Keep vacuum valve open Close valve to to hold fence fixture to table. release workpiece.
38 AMERICAN WOODWORKER ▲ AUGUST 1995
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Easy-on, easy-off. A vacuum template lets you pattern-rout with a flush-trim bit without marring your workpiece with fasteners.
adequate suction for many applications. In theory, a perfect vacuum at sea level will allow about 15 psi atmospheric pressure on the workpiece. In practice, however, depending on material porosity, the tightness of your fittings and seals, and your pump's efficiency, you'll probably end up with about half that pressure. That's still about a half-ton of pressure per square ft.—plenty for most woodworking applications.
A couple of small vacuum plates strategically spaced will hold a large workpiece adequately; you don't need a massive plate. To hold small work, you can create a small chamber on a larger plate by sealing off one section with tape. (Sec photo, page 37.)
Make your clamping plates from any reasonably nonporous, flat material. Panels with a plastic-laminate or impregnated-resin face work well, or you can use sealed, close-grained hardwood or plywood. I prefer plastics such as high-density polyethylene or polycarbonate. Although these materials are relatively expensive, they don't leak air. (Sec Sources, or check your telephone book for plastic suppliers.)
Drill air evacuation holes in a plate where the hose fittings won't interfere with the work or tool. To evacuate air through the edges of plates, you'll need to drill two holes that meet each other. (See Fig. 1.) The hole placement isn't critical, as long as it reaches inside the chamber.
To create a vacuum chamber, use adhesive-backed, closed-cell foam tape. Weather-stripping tape (available at hardware stores) will work, but it tends to lose its resilience rather quickly. I rec ommend the more durable foam tape designed for vacuum clamping. (See Sources.) You can use l/8-in.-thick foam tape if your clamping plates are fairly small and flat. But if they're large, you may want to use V4-in.-thick tape, which bridges warps and irregularities better. Don't use tape thicker than l/4 in. It allows too much lateral movement between the plate and the workpiece.
Clamping plates with vacuum chambers on both sides can secure workpieces to a bench or tool surface. Place the plate under a workpiece to hold it for planing, sanding or routing. To support workpiece overhang, use shims that equal the thickness of the plates and the compressed tape. If you recess a plate into your router table or benchtop, you won't need support shims or a vacuum chamber on the plate bottom. You can connect plates with metal angles or other hardware to create special-purpose fixtures for jobs such as holding work for edge planing. The fixtures shown in the lead photo on page 36 will clamp work together at right angles for assembly with screws or other mechanical fasteners.
You can shape plates for use as vacuum templates. They're handy for duplicate routing with a flush-trim router bit. (See photo, above.) And you can make jigs for straightedge routing, circle cutting, shelf-hole boring and other applications. (See Fig. 2.)
Any rubber, vinyl or plastic hose will work for vacuum clamping as long as it doesn't collapse under vacuum. I prefer a Vfc-in.-i.d., l/4-in.-o.d. hose because it matches barbed fittings and push-pull connectors. Push-pull connectors let you switch fixtures easily. (See Sources.)
Fittings are relatively inexpensive, and many different types are available at hardware and auto supply stores or from mail-order suppliers. Basically, anything that provides an airtight seal between the vacuum chamber and the hose is satisfactory. I like fittings that have barbs on one end to slide inside a hose and threads on the other end to screw into
undersized holes on the plate. (See Fig. 1.) Fve also used metal or rigid plastic tubing for fittings, sealing the tubing into the plate holes with epoxy or silicone. You can buy swivel fittings that prevent hoses from twisting during template routing.
The more vacuum clamping you do, the more uses you'll find for it. But there's one potential danger when vacuum clamping. If you lose vacuum when working, you may lose control of your workpiece or tool. Be sure your hoses are out of harm's way during use. ▲
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