The common knowledge in woodworking circles is that mortise and tenons should fit with a tolerance of 1/64”. In Joe's experience, this tolerance is too great. The most common type of wood glue, poly-vinyl adhesive (PVA), relies on the glued pieces of wood being in contact with each other or near enough to each other to allow for a polar attraction. The range of the attraction is less than .003”. The glue itself has no spanning strength. 1/64” equates to .016”. Thus if there is a 1/64” difference between the mortise and the tenon there can be up to .008” between either side of the tenon and the walls of the mortise. While this amount of glue might seem to be insignificant, it makes a big difference in strength the finished product. Joe has seen glue fail in joints with tolerances over .01" but never in joints with tolerances of .005" or less.
(NCTM Standard: Measurement)
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