Screw Joint

Screw Joint is one fixed by wood screws or screw nails, commonly called screws, which are made of brass, copper, or iron. The latter kind are sold in various thicknesses, and in more than a dozen different lengths, ranging from in. to 6 in., many of which are gimlet-pointed or self-boring. They are used in fixing hinges and other metal work, and in situations where the joint may require loosening either for inspection or repair, as for instance at the seats and risers, etc, of water closets, and in beaded casing to piping. There can be no doubt, however, that the joiner does not employ a sufficiency of screws, for many handsome ceilings, etc. have been injured by the jar of the hammer when the screw driver would have harmlessly and more efficaciously performed the work, whilst thousands of rattling windows betray the absence of screws from the joints between the inside beads and linings of the frames. In making a screw joint, the hole in the piece through which the screw first passes must be large enough to admit freely and easily its whole body or stem. The approximate adhesion of a wood screw in timber in lbs. is said to be found by multiplying the diameter, pitch, and length of thread inserted in the timber all in inches, and the product by 42,000 for soft woods, and by double the quantity for hard; but this must be obviously very untrustworthy in the case of the ordinary sized screws used in joiner's work, owing to the great difference in the holding powers of fibrous and vascular layers, and to the many possible positions of the screws with respect to them. Besides which the tensile strength of oak is less than that of fir, whilst there is much doubt as to their comparative powers of resisting a crushing force when both woods are of good quality.

Scribed Joint

Scribed Joint is formed by scribing, which consists in making a board fit down closely upon an uneven or irregular or moulded surface by marking in the first place upon it with a scribe, or other pointed instrument, the line to which it must be cut. This is effected by placing the board at a little distance from its true or final position, but with one of its finished edges exactly parallel to what will be its own direction therein, and then with a pair of wing compasses, opened to the proper distance, running one point along the profile of the fixed surface whilst the other scribes the required line on the board. Skirtings are thus scribed down to floors and strings much in the same way to stairs, the difference being that a shallow sinking, called a housing, is made in the face of the string after scribing the profile of the step instead of cutting away part of the edge. Similarly, previous to housing the ends of skirtings at a re-entering angle, instead of mitring them, the profile of the moulding has to be scribed, and the joint, though also housed, is often in consequence said to be scribed. Skirtings are likewise scribed to chimney pieces. The way in which the shorter bars of a sash are sometimes cut to fit those which extend through to the frame, as shown in Fig. 133, presents a fair illustration of a scribed joint.