Lathe Test Bar - Someone's in error

[Doozer]

Setting the tailstock set-over just right
is just a standard part of setup when
the tolerance of the job calls for a job
that has to be really straight, without
any taper. Usually the tailstock set-over
is adjusted with the actual workpiece
set up in the lathe, in context.
In the grinding world, the table is always
adjusted for set whenever the tailstock
is moved. Parts can be changed between
centers, but if you move the tailstock,
you must re-adjust the table set.
With a lathe, it is tolerance dependent
to tweak tailstock set-over or not.
Using a tramming bar thingy and hoping
to set it and forget it, is a false dream
of running a lathe.

--Doozer

[J Tiers]

.......

(added) I have a couple of really nice chromed piston rods from a pair of McPherson struts that serve such purposes nicely. Far more precise than any I'm likely to produce.

Of course, with a test bar for that purpose, it is the centers which determine accuracy as much as any other part. They need to be on-center and accurately round in order to have the bar work as intended.

So, if you put in one or both of the centers, you "made" the test bar. It will be as accurate between centers as your positioning of the centers. That is one point that is very much in favor of a bar of the type that requires a test cut, because the centers will be on-center to the degree that the machine you use turns accurately and concentrically. Once you have a condition of same size at both ends (and middle), THEN you have concentricity, and you can switch over to using it with an indicator.

[PStechPaul]

It would be good to also check the height of the test bar at both extremes. It won't have as much effect on turning a given diameter, but it could cause stress problems if you have a rod in the chuck and supported by a live center.