Clearance Between Joint Surfaces

How much clearance should there be between the mating surfaces of a joint? Is .002" enough? Is .010" too much?

Tim Inman: Mating is the operative term. If you can see light through the edge of the joint, there is too much clearance.

John Brock: It's more of a feel thing for me. I want the joint to be a clean, firm fit, with no wiggle or slop before glue-up. Remember, the glue is only there to hold the joint together. The strength comes from the mechanical fitting of the joint, and with most species you can count on a little compression in the wood to help out. On a mortise and tenon joint, I want to have to press the tenon into the mortise with a little pressure. If it falls in, it's too loose. If I need a hammer to test a joint, it's too tight.

Richard Jones: I've never tried to take such a precise approach. I aim for no more than firm hand pressure to assemble cut joints. Waterbased glues will swell the mating parts at glue-up time, thus the joint will be harder to push together, requiring cramps. If the tenon simply drops into the mortise, it's too loose. (Editor's note: "Cramps" is not a typo. In England, where Richard lives, "cramps" is the word for what we call "clamps.")

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