Reply To: Why set mortise gauge pins wider than chisel??
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For a through tenon, you don’t want to see any gaps where the tenon protrudes through the mortise, so it’s common practice to set the mortise gauge just a little wider than the chisel you intend to use. Remember, you’re going to use that same gauge setting to mark the tenons. Just stay between the two gauge lines when you’re chopping. After you finish chopping, paring the sides of the mortise out to the gauge lines will form perfectly straight mortise sidewalls and a tight fitting joint. Some types of wood cut easily and you end up with a pretty neat hole right from the chisel but most likely the sides won’t be perfectly straight until you pare the opening. Other stuff, like oak, will tend to want to leave very ragged edges but that all disappears when you pare the sidewalls.
Appearance usually isn’t so critical for stopped tenons. The tenon shoulders are going to hide the mortise opening so you can be tighter with the gauge setting and not worry about paring the sidewalls of the mortise. Tenons are cut the same way regardless of whether it’s a though tenon or a stopped tenon. In theory at least, if you set the gauge pins to the exact width of the chisel and you use that same gauge setting for both the mortise hole and the tenon and if you cut both the mortise and the tenon exactly to those gauge lines, it should be a perfect fit. In practice, that’s nearly impossible to do so we cut the tenons a little fat (1/32″ or so outside the gauge lines) and then fit them to the mortise hole either by very carefully paring them or by planing them like Paul does using a router plane.