FINDING THE DEAD CENTERS.
The best way of finding the true center is by moving the cross-head a measured distance round its extreme travel, recording the extent of movement on the driving-wheel tire, whose motion is uniform; then bisecting the distance between the marks on the tire, when the dividing line will indicate the true center.
Fig. 18.
Thus: Turn the wheels forward till the cross-head reaches within one-half inch of its extreme travel, as shown in [Fig. 18]. From a point a on the guide-block, extend a tram on the cross-head, and mark the extreme point reached b. Put a center punch-mark c on the wheel-cover, or other convenient fixed point, and from it extend a tram on the edge of the tire, and scratch an arc d. Now, with tram in hand, watch the cross-head, and have the wheels moved forward slowly. When the cross-head passes the center, and moves back till the tram extending from a will reach the point b, stop the motion. Again tram from the wheel-cover point, and describe a second arc on the tire, which will be at e, now moved to the position which d occupied when the previous measurement was taken. With a pair of dividers bisect the distance between d and e. Mark the dividing point C with a center punch, and put a chalk ring round it. When the wheel stands so that the tram will extend from c to C, the engine will be on the forward dead center.
All the other centers must be found by a similar process.