When machining workpieces in a lathe, it is imperative that the ways be aligned to the headstock, and that the tailstock centerline is collinear with the centerline of rotation of the headstock. Accurately measuring misalignment between two rotational centerlines on machines that are not coupled and cannot be rotated at the same time (as is the case with the headstock and tailstock on a lathe) is a very challenging undertaking, unless you have an excellent laser system with special measurement capabilities. See Figure 1.
First, insert a ‘dummy shaft’ in the chuck of the headstock and mount your Laser emitter on it. Next, insert a live center in the tailstock and mount the Receiver on it. Finally, using the special ‘pass mode’ measurement mode, measure the misalignment by simply rotating the laser past the receiver and then the receiver past the laser a few times. Do this twice, as follows:
First, measure the misalignment with the tailstock located very close to the headstock. Now move the tailstock to the furthest position away from the headstock and measure the misalignment again. The four alignment parameters, namely Vertical Offset (VO), Vertical Angularity (VA), Horizontal Offset (HO), and Horizontal Angularity (HA) will automatically be recorded in the Measurement Table for you upon completing each set of readings.
There are two different misalignment possibilities that can exist in your lathe, either separately or simultaneously, and your measurement results will accurately define for you not only whether any misalignment exists but also let you distinguish what is misaligned, how much and which way. Here’s how:
If the four alignment parameters are nonzero but repeat between the close and the far position, the tailstock is misaligned to the headstock. This is the case illustrated in Figures 2 and 3. Note that the slight lack of repeatability in the measurement table between the near and far readings is entirely within tolerance for the 48″ span involved.
If, on the other hand, the Offset values differ significantly between the near and far readings, then the ways are misaligned to the headstock. See Figure 4.
Correct this misalignment first, then correct any misalignment of the tailstock to the headstock. Remember, both types of misalignment may be present in your lathe simultaneously, in both planes (vertical and horizontal); examine your results carefully to ascertain what the problem(s) is(are) and proceed to correct them in the order indicated.
After corrections have been performed, repeat the measurements in both the near and far positions. Once all results are zero or very close to zero, and repeat in both the near and far positions, your lathe is aligned. See Figure 5.