Vacuum table: Which would be better?

[Mayo]

I'm making a second attempt at a vacuum table top.
I have 3/4" plywood as my base, and 1"x2" will be used to frame it in and provide spacers in between the base and the top. All wood has been sealed with KILZ water base primer and I'll be putting a water based clear over that. I have drilled a hole near the center of the base sheet and used bathroom caulk to seal around a PVC pipe which connects to a shop vac.

The top sheet is what I'm wondering about.
Would it be better to use another sheet of 3/4" plywood, and then have the shopbot drill hundreds of small holes in it (I'm thinking approx. 2" apart) or would it be just as good to put on a sheet of 1/2" or 3/4" particle board without having to drill any holes?

After deciding on plywood with holes, or particle board with no holes, I was going to place a sheet of 1/8" or 1/4" sacrificial masonite on top,seal all my edges with duct tape and turn it on.

I'm having doubts if a shop vac will be able to pull enough vacuum through a sheet of particle board to hold anything down. My shop vac is a 3.25 peak HP, with a 16 gal tank.

Incidently, none of the local lumber yards seem to have anything they call LDF, which I have seen mentioned here in the past. Or is that the same as particle board?

I appreciate any comments and advice.

[Wdyasq@yahoo.com]

Mayo,

Vacuum works for a hold down. There are several things that need to be considered.

First, how much vacuum will your shop vac pull? This is a number in "inches", with a second symbol designating if it is Mercury(Hg) or water(H2O). Most shop type vacs are rated in inches of water - with 100" being a very good vacuum. The volume of air at this level of vacuum should not be critical BUT, the volume of air initially will cause the sheet of material to be quickly or slowly pulled down. Then the ability of the vacuum to pull the sheet down with an acceptable force becomes a relevant factor.

There are two schools of thought. One is to use a large area with light (H2O inches) vacuum. The other is to use a higher grade of vacuum(in the15~25" Hg range). The more vacuum power one has, the smaller the area under vacuum needs to be.

A good vacuum hold down system is just that, a SYSTEM. If the why of the system is not understood it is difficult, if not impossible to get it to work PROPERLY. There are several ELEMENTS to a good design and ALL must be correct for the SYSTEM to work PROPERLY.

Ron Brown - wdyasq@yahoo.com

If Stupidity got us into this mess,
then why can't it get us out? - Will Rogers

[Mayo]

I have seen several new Shop Vacs in the Depot and the highest vacuum any of them list is 60" of water. I assume this is a result of the combination of motor amperage and volume of air moved.

Physical size of the shop vac cannister or tank had little to do with the inches of vacuum pulled, judging from there being a very small shop vac (with a 1 or 2 gal. tank) which listed higher inches of water than some of the larger more expensive units.

From the best to the worst there was only a difference of 5 to 10 inches of water which they listed. Not knowing how to figure the math to determine if that's a serious difference in hold down power, I'll get the one with highest vacuum listed, and smallest tank, so it takes up less space.

I now have my base sealed, caulked, and ready for me to put some sort of top on. I got to thinking, why spend the time to drill a zillion holes into a sheet of plywood if I can just buy two sheets of 1/4" pegboard? If I place one on top of another, I effectively have a half inch thick top. This would also allow me to easily replace the top 1/4" sheet whenever it gets chewed up.

I figured on sealing the pegboard to the base using that miracle tool - Duct Tape. Then when I cut parts which are smaller than 4'x8' I will place some rubberized roofing material or heavy plastic to close off the unused vacuum area.

So, is this a SYSTEM that will work or just an exercise in Rube Goldbergian folly? We shall see shortly!

[Wdyasq@yahoo.com]

Mayo,

Good luck on your vacuum table.

Bill Young And I have discussed several variations of the simple vacuum table. One is to use SEVERAL vacuum motors and as the piece is pulled down and the volume of air needed is less, the vacuum motors would shut down.

Once you get your table working, you might think about getting the vacuum motor only. Surplus Center, one of my favorite wierd stuff catalogues, (800-488-3407) has 8 vacuum motors listed with the least powerful one(suction wise) pulling 62" H20 and the most powerful one 90" H20. Prices ranged from $9.95 to $14.95.

I have my vacuum, router and dust system on relays where they can be started remotely. I have been meaning to do a little write up on how the relays interface and keep things simple and easy to use.

Ron

[bill.young]

Mayo,

I hate to be the bearer of bad tidings, but I tried the pegboard idea on my original vacuum table and wasn't happy with the results at all. It seemed like a great idea, but it had very little holding power.

Here's my theory for why. My understanding is that vacuum hold-down works by removing the air underneath that's holding the material UP, so that the weight of the column of air above it is pushing it DOWN. The area of the column of air above is only as big as the area of the air that was removed below. The bigger the surface area of air removed below the part, the larger the pile of air holding your part down.

