'Tools: Working Wood in Eighteenth-Century America" runs through September 4, 1995 at The DeWitt Wallace Decorative Arts Gallery, Colonial Williamsburg, VA. The exhibit features more than 1,500 woodworking hand tools from the 1700s and 1800s. For more information, call: (804) 220-7724.
Using Air Pressure to Hold Your Work by Patrick Spielman
One-sided clamping. Spielman planes a board held by a pair of right-angle vacuum-clamping fixtures. Suction holds the fixtures to the lyench and to the workpiece. The small venturi vacuum pump on the bench is driven by compressed air.
If there is one operation common to most woodworking projects, it's clamping. Hand-planing, routing, sawing, carving and drilling all usually involve the use of vises or clamps to hold the work. Unfortunately, clamps often get in the way of tools. They also won't reach to the centers of wide work-pieces, requiring mechanical fasteners to attach jigs or patterns there.
Vacuum clamping offers an alternative. It will secure flat work without mechanical clamps or fasteners. Workpieces are held from one side-only, presenting the other side for unobstructed tooling. Vacuum-held jigs and patterns for circle cutting and template routing don't require piercing the workpiece with screws or nails.
In vacuum clamping, you create a shallow vacuum chamber on one or both faces of a flat clamping "plate" by applying foam tape around the perimeter of the face. The plate is placed against another flat surface (a work-piece, workbench, or machine surface) and air is evacuated via a hose from the chamber, anchoring the piece by suction. (See Fig. I.) You can combine plates to make specialty clamping fixtures, or you can shape the plates to create templates and jigs.
To set up a vacuum-clamping system, you'll need a vacuum pump, clamping plates, foam tape, hose and some simple fittings. You can put together an inexpensive, low-tech system using a scavenged pump and parts from the hard ware store or you can buy a complete commercially made system. (See Sources, page 39.) Either way, once you find out how easy and versatile vacuum clamping is, you'll be inventing new fixtures for all sorts of woodworking operations.
A variety of vacuum pumps are available but there are two basic types to choose from: electric and venturi. The vacuum flow of both types is rated in cfm (cubic ft. per minute). Most small-shop vacuum-clamping applications will require only 1 or 2 cfm of vacuum flow.
Electric pumps are dedicated vacuum units. (See photo, opposite page.) These can cost upwards of several hundred dollars depending on their cfm rating. They
FIG. 1: HOW VACUUM CLAMPING WORKS
A used refrigerator pump can provide effective vacuum for intermittent clamping jobs such as template routing and assembly work. (See photo, above.) These little units are inexpensive and readily available.
Caution: Don't try to remove a refrigerator pump yourself. Pre-1950 refrigerator pumps can contain sulfur dioxide, and newer refrigerators contain CFCs (chlorofluorocarbons). Both compounds are toxic, and it's illegal to release CFCs into the atmosphere. Buy your pump from a used appliance dealer who can remove it safely and legally.
Another option is an old milking machine. (See photo, below.) Many of these are obsolete and sit unused in the sheds of small dairy farms. They're somewhat inefficient for prolonged use but provide plenty of cfm for clamping large work. —PS.
Clamping 'til the cows come home.
Milking machines salvaged from dairies provide great vacuum for clamping large workpieces.
run quietly and generally have higher cfm ratings than venturi pumps. As a cheap alternative, you can use an electric pump scavenged from an old refrigerator or milking machine. (See sidebar, right.)
Venturi pumps are good options if you own an air compressor. (See lead photo, opposite page.) These small pumps work by shooting compressed air past an orifice inside the unit. The vacuum is created as air is sucked through the orifice toward the stream of compressed air, in much the same way as leaves are sucked into the wake of a passing car.
Venturi pumps are available with various features. Vacuum gauges and air regulators let you control and monitor your clamping pressure. Manifolds with multiple vacuum ports provide hookups for more than one hose. Venturi pumps do hiss loudly in use, but they're less expensive than electric pumps. Depending on features and cfm rating, these units can range from $28 to hundreds of dollars. Most 1/2-HP compressors will provide the minimum 1 to 2 cfm at 60 to 80 psi (lbs. per square in.) required to operate a venturi pump effectively.
Both electric and venturi pumps will also serve for vacuum veneering, but the 1- to 2-cfm units will take a while to evacuate large bags. If you're planning on vacuum veneering as well as clamping, consider investing in a 5-cfm pump. (See AW #44 for more information on vacuum veneering and pumps.)
Making Clamping Plates and Fixtures
Clamping plates don't have to be large to work well. A 6-in. by 6-in. plate and 1-cfm vacuum flow will provide
Downsizing. To hold small workpieces, tape off a section of a larger clamping plate. The hose at right evacuates the chamber on the underside of the plate. Electric pumps, such as the one shown here, don't require a compressor.
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