I have one of the coolest bosses in the world. I love to make things and he pretty much lets me make whatever I want then pays me to do it. He also has a hobby; he likes to work with leather.
So when he got his hands on a leather cutting drag knife (ED: Donek Tool’s Drag Knife) for our lab’s Shopbot he asked if I could build him a vacuum table to hold sheets of leather securely on the machine I was happy to comply. I started with some research on various vacuum work holding solutions, for high precision metal work they used tables with groves cut into to accommodate rubber gasket rings and required an expensive ultra-high vacuum pump. It was way overkill and too expensive. Then I looked at what they use on CNC routers, specialized porous material for the table surface and a very large and powerful workshop vacuum system. Again, overly complex and way too expensive.
The search for a solution
Frustrated that I wasn’t finding a good solution I decided to look on the website were the drag knife was first acquired, http://www.donektools.com. There I found some videos of the drag knife being used on a Shopbot and upon closer inspection of the videos, I saw that they were using what looked like a simple wood box with a perforated top surface, hooked up to what sounded like a standard shop vacuum cleaner. That I could work with. Easy construction and an affordable vacuum system we already had in the lab.
I started off by carefully measuring the Shopbot and I quickly realized that with a suitable air box on top of the Shopbot standard table surface it would use up way too much of the machine’s limited z-axis movement. I opted to remove the extruded aluminum table and have the air-box sit lower inside the frame of the Shopbot, giving me an acceptable amount of movement in the z-axis. This also freed up the some of the original mounting points and made mounting the table to the frame simple.
Next I had to figure out how to attach the vacuum cleaner hose to the air box. Because of Shopbot’s gantry style design, I couldn’t have the hose protruding from the sides or the back. Even mounting it underneath would interfere with the x and y-axis movements, so my only option was to have the hose attach to the air box from the front. Again, because I didn’t want to interfere with the machine’s z-axis, I designed a manifold that hangs over the edge of the Shopbot’s frame. This allowed me to mount the hose on the bottom of the manifold where it was completely out of the way of all the machine’s functions.
The build begins
With that the design was finished and now I could start building. I started out by making the walls of the box. I set up the table saw to the specified height and just cut a bunch of strips of wood. This ensured that all the sides would be the exact same height and not warp the table surface. I marked out all the screw holes as per my design and carefully pre-drilled and countersunk every hole. I didn’t want the wood to split or bulge, and I wanted all the screw heads to sit flush without deforming the surface of the wood.
In addition to screwing the parts together I applied a small amount of wood glue to each joint to help ensure an airtight seal. Next, I constructed the hose adapter and manifold by again pre-drilling and countersinking all screw holes. I had to hand-fit the manifold to the walls of the box to ensure a flush and snug fit, a bit of filing and sanding and it was perfect.
Next came the top and bottom surfaces of the box. I glued the bottom surface, but not the top because I knew I might have to replace it one day. Once the top was on I counterbored the holes for the mounting bolts and drilled a hole all the way through the air box for each mounting bolt. After that I actually mounted the air box in the Shopbot.
Now, I could have drilled all the holes in the top by hand, but that would have been extremely tedious, so I opted to have the Shopbot do the annoying work. I laid out all the holes in the CAM program and let the machine go to work. Half an hour later all the holes were drilled perfectly and cleanly. With that the vacuum table was done, now to test it.
This vacuum really sucks
First I just found a small piece of scrap wood and laid it on top of the table with the vacuum hooked up and on. This had no effect. That wood moved all over. Next, I added some additional pieces of wood and managed to cover every hole on the table. The vacuum started to bog down, but held steady at a lower rpm. When I attempted to move the scrap wood it would not budge no matter how hard I shoved it. I would call that a success. On to the leather.
My boss found me a piece of leather that was slightly larger then the table and we laid it over all the holes. Sure enough, the vacuum bogged down and when we tried to shift the leather it wouldn’t budge. However, when we lifted up on the edges we found it could rather easily be pried off the table.
With that I would say the vacuum table is a complete success. Now on to the next project!