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CNC Vacuum Table Build (WIP)
Thought I'd post my vacuum table build progress here - it'll be a slow WIP due to limited time in the shed.
I'm not a vacuum expert, so I'd welcome thoughts, opinions and discussion here. I'm working things out as I go along.
When I was thinking about playing with CNC's, I had a look at fellow forum member Pippin88's machine, and he casually mentioned that much of CNC thought-life is about work holding. At the time I thought there were so many more interesting things to think about, but it turns out he was right. There's nothing quite like cutting a perfect piece, only for it to slip loose and ping off into a cutter spinning at 20000rpm on the last cut.
Apart from the obvious issues of running the machine into clamps, and slicing up your shiny hardware or destroying your expensive cutter, there are less obvious things like cutting order to think about - cut out the insides before you do the outside, because once you do the outside, the inside is no longer clamped, and it'll move about. Or cut a channel close to a clamp and you suddenly lose clamping force as the wood bows in. Offcut box. Start again.
A solution is a vacuum table to suck those parts down. Particularly useful if you cut up a lot of dodgy cheap plywood with all those 'feature' voids and warps.
For sheer vacuum force, it seems that a rotary vain or liquid ring pump is best:
Attachment 509919 Attachment 509920
Problem is, for a given size, they make lots of vacuum, but not much flow - and we woodworkers need flow in our vacuum setup to compensate for the holes we make in the workpiece - as you make more holes, so you lose suction. A bit like occluding the end of the vacuum cleaner - completely occluded, there's a lot of suction. As you occlude less of the orifice, you loose suction. You can get around this by increasing the gas flow - so stick your vacuum hose on the end of a giant dust collector, and you'll still get suction with only a bit of the orifice occluded.
To get enough flow from rotary vain or liquid ring pump, you have to get something really big, which needs a lot of power, and a lot of cash.
So most woodwork systems use vacuum cleaner motors. They move a lot of air, but don't generate a huge amount of suction (relative to a rotary vain, for example). The solution is to use multiple vac motors. They're relatively cheap, which is good as because they're primitive, inefficient machines doing something they were never designed to do, they lose all their smoke regularly. It seems that people running these systems routinely keep a spare motor on the shelf so they can replace the inevitable failure.
Attachment 509921
Probably wise to have a fire extinguisher nearby too!
The Shopbot forums have some lengthy threads around the original blackbox vacuum concept, which eventually became the Hurricane systems so favoured by youtubers. The bloke who developed the blackbox and started the discussion eventually got fed up and left the thread in a bit of a huff - he seemed upset with people messing with his designs, changing things and building their own interpretations. Not sure what he expected of the internets! He now appears to have a financial relationship with the makers of the hurricane systems, so I think he's still involved in development, and definitely not talking about it on the open source internet.
It's a pretty simple design - mount 4 vacuum motors, connect the suction end to some PVC pipe, and stick the end of the pipe underneath a piece of MDF. MDF is porous, so the vac sucks through it and clamps your work down.
Inside it looks like this:
Attachment 509922
The baffles on the outlet side (I guess) are there to reduce the noise - these vacuum motors wail like a banshee.
A simpler variation uses silencers on the outlets:
Attachment 509923
And another flavour
Attachment 509924
So, my plan is to make something blackbox-like. Melamine-coated something for the box - MDF, ply etc are all porous to some extent, so you potentially lose vacuum. Preferably something not too flammable (possibly why the commercial Hurricane is now made out of metal).
I'll use melamine for the bottom half of the vacuum table too, and MDF for the top. I'll have to play with it a bit as you don't seem to be able to get the ultra-light (more porous) flavours of MDF here in Aus.
Attachment 509925