Mortise and Tenon Primer
How Tight Is Too Tight? (and How Loose Is Too Loose?)
Fitting a mortise and tenon joint together should not be a CTSBTF operation. That's "Cut To Size, Beat To Fit". You should be able to fit them together with just normal hand pressure, no white knuckles, no bulging vein in the forehead. If you have to whack them together with a mallet or a dead blow hammer or Bessy clamp them together then the joint is TOO TIGHT. When you add glue, the wood will swell and things will get a lot tighter, probably too tight to go together without a lot of "persuading". It may not take much "persuading" to split the mortised part - not a good thing.
Here's one "fit test" - the Gravity Test.
You can also shoot for "spit tight". Get the tenon tight enough that if you spit on it and seated it in the mortise the wood would expands enough to make removing the tenon extremely difficult till the wood dries. (thanks to the person in rec.woodworking who described this fit)
Here's a little tip on identifying where a tight tenon needs some work. The pressure points between the mortise and tenon will crush the grain on the tenon. The compressed / crushed grain will be shiny. You may have to hold it up under a good light and rock the piece around to see it but it will be a visible shiny area or spot. Pair off the shiny area(s) and try the fit again.
NOTE: Thanks to the feedback on this stuff which Bill Tindall was kind enought to provide - it must be noted that when using the #2B Pencil Method for finding tight spots in the joint fit - REMOVE THE GRAPHITE BEFORE GLUE UP. As Bill noted, glue won't stick to graphite. This can be done with alcohol or with a wipe with a paper towel followed by LIGHT sanding, with say 320 or 400 grit.
Remember, when it's Glue Up Time, you're going to have more than one mortise and tenon joint to apply glue to, align and push together. If just one of your mortise and tenon joints is too tight you'll be tempted to force it to go together - probably with a good whack with a big mallet or deadblow hammer, or maybe some serious squeezing with a clamp or two.
You may get lucky and seat the joint.
- split the mortised piece
- split off an edge or corner of the tenon which will then prevent the joint from closing
And speaking of Glue Up Anxiety - if you've ever done a glue up which required aligning and then seating a half dozen to a dozen or more mortise and tenon joints, then you know how much "fun" that can be. Well here's a little tip that may save you some grief.
BEVEL THE END OF THE MORTISE AND THE TENON. This will a) make it easier to align the tenon to the mortise, b) eliminate splitting an edge or corner of your tenon and c) leave a little space for glue to go - WHERE IT WON'T BE SEEN LATER. That last point may not seem that important - until you've spent hours getting squeezed out glue off the beautiful piece you spent days, or maybe weeks, making. And if you missed some of that glue squeeze out, when you apply your finish it's probably going to become VERY visible.