How Powerful a Vacuum Does a CNC Router Need?

      A discussion of vacuum pumps, regenerative blowers, and spoilboard techniques for maintaining a strong hold-down vacuum. March 27, 2008

I was wondering if anyone has advice on what size and type of vacuum pump to get for a 4'X8' table. I will be cutting melamine or pre-finished ply 99% of the time. I have heard that a 7 or 10 hp regenerative blower would be adequate.

Forum Responses
(CNC Forum)
From contributor R:
I think it would depend how fast your machine can cut. A friend has a Thermwood cutting at 900ipm and a 20HP vacuum pump, not a regen blower, and has trouble holding small parts. I have a ShopBot with a 15HP regen blower and have trouble cutting small parts. My blower is rated about 14" at 335CFM. So I would think small blowers don't work very well.

From contributor M:
I think the operative word is adequate. You are looking at the bottom end, though, and for that setup to work well, everything else has to be perfect. Keep in mind that someone is trying to sell you a machine and one way they can keep it within your budget is to undersize the pump. Yes, it will work, but this is one place that a few thousand extra is worth it. If I had to make the choice between a drill bank and a larger pump, I would go with the pump.

There are a lot of factors that affect the hold down on your spoilboard. Good gaskets, edge leakage on the board itself, flatness of the work piece, what material you use for the spoilboard (thinner and more porous intuitively seems better, but it is definitely not), how scored the spoilboard is, whether your table is zoned, etc. I worked for years on a 4 by 8 with 10 HP. My opinion is that it is well worth the investment for a larger pump. I work with more than twice that now, and really notice the difference.

All in all, no matter what size pump you use, good machining techniques will make or break you. It will take a good deal of experience to work out how to get small parts off the machine consistently. Large parts like cabinet sides are easy. Drawer fronts that are 4 by 6 inches or even smaller can be done consistently without tabs by onion skinning and gaming the order of cut and the start and end of the toolpaths so that part is still attached to the very last bit of cut. For the most part you are doing things very well if the toe kick notches are all still in place when the sheet is finished.

From the original questioner:
The smallest parts I would be cutting would be nail bars about 3 1/2" X 12". All other parts are much bigger. I will be cutting just cabinet and closet parts.

From the original questioner:
I have been doing research. It seems that there are two factors that come in play - vacuum pressure Hg's and CFM's. It seems that there are tradeoffs, at least the way I understand it. Regenerate blowers have great CFM capacity but only produce 13" Hg. Liquid ring or rotary vane pumps produce 22"-29" Hg, but require a ton on money and HP to produce a ton of CFM. It is obvious that the more inches of Hg that you have, the more pressure of the atmosphere that you have pushing on the piece. I think at 25" Hg the pressure is around 10 PSI; at 13 Hg, it's around 4psi. So it seems that once you start cutting through a non-porous material like melamine, you need some CFM to maintain a vacuum. Does anyone keep an eye on their vacuum gauge? It seems if you did, you would know when you need to surface your spoil board. I have no experience with this technology. Not having a big enough vacuum must be aggravating with part remakes?

From contributor M:
You are exactly right. HP is not really the main factor, just a rough indicator of performance. Flow and pressure are more accurate and more relevant. Keeping an eye on the gauge is a good idea for diagnosing problems. Some machines have a shutoff if the vacuum drops too low. Overall I prefer vane pumps, and taking some care to limit leakage so they can stay efficient. Others see this differently. The higher HP pumps donít necessarily hold the part tighter. They do allow, though, more flexibility in machining techniques and more room for small errors not to affect you so much.

Of course you want to keep air from leaking in no matter what size pump you have. Look at it this way; if no air got past at all, a very small pump would work great. In the real world, air gets by somewhere. This is one reason a thicker spoilboard (to an extent) works better than a very thin one. Pressure in the plenum under the spoilboard stays higher, flow is lower through the spoilboard, but the pressure at the top of the spoilboard equalizes fairly quickly to roughly what you have under the board before you start exposing kerfs, even if you have a fairly thick spoilboard. It's not so intuitive, but it is real.

One guy I knew argued strenuously that letting dust pack in the kerf was a good technique for limiting flow through the spoilboard. I donít think he realized that the density of the dust is virtually zero and doesnít stop any appreciable volume of air from flowing by. It does help keep parts from shifting a very little bit I suppose, but all in all dust under the next sheet is the enemy for hold down.

Onion skinning minimizes kerf exposure especially in melamine, and also creates a framework to keep the parts in place. There is very little side pressure on the last pass; I usually use about 5 - 10 thousandths. Dust on the spoilboard also reduces friction between the work and the spoilboard and creates gaps for air to flow in.

Keeping cut-through to a minimum is also a big deal. With good control of your Z-axis and sharp tools, which you need anyway, about 1-3 1000ths is all you need for cut through. This keeps the spoilboard smooth: less leakage.

Pre-finished ply is more difficult than melamine, primarily because it warps more. If any part of the sheet is out of contact with the spoilboard, it will let air in. Moreover any part cut in that area out of contact will certainly be apt to move, no matter how big it is. I have used edge tape on my spoilboards at times, and finished the edges at others. If you write your fly cutting program correctly, tape works very well; just run around the edges of the table clockwise before doing the middle of the table so the cutter doesnít loosen the tape. If you use 3 or 4 coats of lacquer or conversion varnish, it doesnít matter much how the spoilboard is cut.

