A more "scientific" approach to selecting porous spoilboard materials for vac tables?

[gerald_d]

We read a lot about which material works for who, but there are so many variables that we need to make sense of. I am interested to discus spoilboards from first principles and to try and put some numbers/measurements into the discussion.


5354

First I am interested to collect data from folk who have a vacuum gauge (or water-tube) connected to their suction side (plenum) piping. Can these folk tell us what readings they get when they..

A. have nothing on the table,
B. have half the table covered with a plastic sheet (steal some from the kitchen),
C. have the whole table sealed/covered with plastic?

With these numbers, we can then get into board material types, pump sizes, etc.

The basic theory is that reading C should be the same as the pump manufacturer's spec in psi, water or mercury gauges (it has nothing to do with CFM)

Reading A is the tricky one. If it is close to reading C then very little air is moving through the bleeder (might hold plastic well, but won't hold MDF). If reading A is close to zero then the bleeder is too porous and it will hold big sheets well, but be useless for half sheets.

The question then is; Where is the sweetspot for reading A? 90% of C? 50% of C? What are the pressures that work for you, with your pumps and zones, and with the materials that you cut?

Please note that this discussion does not include gasketing or "pucks" because they serve to force a situation C. My interest is in getting numbers for porosity and bleeding.

(If we do get some reliable numbers, then we can theoretically take a dense bleeder and drill tiny holes through it until we hit the sweet spot on the pressure gauge....)

[beacon14]

Gerald I'd be the first to report back but I have not yet designed or built my vaccuum system. I'm hoping to get started on it as soon as I read the rest of this thread...

[gerald_d]

...same here David

[mikejohn]

Did I read right? Gerald is fitting a vacuum system? Am I going to be the only Luddite left?


.........Mike

[gerald_d]

Mike, at my own workshop I don't have enough power (or hearing protection) for a full-on non-gasketed vacuum table, but I have a neighbour who wants to put a sacrificial bleeder on top of his Italian aluminium table. (His "Van" has a habit of cutting into the alu and he has replaced the top once before at about €10 000). He has a 20kW pump with 100mm piping (noisy beast!), so there is enough power. Snag is to pick a local bleeder material......

The Luddite movement is not under threat.

[richards]

Since I'm still looking for some porous MDF, I can't test anything more than the Fein vacuum and the plumbing to the table. The result was surprising. The Fein vacuum is rated at about 100 inches of water, which should equal about 7.35 inches of mercury, which should equal about 3.6 lbs. per square inch of atmohperic pressure. My vacuum gage showed just 5 inches of mercury, which equates to about 70 inches of water, which equates to just 2.5 lbs per square inch of atomospheric pressure. Even though the Salt Lake area is about 4,500 feet above sea level, I expected better results.

Being a little suspicious, I checked the vacuum reading right at the vacuum, before the hose, at the end of the hose, at the table with all the valves closed and at the table with all of the valves open, but with the orifices blocked. In each case, I got the same reading. That made me comfortable with the plumbing, but uncomfortable with the little Fein. Even though I know that there are limits on what a little vacuum can do, I expected about 30-50% more atmospheric pressure.

On the other hand, my GAST 3/4 hp vacuum pump can hold 25 inches of mercury, which equates to about 12-13 lbs. of atmospheric pressure per square inch. The limitation there, is that the GAST wasn't designed to allow air flow, meaning that anything larger than a pinhole, causes total vacuum loss.

The goal is to be able to cut cabinet panels in one pass with a 3/8-inch single flute cutter at a feed rate of around 8 inches per second. Three weeks ago, when I started this adventure, I found that the Fein wasn't holding the parts securely. I was hoping to fix things by re-plumbing the table , replacing the vacuum platen and spoil board, and still use the Fein. It looks like physics has won again, proving that the Fein, although a great little vacuum, isn't suited for the task at hand.

[evan]

Mike R,
Do you have a #9.77.25 (Fein III,) or a #9.55.13 (Fein II)? I don't have a vacuum gauge or I would let you know what my Fein III is pulling. That's another toy...er tool on my wish list.

[patricktoomey]

Gerald,

That is a great idea, I've also wondered about getting some real numbers for vacuum pressure for different materials and setups. All I've found has been mountains of anecdotal evidence. I just finished my first plenum board and I'm going to surface my first spoilboard (3/4" Trupan) tomorrow. I'll pickup a vacuum guage and take some measurements as you suggest and try to post the results by the weekend. I'll include pics and all my setup specs. Hopefully some other people will do the same and maybe we can get a database going that will give the community a set of real numbers to work with.

