Black Box Vac Source

[Tim Lucas]

David,

My 2 motors were shipped Thursday or Friday from TN also so should be here today or tomorrow. Also called Ward to say I forgot to mention the gaskets and he had them added before shipping. I will be getting my plywood and pvc by this weekend, just too much going on to get it sooner. I have pulled down everything from this thread w/pics too to try to put it all together prior to heading the HomeDepot. Will take pics and post as I get going.

One question, to clear up the confusion, what is the actual recommendation of cutting the zones, I'm assuming the 1"x1" grid? How are they gluing the bleeder board down, by that I mean if you are using the grid, what part is being glued. I hope this part doesn't sound too stupid but I always was under the impression that you just layed the bleeder on top of the grid, is the bleeder board also the spoilboard? I will be sealing the edges.

Thanks for any help with this.

Joe

Hi Joe,

I cut mine at 2" squares but you can cut 1" - the bleeder is the spoil board - I took someone else's advice from this forum and used silicone to glue the bleeder down around the outside edge and between partitions - Hope this helps
Tim

[Kyle Stapleton]

Here are some pictures of when we setup ours
http://www.talkshopbot.com/forum/showthread.php?t=19117
post #2
We just used the stock files from Shopbot.

[barrowj]

Thanks Tim, think I will do that, just silicone the edges and between the zones. I will also seal the edges too.

Kyle, thanks for the info.

Everyone has been more than helpful and I really appreciate it.

Joe

[David Iannone]

Here are some pictures of when we setup ours
http://www.talkshopbot.com/forum/showthread.php?t=19117
post #2
We just used the stock files from Shopbot.

Great re post Kyle. I have read over your setup pics a few times at least. Wish CNC training training was offered when I was in school. You are doing an awesome job.

Dave

[MechanoMan]

The vac source consists of 4 Lighthouse Brand LH-7123 240V motors. These are 10.4 inHg/110 cfm units each. Combined they should pull close to 10 inHg and have close to 400 cfm. Testing has shown this to be a very powerful vac source, and holds over 9 inHg when applied to less than the full 4 zones.

OK, I know this was in 2010, but someone help me out...

The performance of that motor is listed on the XLS here:
http://www.centralvacuummotor.com/lighthouse.htm

7" static is 95" H2O and that motor is rated to move 35 CFM (per motor).
9" is 122" H2O and that motor moves about 13 CFM.

But the 110 CFM free-air rating shouldn't be relevant. Whether or not there's plywood stock covering the bleeder board, the bleeder board is going to make a lot of static pressure drop.

What's the size of the "less that the zones" that this measurement was taken under?

BTW, the check valve thing sounds like an issue. A check value by nature requires an actuation pressure differential. And that pressure differential doesn't go away once it opens- the ongoing pressure difference is what makes the force that keeps it open. The thing is, of course, that pressure differentials are the entire product here, and difficult to achieve in the magnitude we want. So... it's eating up some of the performance.

[Brady Watson]

You would need some pretty sophisticated instrumentation and time/testing to fully answer all of your questions. Vacuum is not always easy to quantify in terms of what is actually going on, especially since in the practical world of CNC routing, things are not always linear.

One possibility regarding less than 4-zones, could be that there is enough leakage (and there should be SOME for cooling of these motors) to move the working vacuum characteristics into a different area of the compressor trim map. This is akin to a turbocharger compressor map - and while ideal is linear, actual is not. Plus, unless you are running a completely sealed setup with gasketing, there is going to be some leakage, such as around the table perimeter, or if you didn't do such a hot job with the plumbing.

Static CFM is very much relevant during the initial suck down when the pump is beginning to build negative pressure - aka getting over the hump. For example - you may have one of these motors capable of 100 CFM static and an air conditioning pump that only pulls 5 CFM, but maxes out at 27 Hg". That pump will NEVER build vacuum without properly sealed and gasketed fixturing. It just doesn't have enough CFM to seal things off. This is where a check valve makes sense in conjunction with a high CFM vac - especially if you are doing something like vacuum bagging. Big CFM vac to remove most of the air; small high suction vac to pick up where the big CFM pump left off. Neg pressure pulls the valve shut & big vac can be turned off.

