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 (mailto: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?

[Gerald D]

Hi Mayo,

I don’t follow your reasoning entirely, but your experiments (and feedback) are very useful! We don’t have a vacuum table for our ShopBot, because we cannot decide what a multi-purpose system should look like – so my ramblings are based on theory and experience of “fluid transfer” in other applications.

Any extractor fan (shopvac) has a performance curve depending on the rate of air flow through the “turbine”. The highest pressure (vacuum) is attained at zero flow (left side of graph) and the highest flow rate is achieved with the lowest pressure (least resistance)(right side of graph)


5451

Okay, I stole this curve from a metric website, but let us imagine the vertical scale is “inches water” PRESSURE while the horizontal scale is “cubic feet per minute” FLOW RATE. So here we have a pump that can suck 210” or move free air (no resistance) at a rate of 80 CFM. But it cannot do both at the same time! Some unscrupulous suppliers will show you this curve and tell you that you have a 210” at 80CFM pump. Lies! In this case 150” @ 30CFM would be correct. (or 50” @ 60CFM, or 200” @ 15 CFM)

The two parameters of PRESSURE and FLOW RATE are important to us in the following way:

1. PRESSURE is going to give us our “clamping down force” if we multiply it with the area of the work-piece. So we want nice high pressures. (Okay, we are on the vacuum side of zero (negative pressure) , but we want to be as far away from zero pressure as possible – the terminology can be confusing)

2. FLOW RATE is caused by leakage, and we saw from the curve above that if we have a high flow rate then the pressure drops off. So we want to work at minimum flow rates, least leaks, to give nice holding pressures. (to the left side of the curve)

Generally a higher horsepower vac unit will have a curve lying above and to the right of low horsepower unit of similar construction. This means that a bigger vac unit will manage a higher flow rate while holding the same pressure. ie. More tolerant of leaks. Different types of pump (vane, turbine, piston, liquid ring) have different shapes of curves and you may find some curve crossovers if you compare them – vanes will be higher and to the left – turbines will be flatter and to the right . . . . .

The size of the canister (gallons) attached to the shopvac is not important at all!!! (Except in special cases where the canister can be pre-evacuated and special valving allows a quick suction on the workpiece to create the initial seal . . . . )

Now to come more directly to Mayo’s question (I hope), we must realise that the pressure inside the 8’x4’ box is the same everywhere in the box. Each peg-board hole has the same vacuum pressure at its bottom end as any other hole. The more air that leaks into the box causes us to move to the right on the curve and the pressure in the WHOLE box drops accordingly. Therefore as you cut through the sheet and introduce leaks, everything deteriorates rapidly, especially since the square inches per part also reduces quickly.

The only way I see around this is to go for big pumps (leak tolerant), seal obvious leaks and keep parts tabbed together (let the holding area of the scrap work to your advantage).

Good luck and keep us informed. (PS next week I am in Germany again, so if I go quiet, it is not because I have lost interest)

[Howard Andersen]

Mayo:
For sure, the key is to apply whatever level of vacuum you can generate over as large an area as possible. The maximum possible is 15 lbs per square inch, not per square foot. I've used a 3 hp shop vac and estimated I got 3 to 4 lbs per square inch (it would pull a column of water in a tube up 6 to 8 feet). The problem with dropping a sheet of plywood over any typical port, whether its just the end of the hose or something else is that the plywood tends to flex and create a seal in a small area around the port and not permit vacuum over the entire sheet. The physics says its all about area. The parts I was holding together were rigid enough and only a square foot or two in area so it worked surprisingly well.

My thoughts on vacuum pumps would be to use a shop vac to remove the initial potentially large volume of air (depending on the particular application) then use an old milking machine pump to get up to 12 lbs per square inch of vacuum. Lots of these are available second hand and at a fraction of the cost of most units designed for the wood shop. Even a milking machine pump can handle an occasional leak especially if a good tank is built, even out of wood, to "store" a reserve of vacuum. I would think it should work well. If anybody has tried it, I'd be interested to know if/how it worked.

[sheldon@dingwallguitars.com]

If you can place 3M stickit strips in 100 grit where you know you are not going to be cutting, the grit alows a slight amount of air to move helping to equalize the vacuum and it helps keep things in place too.

