The Technique of FURNITURE MAKING

Mitre, scribed and scarf joints

If two pieces of wood of equal section are to be joined together without the end grain showing then the mitre cut between them must halve the overall angle. Thus if they meet at 90° then the mitre cut is 45° (168:1); if 120° then 60°, etc. (168:2). If, however, the meeting pieces are of unequal section then a true mitre of 45° will result in a longer cut in the larger section, with the result as shown by the dotted line (168:3); therefore the mitre must be 'faked', i. e. the angle pitched so that the meeting cuts are equal in each piece.

CURVED MITRES

Halving the angle also applies where a curved moulding meets a straight moulding of equal section, but in order to agree either the profile of one must be altered to coincide or the mitre - line must be curved (168:4). Mouldings of the same curvature but curving the opposite way, or those in which one curvature is quicker than the other, must also have curved mitres, and the only exception is two mouldings of the same curvature and with the same direction which require a straight cut (168:4B). Methods of determining the curve are described in Chapter 40, p. 363, Workshop geometry.

MITRE JOINTS

Undoubtedly the strongest mitre joint is the secret mitre dovetail (see Dovetailing, Chapter 19, p. 175) but these are not suitable for small sections or flat frames. Various other mitre joints are shown in 168: the tongued mitre (168:5) (the tongue can be either loose or worked in the solid); the tenon and mitre (168:6); the dovetail and mitre (168:7); the keyed mitre (168:8); the dovetail keyed mitre (168:9), with the direction of entry indicated by the arrow; and the through dovetail and mitre (168:10) often used in cheval mirror-frames, etc,

Applications of mitre joints are shown in the

mitre and lap (168:11), suitable for plinths, cornices or where the joint can be further strengthened by an inner glued block; and 168:12 which is a tongued version which should not require a block. A method often adopted for small boxes and frameworks, and in fact all light mitre joints, is the veneer keyed mitre (168:13), in which the sections are butt mitred, glued and assembled, and diagonal saw cuts run through the corners in which pieces of stout veneer matched to the saw kerf for thickness are glued and inserted, and cleaned off flush when dry. If well done, accurately spaced and with a carefully chosen veneer, it can be almost invisible. Figure 168:14 is the plain butt mitre with a strengthening glue block which is rubbed in position with either hide or resin glues. The joint is strong enough for most practical purposes. In all butted mitres, and particularly in 168:11, 13 and 14, the end grain should be repeatedly sized with weak glue until the pores are filled, enabling the joint to hold together while it is being further strengthened. Tongued, keyed and through mitred edges in which part of the joint is visible should not be veneered over, for the joint will inevitably shadow through the veneer in time. The only exception is the veneer keyed mitre (168:13), which will give no trouble if the slips are fairly thin.