03-2003, 01:14 PM
Simon and everyone else with the enlightenment from above if you can pull 7 inches of mercury with a pump then you will in essence be holding with 494.928 pounds of force on 1 sq. foot of material, this is if the material is non pourous and the seal is good, of course if it isn't then you couldn't pull 7 inches to begin with. David in Wyoming

05-03-2003, 01:47 PM
Exactly David, if you are having a measured vacuum of 7" Hg (mercury) then you WILL be having 500 pounds on 1 square foot, IRRESPECTIVE of leaks, porosity, etc. There are 2 ways to cope with leaks: buy a huge pump, or plug the leaks.

05-03-2003, 04:02 PM
This is all very well when you are dealing with large cutouts. But when you come to cut a bunch of small, closely spaced letters of around 2 inches high and you have cut the first fifty from a panel, there is not much vacuum left to pull down the loose bits cos the air is leaking through the cuts. Specially when the "Active" vacuum areas (Channels or holes) of the table are spaced apart. There simply is no longer the surface area for effective hold down.

05-03-2003, 04:08 PM
So Simon, HOWEVER did you figure that a garage pressure chamber will help?

The surface area is still there, but your leaks have increased. Have you tried throwing old plastic sheeting over the areas that are already cut out?

05-03-2003, 04:27 PM
Gerald - that was obviously a facetious example to illustrate my original point. Instead of sucking, you could blow, and increase your holding power. Higher pressures are easy (if somewhat cumbersome over a shopbot) to get, whereas LOWER pressures are limited by our environment. If we lived on jupiter - well this would be different...

05-03-2003, 05:12 PM
Here is a picture of the pressure chamber idea. I have never posted an image before, so forgive me if it comes up blank.

05-03-2003, 05:26 PM
Looks like a huge project. You would need windows to see whats going on inside. And some sort of hoist to pick up the chamber when done cutting.

05-03-2003, 06:23 PM
Simon you are overlooking the easiest part of vacuum hold down. Just don't cut all the way to the bottom, like leave just enough that you can cut the remaining with a razzer knife. then you really don't loose the vacuum. and if you are bound to this design then you better have one honken compressor to fill up that chamber and keep it at 100 psi and you better really beef up the shopbot table because by the time you build a chamber that big that will be safe enough to hold at least 125psi it is going to weigh about 4times what the entire shopbot weighs. Remember that you can't build it out of wood because the pressure would blow the sides totaly apart. and if you built it out of aluminum then wow the buck dosen't stop here it become the little pink rabbit and just keep on going and going and going........................ David in Wyoming.

05-04-2003, 12:26 AM

Why not just spray the surface of your vacuum table with spray tack? This would a)increase the friction coefficient (which is really what is holding the wood, not the vacuum) b)act as a second "hold down" c) usually does not leave anything on the part*.

3M makes one called "777" Spray adhesive or something like that. Check out either an art supply or crafts store.

Just a suggestion,

Bruce Clark

* I used to use a LOT of spray adheasive in my screen print business. The trick is to spray the surface and let it dry. By spraying on your vacuum table, it _should_ dry pretty quickly. Generally, once dry, it does not come off onto the work (in this case T-shirts). I am sure that same would be true for wood. Just don't use too heavy a coat. Remember, this is to increase the friction, not act as the sole holddown.

05-04-2003, 05:22 AM
"This is all very well when you are dealing with large cutouts. But when you come to cut a bunch of small, closely spaced letters of around 2 inches high and you have cut the first fifty from a panel, there is not much vacuum left to pull down the loose bits cos the air is leaking through the cuts. Specially when the "Active" vacuum areas (Channels or holes) of the table are spaced apart. There simply is no longer the surface area for effective hold down."

Okay Simon, let's start all over again and try to solve your problem. Clearly, you are happy with the amount of vacuum/hold-force when you start cutting the first few pieces. The vacuum drops off until you have cut about 50 pieces and then you have too little force. (If you have a panel full of 2" letters, I guess you have a total of about 1000 pieces). Therefore, you are only 5% done when it all comes loose. And a vacuum pump with at least 20 times your current volume is needed if you don't change any other parameter.

A bubble over the Bot is good lateral thinking, but you are still going to need a huge volume of air to cope with the magnitude of leaks that you are describing. (the bubble could be "jumping castle" material, but your table will not stay flat under that huge force)

To the point of sounding like a stuck record, I again want to suggest our "bread-slicing" method that I have bored everyone with all over this Forum.

If you really want to stick with vacuum, the tacky adhesive would help a lot. But it would also help if you divided your table's vacuum surface into zones and switch these zones on/off to "concentrate" the vacuum in the area that it is really needed. Of course, this requires that you can plan your sequence of cutting, in which case you can also do "bread-slicing" and dump the vacuum system.

Would you mind e-mailing me (mailto:gerald@scapenotes.com) a .dxf file of one of your problematic panels?

05-04-2003, 07:08 AM
I cut out an animal shaped item that is no taller than 2 inches and only 1/4 inch thick. I will cut out a cheet of these at a time with no hold downs other than some srews well placed in my sheet.
I may loose one piece out of 24 to a cutter but even that is rare, I loose more to my dust collection foot. I do not tab my pieces and I use a 1/16 cutter as they are detailed shapes. When the file is done cutting they are ready to be pulled off the table. On occasion I may even leave my dust collection off to pack the cut out with dust so it stays in place and then I will give it a blast as needed. Oh I am cutting Aromatic Cedar (Juniper) not even a nice wood to work with as it is loaded with knots upon knots.