Black Box Vac Source

[Chuck Keysor]

Hello Mechanoman. I would like to mention that I agree with Brady's statement: "BTW, welcome to the SB Forum. Most of us go by a first and last name basis."

This is a very friendly forum, and I think part of it is that people use their real names. I am involved in other forums where pseudonyms abound, and my personal observation is that things don't stay as friendly when people are anonymous. Do what you want, but I hope you will consider using your real name.

Thanks, and welcome aboard, Chuck

[Justin G]

Yet one should not throw up one's hands and declare it incomprehensible because the problem has variables and variable standards.

Clearly there's a wide variation in the achieved vacuums people have worked with. It'd be good to know what people found a useful minimum. Are we saying 9"Hg as a minimum to get "basic" results?

Hmm, how does this work exactly? See a bleeder board might lose 80% of its thickness before being replaced. When "new", a high vacuum underneath doesn't mean high holding force on top. If you fail to resurface the sealed faces of the bleeder board, you develop high vacuum but no CFM.

Conversely, if you had a perforated (drilled-through) conventional vac table, but only used a fan with low vacuum, you could see some high figures for CFM, but this wouldn't equate to holding force.

I'd think "vacuum at the workpiece" would be the appropriate figure, however, there is no appreciable pressure drop at the open surface for a Trupan bleeder board. Or a perforated board, either.

I think what we're looking for is a SEALED surface pressure drop. That is, you have a cup with a rubber seal on the rim, and stick it on the table. How much pressure drop is there in the cup? That should dictate holding force. If you have a low-porosity bleeder board that you failed to surface, you would develop "impressive" vacuum underneath but the lack of vacuum on top would be apparent. Similarly, if you had a perforated bleeder board and didn't cover the unused holes, you get impressively high CFM but again the surface cup test shows the low vacuum pressure indicating poor performance.

The cup would need to be large enough that the sideways leakage- CFM- under the seal does not relieve the generated vacuum. That sort of leakage is going to happen even if the seal were "perfect" and the cup was glued to the board, because the bleeder board is porous by nature.

Yes of course cuts in the surface from prior use will mean you have greater pumping requirements until resurfacing. I do see that. But the test would lead to a proper understanding of the design requirements.

For example, I'd like to be able to know that "for a standard 1/2" Trupan board, properly faced, you need X inches of static underneath, which loads down the pump with Y CFM per sq ft. This will allow you to cut a 1 ft diameter circle of plywood with a 1/4" bit of a certain type at a certain depth-of-cut and IPM".

From there a person could say "I have an impressive 5HP spindle and want to cut faster" or "but I need to cut smaller parts at the same speed" and be able to calculate what the vac pump requirements are to achieve a solid hold under different side forces.

Just, not this guesswork thing. So far I've seen anecdotal accounts going all over the place.

Ah, yes. Science! This is a great question about required hold down force for a a given chipload. I hope you can come up with a table or chart that clarifies this.

[allen00se]

just a quick question...

I am planning on building a 2 motor box where I can use 1 motor at a time. If I am only running 1 motor, what is keeping the running motor from pulling air through the idle motor? Is that what the check valve is for? I have seen plenty of pics of the actual box but have yet to see a "plumbing" diagram.

Thanks in advance.
Krys

[kenkelsey]

I just built the 2 motor version of the BB. I have it plumbed up, but I am still working on the table plenum and positioning of the 2" holes through the table. Based on just a feel test (hand over the vacuum intake at the table, I don't believe one motor will pull enough vac. From my unscientific test, I would say that one vac motor has less vac pull than a 10 gallon shop vac. I did have the same idea as you about using one motor, but I'm now doubting that. I will be up and running in a couple of days and will post my experience. I do have a vac gauge on order for more measurable results.

[David Iannone]

just a quick question...

I am planning on building a 2 motor box where I can use 1 motor at a time. If I am only running 1 motor, what is keeping the running motor from pulling air through the idle motor? Is that what the check valve is for? I have seen plenty of pics of the actual box but have yet to see a "plumbing" diagram.

