Tailstock Alignment

I've been tempted to get one of these precision ground MT/straight test bars, but I'm not exactly sure of proper usage / implementation. Does one set in tailstock & now the (long extension) magnifies tailstock deviation by indicating around the end from headstock mounted DTI? I thought another use might be to insert in headstock (with MT5/MT3 adapter) as a means of validating head alignment to lathe beds & carriage travel or setting up taper turning angle zero datum, but I'm not sure that would give me better results than what I'm doing now.[/QUOTE]

Hi
I thought about one of those bars, but realised that it is not necessary - I normally use a piece of junk plated round bar that was lying around, with quite a few imperfections on it. Any old round bar will work just fine, just as long as it has one clean spot on it.

Here's why. You need to have 3 reliable datums, that is in the ends of the round. Choose a point on the circumference of the round that is clean, somewhere near the end of the round and mark it with a felt pen, that is the third datum. Now, put it between centres, mount a dial gauge on the cross slide, touch the gauge in the centre of the marked part of the round and note or zero the gauge. Now, when you end for end the round and move the gauge to the other end of the round where the marked circle is now and measure it, you are measuring accurately any misalignment. The two centres define the same axis irrespective of which way around you have the round bar, same axis. The marked circle defines a point that is a certain (but unknown) distance from the axis. So the dial gauge measures precisely the distance from the axis aka tailstock alignment.

Here's how. Grab a piece of round and drill centres in each end. Use a nice centre drill, neatness counts. To make a fresh centre for the chuck, grab a piece of scrap and turn a point on it, correct angle of course. This fresh point IS on the axis of the chuck. Put your best centre in the tail stock. Now place the piece of rod between centres and make the two measurements as above. But but but - I want to test the alignment at several points! Simple, use several pieces of rod.

Another win for old age and treachery.

Make a spool.

Locate some CRS about 2 inch diameter and maybe 5 inch long.

Face and center drill both ends.

Place in chuck and remove about 1/2 inch from center area so you have 2 collared spool.

Next setup to turn between centers now place DI in tool post and check roundness of each end by turning on place.

If round done if not take swim cuts until they are.

If cuts taken measure diameter of both.

If different then shift tailstock as needed and repeat until both ends exact same diameter.

Never cut again this part.

Next time place between centers and di in tool post then make sure both ends same distance.

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TQ60, I don't quite follow - what is the significance of removing the center 0.5" section? If tailstock is out of alignment, isn't it going to produce 2 segments of exact same taper angle = essentially the same thing as one long taper with bigger diameter at on end & smaller diameter at the other end to tell you which way tailstock is out. Maybe I missed a step or don't follow your measurement.

If tailstock is 0.020 towards the back way the end supported by it will be 0.040 larger than the end at the Chuck and both will be tapered.

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PE, If you are going to keep this finished piece as a permanent tailstock alignment tool, there really isn't a reason to go to the trouble of turning the center section down to a smaller diameter. Just remember that using a fixed length rod will only assure a centered alignment at the position used. Some lathes will be close enough at other positions unless absolute zero taper is critical . With many lathes, if the tailstock remains locked at one position after being determined to be centered with a fixed length rod, then a shorter or longer work piece is mounted, alignment could be off considerably. Quill travel is a real doozy of a error source with some tailstocks.