Vac Hold Down pump sizing?

[robinsoncr@naxs.net]

Hi Ya'll,

I'm looking at the purchase of a PRT96. I will be processing cab parts out of full 4x8 sheets of prefinished plywood.

I would like a vac hold down table. I have run across a pump and wanted some advice.

It's a Hitatchi pump, with what appears to be a 14" radial fan, and the data plate states it's performance as 10.2kw, 655cfm @ 100". It's the type of pump where the intake and exhaust come off the back, side by side.

How would this work?

It's in nearly like new condition and comes with a filter canister and disconnect. What should it be worth?

I'd like to hear your suggestions and comments.

Thanks,

Chris R

[Gerald D]

Chris, my first reaction is that this is an over-kill. But, if you have 10kW of electrical power available, you could run one heck of a vacuum!

[robinsoncr@naxs.net]

Gerald,

I tried to convert the Kw to Hp and came up with about 13hp/3ph/230v. I can run the motor. I double checked the plate, and it's 655cfm @ 100"h2o. How does that compare to the big SB vac hold down system? If I can use it, I can get it for little of nothing. But it is a BRUTE!

Can you have too much vacuum on the table?

[Gerald D]

Your conversion sounds right (1HP = 0.746kW). If you have too much vacuum, then just make some leaks!

[davidallen]

chris,

it sounds like too much air and too little vacuum.

100" is around 5 psi. if your table is 50% vacuum space and 50% support and friction, then your piece will be held down with 2 psi which isn't much when you consider what a router bit will do. if you increase vacuum space, you decrease the friction surface. that gives you more pressure but less holding area and less support for the sheet.

the best system I've found is the high vacuum (14 psi), air ejector pumps with the hold-down pucks. I use one from West Oaks (see earlier post) and it works real well. I've seen the similar ejectors in surplus catalogs for $10 or so.

they draw 2 to 4 cfm of air at 60 psi. if you have compressor capacity, you can gang several together to increase vacuum volume.

I've seen them used in conjunction with a shop-vac to thermo-form plastic. the shop-vac pulls most of the air out then the ejector kicks in to suck the vacuum down hard. if you really need to pull down a large area, you might consider something similar.

da

[wdyasq@yahoo.com]

Chris,

That is a nice pump. It supplies more than enough volume. It probably supplies enough vacuum for sheet goods. If you are going to do small parts you need to do the math.

I will disagree with David's analysis. The total area should be used for the "area" calculation.

With - let's not get anal on the math - one square foot of area and 5psi there is "only" a little over 900# of holding power PER SQUARE FOOT. That's only like having the backfield of a professional football team on each square foot. Now, when we multiply that by the footage of a panel - 32 sq.ft. - well, it gets impressive.

But, to show the problem - and inforce David's thoughts, we will not get that kind of seal around the sheet. And, when the parts start having holes in them, "breaking" the vacuum, things tend to "go to hell". The small parts created become the problem.

As an example I will use the 160# woman wearing 1/4" square spike heels (forgive me for using a woman but, I don't know any guys who wear spike heels). With only a 2G 'step', she exerts over 5000psi pressure - on the wood floor you just finished.......

In closing, there are a lot of things to consider: volume; pressure; area..... the list goes on......

Ron Brown

[olivier]

Hello David,
Shall I understand from you post that you are using vaccum hold down which are powered by compressed air ?
If this is the case could you be more specific ?
I have been looking for this kind of solution for a while because I do have so compressed air available, but I have never come accross a working solution.

Thanks for advices.

Olivier.

[Robin]

Olivier, here's the link you want:

http://www.wow-woodworking.com/vac_generator_RM1.htm

[wdyasq@yahoo.com]

Harbor freight has the "Generator" only for about $10 Part number 3952.

Ron Brown

[wayne locke]

David,
I am not sure but I think you are confusing psi with inches of mercury(Hg). A 30" Hg vacuum which is theoretically the max, i.e. a complete vacuum, is only about 680 lbs/sq. ft. I believe that 100" H2O is about 5"HG
Wayne Locke

[annetts@mb.sympatico.ca]

What is the best shop vacuum for vaccuum tables. What size is required, horse power, other ratings, noise levels, etc. Specific model reccomedations would be great.

Thanks in advance

Mike

[dmdraper]

McMaster-Carr also has a good selection of venturi sytle vacuum pumps - with detailed specs -available. Go to the website and search for "vacuum generator" (w/o quotes)

McMaster-Carr website (http://www.mcmaster.com)

[davidallen]

wayne,

1 atm is about:
15 psi
30" Hg
405" water.

so 100" of water is about:
7.5" Hg
.25 atm
3 psi.

15 psi is 2160 psf. so about 3 psi is about 430 psf.

da

[davidallen]

ron,

only the area where there is a pressure differential should be used. (i.e. only the area experiencing a vacuum) if you can seal around the perimeter of a sheet, then you can use the full area. but, if you only seal around a 3" hole in the middle, you only have 6.75 sq.in. of vacuum area. that's still about 900 lbs of force on the sheet if you have a good seal and vacuum.

I routinely use a 3" puck to hold cabinet doors up to a couple of feet across. but I have a good seal and high vacuum around the puck and mechanical supports around the perimeter of the door.

da

[davidallen]

oliver,

there are several sources for the ejectors / generators (see above). a few of suggestions;

use a filter between the generator and the vacuum table. a bit of dust will plug them up easily.

use a muffler on their output. it will decrease the performance a bit but increase your comfort a lot.

vent the output from the generator into the cabinet where your SB driver is located. if you can mount the generator in the same cabinet, even better. it'll help reduce the noise and provide dust free ventilation for the driver/computer.

you can use schedule 40 PVC as a vacuum line. if you put ball valves between the line and table, you can use the line as a surge tank to quickly suck down the sheet and seal the table.

vacuum gauges are useful. the more the better. with strategically located valves, you can find leaks easily.

da

[wdyasq@yahoo.com]

Surplus Center (800-488-3407) has a good selection of vacuum motor blowers – like for a “shop vac”. Eight are listed for less than $15 each. The maximum vacuum is 86” – I would guess it is in water and 140 cfm.

I think these could be “sequenced” with higher vacuum pumps to provide a quick ‘pull-down’ and then automatically shut off.

David – on your math 15psi x 144 is 2160 however 100”of water is near 3.75 psi or, .25 atm - for the area 540 lbs.

I am looking at some seals that might be of use in both high and low vacuum clamping systems. I plan on getting some and trying them……….

Ron