Take it easy with clamping pressure
Q: As a newcomer to woodworking, I'm not sure how much clamping pressure to use on joints made with woodworker's yellow glue. Can you give me some guidelines?
—John Burns, St. Paul, Minn.
A: John, the short answer is that a good-fitting joint with the right amount of glue doesn't require tremendous pressure. The clamps just serve to hold the surfaces in contact while the glue dries.
However, let's assume that most joints fall short of perfect, and benefit from enough force to push them into complete contact. Dale Zimmerman of Franklin International, maker of Titebond woodworking glues, recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175–250 psi for hardwoods. When we tested one-handed bar clamps (issue 139), we found that they provided pressure just into the softwood range or a bit less. Squeeze those clamps as hard as you can. But R. Bruce Hoadley, author of the book Understanding Wood, reports that other kinds of clamps, including the bottom three pictured at right, can produce far more pressure than needed. So don't go beyond "snug" when tightening those clamps.
The maximum recommended clamping pressure for most joints is 250 psi. Putting all your muscle into many common clamp styles generates excess pressure that could force out most of the glue and produce a weak bond.
For more in-depth information on gluing and clamping, visit our Gluing and Clamping section in the WOOD Store.