Here is one of my rigs for vacuum holddown on a BT32.
To start with there is the base board, all mounting holes are drilled, as well as C rails installed. All of my vacuum fixtures bolt to the table using the 1/4 20 mounting holes. Below the table you can see the vacuum accumulator.
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The vacuum accumulator allows the part to suck down and make a good seal when the vacuum valve is first opened. It would be hard for a vacuum pump/blower to move enough CFM to do the initial evacuation. The pump is not currently running, when it is it moves over to 28-30in Hg pretty quickly.
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The accumulator is an empty freon bottle I was given by a friend of mine. It fits nicely under the moving table parts and is right where I can reach it while I am holding the part I am about to secure while I hit the vacuum valve to start the evacuation.
The first layer of the vacuum fixture is a channel that routes the input fitting to the individual fixtures. This jig has two hold down areas where I intend to use them one at a time. This fixture was made very quickly, so I didn't worry about machining the outside perimeter of the material. Everything is referenced to the holes for the hold down bolts so the layers line up correctly. The gasket tape is from all-star.
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The top layer is bolted through the bottom layer and the base board with bolts long enough to engage the tapped holes there.
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Now I connect the 3/8 vacuum line to which ever fixture I am using. I have used 1/4 inch vacuum line without any problem, but moved to 3/8 so the initial evacuation will happen quicker.
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I let the hose slide back and forth with the table, it has never pulled out or bound on anything. Just simple.
When my part is on the appropriate fixture half, I open the right hand valve on the accumulator. The part either seals in about a tenth of a second, or something is wrong.
I have made quite a few fixtures like this of various sizes to hold different parts. Making one of these fixtures takes about a half hour to an hour including the time with partworks.
Large wooden parts with rough edges tend to leak a bit around the edges. I still get a minimum 15" hg if will seal at all. I am using a standard AC vacuum pump rated at 5cfm.
Hope that helps-
Dana