Slide rests constructed with every precaution require to be finally adjusted upon the lathe bearers, that they may subsequently turn the true surface and cylinder; this adjustment which merits a few words, is usually readily effected, and with fig. 145 or fig. 146, as follows. The under side of the slide rest fig. 145, its tenon and the lathe bearers, are first studiously cleansed from the accidental adhesion of dirt or turnings; the rest is then placed on the bearers, the tenon screwed up by the tail screw into contact with the further face of their interval, and the rest clamped in position by the bolt and nut below. The top plate of the upper slide, is placed towards the end of its traverse, as in fig. 145, and with a round tool clamped in the holder, the rest is applied to turn a surface.
The test surface turned, should be nearly as large as the lathe will admit, and is generally of hardwood. This material being chosen from the facilities it affords for the preliminary trials, while with care, it may also be made to answer tolerably well for the final tests. In addition, the comparatively slight wear it occasions to the edge of the tool, greatly compensates for the deficiencies of a hardwood, compared with a metal surface, as applied to this purpose.
The surface that is produced by the first traverse of the tool, is tried by a steel straight-edge applied across its center. This, may show it to be not an absolute surface, but an exceedingly obtuse cone, either concave or convex; usually caused by the tenon of the rest not being positively square with the lower slide. To correct this error, the face of the tenon in contact with the bearers is sparingly reduced with a file, or scraper; principally at the right hand end, and diminishing away to nothing at the left hand end, if the surface be convex, and from left to right if it be concave. A very small quantity correctly removed from the face of the tenon, effects a marked difference on the surface produced by the traverse of the tool; for, the actual amount of error in the squareness of the tenon to the slide of the rest, is not only doubled by appearing on both sides of the center, but is also multiplied, by the distance from the center travelled by the tool; making the error much more apparent with increased diameter in the test surface. After the first alteration to the face of the tenon, the rest is replaced and the surface is turned a second time and re-measured; and the correction of the tenon is repeated, until the surface turned satisfactorily meets the test of the straightedge. Some care however is required throughout to avoid over correction, which may easily occur and would throw the error the other way, and also to keep the bearers and the under surfaces of the slide rest free from dirt or chips, the interposition of which, would cause fallacious appearance of error.
The adjustment of the upper slide to turn the true cylinder, follows that for the surface. The tool is placed at right angles to its late position and set to turn a hardwood cylinder. The examination of the result with the straight-edge, serves only to show the truth or otherwise of the straitness of the upper slide of the rest; the parallel position of the latter to the mandrel axis, has to be ascertained by measurement with callipers, applied towards the two ends of the cylinder. One end will probably measure a larger diameter than the other; in which case, the bolts of the circular mortises are slackened, and the end of the slide, opposite the large end of the cylinder is slightly advanced. The distance the end of the upper slide is advanced, is required to compensate only half the difference in the diameters of the two ends of the cylinder; the real error in position of the slide, in a similar manner to that first adjusted to the surface, being also doubled and then increased, according to the length of the cylinder. Therefore towards the end of the corrective process as truth is approached, the requisite advance to be given to the end of the upper slide becomes so very slight as to be easily carried to excess, which then throws the error the other way. The trifling adjustment necessary is conveniently given, by only slightly slackening the bolts and striking gentle blows near the end of the slide with a wooden mallet, the end of a tool handle, or even with the side of the clenched hand. The test cylinder is traversed again between every fresh adjustment, until the callipers show it to be of the same diameter at either end; upon which result, a reading point or mark is engraved upon the upper surface of the plate of the lower slide, in a line with the zero of the divisions upon the edge of the circular movement of the upper slide. This reading point, aiding to replace the top slide to turn the cylinder, or to any angle for conical turning.
In the slide rest fig. 146, the top slide is already square to the middle slide by construction, while the traverse of the lowest is rendered constant by its cradle, screwed up against the bearers. It is therefore only necessary to adjust the middle upon the lowest slide to turn the true cylinder, to ensure the top slide giving the true surface also. The middle slide is adjusted in the manner already described, and its reading point is then marked on the front edge of the plain lowest slide.
Should greater precision be required than can readily be obtained from the employment of hardwood for the trial surface or cylinder, brass or iron may be substituted for the concluding tests; but, it should not then be forgotten that in turning large surfaces or long cylinders in metal, the edge of the tool suffers an appreciable reduction during the progress of a single cut. This is very visible after a near approximation to an accurate surface has been attained; when it may happen that the surface produced will be either slightly concave or convex, as the tool may have been traversed either from the center towards the circumference, or in the reverse direction. In testing the cylindrical position, turning the entire length of the cylinder may also sometimes be avoided. A length of about one inch at each end affords ample space for trial with the callipers, and while more expeditious, avoids the risk of apparent error, arising from the wear of the tool, or, from the springing or yielding of the central portion of the cylinder. To lessen the latter, the cylinder should always be of a sufficient diameter to ensure its stability.