Especially when working with hand-dressed lumber, your stock may not all
be exactly the same thickness. By referencing all measurements from the
show side of the stock, the exact stock thickness doesn't matter. The
show side will be exactly as planned and the back side can be either
left as-is or flushed up as appropriate.
Typically you would set the gap between the knives of the mortise gauge
from the tool used to cut the mortise, be that a router bit, hollow
chisel, mortise chisel, etc. The fence is then set to locate the tenon
on the thickness of the stock.
For the mortise, you can use the same gauge if the members are the same
thickness (door frame) or a separate gauge if they're not (table legs
The accepted drill goes as follows:
Nominate a face of each piece as the datum face (Face side) and an edge at
right angles to it as the Face Edge. Mark with an 'l' type mark with the
bottom of the ell shooting off towards the face edge. Mark the face edge
with a vee that joins the bottom of the ell.
Always work the gauge's stock on the face side.
Set the gauge points to the size of the tool making the mortise. Actually
with the tool just resting between the point at their very very tips. This
ensures a very slightly wider tenon.
To get the joint central to the workpiece, estimate the setting of the
stock, with the gauge against the face side make two dents.
With the stock on the opposite face make dents alongside the others.
Reset the stock so that the points fit exactly between the pairs of dents.
Repeat if necessary.
Jeff Gorman, West Yorkshire, UK
email : Username is amgron
Thank you for going into such detail. If I understand what you've written
it will work great if the two pieces of stock to be joined are the same
Also, the "one marked line" version described earlier may give superior
if the two pieces to be joined are not the same size/width.
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