In some cases a gib is employed, as shown at a in [Fig. 1447], instead of a spring, the advantage being that it is less liable to come out of place when moving the head along the bar.
The trammels should always be tried to the work in the same relative position as that in which they were set, otherwise the deflection of the bar may vitiate the correctness of the measurement; thus, if the rod or bar stood vertical when the points were adjusted for distance to set them to the required distance, it should also stand vertical upon the work when applied to transfer that distance, otherwise the deflection of the bar from its own weight will affect the correctness of the operation. Again, when applied to the work the latter should be suspended as nearly as convenient in the same position as the work will occupy when erected to its place.
Thus, suppose the trammels be set to the crank pin centres of a locomotive, then the bar will stand horizontally. Now the side rod, or coupling rod, as it may be more properly termed, should be stood on edge and should rest on its ends, because its bearings wherever it will rest when on the engine are at the ends; thus the deflection of the trammel rod will be in the same direction when applied to the work as it was when applied to the engine, and the deflection of the coupling rod will be in the same direction when tried by the trammel as when on the engine. The importance of this may be understood when it is mentioned that if the coupling rod be a long one, resting it on its side and supporting it in the middle instead of at its ends will cause a difference of 1⁄50th inch in its length.
Fig. 1448.
Another lineal measuring gauge employed in the machine shop is shown in [Fig. 1448]. It is employed to measure the distance between two faces, and therefore in place of inside calipers, in cases where from the extreme distance to be measured it would require the use of inside calipers too large to be conveniently handled. Its application is more general upon planing machine work than any other, although it is frequently used by the lathe hand or turner, and by the vice hand and erector. It consists of two legs a and b, held together by the screws c d, which screw into nuts. These nuts should have a shoulder fitting into the slots in both legs, so as to form a guide to the legs. The screws are set up so as to just bind both legs together but leaving them free enough to move under a slight friction. The gauge is then set to length by lightly striking the ends e, and when adjusted the screws c d are screwed firmly home. The ends e are rounded somewhat, as is shown, to prevent them from swelling or burring by reason of the blows given to adjust them.
For striking circles we have the compasses or dividers, which are made in various forms.
