Plenum Partworks file

[coryatjohn]

I'm in the process of planning my plenum for a vacuum system. I read here that there are partsworks files for the SB plenums but all I can find is the tool path files. There were no partworks file included when I install the software.

It would be great to start with a design from someone who knows what they are doing so if anyone can post their design files, or send them to me via e-mail, that would be greatly appreciated.

I've got a 4x8 table and I want to do 4 zones.

One more thing... I'm having a devil of a time finding schedule 40 flanges. Should I just make my own?

Thanks in advance for the help.

[Brady Watson]

Attached ZIP.

1/2" bit, downcut works best...Profile toolpath ON vector. Toolpath EACH (Vert/Horiz) separately. Put vac holes where they make sense on YOUR machine. Just pull your tape in X,Y and write it down on the spoilboard. Then change X,Y of each hole in PW. You should be fine making these 0.1875" deep.

-B

[coryatjohn]

Brady,

Thanks!

[coryatjohn]

Brady,

0.1875" deep? 3/16ths? That seems pretty shallow. What is the downside of making them deeper, like 1/2"?

Thanks.

[Brady Watson]

The downside is you'll lose all the rigidity in that layer and essentially have your 3/4" support board with a 1/4" solid layer of MDF with 1/2" blocks on top. Trust me...I've done at least 50 vac systems...3/16" is all you need, even with a big honkin' 25hp pump that moves 600 CFM. Go 1/4" if it makes you feel better ;)

...and there is NOTHING to be gained from using a v-bit, ball end mill or any other permutations of this simple grid from a performance standpoint.

-B

[coryatjohn]

Brady,

I have a 1" MDF plenum board.

Again, thanks for the great info.

[Brady Watson]

3/16" is more than adequate for it to flow enough air. I've tested 1/16-1/2" deep and 3/16" is fine. You'd be surprised at how effective only 1/16" is :)

1" material is even better for stability - good choice.

-B

[coryatjohn]

Plenum cut!

I modified the file Brady posted for layers, to include bolt holes for the flanges and also went a little overboard and added several depths to the plenum.

3/16": Outer ring
1/4": Next ring
5/16": Next ring
3/8": Inner ring

3/16" I'm sure is great for a 25 hp system but I am doing this on a shoestring and have two 2 hp blowers so I need all the help I can get. Having a little more flow in the center of a zone seems reasonable.

http://www.usnaviguide.com/shopbot/plenum.jpg

[Brady Watson]

Looks good. If you really want to be crafty, seal it with 2 coats of polyurethane to seal up the porosity. Make sure you Teflon tape the flange to PVC fittings and silicone (bath tub & tile clear is fine in 'toothpaste' size) around the face of the flange, getting around each screw hole.

The 'well' around the port makes no difference - Vacuum is concentrated from an area of high to low concentration, then distributed as needed via the channels, and the laws of diffusion/equilibrium. It's like a big buffer area in the chamber, similar to the air in your tires. If you get a pinhole in the tire, some air will leak out, and the pressure inside the tire will equalize as pressure is lost, or if you fill it up with air. Channel depth and width only becomes important for a few milliseconds until the plenum can be completely evacuated to 'nothingness'. After this, the pump - regardless of how much CFM it can do on paper, is moving very little air and building negative pressure. Very little air is flowing and channel or well depth does nothing.

Of course it is YOUR machine, and if you need to try something out - by all means! I have done, and have seen, all kinds of experiments with vacuum systems over the years...from round and spiderweb shaped grids, to those cut with v-bits and other shaped bits - trying to leverage the venturi effect. None seemed to have made any difference at all. What DOES make a difference is sealing up those edges equivalent to 108 square inches (10.x" square) of leakage area on your 4x8' machine - √(144x0.75") - That makes a substantial difference.

-B


PS - You can STILL use that as the spare bedroom....a mattress will fit on there just fine! :D

[srwtlc]

PS - You can STILL use that as the spare bedroom....a mattress will fit on there just fine!

I was just thinking the same thing! Crawl in on one end, hit a button and have the covers pulled up over you. Set a timer and it could push you right out onto the floor, ready to go to work! ;)

[coryatjohn]

I got a bit carried away with Partworks. It was a good exercise in design though, so no loss. I certainly can see how it really doesn't make much difference in a low flow rate situation to go to any extraordinary lengths.

Thanks for the advice.

