Techno Vacuum Table Help

[Dave's_Not_Here]

I couldn't hold the corners down on a 4' x 8' x .75" sheet of plywood or on a 4 'x 28" x .25" sheet of acrylic. It holds sheets down, but the corners are off the table; usually at the far end from the plenum/ball valves. I think this is important - as distance seems to be playing a factor.

In some instances the 3hp HSD Spindle can actually move the sheets; especially on the last pass or two. No way I could take a full pass run.

When you say the pump side, I assume you mean between the pump and the main ball valve that controls flow to the plenum?

Thanks,

Dave

[ericscottf]

It isn't typical for flow issues to show up as parts not having as much hold far away from the air intake. Especially in a static system, which yours should be, certainly for acrylic.

A gage near the pump is the best location for it, yes.

We want to see what the pressure is when:
1) all valves are closed (max the pump pulls)
2) all valves are open and table is completely covered with a non porous sheet like acrylic
3) all valves open, table uncovered, but all holes plugged (the difference between 2 and 3 is how much your channel gasketing is leaking)

With those 3 pieces of info, we can figure out what to look at next

[Dave's_Not_Here]

Great.

I will get a vacuum gauge installed and do the test runs.

I am heading out to spend the afternoon with my daughter, so it will be sometime next week, hopefully.

Thanks Eric,

Dave

[ericscottf]

Ok, ready when you are.

Out of curiosity, I wanted to know what use a vacuum booster would have in such a scenario, so I looked up the one you mentioned, and can't find anything about a Fuji VB-89 Vacuum Booster. Can you tell me what it is and what it does for the system?

Thanks
Eric

[Dave's_Not_Here]

Without it, the blower could only run up to 30 seconds continuously,. With it, I can run pretty much indefinitely without overheating the blower in a deadhead situation.

[ericscottf]

That sounds like a relief valve. Critical to know what it is set for.
Also...
Do you own a clamp ammeter?

[Dave's_Not_Here]

I do not own a clamp ammeter.

I have attached the PDF for the Vacuum Booster. The VB-89 is not listed, as the Fuji VFC804-7W Blower I have was replaced. The VB-89 is what they sent with the installation kit. It was also identified by the Fuji National Sales Manager, Bill Maier, from photos I sent him when seeking information on ball valves for the 10 zones.

I also attached a PDF Drawing from Bill, of the Booster and the recommended installation layout. The only part not installed on the layout is the IVF-89 In-line Vacuum Filter, which is being ordered.

[ericscottf]

Yup, that's a relief valve, at least, that's what I've always been told it is. It keeps the blower from working too hard and overheating. They have to be set right in order to work. We need a gage plumbed in, and a clamp ammeter will really really help. They are good to have and you can get a cheap one from plenty of places. Let me know when you've got both and we will proceed.

Eric

[ericscottf]

In your drawing, where is he table hooked up?

Eric

[Dave's_Not_Here]

Here is what they call a relief valve. . . not sure what the difference is. Supposedly set to 100

Both devices are supposed to accomplish the same thing according to their literature.

[CaptainVee]

I couldn't hold the corners down on a 4' x 8' x .75" but the corners are off the table; usually at the far end from the plenum/ball valves. I think this is important - as distance seems to be playing a factor.

Is the plywood not totally flat then?
What happens when you turn the sheet 180 degrees?

[Dave's_Not_Here]

Not always flat. We shy away from really bowed materials.. When we flip it 180° it still won't pull it down. I am wondering if the 180 .5" holes are too many / too large and I need to down size the holes. . .

[Dave's_Not_Here]

I talked with the manufacturer for the Relief Valves and the Vacuum Booster. Here is what they had to say:

"The relief valve is actually designed to break the vacuum at what ever point the valve is set for, to allow air flow and keep the blower cool. Its main function is for situations when a deadhead occurs, like a plugged supply line, so the blower won;t overheat."

"The Vacuum Booster is designed for extended vacuum situations, like a vacuum table, and allows air to constantly flow, while still maintaining vacuum in deadhead situations." The reason the VB-89 is not in the literature, is that they hardly sell one that size.

Lastly, we chatted about what I was experiencing and he ran some calculations. Based on my model 10 HP blower, the maximum number of 1/2" holes that could hold vacuum, is 25; 3/8" holes is 45; 1/4" holes is 100, 3/16" holes is 175 and 1/8" holes is 409.

