Jigmaking Basics

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Making vacuum-clamping jigs involves woodworking familiars like solid wood, plywood, and plastic laminate, and some acquaintances like acrylic, polycarbonate, and phc-nolic plastics. But strangers must be involved, too: vinyl hose, barbed hose connectors and related fittings, and special vacuum tape.

Don't worry too much about the unfamilar supplies you need. When you buy a vacuum system, you'll undoubtedly gel a roll of vacuum tape, a small assortment of fittings, and a length of suitable hose. After you've made a few jigs, you can buy more tape from the original vendor. Fittings and hose you can probably buy for less money at a local hardware or auto pans store. Just match the stuff you got in the original purchase.

Hie familiars and acquaintances are used for the jig bases. This is where the issue of porosity comes up. To create a vacuum, you suck all the air out of the vacuum chamber. To maintain the vacuum, you do two tilings: one, prevent air from seeping back in. and two,continue to draw air out. To prevent air from seeping back into the vacuum chamber, the chamber's pans must be nonporous. Thus,die l>est materials to use for those pans of a clamping jig that form die vacuum chamber are plastics—acrylic, polycarbonate, phenolic, even plastic laminate. The worst arc man-made wood products like particlcboard and medium-density fiberboard (MDF). In between are solid wood and plywood.

Unprompted, you may not think of wood as being particularly porous; I certainly don't. You know, solid wood! But consider wood's structure, and then, well, it does make sense that it would be porous. So when you think of a wood structure as a vacuum vessel, as a barrier keeping air out of the vessel, you can see that air leakage has to lie expected.

'lTiis doesn't mean you can't use wood; it just means that it isn't ideal. You should seal it with several coats of a film finish, like shellac, polyurethanc, or paint, or cover it with plastic laminate. Plywood should be void-free. Apple Ply and Baltic Birch plywood are very good, though lesser plywoods must be sealed carefully. Be especially vigilant about filling voids in the edges. If you use paniclcboard, use a melamine-covered variety.

A final note on the base material: For a vacuum jig. you

Seal wooden vacuum jigs with shellac, poly-urethane, or some other film finish. Thus sealed, jigs made of solid wood and man-made wood products, which all tend to be porous to some degree, will be less likely to leak air. Apply two or three coats to be sure the jig is sealed.

need a flat base. Solid wood docs come and go with changes in temperature and humidity. A loose lx>ard may cup or warp. For this reason, you should favor plywood or laminate-covered MDF or particleboard for a wooden base.

The capacity of the vacuum valve also plays a role here Phase 2 of maintaining a vacuum, after all. is continuing to draw out air. If your valve moves a lot of air—and thus pulls a lot of vacuum—you can succeed with somewhat porous materials. To make best use of a small valve, though, you need to l>e rigorous about using nonporous materials in making your jigs.

In addition, the workpicce is a cast member. It forms half of the vacuum chamber, remember. You don't want to seal it.of course; so the high-volume valve is a plus when you are working MDF and other man-made materials.

To form the recess that, when the workpicce is laid over it, becomes the vacuum chamber, you outline an area of die base using vacuum tape. This is a high-density relative of closed-cell foam weather stripping, with which you maybe familiar. Typically, vacuum tape is % inch thick and !•» inch wide. It has pressure-sensitive adhesive (PSA) on one side.The '^inch-thick tape is the best all-around and is the

VACUUM PORT ALTERNATIVES

Bore a hole through the face of the base.

RICHT-ANCLE PORT Quick-disconnect fitting, male ^^^^^J/acuumtape

Bore a stopped hole into the base's edge. Bore a hole into the base's face, just deep enough to intersect the first hole.

T-PORT

Bore a hole through the face of the base. Bore a hole into the edge, drilling just deep enough to intersect the through hole.

standard tape supplied with most vacuum-system packages. You may find tape that's ' » inch thick; some woodworkers prefer this tape for large work, since it bridges minor warps and irregularities better. But steer clear of any thicker tape, since it can allow lateral movement between the clamping jig and the workpicce.

I mentioned that vacuum tape is similar to closed-cell foam weather stripping. You can use the weather stripping. but it isn't as resilient as vacuum tape, and it won't be as long-lasting. After it has been vacuum-crushed several times, it stays crushed and doesn't rebound when the pressure is removed. So employ it for limited-use templates but not for clamping plates and other jigs that will see a lot of use.

To apply the tape, you simply peel the waxy paper off the PSA backing and apply the tape to the (sealed) base. Obviously, the tape must completely encircle the vacuum area, and any joints between pieces of tape must be butted just as lightly as possible. You'll see. in looking at the jigs shown in the next few pages, thai I try to encircle the vacuum area with a single, continuous strip, so I have only one such joint. It isn't always possible, of course. Moreover, you'll see a couple of instances where the total area is partitioned into /ones, with breaks in the partition strips. A strip of the tape is positioned to serve as a vac-gate; you close the gate lo limit the area of the vacuum, open the gate to extend it.

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