definite distance apart according to the dimensions on the drawings. The trammelling is done diagonally across the frames between points marked on their top edges immediately above the line representing the centres of the axles. This is shown in Fig. 41 in which F F represent the two frames in plan, and A₁ A₂ A₃ the centre lines of the axles. The dotted lines show the
_Steam_Locomotive_Construction_and_Maintenance_Fig41.png)
Fig. 41.—Trammelling the Frames.
diagonal directions of the trammels. By trammelling diagonally the “squareness” of the frames is ensured when the distance x₁y₂ is equal to the distance x₂y₁, and y₁z₂ is equal to y₂z₁. At the same time the longitudinal and transverse dimensions must be also checked by trammelling, and x₁y₁ must be equal to x₂y₂ longitudinally, and x₁x₂ equal to y₁y₂ transversely, similar measurements being made for axles A₂ and A₃. The motion plate f (Fig. 42), the cross stay g in front of the firebox, and the drag-box casting h are then temporarily bolted in place to secure the frames together, the bolts used being smaller than the holes in the frames, so that any necessary adjustments can afterwards be made
