Reply To: Proper use of marking knife
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If this were a simple housing square to the edge, you would knife one side, cut down with your chisel, place the board that is being fitted against that cut and then just mark the location of the second edge with a prick of your knife. You wouldn’t trace along the board. Instead, you’d use the square, registered against the prick. Since it is just a prick, you have a lot of control, e.g., you can reach a little under the piece being marked if experience shows that otherwise things come out loose and you can make sure to use the knife so that the wall lands on the prick and the sloped part of the cut, what Paul calls the bruising, lands in the waste.
The shooting board is a bit trickier since one edge of the wedge is at 45 or 90 and the other is at some arbitrary angle. One approach is to mark both with a pencil, then use a steel rule to knife the line that is at an arbitrary angle (the one that is neither 45 nor 90). Cut that down, place the wedge against the cut, and make a prick with the knife. The second wall will be at exactly 45 or 90, so you can bring the square in now to mark the second side, no need to trace along a wooden batten. Now cut the housing and test the wedge. It is impossible to be loose with a wedge, really, because you will slide the wedge until it is tight, mark the end that overhangs the chute, and then cut to length. You can, however, have the wrong angle for the housing vs. the wedge, but you ought to be really close. A couple light passes of the plane along the wedge will get the angles to match. (Do that before cutting to length).