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Old Sun 20 April 2008, 12:04
Gerald D
Just call me: Gerald (retired)
Cape Town
South Africa
Basic theory of vacuum holddown & pumps

Triggered by JR's post, here is the basics of vacuum holddown:

A blower type pump (shopvac, regen blower, "dry" vane) behaves as per the red lines. Adding pumps in parallel will add the cfm's at the same pressures/inches water. The 3 red lines above show 1, 2 or 3 of JR's blowers in parallel.

A positive displacement pump (piston or "wet" vane (oil-filled)), behaves something like the green line.

The actual table, plumbing and workpiece behave according to the black lines. A well gasketed and sealed table with tight valves will give a steep black line. (Zero leaks is a vertical black line going up from 0,0). Even a perfect "zero-leak" table, with a sheet of MDF being cut on top, is going to rotate the black line down flatter.

Now to the crux of the matter . . . . . .

We want the pump lines (red or green) to intersect the table "loss" lines (black) as high up on the graph paper as possible. Want want max "inches water" for maximum holding power. (Imagine a 100" thick layer of water's weight holding the job down).

For example: Point A could be where JR's system settles with one pump at 30"water read off on the left vertical axis. Adding a second pump will make the system settle at point B which is 47"water. A third pump will take him to 58"water.

If he leaves the plumbing and table/workpiece unchanged, and changes the red pump for a green one, he is going to get only around 15"water of holding power. A green pump will only be beneficial if the black line was almost vertical.

Does this make any sense?
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Old Sun 20 April 2008, 17:32
Just call me: Melissa #83
Brighton (Ontario)
Hi Gerald,

Thanks for that explanation. I've had some experience working with vacuum, but not with a leaky surface like MDF.

Here's an example of the "green" piston pump that I use when vacuum-bagging composite layups (fiberglass / carbon and foam):

And what the part looks like in the vacuum bag:

As the pump is only capable of about 3 cfm, it's critically important to have very few, and preferably no leaks.

To further reinforce the huge difference in application, in the photo above, I'm pulling 24" Hg [mercury]. That converts to 326" of water. Obviously I'll need to buy a different vacuum pump for my MechMate .

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Old Mon 21 April 2008, 05:00
Gerald D
Just call me: Gerald (retired)
Cape Town
South Africa
Thread was drifting rapidly, so I cut back on some words and a link.

The subject here is vacuum hold-down and the pumps used for this purpose.
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