The turning tool is started at the right-hand end of the work and the tool should be adjusted with the left hand when beginning a cut, as shown in [Fig. 10], in order to have the right hand free for calipering. A short space is first turned by hand feeding, as at D, [Fig. 7], and when the calipers show that the diameter is slightly greater than the finished size (to allow for a light finishing cut, either in the lathe or grinding machine) the power feed for the carriage is engaged; the tool then moves along the work, reducing it as at E. Evidently, if the movement is along a line b—b, parallel with the axis a—a, the diameter d will be the same at all points, and a true cylindrical piece will be turned. On the other hand, if the axis a—a is inclined one way or the other, the work will be made tapering; in fact, the tailstock center h1 can be adjusted laterally for turning tapers, but for straight turning, both centers must be in alignment with the carriage travel. Most lathes have lines on the stationary and movable parts of the tailstock base which show when the centers are set for straight turning. These lines, however, may not be absolutely correct, and it is good practice to test the alignment of the centers before beginning to turn. This can be done by taking trial cuts, at each end of the work (without disturbing the tool's crosswise position), and then comparing the diameters, or by testing the carriage travel with a true cylindrical piece held between the centers as explained later.
If the relative positions of the lathe centers are not known, the work should be calipered as the cut progresses to see if the diameter d is the same at all points. In case the diameter gradually increases, the tailstock center should be shifted slightly to the rear before taking the next cut, but if the diameter gradually diminishes, the adjustment would, of course, be made in the opposite direction. The diameter is tested by attempting to pass the calipers over the work. When the measuring points just touch the work as they are gently passed across it, the diameter being turned is evidently the same as the size to which the calipers are set.
As the driving dog is on one end, the cut cannot be taken over the entire length, and when the tool has arrived at say position x, [Fig. 5], it is returned to the starting point and the work is reversed in the centers, the dog being placed upon the other end. The unfinished part is then turned, and if the cross-slide is not moved, the tool will meet the first cut. It is not likely that the two cuts will be joined or blended together perfectly, however, and for this reason a cut should be continuous when this is possible.
Roughing and Finishing Cuts.—Ordinarily in lathe work, as well as in other machine work, there are two classes of cuts, known as “roughing” and “finishing” cuts. Roughing cuts are for reducing the work as quickly as possible almost to the required size, whereas finishing cuts, as the name implies, are intended to leave the part smooth and of the proper size. When the rough stock is only a little larger than the finished diameter, a single cut is sufficient, but if there is considerable metal to turn away, one or more deep roughing cuts would have to be taken, and, finally, a light cut for finishing. In this particular case, one roughing and one finishing cut would doubtless be taken, as the diameter has to be reduced 3/8 inch. Ordinarily the roughing cut would be deep enough to leave the work about 1/32 or perhaps 1/16 inch above the finished size. When there is considerable metal to remove and a number of roughing cuts have to be taken, the depth of each cut and the feed of the tool are governed largely by the pulling power of the lathe and the strength of the work to withstand the strain of a heavy cut. The depth of roughing cuts often has to be reduced considerably because the part being turned is so flexible that a heavy cut would spring the work and cause the tool to gouge in. Of course, just as few cuts as possible should be taken in order to save time. The speed of the work should also be as fast as the conditions will allow for the same reason, but as there are many things which govern the speed, the feed of the tool, and the depth of the cut, these important points are referred to separately in [Chapter II].
Fig. 11. Filing Work after Finishing Cut is taken
Filing and Finishing.—In many cases the last or finishing cut does not leave as smooth a surface as is required and it is necessary to resort to other means. The method commonly employed for finishing in the lathe is by the use of a file and emery cloth. The work is rotated considerably faster for filing than for turning, and the entire surface is filed by a flat, single-cut file, held as shown in [Fig. 11]. The file is passed across the work and advanced sidewise for each forward stroke, until the entire surface is finished. The file should be kept in contact with the work continually, but on the return stroke the pressure should be relieved. The movement of the file during the forward or cutting stroke should be much slower than when filing in a vise. By moving the file slowly, the work can make a number of revolutions for each stroke, which tends to keep it round, as practically the same amount of metal is removed from the entire circumference. On the other hand, short rapid strokes tend to produce flat spots, or at least an irregular surface, especially if the work can only make part of a revolution for each cutting stroke. The pressure on the file during the forward stroke should also be kept as nearly uniform as possible.