vacuum hold down physics

[darvin]

Hello all,

I'm considering my first CNC purchase, ShopBot among them. But my question is related to vacuum hold down.

I'm very psyched about the vac hold down using the four small motors I've read about here on the forums. I'm particularly excited about the Black Box Vac Source. I'm sure I would need quite a bit of help designing that box and not sure if plans are available to assist (?), but mostly I'm hoping to better understand the practical differences between this very low-cost scenario compared to, say, a 10HP regenerative blower system costing quite a few thousands. Correct me if I'm wrong:

Black Box Vac Source may generate 400+cfm of flow, but max vacuum is maybe 9-10 mmHG. By comparison, a 10HP 3-phase blower might generate more like 22mm HG vacuum, but probably much lower cfm (~175 I think I was told). My assumption, based on my experience using a vacuum table press with a Venturi pump and gauges, is that approx. 2mmHG translates to 1psi holding force. So a 10" by 10" square of material being routed is (provided there is sufficient cfm and low leakage) being held in place with about 4.5 - 5psi by the Black Box Vac Source, and maybe 11psi by the big 10HP blower. That would be 500lbs of force compared to 1100lbs for the 10" x 10" work piece. Do I understand things correctly? I also assume that this would only be true up to the cfm capacity of the vac source, and presumably if you have enough leakage you will no longer maintain the above psi values. What is the conventional wisdom on the amount of cfm needed to maintain a given level of vac pressure on a table of given porosity and given leakage? Or am I asking the wrong questions?

The above sample calculation doesn't jive with my experience trying to pry up small parts while the vacuum pump is still running (when visiting a friend's CNC who has a big 20HP vac pump). I seem to remember being able to pick up some small parts (maybe 6"x12" or so) without much resistance.

Can anyone set me straight on the numbers above or point me in the direction of a complete description of the physics of the two approaches to vacuum hold down?

[dana_swift]

Hi Dan- interesting questions!

Let me see here. In very simple terms, total hold down force (in pounds) is PSI * SurfaceAreaInSquareInches.

If you were able to lift a 6x12 (72sqin) easily, it must have only had a total force of a pound or two. That means the vacuum level UNDER the part must have been somewhere around:

PSI = 2 / 72 = .027PSI.

Thats not much pressure. So where did all the horsepower go that is thrashing the air, making heat and noise? Leakage. The air does not go through the part, so it must go around it.

If you have leakage you have to thrash more air to get the same PSI with the extra amount of leakage. As the leak gets bigger the amount of horsepower required rises very quickly. Because the horsepower is not available, or the air is constricted, or some other limit, the PSI drops. Result- is no hold-down.

If you get good seals against your part and no leakage, you dont need much horsepower at all. I have held very small parts with gasketing and a very humble air conditioner pump to reach 14psi. But DONT CUT THROUGH THE PARTS, all assumptions fail quickly..

If you intend to cut through parts, the amount of horsepower required to maintain vacuum rises. To help this, bleederboard (usually trupan surfaced both sides) helps keep the leakage flow down while distributing the pressure to the solid areas remaining. It helps a lot, but not completely.

Alternatively when I have to cut through, I sometimes pause the shopbot and add maskingtape to the cuts to seal off the leaks, then continue. That helps a lot, I can often get 4-5 psi back.

I try to maintain 7psi (14" Hg) minimum on my parts, knowing they won't come lose until the pressure is well below that. Its just my comfort factor, and you will get your own. When it gets down to 7psi, I start switching on bigger pumps to make up the leakage. If I am running my biggest pump, I pause the shopbot and add masking tape to reduce the leaks.

My vacuum hold-down is designed for high vacuum, with 12psi (24" Hg) being normal. That is considered way more than most people need, and is only important if you (like me) intend to cut small parts. High vacuum pressures matter.

Hope that makes sense-

D

[darvin]

thanks Dana.

I'm doing general custom cabinetry nested parts production, mixed sealed pre-finished maple, unfinished MDF, raw veneered MDF, melamine, etc. You're getting deep vacuum at 24mmHG, is that with a large regen blower or rotary vane pump, or with a series of small motors? Do you have a sense of your current draw when you're pulling max vacuum?

-D

[dana_swift]

I hope you have your units incorrect.. if you are getting 24mm of mercury, that is less than 1 inch of mercury which is only 0.5psi. Very very little suction. 29-30 INCHES of mercury is close to a perfect vacuum. Maximum pressure is achieved at the days barometric pressure. Nominally 29.93 inches of mercury at sea level, less if you are at a higher altitude.

My biggest pump is a 5hp Becker (KVT 3.140), it is a vane pump capable of 28 INCHES of vacuum, 14psi, which is limited by its geometry. To save vane life, I don't run it that high. The current it draws at maximum power is approximately 16A at 220v, however start-up currents are estimated at 25. The circuit breaker is 30A, and has never tripped. When it is open, it draws not near as much power, my guess is around 5 amps, I need to check to be sure.

The Becker is not a good air thrasher, having a maximum air flow of just under 100cfm when pulling no vacuum at all. At 15" Hg, it can still move about 50cfm. That allows some leakage, and has been good for my application. I combine that with a 100pound propane tank (propane removed!) to act as an accumulator. That is the other trick to high vacuum.. use an accumulator. But now the ability to hold down small parts exists, as long as I can keep the leakage down to 50 cfm or less.

For your use, the lighthouse blowers are probably the best way to go. Look into the Brady Vac, and other vacuum projects on the forum. They achieve 7 to 8 inches of mercury (4psi) max, and have a nice high cfm to go with it for a whole lot less money. Your description of working with full sheets makes the psi requirement a lot lower. A little experience will help guide you to improve the system to meet your needs better.

High CFM systems dont need accumulators either. Another benefit.

Do a search ( use google, not the built-in search) to search the forum and you will find some excellent options!

D

[darvin]

oops, yup, wrong units. I meant inches of mercury and not mm.

I will search for your recommendations. Thanks again for the info.

-D