[danhamm@abccom.bc.ca]

Check out these easy to make clamps for quick on off, above the table..
http://www.vacuumclamp.com/

[ron_cleaver]

Unless I'm missing something, the vacuum clamp ring system seems to me to have a fatal design flaw. What's to prevent all the 'clamps' from moving?

And $220 for 1 ring. Way overpriced.

[danhamm@abccom.bc.ca]

Ron they clamp both sides to the table and to the
object...

[Gerald D]

I often use V-Rings in my mechanical designs and these seem ideal for making the “vacuum pucks” illustrated above.

A V-ring has a V-cross-section as opposed to an O-ring that is a solid-O. See this link (http://www.jwwinco.com/products/serviceparts/section8/metric_v-rings/page2.htm) for typical metric sizes. Most seal suppliers selling o-rings and hydraulic seals should be able to help you with V-Rings. There are slight differences in the cross-section shape, but the VA type (as illustrated) would work best.

Turn up the pucks from solid wood or plastic so that the V-rings are slightly stretched to go over.


5452

The lip of the ring is very flexible and the weight of the workpiece will make the initial seal.


5453

These pucks will not work with ShopVacs, the pressure is too low. You will need a high-vacuum, low flow system like an ejector/eductor/venturi or vane type.

Let us know if it works – have never tried it myself!

[Wdyasq@yahoo.com]

Resource Conservation Technology, Inc. Of Baltimore, MD has a fairly complete line of gaskets and seals that could be used with either a low or 'high-powered' vacuum system.

They have 'tubeseals' with tongues to hold the seal in place; 'flipper seals' and 'leaf seals'.

I am planning on building my next vacuum table using their products. I am also going to use the same products to seal some windows and doors I need to build.

And, in answer to the next question, they do not have a website I know of.

Resource Conservation Technology
2633 N. Calvert Street
Baltimore, MD 21218
410-366-1146

Ron

[Howard Andersen]

I have had good success making vacuum "pucks" from short lengths of 3" or 4" PVC waste pipe used in residential plumbing . The drawing would be very much like Gerald's drawings above. The seals were just made of self adhesive "closed cell" (this is very critical)foam tape about 3/8" wide stuck around the top and bottom circumferences of the puck with careful butt joints on the tape. One end seals to the table, the other to the workpiece. Its very easy to do, very inexpensive, and the holding power is very impressive. I was using a venturi pump.

[Gerald D]

Howard, your method of closed-cell foam sounds good, and it provides high friction to prevent the workpiece sliding off the puck.

Of course there is no reason for the "Howard puck" to be round. A little square wood box, with no top and no bottom, self adhesive foam on the edges . . . . . .

Some people may wonder what a "venturi" pump is: Well, it is just a chunk of plastic or metal with 3 holes in it! No moving parts, nothing else except this one-piece block.

To get it to work you need compressed air (sometimes quite a lot). Blow the compressed air into the right hole and a vacuum is created at another hole. The third hole is the exhaust where the compressed air plus the vacuum air escape together (sometimes noisily). There is much science (& luck) involved in the size, shape and orientation of the holes in the block - definitely not a DIY job. The bottom line is that a venturi only works if you have "shop" (pressure) air available.

[threadfx]

What type of vacuum system do the "other" CNC routers use?

I am in serious need of a vacuum system. I'm working on signage and need to be able to pull the pieces flat against the table. I've been using strategically placed screw holes, but in some cases, it just isn't enough.

I'm comfortable designing the system, I just need to find the right vacuum pump.

Aaron Savka

[sheldon@dingwallguitars.com]

If you have a lot of surface area, don't overlook the shopvac at least to get your feet wet(or dry)- Sorry


I was experimenting with a new fixture that leaked and my ventury and gast pumps couldn't keep up. Just to see what would happen, I held a shop vac hose up the the 1/4" inlet and was surprised at how much suction it had.

[Gerald D]

Sheldon, maybe you could confirm a point for us:

A ShopVac will give a reasonable hold down for large areas, PROVIDED THAT you do not expect it to do dust/chip extraction at the same time. If you want hold down and dust extraction simultaneously, you need TWO SEPARATE ShopVacs.