There was a battle at one point between my wife (she called it the guest quarters) and myself (I call it a shop) over using the shop for "Unkie" (he's 96). If he does come down for an extended visit, I'll tell my wife about the alternate use for the machine. I hope Unkie likes a hard bed!

[Brady Watson]

I was just thinking the same thing! Crawl in on one end, hit a button and have the covers pulled up over you. Set a timer and it could push you right out onto the floor, ready to go to work! ;)

Nice touch!

I see a new command in the works- ZZZ :p

-B

[coryatjohn]

The 'well' around the port makes no difference - Vacuum is concentrated from an area of high to low concentration, then distributed as needed via the channels, and the laws of diffusion/equilibrium.

Brady,

The hole for the vacuum port at 2" in diameter, would connect to four, or possibly six 3/16" x 1/2" channels. That's a total maximum area of 9/16" square. That seems like a choke point. As soon as a couple of cut-throughs are made, the volume of air passing through that choke point would cause a reduction in vacuum for the entire zone especially if they are near the port. Some might still pass through the MDF on top of the plenum but essentially, it's a restriction point. It seems logical to have at least an area equal to the size of the vacuum port opening into the table.

For my design, I cut a "well" to intercept more channels (12) and remove that restriction. I don't know if this will make any difference or not but it seems like it should.

You are the expert in this so anything I can add is mere speculation (with a dash of math and science).

[Brady Watson]

John,
Use the system you have until you understand it's limitations. I can assure you that the grid depth isn't going to be the weak link.

The conventional laws of fluid dynamics don't easily translate to the types of vacuum systems used for material hold down. This is because you would need some pretty sophisticated instrumentation to be able to map out the compressor (vac motor), it's airflow (CFM) and how much negative pressure (Hg") that it is generating at a given point on the map. The airflow to vacuum pressure generated is typically inversely proportional to each other. Higher vacuum pressures are achieved during lower airflow, and vice versa. When plotted on a graph, they appear much like a curved map you'd see on a professional vacuum pump or turbocharger, they are curved and non-linear, so mapping them isn't just as matter of getting low, mid and high points and interpolating them. Finding where 'X' marks the spot isn't something easy to achieve without the right tools. The 'X' would essentially be the midpoint on suction and the midpoint of CFM. This would give you a good idea about how much your pump could take before you reach the point of no recovery, where it is leaking profusely and the vac just can't overcome it to maintain suction...this means you could lose some parts in some cases to hold down failure.

One thing that you are completely overlooking is your bleeder board. The Trupan/Ultralight MDF substrate is essentially a porous matrix of material that slows down the vacuum loss when there is an open kerf condition. Your machined vacuum grid is only one part of the equation. A 48x96x0.75" sheet of ultralight actually ADDS vacuum plenum volume to your existing 3/16" deep channels. You could probably only go say 1/8"-1/16" deep on your channels and see next to no decrease in vacuum performance. The ultralight is like a great big vacuum 'sponge' and the channels are the water supply. Each of those 2" ports is sucking air through the entire sheet & the path of least resistance in terms of airflow is the grid. The grid helps keep the vacuum distributed across the entire zone or sheet. This happens at low CFM, high vacuum.

Furthermore, when it comes to vacuum hold down, seldom is CFM the problem that people are having or complaining about - for vac powerplants that do 100 CFM+ and 7 Hg"+ of suction. Small Gast-type pumps are exempt from what I am saying because they move <10 CFM. CFM is really only important for about 1 to 3 seconds after you start evacuating air from the plenum. After that, it plays some role in maintaining vacuum pressure as you cut all the way through, but without a compressor map, you can only guess how much you can get away with before parts will break free, using simple surface area calculations. Once the vac starts building negative pressure, CFM slows way down and on a tight system, I would imagine this would be less than 10 CFM.

Those 3/16" channels, plus 3/4" of ultralight is plenty for those little shopvac motors. If you want to mill your channels deeper - go for it. For people doing production with 10 to 20hp vacuum pumps that move 130-600 CFM, the 3/16" deep channels work just fine.

Get out there and start using the machine. You can only be cerebral about so much of it before experience needs to be your teacher. Vacuum is no exception.

-B

[coryatjohn]

So if I understand this right, a 2" line to the plenum is overkill. A 1" line would be plenty. It might take 6 seconds to pump it down instead of 3 seconds. Using 1" PVC instead of 2" would simplify everything. Too late for me though, I already have 2" fittings.

[Brady Watson]

I would run the 2" no matter what. You'll understand why when it comes time to turn ball valves open & closed....

-B