The Heads on Fire Community

Brian set up the vacuum system for the ShopBot and used Brady Watson's explanation as a starting point, He reconfigured it to fit our needs but this definitely provides a great description:

(from the ShopBot forum: http://www.talkshopbot.com/forum/messages/312/18079.html )

First, a vacuum device must have enough airflow to quickly evacuate a vacuum chamber. This is the 1st area where CFM is important. In our case, it must get all of the air out of the vacuum grid and as it does this, it starts pulling the material tight to the bleeder board. Provided that there are a minimum of leaks, as the vacuum evacuates the vacuum chamber, it starts to suck the material down with greater force. Actual airflow past the vacuum motor decreases AS vacuum suction or Hg" increases. The suction will increase until the vacuum reaches it's maximum suction capacity, providing that air does not leak out of the plenum, hoses or any other part of the system. It is *very* difficult to achieve a 100% leak-free system as Hg" increases. The CFM plays an important role in helping the system 'self-seal' against the bleeder or gasketing if you are using any. Low CFM/high Hg" pumps struggle or fail in all but the most leak free setups.

So you have your Fein vac(s) holding down a full sheet of 4X8 plywood and you zero out the tool and start cutting. One thing you want to really think about here is chipload, and ultimately the amount of force that the bit is going to exert on the workpiece. This is not as much of a concern if you are not cutting all the way thru the parts to the bleeder, as the vacuum suction will be maintained. But...if you cut all the way thru the sheet like I do, you really want to pay attention to how many 'open kerfs' you have as vacuum will start leaking out in these areas. This is the 2nd area where CFM is important. The greater the CFM rating, the greater the vacuum's ability to absorb those leaks and maintain a good working level of suction. Just because you have a vacuum system, doesn't rule out that you may have to suppliment the vacuum with strategicly placed screws, tabs/bridges or both. It depends greatly on the shape and number of open kerfs in the sheet layout. Tool geometry is also important. An upcut spiral will fight against the vacuum and exert more vibration into the mix than a straight tool will. Experiementing with different tooling will pay off.

The actual theory of vacuum as it relates to CNC work holding is essentially this: The vacuum device evacutates air UNDER the workpiece. As it does this, it is essentially allowing the entire column of air from your material on top ALL THE WAY UP TO THE EDGES OF SPACE...to get 'shifted down' and apply pressure to the top side of your material. In a perfect vacuum (29.92" Hg), there is 1 Atmosphere or 14.7 PSI of pressure PER square inch of force on the top of your workpiece. Since none of us have the loot for a 29"Hg+ pump that moves 1,000 CFM, we'll have to use what we can afford. I'll use a single Fein as an example. A 9-77-25 Fein T3 pulls down a 4X8 sheet of plywood. It is rated at 126 CFM (at zero Hg") and 7.3" Hg (at zero CFM). Let's say we have a perfect system with NO leakage. To calculate the actual force being applied per square inch, we need to do a little math. Our vacuum max Hg" / perfect vacuum Hg" X max vacuum force in PSI (with perfect vacuum) = OUR psi

So...it breaks down like this: 7.3"Hg / 29.92"Hg X 14.7 psi = 3.6 psi

This means that the total force being applied with perfect vacuum on our 4X8 is an astonishing (48" X 96" X 3.6psi) 16,588 POUNDS of force over the entire piece! To bring this number down to earth a little, that equates to roughly 518 pounds of force per square foot with our single Fein Turbo III vac running full bore with no leakage. (why hasn't our table crushed under the weight? Because for every force there is an equal & opoosite amount of force. This force also pushes up against the table cancelling out the force of the vacuum...it gets complicated...anyway) As we start to cut all the way thru the material, leakage occurs. Vacuum suction or psi force on the top of our material begins to decrease from that 518# number and vacuum CFM begins to take on an important role of helping the vacuum to maintain decent pressure/suction while this is happening. If leakage becomes too great, holding power suffers and parts will move as our inversely proportional vacuum scale starts sliding down from 0 CFM and 7.3" Hg to 126 CFM and 0" Hg. This is where a second Fein would help to cope with the vacuum bleed off and maintain suction by doubling CFM. (just using this as an example could be brand XYZ) With 2 Feins, you would have @ 250 CFM, but still the same 7.3"Hg max suction power. The 3.6 psi of force that they exert is more than adequate for most materials that you cut on a CNC router. Only when you get into severely warped or irregular panels (and very small parts that require gasketing) do you have the need for higher Hg". The Fein has an excellent balance between CFM and Hg", and it's this proportion that is what to look for when considering a suitable vacuum device. As a side note, keep in mind that if you implement a weaker vacuum next to a stronger vacuum, that there will come a point where the stronger vacuum beings leak thru the weaker vac's housing, limiting your max Hg" suction power. If you ran a Fein with 7.3"Hg and a regular ShopVac with 3.5"Hg, your highest Hg" would be 3.5"Hg...so be careful not to do this. There are a few tricks to get around this, but I'll save that for another time.

One more thing I want to add...a high Hg"/low CFM vacuum device, such as the 27"Hg+, 4CFM Gast pump that I have are NOT suitable for CNC work holding UNLESS you are cutting non-porous materials like plastic, AND you have employed suitable gasketing. These pumps are very hard to get 'over the hump' and evacute the air from underneath the workpiece without solid gasketing. They DO work once you have eliminated leaks, and are excellent when you need to hold small parts using a dedicated vacuum fixture. I have a small fixture for doing lithophanes and this works quite well. However, trying to use this setup with a conventional bleeder board cutting plastic is futile, as the pump cannot evacuate the air fast enough to seal it's own system.