I have often been asked why it doesn’t work when you T the vacuum table line into the moving hose going to the router head – the leaks around the cutter are just too big and the ShopVac runs high flow, low pressure (way to the right side of the curve above)

[jorgy.tucson@worldnet.att.net]

Aaron,
My Morbidelli uses center valve actuated vacuum cups 125mm in diameter. The cups sit between parallel support bars. The material is pulled down onto the bars when cups are activated. This keeps it flat. Originally the vacuum source was a Piab venturi pump with a surge tank. It worked well but consumed a fair amount of air. I replaced the Piab with a Busch 5hp rotary pump. This works much better, you cannot move a 600 x 768 panel with 4 cups actuated.

Piab has a large selection of vacuum cups, fittings and venturi type pumps. Busch and Becker are both sources of rotary vacuum pumps. I think all three have web sites.

John Jorgensen

[jorgy.tucson@worldnet.att.net]

Aaron,
My Morbidelli uses center valve actuated vacuum cups 125mm in diameter. The cups sit between parallel support bars. The material is pulled down onto the bars when cups are activated. This keeps it flat. Originally the vacuum source was a Piab venturi pump with a surge tank. It worked well but consumed a fair amount of air. I replaced the Piab with a Busch 5hp rotary pump. This works much better, you cannot move a 600 x 768 panel with 4 cups actuated.

Piab has a large selection of vacuum cups, fittings and venturi type pumps. Busch and Becker are both sources of rotary vacuum pumps. I think all three have web sites.

John Jorgensen

[Gerald D]

Hi John, what is the thinnest (most flexible) material that you pull down onto the support bars which are presumably on 6" (150mm) centres?

What do you do when you want to cut right through the material?

Thanks for the info on the following:
Busch (http://www.buschpump.com/)
Becker (http://www.beckerpumps.com)
Piab (http://www.piab.com)
(just click on the blue words for the websites)

[bill.young]

Gerald,

You're absolutely right about not being able to do vacuum and dust collection with the same ShopVac; too much vacuum is lost to the dust collector. I ended up removing the filter element from my vacuum-table ShopVac, figuring it was just a restriction for a vacuum application. At $100 or so for a ShopVac they're cheap enough to buy a couple. If they just weren't so darn LOUD...!

To make them last longer you'll do need to blead a little air into system to cool the motor on the ShopVac. I've heard that the Fien shopvacs have a seperate cooling fan so you don't have to bleed air. They also pull more vacuum and are quieter; anyone using one?

Bill

[Gerald D]

Hi Bill

A link to Fein (http://www.fein.ca/products/fein/grinding/sanding-dust-extraction/index.htm) - looks like the bigger model has the cooling feature.

(PS, Dudley's first sheets are arriving tomorrow)

[jorgy.tucson@worldnet.att.net]

Gerald,the holding cups and support rails are adjustable in approx .7 x 2.75M plane. 8 rail sets with 2 cups per rail. I can have anywhere from 4 to 16 support rails depending upon the panel size. With this support, deflection is not an issue. The smallest size I can cut is approx 300 x 400 x 3. Thru cuts are not a problem since the top of the rails are consumable MDF. Thru cuts are only .1 to .5 mm beyond the bottom of the panel, outside of the cup area.

If you built a channel for your ring system to ride in you would have a similar system. The height of the rails is approx. 1mm less than the relaxed cup/ring height. When activated the part is pulled down tight to the support rails while maintaining a tight seal. You could even add an outer ring to your system.

For a quantity of very small parts I'll build an intermediate table with cutting platform. A 1.5 hp lab vacuum is the source for this. Not much volume, great suction. Onsrud Cutter catalog (or machining guide for plastic) has directions how to build this cutting platform. The smaller the part the more critical feed speed and cut direction.

John J.

[sheldon@dingwallguitars.com]

Gerald, Yes a separate dust collection and vacuum system would be needed. Can a Shopvac even keep up with the chips produced?

Bill, in Canada at least, Shopvacs are available in a QSP version which is much quieter.

[GD]

A test:

[jkforney@yahoo.com]

Gerald
I have been using a Fein Vacuum on various hold down tables for a while, including the Anaheim Woodworking Show. According to Fein all their models have a separate cooling fan. At the woodworking show we ran the vac for hours on end. Mostly because it is so quiet we wouldn't know it was on. The difference between models is that one has a outlet that will start the vacuum once the tool plug into it is running and stays on for 20 seconds after it is turned off.

