First, the planed surface of the crank will alter in truth so soon as the crank is released from the pressure of the holding devices on the planer or planing machine; second, that surface will again alter in form and truth from the removal of the metal around the surface of the hole first bored; and third, the planed surface will be to some extent sprung from the pressure of the plates holding the crank to the chuck plate, hence the following method is far preferable.
Fig. 886.
If it is intended to plane the back surface of the crank let that be done first as before, and let it be held to the face-plate by bolts and plates as before, while the hole and its radial face at the large end of the crank are turned and finished. In doing this, however, first rough out the radial face, and then rough out the hole, so that if the work alters in form a fine finishing cut on both the radial face and the bore will correct the evil. Then release the crank from the pressure of the holding plates; and it is obvious that however the planed surface may have altered in truth from removing the surface metal, the radial face just turned will be true with the bore turned at the same chucking. Now to chuck the crank to bore the second hole, turn it end for end as in [Fig. 886], and bolt the face already turned to the chuck plate (as at a in the figure) with one or more bolts and strap plates. To steady the other end of the crank, and prevent it from moving under the pressure of the cut, take two bolts and plates b, and place a washer between them and the chuck surface as shown at c, then bolt the plates to the chuck plate, so adjusting them that their ends just have contact with the crank when it is set true. In setting it true it may be moved by striking the outer ends of the plates.
In this method of chucking, we have the following advantages:—
1st. If the chuck plate is not true we may place a piece of paper beneath the crank surface a, to correct the error as in the former method, or if this is neglected, the second hole bored will be out of true to an amount answerable to the want of truth in the chuck, and not to twice as much as in the former method.
2nd. Any alteration of form that may take place during the first chucking does not affect the truth of the second chucking as in the other case.
3rd. The crank being suspended during the second chucking, any alteration of form that may accompany the boring of the second hole will be corrected by the finishing cut, hence the crank will be bored with its two holes as axially true as they can be produced in the lathe.
It now remains to explain the uses of the pieces w in [Fig. 886], simply weights termed counterbalances bolted to the chuck plate to balance it against the overhanging weight of the crank on one side of the chuck plate. If these weights are omitted the holes in the work will be bored oval, because the centrifugal force generated by the revolution of the work will take up any lost motion there may be between the cone spindle journal and its bearings, or if there be no such lost motion the centrifugal force will in many cases be sufficient to spring the cone spindle.