Very small radial surfaces of brass may best be finished by the scraper and polished with emery paper, while large ones may be finished with dry emery paper.
Round corners on brass work should be finished with a spring tool, such as shown in [Fig. 974], but having negative top rake; but if the corners are of small radius a well oilstoned hand-scraper is best.
To enable the smooth and true turning of all radial faces of large diameter, the lathe head should, when it is possible, be steadied for end motion by placing a rod between the lathe centres, but if the radial face is solid at the centre so that such a rod cannot be put in, the end motion adjusting device of the lathe should be adjusted. The slides of the lathe should also be set up to have good firm contact, and the tool should be brought up to the work by putting the feed motion in gear and operating it by hand at the cone pulley, or gear-wheel on the feed spindle. If the lathe has no compound rest, the cut should be put on by this means, but otherwise the tool may be brought near the work by the feed motion and the cut put on by the compound rest, the object in both cases being to take up all lost motion and hold the rest firmly or steadily on the lathe shears, so that it shall not move back as the cut proceeds.
Work of cast iron or brass and of small dimensions and irregular or curved outline should be finished with scraping tools, such as shown in [Figs. 1303] and [1310], polished with emery cloth or paper. But whenever scrapers are made with curves to suit the form of the work, such tool curves should be so formed (for all metals) as not to cut along the whole length of cutting edge at once, unless the curve be of very small length as, say, 1⁄4 inch. This is necessary, because if the cutting edge operates on too great a length it will jar or chatter.
For convex surfaces the curve on the scraper should be of greater radius than that of the work, while in the case of concave curves the tool should have a less radius. In both cases the tool will require a lateral movement to cause it to operate over the full width of work curve.
If the work curves are sufficiently large, and the same is sufficiently rigid that a slide rest tool may be used, the length of cutting edge may be increased, so that under very favorable conditions of rigidity the tool edge may cut along its whole length without inducing either jarring or chattering, but the best results will always be obtained when with a broad cutting edge the tool is of the spring tool form shown in [Fig. 974].
Work of wrought iron or steel of small dimensions and of irregular form, must also be finished by hand tools, such as the graver shown in [Figs. 1285] and [1286], and the finishing tool shown in [Figs. 1289] and [1292], the shape of the tool varying to suit the shape of the work.
Round corners or sweeps cannot on any kind of work be finished by a file, because the latter is apt to pin and cut scratches in the work.
For the final tool finishing of lathe work of plain cylindrical outline, no tool equals the flat file if it be used under proper conditions, which are, that the work be turned true and smooth with slide rest tools, the marks left by these tools being exceedingly shallow and smooth.
A dead smooth file that has been used enough to wear down the projecting teeth (which would cut scratches) should then be used, the work rotating at as fast a speed as the file teeth will stand without undue wear. The file strokes should be made under a light pressure, which will prevent the cuttings from clogging its teeth, and the cuttings should be cleaned from the file after every few strokes. Under these conditions work of moderate diameter may be turned to the greatest degree of smoothness and truth attainable with steel cutting tools, providing that the work makes several revolutions during each file stroke, and it therefore follows that the file strokes may be more rapid as the diameter of the work decreases, and should be more slow as that diameter increases. Allowing the greatest speed of the filed surface permissible, without too rapid destruction of the file teeth, to be 200 feet per minute, and the slowest speed of file stroke that will prevent the file teeth from being ground away or from becoming pinned (when used on wrought iron) to be one stroke in two seconds, the greatest diameter of work that can be finished by filing under the condition that the work must make more than one rotation per file stroke, is about 25 inches in diameter, running about 30 revolutions per minute. The same diameter and speed may be also taken for cast iron, but brass may be filed under increased speed, rendering it practicable to file it up to a diameter of about 36 inches under the above conditions of work rotation and file stroke speed.