Sprocket Wheel

At are shown the guide bar and chain of a chain-mortising machine, two enlarged links of the chain being indicated at A. The chain is similar in construction to the driving chain of a bicycle, with the exception that it is provided with teeth which cut away the timber as the chain revolves. When using a chain mortiser the portion of the machine carrying the chain is fed downwards into the timber, thus cutting a clean and true mortise. If, however, a stump mortise is required it is necessary to pare away a certain amount of timber by hand, because the machine obviously leaves a semicircular bottom to the mortise. To overcome this difficulty the latest types of mortising machines have a square hole-boring attachment fixed alongside the chain. This tool, the working portion of which is illustrated in , consists of a square hollow chisel (E), which is sharpened from the inside, and a revolving twist bit (D) fitted with spurs or nickers, but without a point (one spur can be seen at the bottom of the illustration). This bit revolves inside the shell like a chisel, and bores away the superfluous timber, whilst the pressure exerted on the chisel causes the corners to be cut away dead square. A mortise 3⁄8 in. square by 6 ins. in depth may thus be cut. The portion marked A is the shank of the chisel , where it is fixed into the body of the machine, and the hole at E allows the boring bit to free itself.

Fig. 141.—Method  of Fitting an Interior  Table Leg. Fig. 141.—Method of Fitting an Interior Table Leg.

Fig. 142.—Haunched Tenon for      Skylight or Garden Frame.

Fig. 142.—Haunched Tenon for Skylight or Garden Frame.

Fig 143.—Long and Short      Shouldered Tenon.

Fig 143.—Long and Short Shouldered Tenon.

indicates the method of fixing an interior leg to a table having a circular or straight top rail. The inlaid leg in this case is stump-tenoned into the top rail, and the inlaid portion of the leg is allowed to run through the rail, thus giving continuity of design.

shows the application of the haunched tenon to the making of a skylight or garden frame. In this and similar cases the side rails are rebated as shown in the section, and the bottom rail is thinner than the side rails to allow the glass to finish level upon it.

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