Setting your lathe’s tailstock alignment, photo 1, is something any beginner can do by following a few simple procedures. The first step is to determine how far out of line the tailstock is sitting. The second is to adjust the tailstock using the simple mechanism provided on most lathes. The third is to test the result by turning a bar between centres and measuring the diameter along its length, aiming at the same reading at both ends -- and in the middle of course.
Providing your lathe is in good useable condition, needing minor adjustment and not major surgery, the following steps will soon have you turning parallel. It will also help prevent bit breakages if you have to drill small holes.
The quick way
The easiest way to see what is going on between your lathe’s centres, the headstock centre and tailstock centre, is to place a thin steel ruler between the points and gently turn the tailstock handwheel until the ruler is firmly gripped, then tighten the barrel lock. The tailstock base clamping lever should be locked before advancing the barrel. If the two points are in line, the ruler will stand up straight vertically, photo 2, and lie square across the lathe bed horizontally. This can be judged very closely by eye. If the ruler sits at an angle in either plane, photo 3, the tailstock needs adjusting as described in the second half of this article.
If you don’t have a second dead centre to put in the headstock, you can use my preferred method of gripping a short length of 1/2” (13mm) bar in the threejaw chuck and turning a point on it by offsetting the top slide to 30 degrees, photo 4. The angle is not critical, nor is the concentricity of the piece of bar in the chuck. With the tool bit set at centre height, the turned point will be bang-on true to the spindle axis.
At the tailstock end, don’t use a revolving centre because it can introduce small errors through bearing clearances and machining tolerances. Stick with a dead centre for best accuracy.
This steel-ruler test is accurate enough for initial setting of the tailstock, to the point where you can then go straight to the “real-world” turning test.
The measured way
However, while the steel ruler test is usually good enough to set the tailstock for a final turning test, sometimes you may wish to know exactly how far out it is. To find that we must mount a small dial test indicator (DTI) to the chuck, either by its magnetic base, photo 5, or on a purpose-built bracket that may have less flex.
Either way, the finger of the indicator is brought to bear on the ground Morse taper surface of the tailstock centre as shown. This gives us the truest reading of the alignment of the tapered hole in the tailstock barrel relative to the headstock spindle. The finger may alternatively bear on the angled front face of the centre, but this may introduce a small error due to machining tolerance on the dead centre, albeit usually a very small one.
The DTI readings should be noted at the 9 o’clock, 12 o’clock and 3 o’clock positions, photo 6. There is no need to swing the DTI right around to take a 6 o’clock reading. We are in essence lining up two overlapping circles here: the one described by the DTI pointer as the chuck rotates and the circular ground surface of the dead centre. If the two circles are aligned at three out of the four points within tolerance, the fourth must also be within spec. It cannot be otherwise.
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