Vacuum Capacity, Spoilboards, and Hold-down Effectiveness

      A nested CNC owner wonders if his vacuum pump has enough horsepower. Others chime in with observations about spoilboards, gaskets, and assessing vacuum pump performance. February 13, 2006

Question
Does anyone have experience with vacuum hold down? I would like to know how many HP I need to have for a 4 x 8 table for cabinetmaking and small parts. I have ordered a CNC router with 10HP Becker dry vacuum pump with 172 cfm, and I need to know if this is enough?

Forum Responses
(CNC Forum)
From contributor A:
That doesn't sound like enough vacuum. I have a 5 x 10 table that I generally gasket off down to 4 x 8 and use a 20hp pump and I rarely loose a part.



From contributor B:
We had a 4 x 8 Andi with a 10hp Becker: not enough for many applications! We ended up onion-skinning a lot of parts or creating gasketed spoil boards. Neither of these was a particularly good solution. If you are doing large parts it will work, but smaller parts, especially at higher feeds, will shift. You can keep the small Becker and get a second that you can run in tandem when doing small parts. We are now running a Komo 5 x 10 nested with a 40hp Quincy. This works very well, but even then when we get down to very small parts we onionskin. We can also use a spoil board longer before resurfacing with the big pump. One drawback is that it eats a fair amount of electricity.


From contributor C:
We run a Komo as well, with a 40HP Quincy and I have very little trouble with hold down. We onion-skin our small parts as well and rarely lose any of our parts off of the 5 x 10 table. We also have certain dedicated vacuum boards for different applications, and gasket down to 4 x 8 and 4 x 10.


From contributor D:
We have a 4 x 9 vacuum pod and rail table. For small parts we use a larger spoilboard that sticks well to 4 or more pods and then basically jig the smaller parts in place. It's crude but works great.


From contributor E:
Vacuum is about volume - the more air you can move through your spoil board the better your hold down. We use two Becker 12hp, with 2" hoses. This translates to about 600 meters of air at the table.


From contributor F:
We have a 4 x 8 table and an 18hp Becker at 250cfm. I personally would like a bigger pump, but have learned to make do. I onion skin anything smaller than 10" x 10" or so, and cut the second pass right away. Adds a few seconds per part, but that's OK with me. My control also has an option to cut small parts first. This adds a little more time as the small parts are not always nested together. Using both selective onion skin and small parts first, I rarely have part movement.


From contributor G:
To better understand if the pump is the correct size you need to take a measurement. Locate the vacuum level gauge on the machine and note the reading with a newly machined spoilboard and new sheet, as well as the reading after machining.You want a reading of 25-29 inches of mercury - this is the measurement of the vacuum efficiency.

Are you properly machining the spoilboard during the day? Do you have fresh gasket in the grid? Finally, consider edgebanding or sealing the edges of the spoilboard. We have many customers running a 4' x 8' table with a single Becker pump without onion skinning and with no part movement. If all of the elements are not correct, throwing more HP will not fix the problem.



From contributor D:
One more thing - if your fly cutter is dull it will polish your spoilboard. This will seal the surface. I've seen it more than once.


From contributor F:
Our Becker pump is only rated at 22.5". How can I get 25-29 out of it? I have to admit that most of my part movement is from over-used spoilboard. Don't the before and after readings depend on what kind of material you're cutting? 1/4" raw mdf vs prelam? And the total cut length 2 to 3 big parts vs 40-50 small parts with holes in them? I guess I don't see how that's an accurate gauge of sufficient pump size. Wouldn't throwing more CFM fix the problem? You didn't say what size pump your customers are running without onion skinning.


From contributor E:
I am a technician for one of the leading manufacturers. I have used all the pumps, and all the methods you can think of. As I stated before, its volume (CFM) that will make the most difference when nesting. Pods do not require as much because they maintain an unbroken seal at all times, whereas with nesting you will begin losing your vacuum as soon as you break through the material. I have found that you should be making the differential between an empty spoil board and sheet stock loaded as far apart as possible. You should not be using most of your vacuum to hold the spoil board, with very little to hold the material. I have found that using a 1/4" mdf is best.


From contributor I:
I've been using 1/4" expanded pvc for almost all of my applications. There is 0% leakage so all the vacuum is directed to your part. You can top that by gasketing each part and you'll eliminate losing vacuum. The drawback of the price of the spoil may be offset in your savings on costs - $45.00/sheet.


From contributor E:
I have had no problems with $6.00 1/4" MDF planed both sides, I have had problems with thicker MDF not being able to flow enough air.


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