I think that home depot carries the one without the switch for about $130-$140. My hold down boxes are about 3 high with the 1 1/2 hole on the side. I then cut holes about 1" diameter in a grid and laquered the inside and top and bottom. The holding power is great. At the show we could use it for everything except some 1/8 luan door skins since the vacuum bled right through.

When we have smaller odd shaped parts we cut a hold down in the shape of the part offset to the inside so that we can do all edges. When I get time I am going to try thinner material with supports inside so that we can get the thickness down.
The really nice thing about the Fein besides the noise (or lack of same) is since you don't have to bleed cooling air you get better hold down.


John Forney
p.s. I will be bringing one to Bill's camp shopbot.

[Gerald D]

Hi John, it would seem that other German vacs also have seperate motor cooling fans without mentioning it in the sales literature. Our office carpet cleaner Kärcher has the same. A general observation is that German tools are very quiet, probably due to their strict environmental legislation. Fein also makes a router, but not with a round body.

Our Kärcher will be used to flatten down some marine ply soon and we have just spent $15 and 1 hour knocking together a manifold:



Is anybody laughing? No, it is not on the wrong side! The table has already been pierced and there are spigots sticking through. We have just pre-glued some joints on top before it is flipped over to the bottom. So, a quick photo and post while the glue sets . . . . . .

[Nancy]

5454 Gerald Dorrington's table

[Gerald D]

Thanks Nancy!
Well the glue dried, everything was put into place and it works great! (Remember that we are only looking for something to pull a sheet flat)

Too be honest, it actually works better than expected and I think that all these long discussions on vacuum tables had scared me off from taking the plunge

I hear that most of you are having a long weekend - Enjoy

[Mayo]

Further progress/info on the system I mentioned way above in the beginning of this thread:

It works ok for holding a full sheet.

Once I begin cutting into the sheet and breaking vacuum, like when I just cut 38 pieces from a 4'x8' piece of 1/8" masonite - the kind used in bathrooms... it has a hard white surface on one side, the hold down power is gone.

I had to follow the router around the table with a piece of 1"x2" pressing down on the area it was cutting to make sure it didn't move or lift the piece. The file took an hour and 10 minutes to cut at 1" per second, so I was not a happy camper. And I was covered in masonite dust since I thought it would be better NOT having my separate shop vac turned on for dust collection - thinking that it would tend to lift the parts off the table (which it does at times).

So it looks like the better way to go would be the vacuum pump instead of the shop-vac idea?

Is anyone unhappy with the results their vacuum pump system gives, and if so, what are the reasons?

[mechtron@iafrica.com]

Hi Mayo, hope you have stopped coughing by now, but you must have a sore arm! Some thoughts:

1. Could you cut only part way through with the router (within 0.05" of the table) and trim the job out with a knife?

2. Could you get two clamps onto a long edge and then start cutting furthest away from the clamps so that the parts fall away from the clamps? (a copy of your dxf file will be useful to my mailbox)

3. Have you tried some double-sided carpet tape between the table and masonite to give some extra sliding friction?

4. Instead of your dust shop vac standing idle, have you tried adding it in tandem with your table vac?

Your major headache seems to come from air/vacuum losses when you start puncturing the sheet. IMHO a vane pump will be worse for this application. A vane pump will give you a high intial holding force, but once you let a lot of air leak through the board (via the cuts) then the holding force will drop off quicker than with a shop vac.

What is your table surface at the moment - pegboard or LDF?

To experiment with holding power, I suggest that:

a.) you lay ONE of your cut out pieces on the table, and blank off the rest of the table with plastic sheeting. If the holding power is already too low (with no cutting leaks) you something producing a higher vacuum - more inches of water. (or double-sided tape etc.)

Then:
b.) start peeling back a plastic sheet until you have exposed as much of the table as the cutter will. If your holding power drops off rapidly then you need more volume - CFM. (like 2 shop vacs in tandem)

A vane pump will help with test a. above, but it will be terrible with test b.

Hope this gives you some ideas . . .

[sheldon@dingwallguitars.com]

Speaking of plastic sheet, what about throwing some 5 mil poly sheet over already cut areas to try and block some leakage?

What about using the tabbing function, clamp the edges and just use the vacuum to keep the sheet flat?

[rgengrave@aol.com]

I was asked by a shopbotter today to post how I hold down material when cutting all the way thru.