- Patrick

[mikejohn]

Mike,
At 4500' the air pressure is about 85% of that at sea level.
Day to day air pressure can change up to 20% over a year.

...........Mike

[gerald_d]

Patrick, that "Trupan" which you are using, is it genuine Trupan, and do you know the grade of it? Some Trupan's are more dense than others. Genuine Trupan has the word Trupan printed on an edge of the board. We only get Trupan ST in this country at the moment, which is the STandard more dense grade.

The "anecdotal" evidence says that if MDF is used then the hard "crusts" must be skimmed off to expose the porous inner core.

So, for those that are going to be kind enough to supply some numbers, be prepared for a grilling on the exact type of material/preparation used for the board.


For those using ShopVacs/Feins, you don't need an expensive vacuum gauge. A clear plastic tube hanging from the ceiling into a bowl of water on the floor will do the trick. As Mike showed, the rating is 100" of water, but the reality is less.

(Mike, did you supply the Fein with its max voltage rating?
)

[richards]

Evan,
The Fein is the 9.77.25 model.

Gerald,
Voltage is 122VAC (a little bit higher than rated).

The vacuum gage was not tested to verify calibration; however, it is the same brand used on the GAST vacuum pump and on a ventri system, each giving expected numbers.

[patricktoomey]

Gerald,

Yes, it is Trupan Lite. I get it from my plywood distributor and it has Trupan printed on the side. I'll have to check but I think it says Lite on there also. I tried to get Ultra Lite but my distributor doesn't carry that. Do you know if I should still cut the skin off or is that just required for MDF?

[richards]

Here are the results with my Fein vacuum, 4-zone system (97x49 platten/spoil board) with 1/4-inch unsurfaced (used directly from the store) MDF used as the spoil board. The 1/4-inch unsurfaced spoil board worked better with the little Fein vacuum than 3/4-inch MDF surfaced two sides or 1/2-inch MDF surfaced two sides.

Vacuum readings are all in inches of mercury.

5 - inches at the vacuum (not connected to table)
2 - inches with spoil board in place
3.5 inches with 1/2-table covered with plastic
4.5 inches with entire table covered with plastic

As you can see, the platten leaks a little. Most of that should stop when I coat the edges of the platten with some silicone.

The Fein vacuum with 1/4-inch spoil board easily holds full sheets of 3/4-inch melamine coated particle board and allows single pass cuts to a depth of 0.70 inches using a 3/8-inch 1-flute cutter. The remaining 0.05 inch of material is scored with a plastic laminate scoring knife and the individual parts are trimmed at the router table.

Using the 1/4-inch spoil board lends itself to palleting. A spoil board is covered with a fresh sheet of melamine coated particle board. The melamine is cut almost all the way through. The spoil board and MCP is slid off the shopbot for further processing. A fresh pallet is loaded and the cutting continues. . .

This little system is barely functional. To do production cutting requires much more vacuum, as Eric Lamoray and others have said; however, as a starter system, it works pretty good.

[richards]

Okay guys - those of you who have the big vacuums. How about posting some numbers so that us little vacuum guys can decide whether it's time to break the bank again.

[dvanr]

I have installed a vacuum system on my table using a 2 stage ducted canister vacuum ( central vac) so far so good. ( though it now looks like a blender next to Pierre's 50hp blackhole generator)
It is pretty amazing at the lateral grip the material has on the table while the actual hold down force is not that great. (2.5-3psi)

The whole vacuum system is a very good analogy of an electric cct. I doubt that the analogy is purely linear when trying to achieve full vacuum. Maybe close enough for us.

For an electric cct V = I x R

If you know how many amps ( CFM in vacuum sytems) are flowing through a known resistor ( restrictivity of the wood) then you can multiple the two and come up with the voltage (Pressure)

So I'm thinking it maybe useful to measure an agreed area (square inch?) of material and describe its restrictivity (R) as V/I

10"hg / 300cfm would give .0333"hg/cfm or .0333Ry (restrictivity the opposite would be porosity...conductance) As CFM approachs zero you can see that Ry gets higher if you get to zero then Ry is infinite, plastic , alu, and melamine type materials.

Some testing would need to be done (This has science project written all over it)


Knowing the range of restrictivity of materials being used and the area of the table that is left open when cutting small sheets would allow you use the same basic electric relationships for resistor netwoks to calculate flows and resulting pressures.... (And pick a pump properly sized for your purposes)

I can feel the limb swaying , feel free to start sawing.

[gerald_d]

Dick, you are on the right track. We have a rhyming Afrikaans saying "Om te meet is om te weet" which translated becomes "To measure is to know". Unfortunately us mechanical blokes cannot measure CFM as easily as you measure Amps, so that is why I am asking just for pressure readings where possible.

