A head scratcher here …

What I've learned is that everything is made of rubbery marshmallows.
You think that the 3" of cast iron your lathe bed is built from would be solid. It ain't. It dances like a lady of the night on meth.
At least with a mill, you can see the tram getting off when the herring bone pattern goes away when using the flycutter.
 
I've had great results using Rollies Dad's Method. Everyone should at least learn and try it before spending money on a test bar that will rarely be needed. I just had the headstock off of my lathe for some work, paint and some of mksj's magic. A quick test cut showed .003" taper in 2.5" after bolting everything back together.

I used a random piece of 3/4" cold rolled (in a 3 jaw) that was 18" long to align the spindle. The near end was +/- .004" and the far end showed +/-.015" of runout. After adjusting the headstock, I did a quick 2 collar test with a piece of 2.5"aluminum pipe 12" long, 9" from collar to collar and the difference is within .0005". Takes less than an hour, it's free and it's more accurate than I need to be for most of the work I turn out.

The most difficult thing about aligning the headstock is getting back to where you were before turning the wrong bolt the right way or, was that the right bolt the wrong way?
 
I agree that buying a test bar is not necessary for a head stock alignment.
 
Firstram said:
I've had great results using Rollies Dad's Method. Everyone should at least learn and try it before spending money on a test bar that will rarely be needed. I just had the headstock off of my lathe for some work, paint and some of mksj's magic. A quick test cut showed .003" taper in 2.5" after bolting everything back together.

I used a random piece of 3/4" cold rolled (in a 3 jaw) that was 18" long to align the spindle. The near end was +/- .004" and the far end showed +/-.015" of runout. After adjusting the headstock, I did a quick 2 collar test with a piece of 2.5"aluminum pipe 12" long, 9" from collar to collar and the difference is within .0005". Takes less than an hour, it's free and it's more accurate than I need to be for most of the work I turn out.

The most difficult thing about aligning the headstock is getting back to where you were before turning the wrong bolt the right way or, was that the right bolt the wrong way?
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I agree, this method relies on first principles, not on a mandrel that you hope is accurate.
 
I've been doing a lot of study on this, and my conclusion (yours may differ), is that RDM gets the bed to align with the spindle, no matter what else is screwed up. :)

Your spindle could be pointing at the back wall, and RDM will try to bring the bed into alignment with it so that you get a straight cut. If the spindle isn't too far off, ie a few thou, RDM will be successful at that. But, take a facing cut across a 6" plate, and you'll be able to serve the first course of dinner out of it. Assuming the first course is still soup, of course. If you never do large facing cuts, and you need good tools to tell if you're lathe was out of line to begin with, then RDM was all you were ever going to need.

I have a 16" lathe, that I know I'm going to want to push to its dimensional limits in the near future. After doing RDM, I'm still detecting several thou of dishing. So, I gotta spend more money on tools that will be rarely used. :-(
 
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