On the edges…but what material are you using?
MDF is shockingly porous and won’t hold vacuum unless you seal all of the surfaces quite well. And some plys have a lot of porosity along the edges that will leak vacuum.
I had big plans for a vacuum table on my old MPCNC…then got talked into just doing a quick test with some sheets of foam from the dollar tree.
I wound up using 3 sheets of foam - one with a series of horizontal slots, one with a series of vertical slots, and one with a grid of holes over the intersections of the slots. Added a 3D printed plenum on the side and hooked up the smallest cheapest shopvac I found at walmart for $20.
Almost wasn’t worth drawing it up in onshape at the time but I did:
I just cut slits for the plenum with an xacto. I forget who designed the plenum but I see I had it in onShape as well: Onshape
Actually…just found in the description on the terrible video I did about the vac table that it was @moebeast who did the plenum which is here: Foam Ripper by moebeast - Thingiverse
Video is here - terribly edited (I hate editing video so leave in way more than I should) https://www.youtube.com/watch?v=tyKDV7OIQQs
I had originally planned on only using it for cutting foam and it worked great for that. But I found it also worked great for conventional milling as a “Vacuum pad” more than a “Vacuum table” Since the bottom slots were open to the table it would just suck itself down and hold position. I’d just put my work on top and then toss some sheets of paper over the holes that were still exposed.
I need to make a new one…tossed that one out in March when I started my LR4 because it was really wearing out:
So…point of this long ramble. I started out planning on building a big complex vacuum system into my table. Wound up making a quick proof of concept that cost me $3 and about an hour of effort (would be $3.75 now thanks to DT raising their prices) and found it worked so much better than I expected that I decided the big complex plans weren’t really necessary.
Oh - and just to be really annoying. I’ll finish by somewhat contradicting myself. MDF will leak vacuum like crazy…particle board will too…but…surprisingly not as bad as MDF. My table top I used this on was particle board and it never had any issues holding itself down. Though I did find using it to hold MDF was a lost cause unless you seal the MDF.