Thanks in advance.
Krys

Welcome,

Here are a few pics of my plumbing. Pay no attention to the shopvac filter and 8" PVC. I ran out of room plumbing and am doing a different type of filter. Trying to work on finishing mine this week. The fun part to me was working without a diagram.....(on the plumbing that is) all I did is just looked at my machine and figured out where I thought I wanted to turn on vac box and open/close valves and started cutting and gluing.

P.S. When you build your box, don't forget to seal the vacuum chamber. I forgot to do that and still need to take my box back apart and do that.

Dave

[kenkelsey]

I did finish my plumbing and fired up the black box last evening. I placed a a 3 foot square sheet of aluminum directly on the plenum (no spoil board). Using only one motor in the BB, the sheet was not securely held in place. With both vac motors running, the hold down was solid. Not sure what the result will be when I install the spoil board, but I'm pretty sure that running only one motor in the BB will not be sufficient for most applications.

Your installation looks great.

[allen00se]

I did finish my plumbing and fired up the black box last evening. I placed a a 3 foot square sheet of aluminum directly on the plenum (no spoil board). Using only one motor in the BB, the sheet was not securely held in place. With both vac motors running, the hold down was solid. Not sure what the result will be when I install the spoil board, but I'm pretty sure that running only one motor in the BB will not be sufficient for most applications.

Your installation looks great.

This actually makes me nervous to hear, I would have expected these vacuum motors to be more powerful than your average shopvac. Interested to hear some results with a a vacuum gauge hooked up.

[allen00se]

I just built the 2 motor version of the BB. I have it plumbed up, but I am still working on the table plenum and positioning of the 2" holes through the table. Based on just a feel test (hand over the vacuum intake at the table, I don't believe one motor will pull enough vac. From my unscientific test, I would say that one vac motor has less vac pull than a 10 gallon shop vac. I did have the same idea as you about using one motor, but I'm now doubting that. I will be up and running in a couple of days and will post my experience. I do have a vac gauge on order for more measurable results.

So I was getting worried about this and just googled some performance info on a basic shop Vac
8 Gallon 4.5HP Shopvac
Air Flow: 175 (CFM)
Sealed Pressure: 60 (inches)
Electrical Ratings: 120V 60Hz 9Amps
Peak Air Watts: 320

Compare that to the Motor from the first post
LH7123 240v
Stages:3
AirFlow: 552 (CFM)
Sealed Pressure: 148.8"
Amps: 7.0

Looks like you either aren't using the LH7123 240V, or you have one hell of a shopvac.

[kenkelsey]

I do not have my vac gauge yet, but others using the same black box configuration have reported 8 inches with the recommended vent hole open and up to 10 inches closed with two motors. I do not know what shop vacs pull in inches, but mine is at least as powerful (if not more than) two LH 220v vacs. I should be in operation tomorrow. Sealed the plenum tonight and putting the new spoil board in place tomorrow. Will advise how it well it holds various materials over the next week. CFM is also an important.

[allen00se]

I do not have my vac gauge yet, but others using the same black box configuration have reported 8 inches with the recommended vent hole open and up to 10 inches closed with two motors. I do not know what shop vacs pull in inches, but mine is at least as powerful (if not more than) two LH 220v vacs. I should be in operation tomorrow. Sealed the plenum tonight and putting the new spoil board in place tomorrow. Will advise how it well it holds various materials over the next week. CFM is also an important.

Well I am definitely no expert in vacuums, but I think they were talking about 8-10Hg which is a different measurement than sealed pressure. I have a vacuum gauge and ill hook it to my shopvac tonight and see what it gets.

I am currently running my table off of this old shopvac and I can hold down large sheets of MDF with no issue, was hoping to upgrade to a pair of LH motors and see a significant upgrade so that I can cut smaller pieces without them moving.

here are some pics of my new plenum that I cut last weekend.

Photo-2015-02-21-13-29-08_1646.jpgPhoto-2015-02-21-14-02-31_1653.jpgPhoto-2015-02-21-14-59-44_1656.jpgPhoto-2015-02-21-15-28-08_1658.jpg