Work that has already been turned, but has had its centres cut off, may be recentred as follows. One end may be held and driven by a chuck, while the other end is held in a steady rest such as was shown in [Fig. 802], and the centre may then be formed in the free end by a half-round reamer, such as shown in [Fig. 1190], placed in the position of the dead centre, or the square centre may be used in place of the dead centre, being so placed that one of its faces stands vertically, and therefore that two of its edges will operate to cut. The location for the work centre should be centre punched as accurately as possible, and the work is then placed in the lathe with a driver on it, as for turning it up; a crotch, such as shown in [Fig. 1202], is then fastened in the lathe tool post, and fed up by the cross-feed screw until it causes the work to run true, and the square centre should then be fed slowly up and into the work, with a liberal supply of oil. If the work runs out of true, the crotch should be fed in again, but care must be taken not to feed it too far. So long as the square centre is altering the position of the centre in the work, it will be found that the feed-wheel of the tailstock will feed by jumps and starts; and after the feeding feels to proceed evenly, the crotch may be withdrawn and the work tried for being true. The crotch, as well as the square centre, should be oiled to prevent its damaging the work surface. It is obvious that in order to prevent the lathe dead centre point from seating at the point or bottom of the work centre, the square centre should be two or three degrees more acute in angle than the lathe dead centre. If the work is tried for truth while running on the square centre, the latter is apt to enlarge the work centre, while the work will not run steadily, hence it is better (and necessary where truth is a requisite) to try the work with the dead centre in place of the square one.
In thus using a square centre to true work, great care should be taken not to cut the work centres too large, and this may be avoided by making the temporary centre-punch centres small, and feeding the crotch rapidly up to the work, until the latter runs true, while the square centre is fed up only sufficiently to just hold the work steady.
To test the truth of a piece of rough work, it may, if sufficiently light, be placed between the lathe centres with a light contact, and rotated by drawing the hand across it, a piece of chalk being held in the right hand sufficiently near to just touch the work, and if the chalk mark extends all round the work, the latter is as true as can be tested by so crude a test, and a more correct test may be made by a tool held in the tool rest. If the test made at various positions in the length of the work shows the work to be bent enough to require straightening, such straightening may be done by a straightening lever.
In shops where large quantities of shafting are produced, there are special straightening tools or devices: thus, [Figs. 1203] and [1204] represent two views of a straightening machine. The shaft to be straightened is rotated by the friction caused by its own weight as it lies between rollers, which saves the trouble of placing the shaft upon centres. Furthermore, the belt that is the prime mover of the gears driving these rollers is driven from the line shaft itself without the aid of any belt pulley. The tension of this driving belt is so adjusted that it will just drive the heaviest shaft the machine will straighten; but if the operator grasps the shaft in his hand, the driving mechanism will stop and the belt will slip, the shaft remaining stationary until the operator sets it in motion again with his hand, when the belt ceases to slip and the mechanism again acts to drive the shaft.
Fig. 1203.
[Fig. 1203] represents the mechanism for driving the shaft s, to be straightened, which lies upon and between two rollers, r, r′. Upon the shafts of these rollers are the gear-wheels a and b, which are in gear with wheel c, the latter being driven by gear-wheel d. Motion to d is derived from a pair of gears, the pinion of which is driven by the belt from the line shaft. h is a head carrying all these gears (and the rollers) except d. There are two of these heads, one at each end of the machine, the two wheels d being connected by a rod running between the shears, but the motion is communicated at one end only of this rod, the shaft is driven between four rollers, of which two, r r′, are shown in the engraving.
