CNC Vacuum Problem

      The power you need in a vacuum for hold-down purposes varies depending on your CNC table setup. November 26, 2007

Question
We have a 6x10 table with a grid vacuum top. We normally cut sheets that are 4x8 in 1/8", 1/4", 1/2" and 3/4", so we only have 2 ports open and the area sealed (other times 4x10 sheets and 3 ports open). We have a 5hp Becker pump now and it is just not getting it done at all. What size pump should I have? We would like to cut some smaller items also. Are there good places to buy used or reconditioned pumps? I wouldn't mind trading mine in on another if possible.

Forum Responses
(CNC Forum)
From contributor T:
Rule of thumb is 25 HP for 4 x 8, and 40 HP for 5 x 10 or 5 x 12.



From contributor J:
If your 5 HP Becker used to get the job done, then it may need servicing - the carbon blades wear over time. Otherwise, I would agree with contributor T, you need a bigger pump. If your machine is set up for more than one pump, you may be able to add one to the mix. I would recommend a Traviani or Dekker liquid ring for a flat table CNC.


From contributor A:
Try Ex-Factory or MLS. Plenty of used and new pumps to choose from.


From the original questioner:
Thanks. The Becker pump is almost unused, as the table is; it is just not big enough. I've looked a little on the Ex-Factory site, but am not really sure what I'm looking for. (I'll check out the 2 suggested.) Can I add another pump? And how would this work?


From contributor B:
There are 2 kinds of vacuum pump systems: rotary vane true vacuum and regenerative blower. If you have a 5hp pump, I suspect it's rotary vane, which is a "true" vacuum that pulls a high level of mercury, but with low volume air flow.

If you are doing panel holding with a bleed board, this is usually not the type of pump you want. If you are holding parts on your grid with gasketing for specific holding points, then the 5hp should be enough holding power unless you are getting leaking.

So, are you using a bleed board? That info will help a lot in getting you the right answer. Or are you trying to set up a separate system from your bleed board hold down with a larger regenerative blower?



From the original questioner:
Yes, I am using a bleed board (spoil board). Before I bought the router it was only used to hold smaller parts on the grid, hence the small pump, I guess. We would like to cut everything using a bleed board.


From contributor B:
In that case, I have to second contributor T's general rule of thumb. He is referring to regenerative blowers versus rotary vane true vacuum.


From contributor J:
Have you gasketed the table between the zones that the ports control? Do you cover the open areas of the spoilboard? Are you using regular density MDF for the spoilboard? (Lightweight MDF is too porous and allows too much airflow.)

As far as pump size, you may be able to get by with a 25HP liquid ring pump, if you gasket and control the unused zoned. Otherwise, get a 40 hp pump. One other thing to look at is the size of the main vacuum hose from the CNC to the pump. I like to see 4" diameter.



From contributor E:
Hmm, we use lightweight MDF because we got better hold down than with regular MDF. Different ships, different long splices, I guess!

We run a 12' by 5' bleeder board setup on a 15 hp regenerative blower. We mask off unused areas with scrap laminate and shut off unused zones. All plumbing and valves 4" and use a large vacuum filter. Small parts are tabbed into full sheets, but anything larger than 2 square feet holds well. Of course our feed speeds run in the 480 in/min range. On a few jobs we have used vacuum mask to improve hold down, but that's pretty rare.

Do I wish I had a bigger pump? You bet! But we are about maxed out on electrical service in our current location, so until we get our new shop built, I'm making this work for us. Make sure you have the electrical service to feed a large pump. Make sure you have a motor starter circuit installed by a knowledgeable electrician as well, or besides burned up equipment, you might face a code violation!



From contributor G:
The use of bleeder boards rather than dedicated spoil boards over the last 10 years has brought on some interesting challenges. When 3-5hp vacuum pumps were just fine, I now see 40+hp pumps to be the norm. It makes sense to run your job and then mill off .020" to run the next job. This was eminent because software pushed using full 4 x 8 or 5 x 10 sheets and having everything nested. No one would want to hang onto such large dedicated spoilboards. It would take up too much floor space.

That said, maybe it's time to back up a little on this one and throw a 1/2" MDF board onto your machine, carve in vacuum tracks, and drill holes through to your table. Throw some gasketing or weather stripping to seal each part off and your worries are over. Until you are able to get that 40 hp vacuum pump, this may be an inexpensive alternative.



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