Drilling of Bolt Holes.
Of equal importance is the drilling of the various bolt holes for the attachment of the fittings. It is not always advisable to drill the holes in the spars and longerons before the fittings are applied, but in numerous instances this is possible, and where interchangeability is an important consideration it is imperative. The practice of setting out the positions of the various holes from a drawing and then drilling with a hand brace, is a procedure only justified when a small number of machines of a certain type are to be produced, and ought by now to be obsolete. Under such a system no two spars would be exactly the same, as owing to the influence of grain in the wood, the drill or bit always tends to “run” from the correct angle. Viewed from the aspect of quantity production such a practice is very deficient. It is only by the use of metal drilling jigs of suitable design that anything approaching absolute accuracy is possible. Such jigs should not only locate the hole, but should also form a guide for the drill. In the attachment of the fittings to a properly jig-drilled spar, it should not be necessary to again drill through, although this often occurs. Where this is done, there is a distinct possibility of the brace not being held true, which means that the hole becomes larger than necessary and not infrequently oval in shape. An additional bad point is the impossibility of detecting such a fault after the fitting is bolted on, and it may not be realized until a noticeably slack wire in the complete machine indicates the movement of the fitting. In the foregoing, absolute accuracy in the various fittings has been assumed, but unfortunately in practice almost the reverse is true. Variation generally occurs in built- or bent-up fittings, and is usually the result of jigs of either incorrect or bad design. Where the variation includes a hole out of position, the use of this fitting on a previously drilled wood part is only possible by the bad practice of drilling through with the results explained above. It will thus be realized that the uniformity and accuracy of component production is only attainable by the utmost precision in the manufacture of both wood and metal parts.