From contributor P:
I just purchased a 4' x 8' router a year ago, so do not have the depth of experience of many of the posters, but here's my two bits anyhow. We have a 10 hp regenerative blower which works well on most parts, except when it comes to smaller stretchers and drawer sides - then we lose some. Ours is a two man shop so the router is not running full time, so we cut with a 1/4" compression bit - slower speed, but smaller kerf, thus less vacuum loss. All that said, I just bought two 10hp vane vacuum pumps to get a higher vacuum while keeping the cfm up. Two 10 hp instead of one 18 or 20 hp because I run the shop on a phase converter. Losing even one part is a pain because, especially as with dovetail drawer parts that can't be run on the table saw, you have to go back and set up to run just that part and find a piece of material to cut the part out of and cover the rest of the spoil board with pieces of something, etc. This may be all old hat to experienced users, but is something I hadn't thought of beforehand. The vacuum pumps were used, 4 years old, by the way, no way I could afford new ones. In the process of replacing the vanes now.

From contributor E:
We just purchased a new nested machine and I am looking forward to many learning experiences. Ours will be a 5 x 12. The dealer suggests a 20 hp liquid ring pump. I have talked to several other shops that seem to complain about the small parts issue. We opted to go with a larger 40 hp pump (Dekker) with the variable frequency pump. Think about it... Why should you go to all the expense of a nested machine and skimp on the pump? After all, if you can't hold the part, what is the reason for the whole process? I also contacted our local electric provider and they told me that it would cost a huge sum of money just to start the vacuum pump. That's why we went with a soft start. The nice thing about the Dekker is that you can tone it down to use less HP when you want and still have the soft start. I really enjoy reading the info on WOODWEB and look forward to learning more.

From contributor J:
From an OEM standpoint, regenerative blowers are a very inexpensive way to package a vacuum pump into a CNC router and provide, for example, 20 HP of vacuum cheap. However, they are loud, and are well suited for all applications. I notice when someone is complaining about a lack of hold down, many times it is because they bought a machine that came with a certain pump they didn't know any better so they trusted the OEM to "bundle it in." Vacuum pumps need to be suited to the application. A 20-25 HP liquid ring pump will work well in your application. Probably more money than you want to spend, but it will work properly.

From contributor T:
I find a good rule of thumb is to use 25 HP on 4 'x 8' table and 40 HP on 5' x 10', and higher.

From contributor D:
I have a 10hp Becker rotary vane and sure wish I had two or three times that on my 5x10. We have to work all kinds of gaskets, etc. for smaller parts.

From contributor O:
I'm currently considering buying some Becker oil lubricated rotary vane pumps. Anyone have any experience with these on a CNC? They pump to a deeper vacuum and run cooler than a dry vane pump, but I am worried about the need for perfect filtering to keep MDF dust out of them and if they will emit oil fumes on the exhaust, etc.?

From contributor J:
Contributor D is the classic example of getting stuck with an underperforming vacuum pump that was supplied with his FlexiCAM router. Do not ever use an oil lubricated rotary vane pump on a CNC router. As the vacuum level gets lower, the oil temp rises, fouling the air/oil separators and filling your shop with burnt oil smoke and smell. This is why companies like Becker now supply dry vane systems to OEMS that bundle their pumps like the VTLF 250 with their machines to unsuspecting buyers. This is another some CNC router suppliers recommend 2 of these pumps for a 5x10 table. Great for spare parts business.

From contributor D:
FlexiCam recommended 20hp, but the additional money was outside of my budget. I also look at Reischle claws pumps that are more efficient than dry vanes, but also over our budget. I will add more vacuum in the future that is started and stopped by a pressure sensor. When vacuum level falls, second pump starts to catch up. I am able to hold sheet goods very well and cut multiple nested pieces without issue. We only have problems with parts that are less than .25 sq/ft and onion skinning makes machining parts as small as 1/4" diameter very easy. The dry vane Becker has been easy to maintain. Clean filter and lube bearings once a week. The machine is quieter than the airflow from our dust collection. 10 hp is definitely better than screwing down pieces or fixing with cams like we did on our Shop Bot. I think ideally I would have at least 50hp of total vacuum if I wanted to utilize every inch of the table to mill smaller parts without leaving a skin. For our primary purpose we are only vacuuming down an 8-14" x 8'-10' solid wood plank at a time and for this, 10hp does great. I am happy that I bought the upgraded CNC with a small vacuum rather than the entry level machine with a bigger vacuum. Vacuums are easily added while CNC machines are not that easy to upgrade.

From contributor J:
The difference in cost between a 25 and 40 HP liquid ring pump that we provide is only $1,200.00. Buying any router with too small of a vacuum pump to hold parts down is akin to buying a Corvette with a 4 cylinder engine. I got the car I wanted at the price I could afford but it doesn't work real well, so I'm going to buy another 4 cylinder once I save up some money? I would reduce your machine cost enough and be sure you got a pump that worked.

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