Fig. 2404.
Fig. 2405.
Fig. 2406.
If the brass is to be lined sufficiently to merely bring the key up without respect to the length of the rod we may drive the key home as in [Fig. 2404], and mark on it a line coincident with the edge a of the strap. We then lift the key up to its proper height and mark a second line, so that when removed from the keyway the key will have on it the two lines shown in [Fig. 2405], a being the first and b the second line; and the difference between the width of the key at a and its width at b will be the thickness of the liner necessary to be placed behind the brass nearest to the key. To ascertain the precise amount of this difference (because a very small error as to this amount causes a great deal of extra labor), we set a pair of outside calipers to the width at a; and then passing the caliper points down to b, we keep one of the points even with the line b, and insert a wedge until it just fills the space between the other point and the side of the key, as shown in [Fig. 2406], c being the wedge, which should be chalked along its surface so that, when inserted until it touches against the caliper point, the latter will leave a mark on the wedge, denoting exactly how far the wedge entered, and hence the exact required thickness of liner.