The thing to remember with these vacuum systems is that they are designed to be a shoestring solution to more expensive vacuum pumps. The original idea was to employ these bargain setups and make some money until you can afford a proper commercial/industrial grade setup. Some users don't have 3 phase or simply can't afford (or don't need) a pro setup - so certain provisions and allowances have to be made...as in, 'you get what you pay for' & 'everything has a price'. So...some questions probably cannot be answered without either paying someone to do the research/study or you have to do this sort of thing yourself.

-B

[MechanoMan]

Hmm I think it may boil down to this-

With a well-designed bleeder board, how many inches of vacuum is required to get a useful holding force on a small object, and how many CFM per sq ft will occur at that pressure?

Now the CFM situation drops when you throw a whole sheet on it and vacuum increases its hold. But once you're talking about a whole intact sheet the holding force is absolutely tremendous and holding is not a problem if you could hold a small object. The ability to hold a small object drives the specification for the job.

I don't know what the most practical spec to set up for a small-object is. Like I could ask "can I cut out a 3" circle from a 5" square of plywood and have it stay in place?" But then again if I didn't have vacuum table I probably would never try to use Raptor plastic nails to secure a little 3" circle, I expect a person would leave it tabbed together and break it out later.

The inches of vac needed to hold a small part should be the same whether the rest of the zone were covered or not. The question becomes whether that inches-of-vac spec can be met under the CFM flowrate of a mostly uncovered bleeder board zone. Which can readily be predicted by blower spec sheets if there's a spec for what CFM-per-sq-ft should be through a bleeder board at the minimum static pressure spec for holding small objects.

[bobmoore]

An interesting study Mechanoman. Please keep up the study and post your results. I used to run a plant with komo routers with 40 horse becker vacuum pumps and we would never have dreamed of trying to run a 5" square with a mostly uncovered spoilboard. Becker pump could pull nearly 27" of mercury. I don't remember how many cfm but it was substantial. Bob

[Brady Watson]

An interesting study Mechanoman. Please keep up the study and post your results.

I would also like to hear what you've discovered about this. BTW, welcome to the SB Forum. Most of us go by a first and last name basis. Feel free to introduce yourself. I know a few SBers down in your neck of the woods.

With a well-designed bleeder board, how many inches of vacuum is required to get a useful holding force on a small object, and how many CFM per sq ft will occur at that pressure?

You have to run some calculations. There isn't a hard and steadfast yardstick because many variables are at play; such as: tooling geometry - is the cutter defeating hold down with upward helix/lift? What is the chipload and in turn, the amount of force being exerted upon the workpiece? Does the bleeder board have shallow kerf marks in it coincidental with the current toolpath (you cut a sheet of circles with freshly flattened bleeder...now parts don't hold on your 2nd sheet) ? There are other things too.

In an ideal world, CFM would equal 0 or very close to zero when holding down a given part. However...this is hard to quantify since the first part that is completely cut out of a piece of material has full vacuum force holding it down - as the kerf starts to bleed out (or in), suction is reduced and airflow increases. This is with a bleeder board...not a gasketed setup.

So...Surface area is going to be the yardstick here. If you have a vac capable of delivering a solid 9 Hg", then you'd be a little more than 1/3 atmospheric pressure, 14.7 psi, so we'll say 5 psi. Can your surface @ 5 psi total resist the force of a cutter going XY,Z speed at fC (ChipLoad)?

It's not the pump that is the most important...It's how you use what you've got. I've seen plenty of shopvac setups trump industrial pumps in their usefulness because the operator knew how to use the vacuum he had at his disposal. Universal bleeders are not the end all be all. They are meant to be a 'throw a lot of vacuum and airfow and see what sticks' solution. They waste so much energy compared to a puck system with gasketing and a smaller pump - provided the material isn't too porous...but they sure are convenient if you mainly do full sheets.

It would be lovely to quantify vacuum and say for sure what will and won't work. When it gets down to the nitty gritty, there's too much room for variation. So I'm afraid for now, the best we can do is guesstimate, take our own readings and do our own testing in our own shop. Something as simple as a material vendor (such as Trupan UL vs Sierra Pine) could mean a substantial difference in performance with identical hardware...so like most other things in life, YMMV.


-B

[MechanoMan]

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.