[dirk]

The CFM could be calculated off a performance graph supplied by most vacuum pump manufactures. You could simply look up the CFM for a given vacuum.
Sorry I haven't contributed as far as my vacuum pump setup is, I'm in the middle of some renovation in my shop and everything’s blocked in.
Dirk

[dvanr]

Gerald,

Measuring CFM may not be that hard, automotive guys do it all the time as part of engine monitoring systems. It can be as simple as a vane mounted in a tube with a spring and potentiometer. Calibrating it might be the tricky part .

A good pressure gauge combined with Dirk's idea maybe all that is needed to calibrate a shop made one.

The vacuum problem has me interested, right now I'm flat out with work, bot work, and a bot mod, I'm hoping things will calm down so I can try my hand at building a flow meter.

[gerald_d]

Yeah, a good pressure reading combined with a lookup against the pump curve is a practical starting point......but so far we are battling to even get some more pressure readings.


Mike Richards has given the only set of readings so far and he is as curious as the rest of us to see more readings.

[patricktoomey]

Sorry for the delay guys, but I finally had a chance to get a vacuum gauge and take some measurements. I just got a standard automotive vacuum gauge from AutoZone and it seems to work well.

Here are the details:

8 Zone table broken into 2 sections, each 4x4 section is powered by a separate 6.5 HP ShopVac. My measurements are all for one section so that would be 4x4 with one vac. Spoilboard is 3/4" Trupan Light surfaced on both sides and edge banded with PVC edge tape. I will post a detailed description of my system with pics and cut files on my website soon, I'll add a link in this thread when I get that stuff posted but for now here are my measurements...

All zone valves turned off (Vac Max Pressure): 4.6"
Nothing on spoilboard: 3.2"
Table half covered: 3.6"
Table fully covered: 4"

With very flat plywood I have had excellent results with this system, nothing has slipped. The problem has been with even slightly warped panels, they just don't get pulled flat. I'm suspicious about the fact that the numbers are so close between the spoilboard only and the fully covered spoilboard. That would lead me to believe that the airflow must be low. I'm wondering if that may be affecting the ability to pull material flat? I may try to surface the spoilboard down to 1/2" and see what happens.

[gerald_d]

Hi Patrick, many thanks for that!

For pulling material flat, you need a high airflow until it is flat and a seal is made. A more porous spoilboard will help with that.

Your empty table vacuum is 80% of a full table - Mike's table is at 44%. You are thinking of thinning your spoilboard and moving towards Mike's figure....

I am curious about the difference in the the 4.6" and 4" reading - any idea where that 0.6" is getting lost? (leaks?)

[patricktoomey]

Gerald,

That's a good question about the loss of 0.6". Since my plenum board was coated pretty well with shelac on the top, bottom and edges, and my spoilboard is edge banded all the way around, I'm guessing it's a loss at the intersection of the plenum board and the spoilboard. I don't have the spoilboard screwed down and I don't have any gasket material between it and the plenum board so I think I'm losing something there. I don't know what would be the best way to get a better seal there. Once I resurface my spoilboard to get it thinner I'll post new numbers and let you know if the system pulls warped panels down any better

[mnrite]

I ran some tests this weekend
I took a 5" square of corian, did a 4" dia x 1/8" area clear on the bottom, mounted a vac gage in the top and sealed it with allstar 1/4" x 1/8" gasket.
On my plenum which is 1/2" MDF (unsealed) with 1/4" groves 2" O.C. I used 1/4" rubber cord to seal off a 4" square around my shopvac intake.
With my gage directly over the intake the gage read 3-1/2 in/hg, 1/4" MDF spoilboard 2-1/2 in/hg and with 3/4" MDF spoil board 1-1/2 in/hg (6-1/2 hp RIGID vac)
Today I took the gage to work where we have a 20hp becker pump similar grid surface made from phenolic. Gage directly over intake 21 in/hg, 1/4" MDF spoilboard 12-1/2 in/hg and with 3/4" MDF spoilboard 7 in/hg.

Mike

[gerald_d]

Interesting tests Mike, but it is a different approach giving results that can't be compared to the others above. Still a good test though!

[mnrite]

Gerald,

I wanted to see what kind of vacumn I was getting right at the parts. Also I can easily check any variable of my system, new vac, different gasket, different sealers on the plenum, LDF spoilboard, MDF spoilboard, Trupan, 1/4", 1/2", 2 layers of 1/4", larger spoilboard area etc.

Thanks
Mike