With pegboard you only have one 1/4" hole every square inch or so, which is less than 5% of the surface area. You're really only getting about 1.5 square feet of hold-down for 32 square feet of table top.

Ron Brown has a much better handle on how all this works than I do; hopefully he'll correct the parts that I have wrong.

Bill

[dan@burningimpressions.ca]

Firstly I would imagine friction would play a large part in holding due to movement of the part,
I have seen Septic tank trucks with a little 5hp.
honda running a compressor on the suction side,
suck up 500 gals of crud in one go,they reverse it to blow it out..
Why couldn't the same principle be put into use
in a small shop,... large vacuum tank, small vacuum pump.. with a vertical ball check in line to stop any huge gulps of vacuum? just my 2bits..Dan H

[Wdyasq@yahoo.com]

Gee Bill,

Thanks a LOT.

There are many factors in getting vacuum system to work. There is the
vacuum; seal; volume(CFM); porosity of the material........

The list goes on. A boat hull that is water tight may not be sealed enough
for a vacuum bag operation. A leak you can stop by putting a .5mm pencil
lead in the hole can supply enough air to ruin an elaborate gluing
operation. Or, since we are talking about holding parts, ruin the part.

THAT SAID, I will take notice of the vacuum tables ShopBot sells. They have
HUGE vacuum motors. They have BIG pipes and plenum chambers and BIG holes
in the table. They lessen the problems of precision with a high volume of
airflow and good (relatively speaking) vacuum holding power. The small
pinhole or porous panel will not have a noticeable effect on the ability to
hold the panel while milling. STILL, there is a policy of either not
milling all the way through a panel or making a barrier or seal so the panel
is not released by the router bit cutting through the material and venting
the vacuum, therefore ruining the holding power. One should NOTICE the
SMALLEST pump will pull 8"Hg (about 100" water column) at 100CFM - that's
about as good as the BEST shop vacuums do at highest CFM and maximum pull.

One should also notice ShopBot is reccommending 5 to 15 Horsepower to hold a panel they use a 3 1/4 HP router. I still believe 3 1/4 horsepower is optimistic for a P-C router, I don't think one will pull a continual 20.2 amps at 120 volts.

The "PODS" are an attempt - and a good one- to lessen the loss of vacuum
and ease the task of 'masking' the unused area of the table. There are
still the problems of: sealing the material; porosity of material. You will
be amazed at how much 'air' can be pulled through a sheet of plywood.

I have some ideas of what might work for 'cheap' vacuum pods. I haven't
been to the places where I might get the wooden balls I would use or looked
for the right size PVC parts. I will probably use wooden springs. But, a
steel or plastic spring inserted from the top and the ability to rebuild a
pod hit by a router may be more practical.

Ron Brown -

If Stupidity got us into this mess,
then why can't it get us out? - Will Rogers

[Gerald D]

Mayo

Just a couple of random points that factor into the whole vacuum equation:

1. How porous is the material that you want to hold down? Would bet that nobody has a system that sucks down open-cell foam . . . . .

2. How small are the pieces and how hard do you need to hold them? Bigger pieces hold easier. Thin boards with low cutting forces are easier than hardwood jewellery boxes . . . .

3. We all agree that we want leaks to be kept close to zero - but at zero leakage the CFM of the pump is irrelevant because there is no flow (in fact you hear the motor speed up, because it is not moving anything!).

4. Some vacuum motors depend on the leakage flow to cool the motor, so be careful to aim for zero-leakage if your motor is going to cook.

5. Why don't you experiment with much smaller pieces of "peg-boards" first - say about a foot square?

[Gerald D]

A rough calc:

60" to 100" of water column is about 2 to 3.5 lbs/sq.in. Therefore the clamp pressure on 4"x4" square would only be 32 to 56 pounds - about as much as a tight hand grip!!

But, if the area was pushed up to a 8'x4' sheet, then the clamp pressure is 9000 to 16000 lbs! Okay, your table won't collapse, but make sure that you have plenty of bearers between the upper and lower skins of your vacuum box to prevent the "hungry-horse" look.

[Mayo]

Preliminary report:

I've placed two layers of 1/4" pegboard on the shopbot table to serve as my vacuum top. Underneath it I have 1"x2" lumber framing the edges and also every foot going in the "Y" direction -

I sealed the edges with Duct Tape and turned it on. When I placed a 18"x24" lightweight piece of corrigated plastic on it, it did absolutely nothing since all the space around it was open.
However when I placed a full 4'x8' piece of the same material on it and turned on the vacuum, I could not lift it or slide it at all.

Gerald D: thanks for the specifics. From this I can see that if I want to cut parts that are say, 5"x18" out of a 1x6x8, once the part is cut on the outer edge, how do I figure how much clamping is really holding the part since the vacuum is going thru 1/4" holes that are spaced every inch?

Does this mean you have to divide the resulting pounds of clamping by 16 since there are technically 16 1/4" holes in a square inch and only one of them is pulling vacuum?