Fig. 2504.
Fig. 2505.
Placing two Cranks on a Shaft so that their Centre Lines shall stand at a Right Angle.—It is obvious that the keyways in both the crank and the shaft must be cut accurately in their proper positions, because it is a tedious operation to file out the sides of the keyways when the cranks are placed upon the shaft. To mark the keyways in the absence of any tools or appliances specially designed for the purpose we proceed as follows: Placing the shaft upon a marking-off table, we plug up the centres upon which the shaft has been turned by driving a piece of lead in them, leaving the surface level with those of the shaft; and then from the perimeter of the shaft we carefully mark, upon the lead plugs, the centres of the shaft. From this centre we describe a circle whose diameter will be equal to the required widths of the keyway, and then taking a square we place its stock upon the face of the marking-table, and bringing the edge of the blade even with the edge of the circle, we mark a perpendicular line upwards from the circle to the perimeter of the shaft, and then draw a similar line on the other side of the circle, as shown in [Fig. 2504], in which a represents the shaft and b the circle, c the perpendicular line struck on one side of the circle, and d the square placed upon the marking-table e, in position to mark the line on the other side of the circle, f and g being wedges to keep the shaft a from moving its position upon the table. We next mark with a scribing-block or surface gauge the depth of the keyway as denoted by the line h, and the marking at that end of the shaft is completed. Passing to the other end of the shaft we find the centre of the shaft, and describe around it a circle equal in diameter to the required width of keyway, and from the edges of the circle to the perimeter of the shaft draw two lines with a scribing-block, as shown in [Fig. 2505], a representing the shaft, b the circle, c d the breadth of the keyway, e the marking-off table, f and g the wedges, and h the depth of the keyway, which must, in this case, be marked with a square resting on the table.
If, however, the shaft is too heavy or large to be placed on a marking-off table, we may proceed as follows: Strike as before the circle b, [Fig. 2504], equal in diameter to the required width of keyway, and adjust a straight-edge held firmly against the end face of the shaft, so that its upper edge is coincident with the perimeter of this circle, while the straight is horizontally level-tested by a spirit-level. Draw a line along the shaft face, using the straight-edge as a guide. This will give us the line c in [Fig. 2505]. By a similar process the line d, [Fig. 2505], may be drawn. At the other end of the shaft similar lines, but standing vertical, may be marked, which will give the positions of the keyways.
Fig. 2506.