The conditions, however, are so uncertain, that it is always advisable to be on the safe side, and either wind the tool out from its cut before winding the rest and carriage back (thus destroying its set for diameter), or else to stop the lathe and remove the work before traversing the carriage back as already directed. If the latter plan is followed the trouble of setting the tool is avoided and much time is saved, while greater accuracy of work diameter is obtained. It is obvious that this plan may be adopted for roughing cuts in cases where two cuts only are to be taken, so as to leave finishing cuts of equal depths; or if three cuts are to be taken, it may advantageously be followed for the second and last cuts, the depth of the first cut being of less importance in this case.
The following rules apply to all tools and metals:
When the pressure between the tool and the work is sufficient, from the proportions of the work, to cause the work to spring or bend, the length of acting cutting edge on the tool should be reduced.
As the diameter and rigidity of the work increases, the length of tool cutting edge may increase. The cutting edge of the tool should be kept as close as the work will conveniently admit to the slide rest tool post, 1⁄4 inch even of this distance being important.
The slide rest tool should always be resharpened to take the finishing cut, with which, for wrought iron or steel, soapy water with soda in it should be used, the soda serving to prevent the dripping water from rusting the parts of the lathe.
Cast iron will cut with an exquisite polish if finished at a very slow rate of cutting speed, and turned with a spring tool, such as was shown in [Fig. 974], and water is used. But being a slow process it is not usual to finish it in this manner, though for round corners, curves, &c., this method is highly advantageous.
For cast iron the tool should be as keen as the hardness of the metal of the work will permit. If an insufficiently keen tool, or too deep a cut, or too coarse a feed be taken, the metal will break out instead of cutting clean, and numerous fine holes will be perceived over the whole surface, impairing that dead flatness which is necessary to an even and fine polish.
To remove these specks or holes in cylindrical work, the file may be used, but for radial faces hand-scrapers, such as shown in [Fig. 1295], are used, the work rotating in either case at high speed. Such scrapers are oilstoned and held with the handle end above the horizontal level.
The rest should be so conformed to suit the shape of the work, that the scraper will be supported close to the work, which will prevent chattering, and a piece of leather should (as a further preventive of chattering) be placed beneath the scraper. A very good method of using a scraper is to adjust it to the work, and holding it still on the rest, traverse the slide rest to move the scraper to its cut.
After the scraping, three methods of polishing radial faces are commonly employed; the first is to use emery paper only, and the second is by the use first of grain, emery, and oil, and the subsequent use of emery paper or cloth, and the third is by the use of emery wheels and crocus cloth.