Test bar versus RDM - what's the real difference ?
I successfully realigned my lathe using RDM. Just used a length of BMS 38mm or so. It had a ground finish with uniform diameter and was round enough for RDM purposes. It was mounted in the 3 jaw with about 800 mm stickout. RDM is a bit tedious because you need to find the high and low each time then average it out with a calculator, and write the results down so you can see how you're going, but it's just a matter of being a bit organised. RDM does not require the bar to be straight, parallel, round or anything like a precision piece. The averaging method takes care of all that. In my opinion it's more accurate than using a test bar, given that a test bar will have some runout and there will also be some inaccuracy in the spindle taper.
If I was using a test bar I'd use it to get "good enough", then use it with RDM to do the final adjustment. The better the bar, the easier it is to use RDM, you'll still need to do the averaging for runout, but you shouldn't need to compensate for diameter difference between your two chosen test points.
If I had to do it again, I'd have a laptop with a spreadsheet open, that'd speed things up enormously.
Where the test bar fails is with vertical alignment, over a couple of feet with a solid bar there's considerable sag, a surprising amount in fact. You can calculate that and allow for it fairly easily, but if you're doing that, then it's not much different to RDM time wise, except that RDM requires a high/low measurement each time. With a test bar you've still got the runout problem, which you'd use RDM to compensate for, so then the method becomes virtually the same (except that with a test bar you'd probably not have the diameter difference calculation, but then if it's in a spreadsheet that's automatic anyway)
Timewise I don't think RDM would be much different to using a test bar, and theoretically the result is perfect.
The big problem I had was doing the job singlehanded, making the adjustment without being able to watch the indicator is a real pain, I guess I could use my little webcam and laptop to watch the indicator remotely, that's something I'll do next time, along with a spreadsheet.
Having said all that, if anyone out there has a 7MT test bar surplus to requirements, for less than the Greek national debt I'd be very interested.