Newbie- Vacuum Table

[vivagolf]

Hi All,
Have a question on a 4X8 vacuum table.
We built our own table from a progam a friend gave us.
We used a 1.5Hp blower motor, used 1 1/2" pvc and drilled 1/8" holes in our sacrificial top.
We wanted more suction so we got a 4.5hp blower motor and were wondering if we should go to 2" pvc and should the holes be bigger in the sacrificial board????
Thanking anyone in advance.
All the replies I've received in the past have been most helpful.
Larry

[johnmac]

Larry, do you cover up the holes that are not in use? Doing that will help, maybe a lot. I don't want to say you should drill out the holes, but remember that the work piece is held down by the holes, not the table between them. The larger the holes, the more the suction will hold the work piece down.

John

[vivagolf]

Yes, we have the table sectioned in thirds. So depending on the size of the piece we are using we can shut off sections.
We were just trying to figure out exactly what you were saying. The larger the holes the more suction to the piece we were trying to cut. Also the size of the pvc, should we go to the 2" instead of the 1 1/2"?
Thanks for the reply and I hope you have a great Thanksgiving.
Larry

[johnmac]

Yes, the larger holes in the bed, or more small holes give more surface area that the vacuum can suck on.

For example, if you have ever seen a commercial for a vacuum cleaner, and they are showing its suction power by picking up a bowling ball, take a look at the adaptor used. It covers almost half of the ball. Now try it with your shop vac and 1 1/2 or 2 inch hose and see what happens. The one in the ad had much more surface for the suction to pull on.

Think of it as psi, pounds per square inch, only in reverse.

Even with the unused sections of the bed closed off, try covering up exposed holes around the work piece. It can make a difference. It will also keep dust from being sucked thru the bed and suction fan.

The only way I see a bigger PVC pipe helping out is if the big vacuum blower needs to overcome a lot of vacuum leakage.

Have you measured the amount of vacuum the unit can draw? Not the volume, but the inches of vacuum? I have seen big suction blowers that have a lot of volume, but little sucking power. Kinda like a dust collector vs a good shop vac.

John

[Geof]

Is the blower just a centrifugal blower, like a dust collector or vacuum cleaner, or is it a very highspeed multistage blower?

You will not get adequate vacuum from a single stage centrifugal blower; these things are often rated up to 100 inches of water lift which might sound like a lot but full vacuum is something like 380 inches (32 feet) of water. Vacuum is normally measured as inches of mercury (Hg) and full vacuum is about 30"Hg; to get adequate holding you need to get at least two thirds of full vacuum which is about 20"Hg. The only practical way to do this is with a positive displacement pump.

[vivagolf]

Geof,
It's a Gast 4.5Hp 3 phase motor.
Larry

[Geof]

Originally Posted by vivagolf Geof, It's a Gast 4.5Hp 3 phase motor. Larry

What kind of pump is the motor driving; vane, lobe or regenerative blower?

Have you measured the vacuum the pump can create when the pump inlet it completely blocked, not connected to the table or anyother pipes?

[vivagolf]

It is a regenerative blower.
I haven't measured the vacuum.
Larry

[Geof]

Originally Posted by vivagolf It is a regenerative blower. I haven't measured the vacuum. Larry

I think that is why you are not getting enough vacuum. I looked up on the GAST website and maximum vacuum I could find was 183 inches of water lift for a 6hp unit; this is marginal for a vacuum table. Using bigger holes and larger piping may help; if you are holding down area of at least 3 square feet and have no leaks you might find it adequate.