Accurate Part Production.
In the production of the various struts, longerons and fittings of the fuselage, the wing spars, compression and interplane struts of the planes, the utmost accuracy must be observed. Although tolerances are permissible with regard to the overall dimensions of the struts, spars, longerons, etc., the lengths particularly of the fuselage struts should be absolutely correct to drawing. The bad effects of a strut, say 1 millimetre short, are not restricted to the particular component of which it forms a part, but are noticeable in one way or another in the complete structure. Similarly the ends of struts which are required to be square should be dead square, and those which are cut to a bevel should correspond with the correct angle. The result of the slightest discrepancy in this respect becomes speedily apparent when the defective struts are assembled, as the tension of the bracing wires will result in the strut becoming bowed or bent, this being due to the bedding down of the strut end in the socket or clip. It is also advisable to trim the ends in a machine after being sawn to something approaching the correct length, and the practice of sawing to dead length should not be permitted. The surface of a sawn strut end is formed of a number of more or less ragged fibres, which in position in the machine and under pressure of the bracing wires tend to gradually flatten down, this resulting in slack wires and loss of alignment. Absolute accuracy and uniformity of part production can only be obtained by the use of jigs, preferably of metal, and some form of jig should certainly be used for cutting the various struts to length. Referring again to the necessity of the strut ends being of the correct angle, it is surprising to note the effect of the smallest inaccuracy. The writer has frequently noticed fuselage struts considerably out of straight, the grain of the timber being sometimes advanced as the reason. However, the removal of the defective strut always resulted in its return to a straight condition. It should be realized that the effect of an initially bent strut is a reduction of strength, and as this may prove a source of danger, it is in itself sufficient reason for the rigid observance of length limits.