Forgive the newbie - question: is it usual to utilize separate vacuums for dust extraction and hold-down, or could they be run off the same source?
A proper dust collection system is not suitable for hold down because it is a low vacuum but high volume flow; the negative pressure it can generate may be only a few inches of water.
A vaccum cleaner system, portable or built-in, will produce negative pressures in the several tens of inches, 80" to 140" (optimistically), so if you are dealing with surface areas greater than 2 or 3 square feet you can get adequate hold down.
When you are dealing with surfaces areas less than 2 square feet you have to start moving to proper vacuum systems which can generate negative pressures in the hundreds of inches of water and normally these are rated in inches of mercury.
There are other details such as the porosity of your material and whether you want to do spoil board work with a solid sheet held down on top of a porous sheet and these things influence whether you should have a high flow high vacuum, low flow high vacuum, etc.
If you tried to use the same vacuum at the same time for dust extraction and hold down you would loose all of the vacuum pressure on the dust extraction side.
DIY plans for Vacuum tables
whats sort of details / factors would you need to take into consideration if you were going to build a vacuum table (saY 8' x 4' ) evenbetter still does any body have plans etc...
I have had a look at quite a few systems and I think I will have to build one. I have decided on 25mm thick HDPU (that stuff they make plastic chopping boards out of) it will have 5mm grooves cut using an end mill spaced every 20 or so mm apart in both the x and y direction, then every 4th or 5th intersection I will drill and tap a hole thru to the bottom.
the under side will have the same 5mm grooves but only in the x direction
at each end there will be a cutout that will form a plenum chamber and a pipe to which is attached the vacuume pump
in order to seal the whole unit I will attach a sheet of thinner (3-5mm thick) material. the hole lot will be screwed glued and tatoo'ed to my table. I will then machine the whole surface to ensure it is flat to the tool.
I have some rubber strips that I will use to create zones under the job to be machined
And remember earlier i mentioned drilling and tapping some holes ever 5 or so rows... well these will have grub screws in them that I can add or remove to creat the inlet for the suction
based on this theory I shouldn't need a great deal of vacuume presure to hold down my work