I think that the nature of a maker is to improve and build on other's ideas to come up with your own. This is an instructable designed to show you how I made this vacuum table so that you can adapt it for your CNC. Because CNC machines come in all shapes and sizes, the size and shape of vacuum tables will vary greatly so I will try to make it as versatile as possible.

I made this vacuum table to fit the Rascal CNC and specifically to cut 1/4in foam or cardboard that is about 20 by 30in; however, it can be used for many other materials as well. It is split into quadrants so it can be used for smaller pieces as well. Also, because it is made from foam, your bit can cut into it and not be damaged.

To build it you will need to be able to rip some lumber with a table saw and CNC a bracket for the hose. Otherwise it just require regular garage tools like a drill, drill bits, and screws.

It works best with a common shop vacuum. If you suck too much air you risk crumpling your foam.

Supplies

1in XPS foam insulation (Menards or Home Depot)
PVC about the size of your vacuum hose
3/5in 1 by 4 pine board
Gorilla glue
1-1/4in wood screws
Small scrap of plywood

Step 1: Design

Sizing

The first thing you want to determine is how big you are going to make your table. I wanted it to cut 20 by 30in foam board. My machine has a bed of about 35in by 30in so this was perfect. I also left some margin (~2in) just in case.

Fastening

My CNC table has threaded inserts every 4 inches so I designed my margins to match up with the holes so that the wood edging can be fastened, down providing excellent stability.

Top Grooves

The grooves were cut with a 1/4in bit and left 1in squares in the middle. This seems to work just fine. It needs air to move through, but it also needs to support the material especially if it is foam or cardboard.

Bottom Air Channels

The bottom air channels are what actually supply air to the grid. I cut them 1/4in deep and 1-1/2in wide. This seems to be a good size. Again, its more about getting a vacuum than the quantity or pressure of air. Also make sure that the channels don't line up with any of your threaded inserts because they will let air in and destroy your seal.

Toolpaths

I use Fusion 360 for almost everything I do CNC. I use three types of toolpaths for this project:

2D Contour - for the holes that go all the way through cutting halfway through on both sides. I also recomend adding tabs even though its just foam. I had a chunk go flying. It was exciting. I also used this for cutting out the bracket at significantly lower speeds.
2D Trace - for the grooves. Fusion has trouble cutting a slot the same thickness as the tool.
2D Pocket - for the channels. I also included a slower cut for cutting the channel in the wood boundary.

Step 2: Assemble

Cut wood frame

Rip the 3/4in pine into the thickness of your foam. To do this accurately you can use the foam as a reference against the fence of the table saw. You want them to be as close as possible erring on the side of a slightly shorter wood frame. If the frame is too tall it will create a lip above the foam. To check set your foam and wood next to each other on the table and see how they match up.

Drill holes for fastening system

Drill the holes where they line up with your fastening system. For me this was every 4 inches. I then test fit it to make sure. Also consider insetting the holes for the bolts so they don't stick up into your stock.

Screw wood frame together

Clamp and screw the frame together making sure it is all still square. I used 1-1/4in wood screws, but whatever works.

Cut foam to size

This is the most difficult step. I found that a table saw works pretty well, but it is still difficult to cut squarely. You can cut it with a utility knife blade and then dress up the edge with a sanding block and wood guide. (see pictures) Test fit often to make sure you don't take too much off.

Glue

After everything fits nicely, fasten the frame to your CNC table and mask the bottom edge where the foam and wood meet. Apply a very thin bead of Gorilla glue to the edge of the foam and spread it out. Spray the frame with a little bit of water to activate the gorilla glue. Then slide the foam into the frame and set weights on it. You might also consider placing thin spacers under the frame just to make sure the foam makes it all the way to the bottom.

Step 3: Cut

Cut Top of foam

Cutting this foam is a breeze. The toughest thing is flipping the board over to do the other side. I made the origin on one of my bolts because this was an easy reference point. If it is not lined-up very well, don't worry about it because it doesn't matter in the long run.

Bit = 1/4in 2 flute straight. Feedrate = 80-100ipm. Spindle = 15,000rpm. Stepdown = 1/4in

Cut the bottom

When you cut the bottom you will want to cut a channel through the wood frame. Make sure you slow down your cutting when you cut wood.

Step 4: Add Bracket

Cut

I used a 1/8in endmill for cutting the bracket. I made a rooky mistake when I left the feedrate at 80ipm but I got lucky and the machine cut it fine. Always check your feedrates. I included a dxf of the bracket as well as including it in the Fusion 360 file. For the dxf, the semi-circle is half the thickness of the plywood and the rectangle at the bottom cuts all the way through. Test fit your PVC pipe onto the bracket to make sure it still sits flat with the pipe installed.

Drill and screw

Line the bracket up with the channel on the front of the frame and drill holes for your screws. Screw it on!

Add the tube

My tube is currently just friction fit in the bracket. It needs to be able to support the weight of the vacuum hose. If this doesn't work, Gorilla glue works great, just use it sparingly.

Step 5: Use

Using this vacuum table is very easy. You simply lay your material on, block any unused quadrants with extra foam or wood cut to size, and turn your vacuum on.

If you don't build the valve, leaving a small gap around the edges makes it easier on your vacuum.

In the picture you can see that I cut some Adams foam board. I am cutting a Flite Test Simple Cub RC airplane. It works great for this kind of stuff.

Step 6: Vacuum Adjustment Valve

Credit to the comments for this idea. When you run the vacuum table with the vacuum directly hooked up, it tends to put undue stress on the vacuum so I wanted to make a valve to adjust the airflow. The idea is pretty simple but very effective. It has a sliding tube that covers and uncovers a slot in the vacuum tube. All it needs is a straight fitting the size for the tube.

Cutting Fitting

You need to cut the fitting so it fits around the tube without hitting the table. Mark the fitting with a sharpie and cut with a handsaw or Dremel.

Sand Inside Lip

The fitting comes with a lip inside to make the tube stop half-way. I was able to sand it off really easily with a Dremel. It sands so easily you could probably do it by hand. Make sure you sand enough off to make it slide easily on the tube.

Cut Slot

Use a large drill bit to start the hole and touch up with the Dremel or sand paper.

Use

I try to allow just enough pressure to keep my material securely down but no more than that. There might be a tiny bit of leakage around the fitting so you might consider skipping this if your vacuum isn't very big.

Step 7: Final Thoughts

This project is 100% opensource. If you have suggestions, please comment and let me know. Future ideas include better plugs for the quadrants, and maybe even changing the grid pattern for improved air flow.

Thanks for reading this instructable. It has really helped me to organize my ideas and I hope it will inspire you to make your own vacuum table.