I do not need or use a Vacuum table or pump, I use T-Track to hold my material in place.

I lay a 1/2 foam board 4x8 on my table and lay the piece I want to cut on that, I now slide the holding blocks to meet the wood and foam.

This will keep the material from sliding up and down and left and right.

Now I just run my files and cut out anything 1 want, from 1/8 to 3/4 in material.

I use and up cut or down cut bit, does not matter, with my dust skirt on and bit ready to go I leave the shop-vac off so the sawdust stays on the table and not in the air, but you will get some floating around?.

If I am going to cut thru say 1/2 material I will set my cut depth to -.60, this will let me cut all the wat thru, now I just run the file and let it cut, what happens is as the bit cuts the dust now fills in where you cut and will hold it in place when it meets the end of the cut, now you have a nice clean cut and the material did not move.

The trick to this is the dust and foam, when you cut thru material and foam you will melt the foam a little because it has a plastic film on top, when this melts it sticks to the material and holds it in place, now I have tried this both ways with the shop-vac on and off and get the same cuts.

Now lets say you do use the shop-vac to collect the dust? you will still get the same cut but now the bit will cut around the part and when it meets it will cut clean or push your material out of the way of the bit, but if you have the dust skirt rubbing on the material if might push it into the bit and gice you nicks where it joined.

My machine is in a room all by itself, so when I am cutting I do not have to worry about dust or flying material that hits the floor or me...lol.

Last year a shopbotter asked me how he could do this in a big room, well after a few cold ones we went to Builders square and bought some 1x2s and some 4x8 sheets of foam, we built walls around the machine and put on a celing too, we used the foam to cover the walls and celing, we cut a 1 1/4 hole in the foam and inserted his shop-vac, then we made a wide door 10 long and used drop down plastic film like they use to cover furinture and for painting or to cover material from getting wet, we use the clear film so you can see thur it.

Now we ran a file and turned on the shop-vac that is connected to the wall and sat back and watched, the sound was cut in half or more and no dust moving out of the room we made, when the file finished the room was cleared of dust.

Now when done for the day all that was needed was to just blow everything to the floor and picked up with the shop-vac of a broom.

Total cost to make the room around the shopbot under $100.00 including the shop-vac.

This might not be for eveyone but it does work and works well,I cut Fret Pattern all day long and never have to worry about my material moving or chipping when it meets. but like I said my machine is in a 24 x 24 room.

So you are in a garage? have you ever been to a hosiptal? ever see how they pull the drapes around the bed? got any old sheets?

Ron V

[jorgy.tucson@worldnet.att.net]

Mayo, a vane pump to cover the volume of leakage you described would have to be a monster.

Presuming you run this file on a regular basis, try this.
a) Support sheet with blocks above table so parts fall away after cut complete.
or
b) Make spoiler board for entire pattern. Set z to mark outline on board, add foam tape and vacuum holes to hold parts.

Some ideas.

John Jorgensen

[Mayo]

Well I guess the foam and clamps will be the next thing I'll try since several people thought the vacuum pump won't overcome the effects of the leaks either.

I do use carpet tape for some of my other projects and it works well. My only complaint about it is the cost. I pay around $8.00 for a roll at either the Depot or Menards. It's the "Professional Cloth Carpet Tape". It comes in a 2"x75 feet roll.

This current project involves pieces that are approx. 13"x14" and nested so that I get 38 out of a sheet. This would mean I'd have to run tape about every 8 inches or so, to make sure I had them all held by tape. Another option is to use spray glue but it's incredibly messy and then I'd have sawdust stuck to the sticky backs. Not an option.

Thank you all for the advice and suggestions :-) That's what makes this bulletin board such a great place! I'll keep you posted how the foam idea works for me.

[Gerald D]

What is a shop vac?

There seem to be two styles of dust collectors for shop use:

Style A: (http://www.metabo.com/metabodaten/bilddatenservice/thumbs/0241700s_10.jpg) Where the filter and tank are under pressure (click blue link for typical picture)

Style B: (http://www.metabo.com/metabodaten/bilddatenservice/thumbs/0204400s_51.jpg) Where the filter and tank are under vacuum.


To compare these two for vacuum table application, type A normally has only about 20% of the vacuum pressure (inches water/mercury) of type B. Type A moves more air (CFM) but is generally much less powerful.

