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Shop Built CNC Through Vac Hold Down

Listing #87   Listed on: 10/08/2012

Attached are a few photos of my new bleed board vacuum hold down system on our CNC.

I purchased this CNC router without a through vac hold down because 99% of what we cut is pod held solid wood parts. We screw these pods down to the MDF full table surface on the router. This is a fast and efficient system for cutting our hardwood parts.

Now and then though we need to cut a some panel stock. In the past we've just located hold down screws or plastic nails strategically to hold the sheets. This new hold down system will eliminate the need to do that.

The hold down panels are comprised of four individual 24" x 48" units. These are made of 3/4" MDF machined as can be seen in the first photo (there is also a back relief to maintain even front/back stress on the panels). The MDF is painted with two coats of primer and two topcoats. I considered using phenolic for these units but thought the MDF would be lighter and easier to install and remove as needed.

The vacuum power is supplied by 4 vac motors purchased from Central Vacuum Motor ( LH-7123-13. They are plumbed together as a single unit in my installation but could be isolated as individual vac motors for each of the four table sections.

I've done a number of projects with this system now and it's working as one would expect from a bleed board vac system.

Thanks to Jim McGrew for sharing photos of his similarly designed hold down system.

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Shop Built CNC Through Vac Hold Down

Viewer Comments:

Posted By: Dropout     [10/08/2012]

How many inches of vacuum are you pulling?

Posted By: B.H. Davis     [10/08/2012]

To be honest I don't know. Since it's through vac all I really paid attention to was if the parts stayed in place after being cut free from the sheets.

On our rotary vane pump true vac pod system I have the pump set to cycle at 24". It can be pushed up to 26" but needs to run noticeably longer to hold that reading. The lower setting of 24" holds the curves on the pods just fine so there's no need to push it higher.

Next time I set it up I'll try and remember to put a vac gauge on the system to get a reading.

BH Davis

Posted By: Ed     [10/09/2012]

Please excuse my ignorance!
I love your setup & I want to adapt it to a CNC mill. What I can't figure out is once the bit penetrates your 4x8 sheet isn't the draw of the vacuum significantly diminished and therefore considerably less holding power? And once the bit cuts into the MDF however slight what happens to the holding power of the vacuum for the next part of a different shape. I don't know if your system would work on what we're doing but I would love if it did. Attached is a picture of some of the crazy stuff we do. Any insight on vacuum hold down on these kind of parts is appreciated.

Posted By: Tim Schultz     [10/19/2012]


That's nice looking work.
I think I would use a $20 bill to provide a sense of scale.

Posted By: Ed     [10/19/2012]

Thanks, actually on those parts $100.00 wouldn't begin to cover it.

Posted By: Mike     [11/13/2012]

I have a pair of the 110v lighthouse motors connected to a 4x8 plenum, with 5 zones. These are basically the same motor BH Davis is using. I have then separated with PVC swing check valves so I can run one or the other, or swap between the two when running for extended periods.

What makes the central vacuum motors work is that they will pull a lot of CFM especially when used in a parallel configuration, while maintaining just enough vacuum to hold parts. You won't have 15-20" of vacuum but you will have plenty of CFM. This is what enables you to cut through in comparison to a smaller pump.

The inherent trade off is that you are not going to cut a sheet full of small parts in a single pass, at 25 IPS .

The key is that you will need to step down to an onion skin of about .05 before cutting completely through the part you can easily hold a full sheet of small to mid sized parts at moderate cut speeds. With 4 motors, you probably won't have to worry. I use two, and get by just fine.

You will also want to use ultralight MDF as a bleeder board (Trupan is the best).

The bleeder board should be glued to the plenum. A dry laid spoil bored will work with a huge blower, but costs much CFM.

Efficient plumbing minimizing elbows is critical. Each elbow according to some will cost you about 10% of your total CFM.

2 motors work surprisingly well, and 4 will hold whatever you want within reason.

When using 2 or more motors, they should all pull through a common manifold, rather than hooking each motor to its own table zone as some have done in the past. This results in greater CFM.

With my system the working vacuum ranges from about 7-9.5" H20 When my spoil board gets trashed up the low end drops to about 6". If you are careful and cut slow, you can get away with 4-5" of vacuum, but I have never had my gauge drop that low.

The ShopBot style motors are not a big iron solution. For a small shop that lacks 3 phase power, or the funds to purchase a 15-20hp blower they can be a real game changer.

When I bought my CNC, I was told universal vacuum and nested cutting was impossible.

I have been surprised as to what I have been able to hold down. I almost always cut completely through by an additional .015 fully releasing parts, unless I am cutting something exceptionally small (under 4"x6" )
Then I might leave a slight skin, or a few tabs.

If I am cutting a few really small parts I have a 24"x24" zone at the front of the table.

Posted By: B.H. Davis     [11/14/2012]


We are doing basically the same thing as you. The only real difference I see is that I've run a 1/4" closed cell foam gasket around the perimeter of my milled grid panels. This eliminates the need to glue down the spoil board. We do shoot a brass screw into each corner of the spoil board so it can't slide around and will stay with the grid when removed from the CNC.

BH Davis

Posted By: Mike     [11/15/2012]

I would like to add a correction to my previous post. I mistakenly referred to 7-9" H20, or water column. 7-9" Mercury or HG, would be the correct unit of measurement.
There is a big difference.

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