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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
If the shop vac is pulling pressure down 2 psi (-2 PSI gauge or ~12.6 psi absolute ) then your delta from atmospheric is 2 psi ;) And your surface area that you use would only be the area of the vacuum distribution channels in the picture I linked. The squares which support the work piece dont contribute to surface area sucking down the part (until it starts to lift up, but thats generally a bad thingâ„¢ and you dont want it lifting up at all)
So you need to balance your percent support area to vacuum channel to get the support you need to keep the workpiece flat, vs the hold down force you need to get the static friction to resist the side load.
And as far as tip angle vs flatness, of course an increased angle needs a flatter work piece. a 170 degree tip would only need a little bit of travel to go from the tip to the full shank diameter, and a 10 degree tip would be quite long, so the kerf width vs depth wouldnt vary as much.
Since a vacuum cleaner is a high flow, low vacuum sort of vacuum pump, your fixture should be high flow to suit it. I.E. rather then small channels and a few holes like in the picture I linked, you should probably have a plenum underneath your table, and holes drilled into the plenum at every intersection of the vacuum channels. Perhaps you can find some sort of spray adhesive to apply to the fixture that would make it tacky, to increase the COF of the part sitting on it.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
sorry, i meant to type down to 2 psi, not down 2 psi... i've get to start proofreading my posts...
anyways, the site that is pretty unclear so it might as well be -2psi. i know how inefficient it is to use a vacuum instead of a vacuum pump, but this is just for testing purposes. if it proves viable then i'll invest in some real pumps.
i know that the wider the bit angle the wider the cut if the tables *not flat, but then why are people implying that they work better than narrow angle bits as long as the table *is flat? simply makes no sense. just go for a 10 degree if you ask me.
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vacuum hold down table
