Making the Best of an Underpowered Vacuum

A better vacuum pump isn't in the budget, so a CNC owner asks for advice on improving hold-down performance with spoilboard improvements and other workarounds. November 5, 2013

Question
I’m looking for a little advice on parts management on the CNC. My regenerative blower vacuum, at 7.5 HP is undersized for small parts but it is what I have at this time to work with so I am looking for a few tips on making the best of it and maximizing the efficiency of the unit. I will give some starters. The table I have is 72x144. I don't need all of that area and have zoned it down so that I am only working with a 4x8 area and have concentrated the vacuum to this area. It has helped greatly but I still get errant parts moving on me that I need to fix.

I have been full sheet skinning the parts using a 3/8 compression that I have set to go at 400 IPM. I have the small part size set to 144square inches and minimum dimension at ten inches. I have the cutting controls set at a thickness of .1 inches and have a 50% reduction in feed-rate for the last pass. I also have the bit set to do the final through-cut at .031 inches, which may be excessive. I was having trouble with the final pass not going deep enough until I switched cutters to this current compression and now I feel I can reduce that but would like to know what a common cut through depth would be.

Also, for resurfacing spoilboards, how often is that typically recommended to do as far as sheets through the machine before resurfacing and what is the depth you take off of the sheet each time? Would using a 1/4 inch cutter instead of a 3/8 inch cutter be a better choice for any reason such as having less vacuum loos or anything like that? Any suggestions that can help me tweak my procedures to be as effective as possible are greatly appreciated.

Forum Responses
(CNC Forum)
From contributor J:
You're fighting an uphill battle with the regen blower. You really need a rotary vane for this type of work. Just keep an eye out for a good deal - check out some auctions or look for local businesses closing. There are a couple of things you can try. Use a 1/4" down shear for the small parts, space them out leaving .5-.75" of material left between them after they are cut, creating a frame to cage in the parts. Leave a thicker onion skin, between 1/16 and 1/8 of an inch. The down-shear will pack in dust and the thick skin will hold the part until it is completely cut free; it also won't lift the parts. The smaller the diameter of the down-shear the less movement is possible, there is nowhere for the part to go. I wouldn't use anything smaller than 1/8 inch though. Might not leave the best finish on the bottom surface, but should be sand-able if the bit is sharp. This will be harder on the tooling but will get you through until you upgrade your pump.

Surface your board whenever it starts looking like a circuit board. Don't cut too deep and you'll make it through a good 10-20 sheets before you need to do it again and you won't have to remove much when you do surface the board. If you leave a .75 inch border around the sheet you can screw it down or use some aluminum nails if you're afraid of ruining a bit. Before you fasten it down you can run some strips of cheap packing tape across the flip side that will act as tabs. If you use this with the cage method you don't even need vacuum. Other than that you can try some 1/32-1/16 thick support tabs or some double stick tape or spray adhesive if possible. If all of that doesn't work you can always leave the skin on and finish with a trim router table as a last resort.


From Contributor E:
It might be superstition, but in the past when I was running a router for another shop we would edgeband the spoilboard. I still hated that router though, it was always throwing parts off the table and it had two massive rotary vane pumps. You mentioned slowing the feed rate for the last pass, this may not be optimal. I've found Courmatt Int'l to be very helpful when it comes to tooling, feeds, and speeds.

From contributor J:
That is a good practice Contributor E - there is a lot of bleed through the edges. I have a four zone and I have each zone sealed so I don't transfer vac from one zone to another. That made a big difference.

From Contributor W:
Is it possible in Moziak to ask it to cut smallest parts first? This is when vac is at a max and it needs far less for smaller parts. On another program I have used we could select part cut order this made a major difference. I have cut with ten hp regens and it can be done but you are at or slightly below the margin line and another vac pump may be required. On the new machine I have a 15 hp regen that I have been told had far better results and I am always on the lookout for a good pump!

From the original questioner:
Well, this weekend I tore apart the top and resigned it to a 4x8 table size. I caulked every seam in the grid board and then used this silver metallic tape used to seal duct work (not the common duct tape) and also sealed the edges of the spoil board with that tape. I then surfaced both sides of the new spoilboard and tried it out. There was a huge difference in the holding power and I thought we had it licked, but the first sheet of BB ply we put on for drawers had parts move right away. I do have Mozaik cut the small parts first and they still don't hold so we are scouring the area for a second pump and found some interesting prospects in places I had never thought of.

So now we are looking at adding a second 10 hp pump that would give us the option of a smaller vac for bigger parts and a larger one for times when we need more. The pumps we are looking at can be fitted with a variable speed drive just like the CNC spindle to be able to tune the vacuum to the amount of flow you need. Has anyone ever tried that before? Does anyone have experience using two units for vacuum instead of one and how the plumbing needs to go?


From contributor G:
We use two Busch rotary vane pumps and have a check valve on the input of each pump so if you are running only one you don't back draw the other. We also have a vacuum gauge on the manifold close to the controller so we can track the amount of vacuum we have. We often start with one pump but if cutting lots of small parts will kick the second pump in to maintain good vacuum. We do seal the edges of the spoil board and mask off the sections of the table we are not using. Our controller allows us to set a minimum size to cut that is both onion skinned and cut before the larger parts. Being able to view the vacuum reading gives you the information you need to decide when you are going to see parts move.

From the original questioner:
To contributor G: It did help - thanks.

From Contributor A:
I have two suggestions: First, cutting the smallest parts first is great but you also want to pick your start points if possible. Your start/end point should be located so that the part is attached to the largest part of the sheet for the longest amount of time. You should never have your start/end points on the edge of the sheet or against another part that has already been cut.

Second, if you are cutting the same parts in the same locations consistently, drill one or two 2" holes all the way through your spoilboard everywhere there is a part. This will give maximum vacuum directly to the part. However if you are nesting and running different parts all over the spoilboard this will not work.


From contributor D:
Here are a couple of tricks I have used due to the problems we used to have.

1. Instead of edgebanding, I use wood glue and roll it on. Several coats will seal the edges well. Latex paint helps and makes it look better. I only started doing this because I had problems with the banding chipping off when surfacing the table and/or flipping the sheet when first placing it.

2. A thicker spoil board will help. There are lots of people who say it won't but it does. I am not sure if I would use it on a small pump but your holding power is increased by the cubic volume of air, not the square footage. We use 1.25" MDF because it is readily available and I can tell that I begin to lose power as it gets thinner. It also reduces the number of times you switch out your spoil board which can be a lengthy process.

3. The size of pipe from the pump to the table is usually optimal around 4". You might check yours with the manufacturer. Again, this allows a higher cubic volume of air to pass through.

4. Raise your brushes or hood on the final pass. You would be amazed at what a suction your dust collector can generate when funneled straight down. Please ensure that you do this as safely as possible!

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