After making all of the saw cuts, you can start chopping out the waste. One way to do this is to hold the chisel absolutely perpendicular to the board, and chop straight down on the base line. But there's a problem. The natural tendency of the chisel is not to go straight down, but rather to undercut the shoulder.
No matter how careful you are, or how gentle the first cut, the beveled front edge of the chisel will act as a wedge to force the cut back, away from the base line. One solution to this is to carve out a small V-shaped section along the base line. This removes the wood on the bevel side of the chisel, and prevents the wedge action.
An even better way (for me) is to clamp a backing fence along the base line, Fig. 7. Although this is time-consuming, it provides a sure rest for the chisel and prevents undercutting on the first few strokes. Now, chop straight down at the base line. Don't get carried away. One or two medium size taps are all that's needed at first. Do this along the base line of each waste section.
clean out waste. To clean out the waste, hold the chisel (bevel up) on the end of the board and make a small tap, Fig. 8. This should cleanly remove a little chip of waste (about Vie" thick).
undercut. After chopping straight down for about Vs", you can start undercutting. Hold the chisel at a slight angle (tilt the handle toward you). This forms a slight V-cut on the bottom of the shoulder. Fig. 9.
other side. After chopping down about %'s of the way through the board, flip it over. Position the backing fence once again, and proceed as before, Fig. 10. However, be very gentle with the hammer — too hard a whack may slice through to the "OUT face.
clean out corners. Since the saw kerf may not have gone all the way to the base line, and because the V-cut of the chisel is actually below the base line, there will be little splinters of waste in the comers. These "dirty" corners can cause a lot of problems, so it's important to clean them out with a sharp chisel, Fig. 11.
checking the pins. After all the cuts have been made, hold a try square along each cut line (both front and back) and make sure the edges of the pins are perpendicular to the end of the board.
If the pins are not straight, I mark a slightly heavier line (as in Fig. 4) and correct them by paring down with a fairly wide sharp chisel, Fig. 12.
You can also check the pins by eye, sighting down the length of the board to see if the pins are all straight and parallel. Next comes the tails.
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