This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
In setting out dovetailing first set out the shoulder lines on each piece; if the ends are shot true this may be done by a gauge. Mark off the centre of each socket, and then half the breadth of the sockets on each side as at A (Fig. 1). Make a template as shown at Fig. 2, the edge A B being square to AC; AD and C E should be about 80° to the edge AC. Then mark out the sockets with a template and a sharp pencil (or awl) as indicated at Fig. 2. Saw carefully in the waste parts; then place the socket piece on the pin piece, and mark the shape of the latter by using the end of a saw placed in the sockets (see Fie 3). A Fig- 1 F, Fig. 2, and A, Fig. 3, refer to the same side.

Fig. 1.

Setting Out Dovetailing.
 
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