10 Tips for Clamping Success

Expert advice for great results with your woodworking projects every time.

Because clamping seems self-explanatory, many DIYers unwittingly wind up creating out-of-square cabinets, out-of-plane frames, twisted panels and weak glue joints. You can avoid these disappointments if you use proper clamping techniques. Here are 10 tricks for using clamps and avoiding sticky situations.

1. Fit before you glue
Here's a scenario you may have experienced: You're eager to assemble your project, so as soon as you've cut the parts, you apply glue and begin to clamp. Then you discover (too late) that the dowel holes are too shallow and the joints won't close tightly. You can avoid this disaster with a dry assembly.

A dry assembly offers several benefits. First, it verifies that the parts fit together as intended. Second, it allows you to organize the clamps and parts for speedy gluing. Third, it gives you the opportunity to reduce a single complicated assembly into a series of more manageable subassemblies. Finally, it helps you determine whether you'll need an assistant to complete the project. Don't skip this step, it'll save you time and frustration in the long run.

2. Distribute sufficient pressure
For maximum joint strength, clamps must draw mating surfaces to within 2/1000 in. The best way to accomplish this is to start with clean, flat surfaces. Take the time to make good joints. This sounds obvious but is often overlooked. Don't assume that your clamps can fix every irregularity.

Choose clamps that apply sufficient pressure to close the joint and squeeze out surplus glue. The better the joint fits together, the less pressure is required. If you are gluing two or three seams at a time, double or triple the amount of pressure that's required. Screw clamps work best for glue joints. One-hand squeeze-style clamps are useful for many shop operations, but they deliver less pressure than their screw driven counterparts.

Pressure should be distributed as evenly as possible across the entire joint. A clamp will deliver its maximum force in a line between the clamp pads. The pressure the clamp provides diminishes on either side of this line. You can distribute the pressure by using several clamps close together or by using cauls, which are additional boards placed between the workpieces and the clamps (photo below).

Clamping force is narrow and concentrated near the pads. It becomes weaker and more diffuse farther from the pad.

3. Keep panels flat
Pipe and bar clamp pads sometimes wiggle and flex under pressure, arching the panel. One method of clamping panels flat is to elevate the panel so that the clamp's screw is in line with the panel and the center of the opposite clamp pad (photo A). This way, the wiggling clamp pads impart no sideways thrust.

Prevent the panel from arching by using spacers to align the panel with the clamp screw.

A second method used by many woodworkers is to clamp pairs of battens - straight strips of wood used to keep assemblies flat (photo B). Remember to put wax paper or packing tape on the battens so they don't stick to the panel.

Clamp battens on both faces of the panel to hold it flat during clamping. Apply packing tape to the edges of the battens to prevent them from sticking to the panel.

A third method of counteracting this force is to place clamps on both sides of the panel with the boards to be glued directly on the clamps' bars (photo C). The contact with the bars as well as the opposing forces of the top and bottom clamps help keep the panels flat. Place wax paper between the bars and the workpieces to prevent staining.

Place clamps on both sides of a panel to help prevent sloping clamp pads from arching the panel.

To avoid the wiggling pad problem altogether, use bar clamps that feature clamp pads specially designed to stay parallel under pressure (photo D).

These K-Body clamps from Bessey feature parallel pads that will not flex or twist under pressure.

5. Extend your reach
When your clamps are too short for a project, link two pipe clamps by hooking their lower clamp pads together (photo below). Pipe clamps are especially well suited for this application because their clamp pads can rotate 90 degrees to engage one another.

Extend your clamping capacity by linking two pipe clamps or building an extender out of scrap wood.

This technique does have three potential pitfalls. First, because the line of force exerted by the clamps will not run through the panel's centerline, the panel may bow. Second, the linked clamp pads may mar the surface of the panel, so slip a square of cardboard under them. And third, the worn pads can slip. Make sure they're in good condition and make good contact to avoid slipping under pressure.

5. Make assemblies square
To ensure a square assembly, measure the length of the diagonals. If the measurements are not identical, your assembly is not square, a problem known as racking. There are two easy solutions: You can try repositioning the clamps (photo A), or you can place another clamp across the assembly's longer diagonal and draw it back into agreement (photos B and C).

To maneuver the case so it is square, shift the clamp off the centerline of the assembly.

Check for square by measuring the diagonals.

Make the assembly square by clamping across the longer diagonal.

6. Extend the reach of C-clamps
Sometimes a C-clamp cannot reach the area you need to clamp because the throat is too shallow. Extend your clamp's reach by creating a bridge (photo below). Tighten the C-clamp down on the bridge piece positioned between the edge of the panel and the point to receive pressure. The penalty you will pay for this increase in reach is a reduction in the pressure that is delivered.

Extend a C-clamp's reach by bridging the gap with a scrap of wood. The clamping force will diminish the farther it extends away from the C-clamp, but the pressure will still be considerable.

7. Distribute pressure across long joints
When clamping a deep cabinet together, how do you draw the sides tight against the shelves? Clamps easily draw the front and back of the joint closed, but what about the middle? Even if you use a stout straight board as a caul, the pressure on the front and back edges makes the center area of the joint spring open.

The solution is to crown the face of the caul with a hand plane or sander. The caul's center crown will close the middle region of the joint first. Then as the clamp pressure increases, the caul will flex until the front and back areas are tightly closed as well (photo below).

Crowned cauls bow under pressure, distributing clamping force along the entire joint, starting at the middle.

8. Reach the center
Sometimes you need to apply pressure to the center of a surface that standard clamps can't reach. In this situation I place three cauls on top of a particleboard panel. The center caul stands 1/8 in. taller than the cauls on either side of it. I then clamp straight hardwood cauls across the spacer boards. As the clamp pressure increases, the top cauls flex until they grip each of the bottom cauls, and the clamps impart uniform pressure on the center area of the surface (photo below).

Hardwood cauls flex across the taller center batten to reach the outside battens. The force then transfers to a piece of particleboard that acts as a caul over a large surface.

9. Shape cauls for shapely edges
Clamps cannot firmly grip on a curved or angled edge. So how can you apply adequate clamp pressure when assembling these parts? The best way is to make a caul that matches the curved or angled edge. Trace the edge profile on a piece of plywood. Saw along the line so the plywood caul fits snugly against the edge of the part. The other edge of the plywood should be sawn straight and parallel to the opposing clamping edge (photo below).

Cut cauls to match the profile of the part to be clamped - in this case a table leg.

10. Clean up excess glue
Even the best assembly can be ruined if stain reveals dried glue residue. More is not necessarily better when it comes to the amount of glue you apply to a joint. A little squeeze-out is to be expected, but a lot only leads to a mess, not a stronger joint. Apply just enough to lightly coat the mating surfaces.

Wiping excess wet glue with a dry rag forces it into the grain, sealing the wood and preventing it from receiving most stains. A better method is to wait for the glue to partially dry or form a skin and then shave off the glue with a chisel (photo below). If the glue gush-out dries in place, it can be removed with a sharp chisel, a scraper and sandpaper.

Use an old chisel to shave off partially dry excess glue. Work with the beveled edge down to avoid marring the wood.

Even though you may never have enough clamps, getting the best performance from those you own requires the same care and precision as making joints. Don't undermine your projects by discounting the importance of clamping. As with any other woodworking step, taking your time and proceeding methodically will help you achieve optimal results.