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 marking off the ends of two pieces of cornice moulding which are to be joined at right angles, the procedure is as follows. Let the section of the moulding be as shown in Fig 1 Draw the plan of the mouldings and. mitre as at Fig 2. Then set a bevel to the mitre line C D. This will be the bevel to apply to the top edge, as indicated by the line CD (Fig. 3). For the bevel for the sloping back, through the angle at A' (Fig. 1) draw A' B. With A as centre and C as radius, draw the arc C B . JNow draw B' B parallel to the lines in the plan, as shown, and C B parallel to A' B'; then join B to A. Set the bevel as indicated, and apply it to the sloping back of the moulding and mark it. This will give a line as indicated by A C (Fig 3) As A' E' is a vertical surface, the line A B indicated at Fig. 3 can be drawn square. This principle can be applied for mouldings meeting at any angle. If there are several mitres to be made and all meet at the same angle, a simpler plan, and one that will save much time, is to construct a mitre box which will hold the moulding to the exact angle, as shown at Fig. 4. and the mitres can be cut in the manner illustrated and described on p. 136.
Mitreing Cornice Moulding.
 
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