I finally finished my vacuum table. I have a bottom sheet of 3/4 MDF with vacuum galleries carved out and I have a 15 mm sheet of MDF screwed on top. The MDF started out as a 3/4” sheet but was skim cut on both sides.
I am using my house central vac system for these tests as it seems to have more suction than two shop vacs I tried.
Anyhow, I have around 6 mmHG of suction both with sucking through the vacuum table and only slightly more if I just measure deadhead suction on the hose.
As it turns out, that is definitively not enough vacuum to hold a reasonably large work piece secure enough for milling.
I have two questions:
If you are using a shop vac for a vacuum source, what kind of negative pressure do you see? Based on what I could find on the net, the better two stage vacuums produce between 6 and 7 mmHG maximum.
If you are using a shop vac, are you sucking air through an MDF spoil board or do you place your work piece directly over the vacuum galleries with no MDF in order to get reasonable holding power?
Yes (mostly) …. The underside of the bottom sheet was painted and the edges of the bottom sheet had solid wood edging glued on. If you put your hand onto the top MDF sheet you can feel the suction and also the air flow. It really surprised me ….
First a correction - with the vacuum connected to the approximately 25^44 inch vacuum table, I only have about 3 mm Hg vacuum and not the 6 mmHg indicated earlier.
It occurred to me that maybe the stock I was trying to hold was not big enough. After all, even the commercial tables have issues holding smaller pieces of stock. I took a sheet of MDF about the size of the vacuum table and tested that - I was unable to move the sheet!
I tried a half sized sheet (in this case particle board) and it too was held probably sufficiently well for normal cutting forces.
I might be able to go even a bit smaller if I reduce the cutting loads a bit.
If I remove the MDF spoilboard and place the stock directly on top of the plenum chambers (with a seal around the work piece to prevent leaks), I seem to get even better holding power (although this is subjective)
I have placed some EDPM sheets (thanks @dehoutwinkel) in my Amazon shopping basket and will give that concept (ie as per AirWeights) a try once the order is high enough for free shipping.
I have several shop vacs and I will see if can improve holding power if I combine multiple vacuums.
Two shop vacs daisy chained together can give me a full 6 mm Hg but unfortunately, with the vacuum table empty, there is so much air flow that the passages restrict things too much. The back end of the table is unusable at that point. I would have to reduce air flow by reducing the open areas over the MDF.
This is where my next test comes in - I will give the EDPM sheet a go and see if it can reduce the air flow through the MDF enough to make a difference. Covering the unused surface of the MDF spoilboard would also reduce air flow but I want to give the EDPM a try.