I have been vacuum veneering for years now and have several vacuum compressors. I would like to use them for hold downs and wondering how some of you have done this and what kind of fixturing did you build that worked. Any other ideas for use with vacuums?

Pretty much anything involving work holding can be done with vacuum. I even use vacuum for clamping a fence to the middle of my workbench (pneumatic fence on the edge). Boards (pic is corian) with seals on both sides are handy for all sorts of tasks. In simple applications 1/8-3/16" closed cell foam tape works fine.

For CNC router work holding I use 3mm round EPDM in groves. Two pieces of 1x 1/4" melamine (with routed vacuum channels) glued together works well - 2x 1/4" for second level clamping (pic).

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Vacuum clamping is great for non-marring work holding especially for edge routing and the like. I have several simple fixtures made of a single layer of mdf, banded with p-lam and sealed to the bench and workpiece with 1/8" or 1/4" foam tape bedded in grooves around the perimeter of each face. A hole drilled in the edge and lined with a short section of vac hose to accept the vacuum hose nipple intersects with a hole drilled through the faces. For larger fixtures routing a grid in the faces is a good idea .

I tried using a similar fixture on my cnc spoilboard but could not get it to pull down on the ultralite mdf surface, so I put a flange under the fixture, screw it down to the spoilboard, and pull vacuum on the workpiece only. This is good for pieces too small to hold with the low suction generated by my home-made vacuum table.

Vacuum passes pretty freely through unsealed MDF, some folks claim it's enough to provide a suitable gasketless clamping surface. For airtight gasketed fixtures, coated MDF minimizes the amount of (PVA) sealing I have to do.

All my CNC router fixtures are mounted via m5 inserts in the spoilboard. While I haven't noticed a difference, even small part holding could benefit from topside channels.

To overcome the problem of holding large porous sheets, it isn't a problem of the pump, but the limitations of the air hose size. The math is simple in that each doubling of diameter, is a quadrupling of the area, so maybe take a look at plumbing.

A few years ago I had some pretty large template routing to do. Almost all shop vacuums pull -2psi, so over a large area that adds up pretty quickly. For instance, a square foot = 144" x 2 = 288#, and the simple fact that a 2" hose allows for some significant leaks without causing much of a drop.

I'm a small shop, and don't have a CNC router, so I'm not comparing a shop vac to the big guns some of you have.

Good shop vacs draw ~150cfm and may be able pull ~7 in Hg (if I did the 100 inch water column conversion correctly). My (tiny by industrial standards) veneering/clamping pump draws 3.5cfm and can pull 25 in Hg.

While clamping at ~7 in Hg may be enough in some applications, I believe the standard for veneering and clamping is ~20 in Hg.

The CFM needed depends on leakage and how fast the air needs to be evacuated. Note: Pump CFM ratings are at zero vacuum, e.g. 1.24 @ 0 down to .16 @ 20 for my tiniest pump (why it only clamps good when there's zero leakage).

There's a balance among the force needed for clamping vs the force exerted by machining, the vacuum level (often expressed in in. hg) and volume pulled by the vacuum source, and the area subject to vacuum and its porosity.

With a small pump pulling a moderately high vacuum like my Gast rotary vane pump at 24" hg you can hold a small part (<.5 sq. ft) securely for routing as long as it is not porous and has no holes. With my home-made cnc vacuum table using central vacuum motors pulling only about 7" hg pieces that small would break loose under typical cutting pressure, and even large warped pieces may not be pulled flat to the table. A melamine coated panel will be held down better than a piece of ultralite mdf. More power is better, but a low volume or low vacuum level can be adequate depending on the situation.

I would love to see some pictures or diagrams of how you made the fixtures you use. i love this site for all the sharing that we get to do. I think everybody benefits when we collaborate on ideas. This is what gives this trade a future in my opinion.