Maintaining Vacuum on CNC Equipment

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Considering pipe sizes, pump setup, and a vacuum reservoir tank. June 30, 2005

When installing a 20 hp vacuum pump to a 4 x 8 CNC, should the PVC pipe from the pump, which is 4", be reduced to a smaller pipe?

Forum Responses
(CNC Forum)
From contributor D:
As far as flow is concerned, you would probably be okay with 3 inch pipe. I doubt that the ports on your machine are any larger than that. The 3" pipe and butterfly valve are probably a little cheaper.

One advantage to the larger pipe is that it provides a vacuum reservoir when snapping down warped panels. When you crack the valve at the machine, you get a brief inrush beyond the capacity of the pump. The larger the volume of the pipe, the bigger the reservoir. I have even seen installations where a length of very large diameter pipe (12"+) is used as a "tank" just behind the machine.

From the original questioner:
The system I am using has a 4" outlet. It is connected to a 3" manifold with 4 valves. It is then reduced to a 2" web of pipe to run a 4 zone table. The vacuum holds the parts okay, but it will not snap down the 4x8 plywood like you described. Also, where the 3" connects to the 2", the sweeps on the T's are not with the flow from the pump. Would this reduce vacuum?

From contributor D:
As I understand it, your router has a 3" pipe off of which come four 2" pipes. The four pipes run to four zones and are individually valved. In order to get your part to snap down with a 20hp pump, you will need a reservoir as described in my previous post. You will also need all of the four valves to open at the same time. I really don't think that the sweeps being backward on the tees is a big issue, but if you are replumbing, I would certainly turn them around.

From the original questioner:
Yes, you understand right. After looking at it closer, there does not seem to be enough room to replace 2" with 3", but I could build a reservoir. I assume I could place 1 3" valve in front of the 4 2" valves to switch on at once. Once the vacuum snapes the plywood down, does it stand a better chance of holding it?

From contributor D:
Once the part is pulled down, the system will reach equilibrium and the reservoir will have no effect on the end vacuum. The only way to maintain deep vacuum with bleed boards or other leaky fixturing systems is by application of sufficient horsepower. In your case, a 20hp screw, vane, or liquid ring pump is more than enough for a gasketed fixturing system, will work well with a bleed board for medium to large parts, but may be a little weak for nests of small parts on a bleed board.