Part 15 (1/2)

[Ill.u.s.tration: Fig. 325.--Use of Plane and Shooting Board for Mitreing.]

[Ill.u.s.tration: Fig. 326.--The Screw Mitre Trap.]

A simple method and one that should always be remembered because it is handy when working without a shooting board is shown at Fig. 328. Set the marking or cutting gauge to the thickness of the wood to be mitred at 45 degrees; then gauge this distance on the wood, as shown at B; draw from the line to the edge, as shown, and saw and plane to a finish. The diagonals of a square give 45 degrees, and this is the method used to mark out the work. The end of the wood must, of course, be square with its edges before marking out in this manner.

[Ill.u.s.tration: Fig. 327.--”Donkey's Ear” Shooting Board.]

[Ill.u.s.tration: Fig. 328.--Gauging for Mitres.]

[Ill.u.s.tration: Fig. 329.--Narrow Inner Moulding.]

[Ill.u.s.tration: Fig. 330.--Wide Mitred Moulding.]

Fig. 329 shows a bevelled framing into which has been mitred a narrow moulding M so as to show a correct margin around the panel.

[Ill.u.s.tration: Fig. 331.--Door with Curved Mitres.]

[Ill.u.s.tration: Fig. 332.--Method of Setting out for a Curved Mitre.]

Fig. 330 shows a similar framing, but with a wide moulding M mitred around it. To obtain a correct intersection of this moulding, the angles A and B are bisected. The bisection of the angles meets before the width of the moulding is cleared, therefore the angle C will again have to be bisected, and the finished joint will appear as shown. One of the simplest of mouldings with a large flat face has been chosen to ill.u.s.trate this. The moulding could be all in one width, as shown, or it could be built into the framing in separate pieces, the wide flat and the piece carrying the mould.

CURVED MITRES.--We now come to what are probably the most difficult of all mitres, viz., curved mitres, and the writer well remembers in his apprentices.h.i.+p days his first experience of attempting to fit the mouldings around the door shown at Fig. 331 by using straight mitres at A. This, of course, is impossible if the mouldings are of the same section and it is desired to make all the members correctly intersect. If straight mitres are used the section of the curved moulding will have to be of a different shape from the section of the straight moulding, and in these days of machine-made mouldings this method is seldom resorted to.

It is better, cheaper, and easier to make curved mitres when the necessary machinery is at hand.

TO SET OUT A CURVED MITRE (see Fig. 332).--Draw a section of the moulding full size, A, as shown at the left hand of the ill.u.s.tration, and project lines round the framing, as shown V, W, X, Y and Z. Where the lines V, W, X, Y and Z intersect at the corner D, it clearly shows that a straight mitre will not cut all the points of intersection. A curved line will cut all the intersections, and a template made of cardboard, sheet zinc, or veneer, should be made to this shape. At the left-hand side the geometrical setting out is shown for obtaining the curve without having to resort to drawing it freehand.

Take half the width of the moulding, as shown by dotted line A, and where it cuts the approximation of the curved mitre place the point of the compa.s.ses and strike out a circle as shown; with the same radius place the compa.s.s point on B--that is, the inside point of the mitre, and cut the circle on the right and left with the small arcs shown at _aa_. With the same radius put the compa.s.s point at the junction of the circle and mitre line, C V, and cut the circle at right and left, viz., _ee_.

[Ill.u.s.tration: Fig. 333.--Template for Mitreing.]

Now rule a line through _aa_, and another line through _ee_, and where these lines cut each other it will give the correct radius of the curved mitre. The advantage of knowing the correct radius of a curved mitre is of great benefit to the skilled machinist, as it enables him to set up his machine so as to give a definite result.

MITREING A MOULDED DOOR FRAME.--Fig. 333 ill.u.s.trates the method of mitreing the moulded portion of a door frame where the joint is dowelled, not tenoned. A small wooden template is made out of beech or other hardwood, having its ends cut at 45 degrees. This template is placed on the rail, as shown, and held in position by placing both the rail and the template in the vice. The face of the template forms a guide for a wide chisel, and enables the worker to gradually pare away the moulding to the correct angle.

[Ill.u.s.tration: Fig. 334.--Using Panel Saw in Mitre Box.]

For sawing the mitres on large mouldings such as are used on the lid of a gramophone or wireless cabinet, a mitre sawing box and a panel saw may be used as indicated at Fig. 334.

JOINTS FOR CURVED WORK

Fig. 335 shows a circular frame made up in two thicknesses, the segments being screwed to each other and the joints crossed in two layers. This is a very strong method, and it is used for making circular frames and curbs up to 15 ft. in diameter. The segments can be either long or short, the only important condition being that they must be marked out and sawn to the correct radius. Fig. 337 shows a board marked out in segments for this cla.s.s of work. The longer the boards the better will they cut up, as it gives more opportunities of cutting one piece out of the other as at A A.

[Ill.u.s.tration: Fig. 335.--Circular Frame in Two Thicknesses.]

[Ill.u.s.tration: Fig. 336.--Circular Rim in Halved Segments.]