So, when we talk of shop vacs used in vacuum tables, we are surely only considering type B?

[Wdyasq@yahoo.com]

Style A = Dust collector

There are many types of air handling devices.

For low-power vacuum tables I use a motor alone I can buy from several sources for less than $20.

I eliminate the filter bag and hoses - two restrictions and possible sources for loss of vacuum.

It is also amazing how much air can be "pulled" through what appears to be solid material.

Building a vacuum table is a simple process. But, there are a lot of little details that can bugger the work. If you don't know how and why vacuum works learn. Build a small vacuum table - say 2'x3' and play with it till you learn what you can and can't do.

The most powerful 'Shop Vac' I was ever around had the tank under vacuum and the filter under pressure, a configuration not shown in the previous message.

Ron

[Gerald D]

Not to prolong this subject too much, but to support Ron's comments above:

Vacuum motors are reasonably easy to find as spare parts from appliance repair shops. This Slowenian site gives nice illustrations (http://www.domel.si/domelwww/004923778000.htm) and data. (http://www.domel.si/domelwww/Karakteristike/004923778000k.html)

In the data section it is interesting to see the performance curve. Particularly how the presure drops off according to the hole (orifice) size through which it is sucking.

I bought a similar one yesterday for $30. Just before the sales guy handed it to me, he took a rubber stamp and marked the invoice "Warranty only for household use" - now to move the SB into the house?

[danhamm@abccom.bc.ca]

Has anyone tried to series these vacuums to create more holding power? you probably would lose a bit on volume but it should increase the
suction some..

[Gerald D]

Dan, if you simply put 2 of these vacuum motors in series, then the second motor has to handle more compressed air (more dense) than the first, so they become mismatched and very inefficient. Two identical motors in series may only give you 10-20% more holding power.

In fact, the motor illustrated above already contains 2 turbines in series (that is why the can is so long). Some also have a 3-stage turbine. If you break open these motors, you should see that each stage has a different turbine, according to how the air is being progressively compressed.

Here (http://www.domel.si/domelwww/wet_vacuum_motors.htm) is one of the index pages where you can see 1-stage, 2-stage and 3-stage examples.

BTW a perfect vacuum is about 1000 mbar, so 300 mbar is about 100" water.

Most shop vacs contain these very same Slovenian/Italian motors, and it is interesting to see the OEM specs at a site like www.domel.si

[kitman]

Iwould like to see plans for a vacuum holddown table as i have a thermofoiling machine an think i can build mine with the same vac design Iwould be glad to share my plans THANKS RICK

[Ronnie Smith]

I found some Vacuum Clamps that work great and do not cost alot. I found them on Ebay. They sell them regularly. I use them on the CNC and for doing cabinet work. Just go to Ebay and do a search for Vacuum Clamps.

[Mike Schwartz]

If you're cutting an entire sheet I have some experience that may be of help. I've been using my ShopBot for about three years with various vacuum hold down systems. Almost all that I cut is marine plywood with thickness ranging from 3mm to 12 mm. I've tried the peg board idea and as you cut and expose holes you quickly lose vacuum. I've gone to a ShopVac and fiber board insulation as a sacrificial surface. The fiber board insulation can be planed to a very uniform height so you have excellent vertical control. In conjunction with using the tabbing feature, this works extremely well. The key is to plane the surface of the table even before use so that your tabs are strong enough to hold up.

For the base of your table, try using the plastic lumber that has open channels on the inside and cut holes in the top. This way you can use clamps to hold smaller pieces and use a vacuum with a sacrificial surface for sheet materials.

[David Fisk]

