This is an excerpt from “The Woodworker: The Charles H. Hayward Years: Volume III” published by Lost Art Press.
This is a common fault even amongst experienced men who should know better. The wood between the dovetails is torn out, leaving an unsightly gash which robs the joint of much of its strength. In nine cases out of ten it is concealed when the joint is assembled, and this is probably the reason why so many do not take the trouble to avoid it. There are cases, however, when the blemish is seen, especially when the wood tears just below the surface. When the joint is levelled it is easily possible to plane into it with the result that an unsightly gash is disclosed. The fault is easily avoided as explained below.
Let us first consider the reason why the wood tears out in this way. There is first the downward chop short of the gauge line across the grain, as at (A), Fig. 2, followed by a horizontal cut which splits away the waste piece. Next is another chop right on the line and a second horizontal cut. So far no tearing out has taken place, yet it is this preliminary cut that is the cause of all the trouble. The wood is now reversed and a similar chop made, as at B. It is easy to see what happens. The shock of the blow causes the unsupported waste to bend over, and it tears out the fibres from the shoulder as shown. If the chisel happens to be blunt the defect is so much the worse. It is all due to the projecting waste piece having no support when the wood is reversed and the second cut made.
The remedy is simple. Begin by chopping down across the grain short of the gauge line as before, and then make a sloping cut to meet it as at (B); Fig. 3. Make a second cut a little nearer the gauge line followed by a second sloping cut, and finally right up to the line, as at C. Note that sloping cuts leave a short piece of uncut wood at the corner. On no account cut away the waste horizontally from the end. If now you reverse the wood and chop down, the grain will not tear out because the waste piece is supported. You can ease the work too by splitting away the waste at the end. It does not matter once the wood has been reversed.
It will be realised that in working in this way the removal of the wedge of wood enables the chisel to penetrate easily when the second chop is made closer to or right on the line. The idea is shown pictorially at (D), Fig. 3. In thick wood or extra hard wood it may be necessary to make several cuts, easing away a wedge of wood after each, as at (C), Fig. 3. The great point in avoiding tearing out, however, is to leave the wood untouched at the corner so that it is not forced downwards when the wood is reversed. The corner supports it, as shown in Fig. 4. Chopping in with the grain at the end after the wood has been reversed however enables the chisel to penetrate more easily.
If for any reason you have cut away the waste right to the end in the first chopping, you can still prevent tearing out by inserting a little wedge of wood beneath the overhanging waste as in Fig. 5. This gives support and prevents it from bending over under the force of the blows and so wrenching the fibres.
— Meghan Bates