Anybody with experience with holding down stock with a vacuum?

I want to mill something without flat sides (imagine something shaped and sized roughly like a rubber ducky (maybe 2.5"x2"x2"). It’s bigger than the flip jig (I think - I haven’t received mine yet!), and I’d rather not do the thing where you leave little tabs attached to an outer frame of the stock - I’d rather just have it mill it out totally.

Somebody on reddit pointed me towards making a block with a cavity that’s the negative shape of the ducky with holes in it (so it’s a custom formed vacuum table), then milling one side of the duck, flipping it over, inserting it into the cavity, and using vacuum to hold the piece steady inside the cavity, and milling the second side.

I have no experience with this at all. I see that somebody (@cpignol, if you still come to this forum!) has done something similar here, but that’s with a flat vacuum table and a flat PCB blank. Does anybody know if that is feasible at all for a rounded, sculpted shape? Will it work even if my negative shape block isn’t exactly perfectly fitted to my positive work piece? What size/power pump do I need, do you think? I see both multi-hundred-dollar vacuum pumps for sale and then hobbyist 12v batteries for tens of dollars. If I have a fairly close fit and a 12v pump, will I be able to hold this piece steady down in the cavity?

Any other hints as to the construction of this thing would be helpful as well; where do I find a gasket that will bridge the gap between pump nozzle and vacuum block? Is this even feasible or is this a terrible idea? Any other alternatives for milling out a non-flat shape without using tabs?

@kjl, is there a reason you don’t want to use supports? They are a great way to self-fixture a part into the rawstock, and I think they will be essential when making parts using the flip jig. The only downside is that they require a little handwork to trim and sand flush after the machining. I have zero manual skills–I can injure myself eating salad with a fork–but I’m enthusiastic about using supports.

I showed some examples at Splines for 2-sided milling and included a few links over to the MeshCAM forum. They key to using supports is to place them on the widest or “equator” line of your geometry so they don’t shadow areas beneath them. That keeps the area to trim to the cross-section of the support itself.

@Randy: It’s the part about trimming and getting it to look right afterwards. I feel like my piece is shaped such that I wouldn’t trust my ability to get the tabbed areas to look right with hand tools; also, I don’t want to :smiley: . Also, if I wanted to switch materials to something else (aluminum?) I actually wouldn’t have the tools available to hand finish it at all.

I think my “backup” plan if the vacuum thing turns out to be a stupid idea is still to make a negative-space shape block that I can seat the stock in after I’ve milled one side, and then clamp it down and finish in multiple passes (e.g. clamp down on left and right and mill out the middle, then remove left and right clamps, clamp the middle and mill out the left and right).

Vacuum is common on moving gantry machines (i’ve used it many times) - the table has a vacuum surface that can hold data sheet goods with amazing amounts of force. Vacuum holding is very difficult with a moving bed and/or small parts - there must be sufficient area to have sufficient “grab”.

Vacuum holding can also be quite noisy.

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I am still using my vacuum table for PCB. The PCB should be flat. The main advantages are the rigidity since the force pressing on the board is important and all other the board, and the convenience. Start the pump, gently push down the board and watch the vacuum gage if you have installed one. The board is locked. Yes vacuum pump especially the small 12v is noisy.