centering a shaft for lathe centers

If you have no DI, you can also find center with a tool bit in the lathe if you can spare a few thou off the diam. Chuck work in the four jaw, ease tool up to within about 1/8" of the work. Rotate chuck by hand until it catches on the tool and move jaw on that side inward until it clears, repeat until chuck clears on all sides. Move tool within 1/16" and repeat. 1/32", 1/64", etc.. until you are close as your eyes can get.

Now rotate the work and see where the chuck grazes the work. Move that jaw in just a fraction of a turn (often it is a matter of tightening torque changes at this point). As you keep working this way, you will see the grazed spot wrapping further around the work. When it is 3/4 of the way around, you are within a couple thou at most.

This is actually a lot faster that it looks. 3-4 minutes and the work will be running true. Once you get the hang of this, you will throw your three jaw chuck in the drawer and it will likely hardly ever see the light of day again.

duhhhhh.

Do not have a center? The traditional way was to use an odd leg center and draw a series of intersecting lines near the center, and then center punch the area in the middle.
You can do something similar with a surface plate- but if you are asking the question I would guess you do not have one.

I will confess that I have used a pair of old vernier calipers with a relativly sharp tip like the odd leg caliper would be used. It works- but would not do this with a "good" one