CNC made Vacuum table project.

Sooner or later everyone needs a vacuum table, but where to start?

In the picture below you can see the main parts of this project, at the bottom, the tray, above that the base and three identical inserts.

This whole project should cost less than 20 UK pounds for acrylic.


This project features objects that can be made equally well out of acrylic or aluminium, with a total vacuum area spread over three inserts, each insert being capable of generating in excess of 25 Kg (60 lbs) of holding force.

Each insert is just under 100 x 150 mm, and each insert can be flipped upside down so when it get's damaged simply flip it over. This "so what" attitude to the vacuum table being damaged is IMHO essential, and so the vacuum table needs to be made from such materials, and to such a design, that such "expected" damage has the minimum possible impact.

You will note that there are three inserts, note that the corners are radiused, as the recess that they sit in will have radiused corners from the 4mm endmill (suggested) which will be used to make this project from flat "optical" or window grade acrylic. Incidentally, while it could also be made from aluminium, if made from acrylic the vacuum passages are visible, which might be handy with swarf etc.

You will note that when under vacuum, there is a greater area of insert subject to being held down, than there is available to hold down the actual work, specifically the spaces between the slots contain more area than the holes.

The vacuum holes are 6mm diameter, I'd  suggest using a 4 mm diameter end mill for this whole job, even if you don't have a "centre cutting" mill it will circular pocket down nicely, we want these holes clean, so I'd suggest all machining operations work in slices 0.5 mm deep.

The radiuses at the corners of the inserts are 6 mm diameter, so the 4 mm end mill can produce them nicely.

Although there are three inserts, you could always make a couple of blanks. It is assumed something like neoprene rubber or 1mm acrylic will be used to blank off any holes that are not required.

NOTA BENE, when finished, the inserts should NEVER sit so deep in the pockets in the tray that they are recessed.
0.1 - 0.2 mm proud is fine. In extremis a sheet of 80 gsm paper under the insert will do.

Don't forget, once you make provision for alignment between the various sections of the vacuum table, the vacuum itself will keep the whole thing together. One thing to watch out for though, if you make lots of parallel, partial depth, slots in acrylic, there is a tendency for the material to stress relieve itself and curve around the slots, especially if you run the cutter hard. If this happens a hot air gun or IR lamp is your friend.
Please remember that it is quite possible to vacuum form 10 mm thick acrylic with a surprising level of fine detail.

Don't forget, hold down force is a function of area x pressure differential. If you are lucky enough (like me) to have an old speedivac piston type vacuum pump you can get near as dammit 75 Kg of hold down force, even with a shop vacuum cleaner you should pull 30 Kg, and the fact is that anything that can shift friction caused by 30 Kg of clamping force is probably going to overcome the tensile strength of the acrylic itself, particularly where / how the acrylic is attached to the mill table. It doesn't need fancy seals or anything else because the actual vacuum inside the table keeps it sealed.

Attaching the acrylic to the mill table is something I have left blank, in mine I simply use six off 4 mm socket screws into drilled and tapped holes in the machine plate (a 10mm alloy plate covered in an array of 3, 4 and 5 mm drilled and tapped holes) that I sometimes attach to the mill table.

While this design is suitable for a machine will a working envelope in X and Y of 300 x 150 mm, this is a design that can be scaled up or down at will. You are asked to note that with the application of only a little brain work it is quite possible to make the entire vacuum table on the machine in question, even though some of the pieces exceed the work envelope, none of the actual maching operations do, because only certain areas of the largest pieces require machining, or because certain areas can be done in multiple stages.

The basic materials are 10 mm thick acrylic and 5 mm thick acrylic and one small piece of 2 mm thick acrylic.

Working tolerances of 0.1 mm is good enough for this project.

The basic tool is a 4 mm diameter end mill, ideally centre cutting but not required, HSS is fine, the sharper the better.
Definitely rig up something to blow a jet of air at the cutting zone to keep the cutter clear.

The objects in stl format are here, and dxf format are here. Only one of each object is included, though you must make 3 inserts,
  1. The base is pretty straightforwards, just some slotting at one end and a bit of pocketing. Apart from that there are the 4 off 3 mm diameter holes to locate the tray. The small seal is anything you like, it just exists to seal the troughs away from the gap between the tray and suction pieces.

  2. The suction piece is anything to mate to your vacuum source, now unless you've got a lot of leaks somewhere it doesn't take a great deal of volume to get a good vacuum. 1/4" NPT barb works for me.
    NB vacuum cleaners don't like sucking a hard vacuum, they use that same air to keep the motor cool... you can imagine it, half way through cutting that part and the vacuum cleaner motor thermal overload trips...

  3. The tray is quite complex, although it might seem like a waste of time I'd consider circular pocketing ALL those holes all the way through first, it stress relieves the acrylic evenly, then pocket the top, then slot the bottom. Up to you.

  4. The inserts are straightforwards enough, don't forget the radiused corners.

  5. The tray and inserts are what CNC is all about, boring repetitive stuff, so let it do it, just use air to keep the cutter clear, and don't push too hard and fast.

  6. The most important thing to watch is that when finished the inserts sit dead level with the top of the tray, or fractionally higher, NEVER LOWER, or it won't work. Think about it. If you do screw this bit up go and buy some 6 mm thick and make some more inserts.
So, some tips
  1. I'd suggest you empty your vacuum cleaner before you start, this is going to make a LOT of plastic swarf.

  2. I'd suggest you rig up something to blow air over the cutter to keep it clear.

  3. P A T I E N C E, let the CNC robot slave do what you built it for.

  4. The sharper the cutter, the better, ideally a brand new or reground 4 mm end mill.

  5. When setting Z height fine jogging with a cigarette paper in between the end of the mill and the top of the work is useful, when it grips the cigarette paper there is less than 20 microns gap. Annoyingly I just gave up smoking.

  6. For use with the vacuum table try to cobble together something other than a vacuum cleaner, they are noisy and prone to overheating when used in this manner, plus they burn a lot of power, 2,000 watts or more. Old compressors and small 150 watt electric motors make useful vacuum pumps. Much more economical, much cheaper, much quieter, much longer lasting.

  7. The actual suction / clamping force achieved is exactly related to the level of partial vacuum achieved, and that is exactly related to the ratio of your vacuum pump capacity and how many leaks your vacuum table assembly has, keep it clean, especially mating surfaces, and you'll be surprised. Obviously whatever is being held down needs to have a smooth surface too, but with all acrylic vacuum tables holding acrylic work pieces I've managed to pull what basically amounts to a hard (2nd stage of a 2 stage speedivac) vacuum without even trying

For reference I use Rhino for CAD, MeshCAM for CAM and Mach3 for CNC.

You can always try to contact me via https://surfbaud.dyndns.org/ (self signed cert)

December 2008