For months I have been bantering around the idea of a vacuum table for the CNC router, but each time decided that screws or pins were easy enough, and the whole issue stayed in the too-hard basket.
As I have been doing quite a bit of nesting work recently, it gave me pause to thought – for a one-off, a few screws are all very well, but the combination of that, and the significant time wasting of using tabs to restrain the cut components (both drawing them, and then physically having to cut and remove them) was proving an incredible time waster.
So I finally was pushed into addressing the whole material hold-down issue.
I started doing a bit of research online, but the results were less than helpful, and I felt as a whole, a lot more complicated than necessary. So instead, I decided to build an idea I had, and just see if it worked.
I did use the CNC for the following steps, but that is certainly not necessary, and secondly, while I am using this on the CNC router, there is absolutely no reason this cannot now be applied to other areas of woodworking. Nor do I expect I have come up with anything novel, but in going back to first principles, hopefully I have significantly simplified the solution.
So to start, I took a thick piece of MDF (22mm or so, which I had to hand. I would have used thicker, but the 32mm MDF I bought last time from Bunnies was only some of their promotional stock. Not sure what they were promoting, because they don’t stock it otherwise). With a 1/2″ ball nose router bit, I cut a matrix of tracks, 5mm deep, and about 20mm apart, both horizontally and vertically, stopping about 10mm from the edge.
Next, the edges of this board were sealed. I know people use some edge tape, or shellac for this, but I thought PVA glue would suffice!
This board was then screwed down to the bed of the CNC machine, and a hole just big enough for the end of a vacuum hose was drilled, all the way down and right through the table. The hose of the vacuum (connected up to a cyclone separator) was jammed into this hole from underneath.
A second thick board of MDF was laid on top of this bed, and the vacuum switched on.
Test one – does it suck? Yes it can! The first proof of concept is a winner.
Into this second board I cut the same matrix of slots. By then flipping this board over, each of the passageways is doubled in size (adding together the bottom and top halves), and also exposes a significant area of the soft, porous core of the MDF. Each passage is now 10mm diameter, so that gives significant passageways for the air to pass through.
The vacuum was switched on again, and the top surface of this second board (the sacrificial board) was machined away with a surfacing bit (otherwise called a spoilboard bit). And that is what this upper board actually is – a spoilboard. When it gets too badly cut up, it can be flattened again, and this repeated until it is too thin, when it is then thrown away and a new board takes over. By planing away 0.5mm of the upper surface of the spoilboard, the hard, compressed (and more non-porous) upper surface is removed.
Now I have seen a number of vacuum tables, and spoilboards with a large matrix of holes drilled though it. Don’t need it. The core of the MDF is so porous, that the vacuum can draw air directly through the MDF. And that in a nutshell, is my vacuum table!
I’m using a basic Shopvac for this, so I do have a bit of a concern that this will shorten the life of the vac. I possibly need a vacuum pump, but this will do in the meantime. The cyclone separator is to try to keep as much MDF away from the vacuum, to try to stop it being killed even more prematurely.
The proof is in the trial.
With a sheet of 3mm MDF laid on top, the vacuum switched on, and voila – it sucked big time, right through the MDF. The board to be cut was held firmly, enough to run a trial nesting job.
It was a complete success. Other than the noise of the vacuum cleaner, I could not fault the process. The vacuum will soon find itself in the shed next to the workshop, and switched on and off with the remote power switch I happen to have in there (the actual switch is right near the CNC as it happens).
I cut out about 5 patterns in total, and each time it worked perfectly.
Next, I tried another idea. If the only reason for the material between each piece is to support the piece as it is being cut, it is really necessary if the piece is supported by the vac?
So I ran a large job with a full sheet, no tabs, and only 2mm between each piece (or more precisely, between each path the CNC was trying to follow). And 5mm from the edge.
Pretty much nothing left, what is gone is the project, leaving this sad skeleton.
So there you have it – my poor mans successful attempt at making a vacuum table.