It is fair to point out that unless you're concerned with alignment of the spindle's MT taper to the spindle axis, an accurately ground bar (e.g. 3/4" Thomsen case hardened linear shaft, $12 at ENCO) can provide the same measurements as a more expensive test bar.
Or if you have an accurately ground bar you can make acceptably accurate measurements for alignment of a 7x12 with that. If you're willing to work at it you can lap a straight 1" round steel bar (preferably a tube), use a 4 jaw and set the runout near the chuck to nil, then set the runout at the far end to 4/10 or less by tapping it in the right direction as you incrementally tighten the jaws. It takes a few minutes to do this but at that point you have the test bar aligned exactly on the spindle axis. This should perform just like a commercial test bar. Of course, you should use RDM to correct for any minor runout remaining, just like when using a real test bar. Or, you could chuck the lapped bar in a 3 jaw, accept a thou or so runout, push the high side on the far end as you tighten the chuck incrementally and get runout to a thou or so, then use RDM - my guess is there will be little difference in results.
More thoughts on 7x12 alignment here: MiniLathe Alignment There's more to it than a test bar, a precision level may also be helpful, upping the cost for alignment tools.
John
Posted by: John@GadgetBuilder.com
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