I’ve been interested off and on in vacuum fixturing for quite a while. My brother is in the design stage of a big CNC router table, and wants to build in a vacuum table capability. I had been dimly aware that it is also applicable to CNC milling operations on metal, but hadn’t really delved into it too much. Then I came across a great article showing how to build a vacuum table over on the MicroSystemsGeorgia web site and it was the impetus for several hours spent researching this method of fixturing.
Here is the table design by Chris Kokourek that got me going:
Here is the vacuum table mounted on the VMC table. That outside groove was done with a 1/8″ ballnose and is used to hold rubber cord that seals the edge. The round vacuum ports are milled 1/16″ deep and each one has a hole in the center leading to the passages behind…
Here is the vacuum pump. It takes compressed air (20 CFM) and uses a venturi to generate the vacuum. In addition, it tries to silence the noise a bit. Not cheap, but some of these are available on eBay!
Here are the air passages on the back side of the vacuum table…
The air passages are sealed with a simple plate…
These are the changes Chris made to get to his 2nd generation vacuum table:
Fewer holes. Originally there were holes around every edge to use additional clamps. The clamps are needed to prevent the work from sliding sideways from milling forces. But, he discovered the two clamps shown on the table were sufficient to prevent the sliding. Filling the holes means one less place for liquid coolant to collect and splash back if you use an air hose to clean the chips.
He re-routed the groove for the outer gasket so there are no open bolt holes, hence it is inset around the holes. If you don’t do this, you have to tape the holes on the workpiece or you have a vacuum leak.
Chris has provided some dowel pins to repeatably locate the workpiece on the vacuum table. You’d think these would help minimize the sliding tendency as well. Or, you could put the dowel pins along two edges and butt the workpiece up against them to locate it.
A nice vacuum table like this could save you a lot of time loading and unloading your work–perfect if you do a lot of work with large plates!
No sooner had I finished absorbing this interesting article did I head over to eBay to see how much more cheaply I could lay hands on a venturi vacuum pump. There were some pretty decent deals, but I didn’t pull the trigger. It did get me poking around though, and I found this:
ClampuSystems Vacuum Clamping Kit…
This is a vacuum clamping kit. You get 4 puck-shaped clamps which you presumably secure to your table and set the material on top. They sell these for cabinet makers to use with routers and such. The kit is very reasonably priced and might be a good way to get started experimenting with vacuum fixturing. They sell a variety of vacuum tables, clamps, and accessories on their web site, including vacuum venturi pumps that are a lot cheaper than the ones listed on McMaster Carr. I suspect they may be louder, but the other specs seem comparable.
Next, I jumped over to CNCZone to see what posts about vacuum fixturing might pop up. As usual, there was a lot of material to go through. These were my findings after going through 10 pages of search results:
– One post suggests using Gast rotary vane vacuum pumps. This may be a lot quieter than a compressed air venturi pump and a cursory look at these pumps on eBay suggests they don’t cost any more. You’ll probably want to pair one up with a tank so there is a little extra capacity, so that would increase the cost a tad. If you do use a pump on a machine with flood coolant, you’ll also need to figure out some way to keep coolant out of the pump.
– The same thread suggests All Star Adhesives for gasketing supplies to use in conjunction with vacuum fixtures.
– From the same thread, here are some pix of a vacuum table used for diamond drag engraving small plates:
The white material is an open cell foam used for gasketing. Note that it is also in the center to support the workpiece from bowing. Each pocket holds a workpiece…
– Long-time contributor Geof converts a compressor to a vacuum pump for some serious capacity. Here is his vacuum table with improvised coolant trap (important to avoid screwing up the pump!):
Household water filter serves as a coolant trap for the vacuum line…
Converting a compressor like this results in a much more efficient vacuum pump than the venturi systems, but you do have to be pretty serious about vacuum fixturing and have a spare compressor laying around somewhere! Interesting thought: to pull a vacuum, the compressor must overcome a pressure differential of 14.7 lbs per square inch. To compress to normal shop air pressures, it needs 10x that much pressure differential. Doing duty as a vacuum pump is much easier on the compressor. Geoff suggests a 2HP converted to vacuum pump is plenty. He just happened to have the 5HP available.
– Vac-Magic is Mitee Bite’s vacuum pallet system:
– Advice from TXFred on CNCZone about using vac tables:
1. Only a single vacuum port is needed unless you plan to set up multiple pieces at a time. I put the port near the bottom left corner, and made that corner the origin for all my programs. You can see that I put in lots of holes for ports. This was a mistake, because I had to divert the gasket around each one of these holes.
2. I used 1/8″ closed cell foam tube from MSC as my gasket material.
3. The grooves were milled with a 1/8″ end mill. Feedrate was kept low to create the smoothest possible surface.
4. Face the top after the grooves are milled. Then chamfer the edges of the grooves, or they’ll tear up your seal. Round the corners of each raised square, for the same reason. Once that’s all done, flip it over and face the bottom.
5. The grooves should be spaced so that the distance from the outside edge of one groove to the opposite edge of the next is a fraction of an inch. Your stock will likely be measured in whole inches. You need to be sure that the stock completely overlaps the seal. If your grooves are on 1/4″, 1/2″ or 1″ centers, this won’t happen.
6. The grooves should be about .08″ deep or less. The foam compresses a lot, and you want a good seal. I made my grooves 0.1″ deep, and sometimes had a hard time getting a good seal. Put a vacuum gauge on your pump so you can verify the seal. Also put a ball valve on the pump, so you can quickly apply or release vacuum without power cycling the pump.
7. The Harbor Freight vacuum pump works fine, but does put out some oily smoke. Also, it is not compatible with flood coolant. It will inhale any coolant that leaks past the seal, and that coolant winds up in the pump’s crankcase. [Fred also mentions filling his coolant trap with flood coolant very quickly, about 1 minute!]
8. When programming, touch off your Z to the table, then jog up to set your zero. Since your zero is relative to the table, you can do clever tricks like cutting almost all the way through material without breaking the seal or milling into the table. And when you’re done you can break the material into pieces along the lines that you milled.
At some point, I need to build a vacuum table to play with. These notes and references will serve me well when that time comes.
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