How to Glue-Up Joints: Different Woods Need Different Clamping Pressure
Many woodworkers are underclamping their joints during glue up. To help avoid this, professor Roman Rabiej developed some simple keys to clamping success. (Read the full article on the topic.) One of the keys is that you should apply the right amount of force for the type of wood you’re using since different species require different clamping pressure. Wood orientation (flatsawn or quartersawn) is also important.
Why correct clamping pressure matters
Optimum clamping pressure creates strong glueline joints in several ways. First, it overcomes the viscous resistance of the glue and forces it into a thin, continuous film in contact with the wood, which is necessary for a strong joint. Second, as the glue releases moisture, causing the wood to swell, clamping overcomes this pressure and prevents the joint from opening up. Third, it overcomes minor surface imperfections between mating surfaces. And fourth, clamping holds parts in position until the glue cures.
Too little pressure will fail to achieve any of these benefits. Conversely, extreme pressure can produce weaker joints, however, this is unlikely with common woodworking clamps. Because modern glues are stronger than the wood fibers, a good glue joint should break in the wood, a process known as wood failure, rather than along the glueline.
So rather confusingly, the higher the percentage of wood failure, the better the joint. The wood-failure percentage starts to diminish as clamping pressure is increased beyond a certain point, because excessive pressure begins to starve the joint of glue and also to compress the wood and reduce its ability to absorb the glue.
|Wood Type Matters
|In general, dense and tight-grained woods require the application of greater force. On hardwoods, glue joints between radial or quartersawn faces require half the pressure of tangential or flatsawn face joints. This is because on hardwoods, the quartersawn face has half the compression strength of the flatsawn face, so the fibers are more easily crushed. On softwoods, the reverse is true, with the quartersawn-face gluelines requiring twice the pressure of the flatsawnface gluelines.
|Recommended Clamping Pressure
(pounds per square inch)
The chart simplifies the science
The chart above shows the recommended glueline pressure for selected furniture woods. The optimal pressure is roughly twice as high. This peak pressure is the point just before the glueline is starved or the wood fibers are crushed.
For most hardwoods, however, normal woodworking clamps can’t get close to these levels of force. But joints clamped at the recommended levels will be quite strong enough, with the glueline being stronger than the wood itself. You’ll achieve a glueline thickness well under the recommended maximum, which is about 0.004 in. To give a point of reference, the cover of a (like Fine Woodworking) magazine is 0.005 in. thick.
When determining whether a joint is flatsawn or quartersawn, consider the two surfaces to be glued rather than the visible surface.
The next step is to find out how much pressure you are applying with each type of clamp.