Hello, I was wondering how the vacuum power distributes on the big CNC.
Is it a max pressure distributed evenly along all open spaces, or is it constant, independent of number of spaces open?
E.g if I only have A open, is it double the pressure in the A section compared to having A and B open? It makes sense that it would be the former, but I wonder if its better if I have a piece that overlaps most of A and some of B if its better to only have A open and keep B closed than waste pressure over a lot of B
Also what happens if I open a and b but don’t cover all the unused B? Is it massively lossy through the uncovered area?
It is not constant. @jamesfreeman @Jon or @mkmiller6 would be most knowledgeable on answering the technical bits on this. It is definitely best to have the areas you have turned on fully or mostly covered, but you can use simple/cheap materials to achieve this.
The way the progressive relief valves work provide constant-ish pressure until you hit the volumetric limits of the pumps. Once the table’s covered enough that the valves start to open, covering more table just causes the valves to open further instead of increasing vacuum pressure (approximately).
It’s fairly rudimentary as far as control systems go, but it gets the job done.
Jon’s totally right. What that means in practice is that no, you won’t get double the vacuum in A if you only have A open compared to if you had A and B open, assuming both sections are completely covered. In both cases, the max system vacuum is ~-300 mbar, since that’s the set point of the relief valve. You may (and probably will) get much more vacuum in only A compared to A and B if a good chunk of B is uncovered, since that uncovered chunk is very lossy and probably won’t allow the vacuum to reach a level where the relief valve opens. If you have a piece that’s only partially covering a section but you’d like to draw vacuum in that section, I would definitely do your best to cover the rest of it.
You might wonder why we don’t just increase the maximum allowable vacuum. The answer there is that the “vacuum pumps” aren’t true vacuum pumps, they’re regenerative blowers that aren’t designed to reach medium to high vacuum. There needs to be a minimum air flow to prevent the blower from overheating. Most true vacuum pumps (especially the “cheaper” oil-sealed rotary vane kind) aren’t designed for prolonged operation in the low vacuum ranges you’d be able to achieve in a CNC vacuum table. And two of those “cheap” pumps that could move the same volume of air as the regenerative blowers would cost more than the entire CNC.
Initially (before getting gouged up), even with a single whole zone uncovered on a single pump, the vac was high enough that the relief valve was already open. Covering the unused area within a zone when it’s in this regulated vac mode should not actually increase the vacuum at the pump, just open the relief valve more, but in practice it does improve the hold as it limits the air bleeding sideways in and under the bed.
With the relief valve is already open running on one pump to limit the vacuum, then turning on the second (which also has its own relief) should not increase the vacuum.
When would the second pump be used? I’m sure with multiple zones open and uncovered, the single vac’s relief valve will close and its vac will go down so the second pump could then increase the vac, but if you have that much uncovered area then you should cover it, which should then reach the vac relief limit on a single pump, right? Or is a substantial amount of air flowing through the area of the stock itself creating more work for the pump?
Regular 22"x48" posterboard is very easy to use for covering unused area. It can be peeled off the bed even with peak vac underneath, and- very important- unlike plywood or cardboard, it overlaps without making a gap along the seam. And light/cheap/reusable.
I talked with @jamesfreeman about putting a holder on the back of the control center to keep posterboard handy.
How do we know we’re at ~300mbar? Can we install a vac gauge to see how it’s performing?
I agree with being able to read the vacuum pressure. It would be nice to have that functionality. Interestingly, swico has an auction for something that maybe could work? Tbh, I don’t know exactly what it is for but it seems relevant haha. Trautwein 7 positio vacuum/pressure regulator - Swico Auctions
I am a bit confused at the explanation @mkmiller6 provided.
What that means in practice is that no, you won’t get double the vacuum in A if you only have A open compared to if you had A and B open, assuming both sections are completely covered.
Does this mean that if one section was completely covered at 300 mbar (~4.35psi), you’d get ~5000lbs of holddown force (48"x24"=1152 in^2*4.35psi), and at two sections you’d get 10,000lbs of holddown force? or at two sections you’d still get 5000, cause the pressure is split between the two? Please correct my understanding, especially if my math is wrong haha
Wondering if a roll of saran wrap would be useful, happy to buy one and leave it if its approved, I often have weird shapes that cutting posterboard seems wasteful. Tried using shop paper and it didn’t work that great.
The -300 mbar is at the blower inlet, not at the table surface. So you’ll be seeing something much less than 4.35 psi of hold down at the table bed. But just to keep the example consistent, you’d either be getting 5,000 lbf in one section or 10,000 lbf in two. You wouldn’t be splitting 5000 between the two. You’re either getting 4.35 psi over a smaller area or a bigger area. Again all assuming that both sections are covered and you aren’t getting a bunch of losses through a section. So that means you’ll want to open as many vacuum sections as you can to get the best hold down for your workpiece.
Danny’s point about losses through the workpiece itself is an interesting one though. It might be fun to test out with some different materials. An extreme example is the spoilboard itself. It covers up the entire vacuum bed, but obviously you’re still getting airflow through it because it’s so porous (that’s the point).
Since we don’t have a vac gauge on the system, I don’t know precisely what the actual vacuum is. -300mbar is just the manufacturer’s rating on the relief valve, so I’m assuming it’s something close to that. I agree it would be nice to have a gauge. The one you linked is a bit overkill, and will actually regulate the vacuum level vs. just give a readout. But looks like the max vac on those regulators is something like 25 in. H2O, so wouldn’t work for this system. A simple dial gauge on the blower inlets would be the easiest to add.
Thanks for the explanation, that makes more sense! I’d be curious what the actual pressure that it is applying at the bed is. And yeah, I figured that it would probably not work I just came across it at the same time as I was reading these posts, so thought it was interesting.
Currently with whoever messed up and cut really deep >1cm ish into the spoilboard, the vacuum on areas A and B is already really lossy since theres these huge channels that run all around the pieces.
Shop paper didn’t work well to cover the gap, and all I had was scrap wood so it was kinda janky. I ended up moving my board to C.
It would be interesting to know. In a perfect system where the piece is non-porous and perfectly sealed to the bed, it should be the same as the blower inlet, but there are surely losses in this system. Maybe some kind of pressure pad setup could give you an idea.
There are vacuum chucks in the machining world that are rated for almost full vacuum (~14 psi of holding force), but that’s a different ball game.
Regular MDF is not as permeable as that spoilboard. Unless planed off, the faces are a denser, glue-packed low permeability. But over a large area, there may be significant infiltration through a big sheet of something.
A dial vac gauge isn’t too expensive. It’s also practical to use flow meters with a small orifice sucking in ambient pressure that lifts the ball. The inflow shouldn’t be significant enough to load the pump down.
Vac gauges come in all types- some detect just inches of water column for HVAC, others go all the way to “0” PSI but they may be crappy about hundreds of millibar of offset error that is comparable to what we’re trying to measure.
Also, you should be able to use a vac meter on the surface. Just a cup with a seal and a port to an appropriate vac gauge. And on the surface of MDF stock placed on the board to see if air is flowing through the stock.