I thought my holes were 1/2" but they are not. I never finished making them larger. So, with my 190 holes being a handful of 3/8" and mostly 1/4", I maybe could maybe run 80-90 total holes. . . not the 190 I was running. Even with only 48-50 holes open for the 2' x 4' acrylic I was cutting, it was over maximum by several holes.

He said that the 2.5" supply port equated to 4.91 Square Inches and that the total square inches of all open holes should not exceed the total square inches of the supply port. So, with the 4.91 square inches in mind, I have to set about reconfiguring my table layout. If I want more holes, they have to be smaller. . . simply needs the velocity to achieve the vacuum. I could plug half of the 190 holes, or make new strips. Plugs would be less costly, as long as I can make a good determination on a pattern of holes which need plugs so I can still get good hold-down.

I will also plumb in a vacuum gauge, because we need to know how efficient the holes and plugs are working under load and without a load. I may have leakage between the strips and the table and the plugs might not be sealing like they should.

[CaptainVee]

When we flip it 180° it still won't pull it down

That suggests that the location on the table does not make a difference.

I find vacuum tables a bit of black magic.
But the basics are quite simple:

There is an absolute maximum of clamping pressure which is based on the weight of the column of air above the workpiece.
The stronger the pump, the higher that pressure.
More power does not mean the pump creates a stronger vacuum
The friction between workpiece and is a combination of the achieved pressure and friction coefficient of the materials.
The bigger the flow the pump can achieve, the higher the allowance for leakage

[Dave's_Not_Here]

The bigger the flow the pump can achieve, the higher the allowance for leakage

That's why I am going to try and reduce my holes to around 80, even though the calculations show that the pump will handle up to 100 1/4" holes. Running at 80% of maximum, should help recover any leakage I might find.

Another thing I find intriguing, it the whole issue of velocity. . . how fast the pump can evacuate the air through the holes. Smaller holes evacuate faster, which should minimize vacuum loss due to leakage.

I am seriously thinking of trying 1/8" holes, which would max out at 409 holes, or about 40 holes per extrusion/table strip. That's about one hole every 2.5" of length. With my 1" grid pattern, and 1/4" gasket spacing, I would get an 1/8" hole, center-to-center in every other 1" grid.

However, If I plug and re-drill my existing holes with 1/8" holes, I would cut my hole count from the maximum of 409 to the current 190, a reduction of 54%, so the blower shouldn't have any trouble maintaining vacuum even with leakage. It will be interesting to see what the vacuum gauge reveals.

If I didn't have the full table of extrusions and had an open base to play with like a lot of other users do, I could run two 4' x 4' grid sections with a 1.75" hole in the center and barely max out the pump. 1.5" holes would save 28% of the blowers capacity for any leakage and still get the job done.

But, since I have a solid table with a centered ball-screw down the middle underneath, I have to work with 10 extrusions and keep the velocity up while retaining a decent amount of blower capacity for leakage.

[ericscottf]

Dave,

With regard to hole size, don't you have them all plugged up? If they're plugged, or the table is covered with a non porous material, hole size is irrelevant.

Hole size only matters if you have flow, which you don't have if I understand your situation correctly. If I don't understand it, please elaborate.

Before you go redesigning anything, I highly recommend plumbing in the gage, and getting a clamp meter.

Every vacuum relief I've ever seen comes from the factory set really low, so if you didn't set it right, you probably aren't pulling any real vacuum at all.

The clamp meter is a backup tool for making sure that once the relief is set, he pump isn't drawing too much amperage. Having the gage and meter are really important tools for getting this right.

Eric

[Dave's_Not_Here]

That makes sense. Perhaps the leakage is more severe than I thought?

The max amps the blower should draw is 23-26. It's on a 70 AMP breaker.

I use to have a clamp meter but gave it to friend who was an apprentice electrician. Working on getting another or borrowing one.

[ericscottf]

Why is a pump with a max draw of 26a on a 70a breaker?

Regardless, we will know infinitely more useful things once you have a clamp meter and a vacuum gage plumbed in.

[Dave's_Not_Here]

It's a 3 phase blower running through a VFD programmed to ramp up the start for about 5 seconds. The 70 Amp was needed in case we didn't use a soft start. Because of our location, we do use the soft start to keep from browning out other people during start up.

Should have the clamp meter tomorrow.

[ericscottf]

Ok, don't forget the gage