A reader recently posed the following question regarding clamping a glue-up joint with woodworking clamps: "Just how much pressure is enough when gluing wood together? I have attempted to find this answer through reading various articles on gluing wood but none seem to address how much pressure to apply to a glue joint. Is there a rule-of-thumb that can be used? I look forward to hearing from you with some sage advice on just how much clamp pressure to use."
The obvious reason for using clamps to hold together a glue-up is to keep the boards in place while the glue dries, but there's a little more to it than just keeping the boards aligned. Ideally, you'd like just enough pressure to hold the boards securely in place along the entirety of the mated surfaces, but not so much pressure that you squeeze most of the glue out of the joint.
Types of Clamps
With some types of clamps, regulating this pressure can be a little more difficult than with other types of clamps. For instance, a number of spring clamps on a joint may hold the joint securely, but because you cannot adjust the pressure on the clamp, you can't directly determine the amount of clamping action applied to the joint. For this reason, reserve spring clamps for glue-ups where other clamps simply can't be utilized.
A better choice for most joints is a bar-style clamp that allows for adjustable pressure. A single-handed grip-style clamp is great for smaller joints, whereas a longer adjustable bar clamp is ideal for holding panel glue-ups and larger joints.
When applying glue to the joint, you'll want to be sure that you have an even application of glue along the entire face of the joint. A glue roller or a small brush are both great ways to ensure that the glue you apply to the joint is evenly spread across one of the mating surfaces before they are aligned and clamped. Keep in mind that some glues may require you to apply glue to both surfaces and allow the glue to set before clamping. Read and follow the instructions on the glue to understand the required application method.
When positioning the clamps onto the joint, you may need to alternate the placement of the clamps to allow one clamp to counteract the pulling of the neighboring clamp, to keep the joint from twisting. For instance, for a panel glue-up, you might want the first clamp on the underneath side of the panel, with the next clamp on top, continuing the alternating clamp positioning until you have covered the entire length of the joint. By tightening the alternating clamps evenly, you should be able to apply even pressure to the joint.
When applying pressure to the joint, as noted above, you'll want to apply just enough pressure to align the mating surfaces evenly and completely, just enough to keep the glue from squeezing out of the joint (provided of course, that you didn't apply an excessive amount of glue before mating the surfaces). Too much glue can cause glue run-out problems that will likely cause other issues at the time of finishing the project. Proper clamp pressure will also compensate for any imperfections between the two mating surfaces.
Additionally, consider how much the moisture in the wood will cause the wood of the joint to swell. This isn't a huge concern, but it can be a factor, but even pressure from the clamps will help reduce the swelling that can occur in the joint.
The type of wood(s) being used in the joint can also be a factor to consider. As a general rule, hardwoods (particularly those with tight grain patterns) require a bit more clamping force than soft woods. Because the wood is denser, hardwoods will be a little less forgiving when clamping than more porous soft woods.