Two more accurate means of aligning centers are to use a test bar and to machine and measure. A test bar is simply a shaft that has true centers (is not off center) and has no taper. Some test bars are made with two diameters for convenience.No dog is necessary for checking alignment with a test bar, as the bar is not rotated. A dial indicator is mounted, preferably in the tool post, so that it will travel with the carriage (Figure I-224).Its contact point should be on the center of the test bar.
Begin with the indicator at the headstock end, and set the indicator bezel to zero (Figure I-225). Then, move the setup to the tailstock end of the test bar (Figure I-226) and check the dial indicator reading. If no movement of the needle has occurred,the centers are in line. If the needle has moved clockwise, the tailstock is misaligned toward the operator. This will cause the workpiece to taper with the smaller end near the tailstock. If the needle has moved counterclockwise, the tailstock is too far away from the operator, and the workpiece will taper with the smaller end at the headstock. In either case, move the tailstock until both diameters have the same reading.
Figure I-226 The carriage with the dial indicator is moved to the measuring surface near the tailstock. In this case the dial indicator did not move, so the tailstock is on center.Because only a minor adjustment is usually needed while a job is in progress, the most common method of
aligning lathe centers is by cutting and measuring. It is also the most accurate. This method, unlike the bar test method,usually uses the workpiece while it is in the roughing stage(Figure I-227). A light cut is taken along the length of the test piece, and both ends are measured with a micrometer. If the diameter at the tailstock end is smaller, the tailstock is toward the operator, and if the diameter at the headstock
end is smaller, the tailstock is away from the operator. Set up a dial indicator (Figure I-228) and move the tailstock half the difference of the two micrometer readings. Make another light cut and check for taper.
After the cut is made for the length of the workpiece, a micrometer reading is taken at each end to determine any difference in diameter.