Spindle runout matters but is easy to measure and compensate for… You compensate for spindle runout with the 4-jaw or spider chuck…. but even if the spindle nose itself has zero runout, whatever chuck you put on it will have runout up the wazoo (technical term) unless you can dial it out. I use Set-Tru or Adjust-Tru type chucks for this reason when I’m “making parts”; I can dial my 10” Bison in at a given clamping diameter very quickly and the repeatability is excellent, way better than most would think a 3-jaw is capable of. Same with my 10” Buck 6-jaw, and the 6” Buck on my HLVH clone.
But for barrel work you want a 4-screw spider chuck with rounded brass or copper ends on the screws, and another spider chuck on the back end of the spindle so that you can adjust for both concentricity and axial alignment to the spindle CL. If you only use a 4-jaw, you can dial in the concentricity but it’s not going to be exactly longitudinally coaxial to the spindle CL unless you skim cut the chuck jaws themselves “in situ”with the chuck mounted on the spindle… and that can be tricky, depending on the chuck.
Measuring spindle nose runout is easy, although it can change with how warm the bearings and spindle are. Assessing the main ways, and the crossfeed and compound ways and leadscrews, is trickier. One way to get a read on the functional condition of the main ways is to cut a cylinder test and check for taper. Take a light roughing cut, then a light finish cut, using a sharp cutter on an unsupported bar of material sticking as far out of the chuck as you can get away with without it singing (chatter). You want a stout piece of material here. I have a left over piece of 1-5/8” T303 about a foot long I use for this, although I’m running the test using a live center to verify the tailstock is dead nuts for precise shaft works….. different test, same idea. Doing the test with the stock unsupported shows you if the ways are worn enough to drop the carriage relative to the CL of the work, which will change the resulting diameter of the test bar. Often lathe ways “dip” up close to the chuck due to normal wear. The difference in diameter is subtle, unless the ways are really worn, which is why you measure with a micrometer, not a caliper.
High quality floating reamer holders cure many ills……..