I've been using a closed cell pvc foam 1/16" thick (it comes in rolls 5'by 10'long) wrapped around 1/4" mdf. The Mdf serves as a jig or large puck that holds my small parts. On the table I mounted 3/8 acrlyic sheet and cut channels into it. The bottom side of my mdf/foam jig gets sucked down to the acyrlic grid with a Gast rotary vain pump capable of 25" of mercury run with a 3/4 hp motor. It dosn't pull a high volume of air, but I've found the combination of the mdf and the foam work great at slowing and sealing air leakage. I added an old water heater as a vacuum storage tank that helps with the initial pull down. I've only successful used about 1/4 of my table at a time using this method. With no wood sitting on my jig, my pump pulls between 18-20" of mercury. When I put my plywood on top the loss is the same, but holds the parts fairly well. In the top of the jig I cut vacuum chambers under each part. The jigs line up on the table with dowel pins for quick locating. I have drilled (to China)through my jig and into the acrlyic, and I've found that hot melt glue is the fastest way to seal it back up. I'm thinking of adding a bunch more old hot water tanks to increase the holding reserve so I can use more jigs at the same time. I would like to be able to use at least 1/2 of my table like this with four 1'x4' jigs. Will adding a bunch more tanks help me to to reach this goal. Or should I look toward a bigger pump? Some one told me to put a blower fan into a 55 gallon fiber barrel and that will work as good as the big pumps like shopbot sells. Does anyone have any experience with doing this?

[gerald_d]

David, to answer your questions directly:

You will need a bigger pump. (tanks only help for initial pull-down).

Don't waste your time with a "blower fan into a 55 gallon fiber barrel". (again, the barrel only helps for initial pull-down. The shopbot pump will "implode" a barrel, it has that musch more "power" than a blower fan)

[David Fisk]

It dosn't make any sence to me but the person who told me about this said to use the blower to blow into the fiber barrel with the vacuum line going directly into the blowers intake. I guess presurizing the barrel. He said he used it for some big jobs he had for some big companies. Is this crazy or what? I just wanted to bounce the idea off this forum so I don't waste my time/resources. Thanks for any input on why/why not this would work.

[gerald_d]

He seemed to be talking of a dust collecter system rather than a hold-down system. Then the barrel would need a filtered exhaust.

Blowing into a closed barrel is the same as blocking the blower outlet - no air flow at all, and definitely no vacuum either.

[sales@vac-clamp.com]

Something to consider is a Vac-Clamp. I think that it may solve more than a few problems for a lot of Shop Bot users. Look in the "Cheap Vacuum Clamps" forum or look into http://vac-clamp.com

[johnthumph3@aol.com]

Will a 1900 cfm @7 hg work for holding down a 2.6sqft piece of wood .There will be 10 pieces on a 4'x8' table top w/a dicated spoil board,or should I use a 117 cfm @ 26 hg,or will eather work .john t humphrey
johnthumph3@aol.com (mailto:johnthumph3@aol.com)

[the@26]

i would go with the@26 (mailto:the@26) hg . where are u getting it at ? that is a good cfm at that hg

[ron brown]

John,

Both of those are more than enough power. As larry said, that is a large CFM at 26 inches of mercury. The 1900 CFM unit has good suction too.

On my old vacuum table, I would reguarly hold 1/3 square foot pieces with a gasketed surface and only using 80 inches of water vacuum. More may depend on your setup than the vacuum unit itself.

Ron

[gerald_d]

John, you are asking a trick question. As Ron said, both of those options are probably okay. But the wood may be so porous/rough/warped and your gasketing may be so leaky that neither of them is okay.

Your next jobs may be much bigger (area wise), or much smaller, and you may head in the wrong direction with your current choice.

I would also be interested in factors such as price, electrical power consumption, and availability of spares (if sliding vanes are involved for example) before committing to a decision.

[daniel]

Hey Guys, Has anyone tried to cut the 2 or 4 zone vacuum file offered by Shopbot. I have .sbp part file, but I don't know the bit radius to use, Or the plunge depth, Or any other deminsions. Any info would be a big help.

[scott_smith]

This is from the S_VAC4.SBP file
The program cuts 3/8 deep below z-zero
If you open the file there is lots of info in the beginning.

' Use a 1/2" straight or up-cut spiral bit and zero it to
' the top surface of the material.
' Zero the X & Y with the center of the bit on the corner
' of the plenum material.

[daniel]

Hey Thanks Scott, I'm still kinda new to this, and I don't know much about programing, I never thought to open the part file edit! Thanks!

[arcturs]

Hi Guys,
newbie here. I am in the process of fitting our fein turbo 3, and can't seem to get the S_Vac4.sbp file online. Do I have to do anything to have access to the ftp?

Thanks -- John

[richards]

Download the latest controller software. The s_vac4.sbp file is part of the files included (at least for the Alpha).

Mike

[arcturs]

Thanks for the Info Mike

John