Master precision Machinist level

Dabbler said:
@Ben17484 I have some negative-Nelly news for you (what an alliteration!). Unless you bought a M&W or B&S MT test bar, you are best off assuming it is junk. Get a piece of ground stock - it is referred to a O1 precision ground, Drill Rod etc. Get a short piece in 1" and use that to set up your headstock and measure the twist in your lathe. The ground stock won't break the bank is far better than any offshore test bar you can buy, I've "proven" 5 of them so far. 5 for 5 were junk.
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Thanks Dabler. So to make sure I understand, this isn’t used to align the tailstock, just to put in a chuck and then sweep with an indicator places on the saddle (so no tailstock use at all)? That will show if the headstock is aligned without having to take any actual cuts?


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@Ben17484 the problem is the same, tailstock or headstock. These test bars are so poorly ground that the MT taper is not coaxial to the ground straight side. It will give you erroneous readings, and can be 'proven' by measuring the runout of the bar. I've even seen bent bars. I have tested 5 of these bars that other people have purchased.

Use a ground bar in your chuck instead. (this is a longer answer, but not in this post)

The RDM method is a little more coarse, but is very accurate. It is easier to practice for a novice.
 
Dabbler said:
@Ben17484 the problem is the same, tailstock or headstock. These test bars are so poorly ground that the MT taper is not coaxial to the ground straight side. It will give you erroneous readings, and can be 'proven' by measuring the runout of the bar. I've even seen bent bars. I have tested 5 of these bars that other people have purchased.

Use a ground bar in your chuck instead. (this is a longer answer, but not in this post)

The RDM method is a little more coarse, but is very accurate. It is easier to practice for a novice.
Click to expand...

My questions was more around how I would use a ground bar for testing tailstock alignment (which is what I thought you were initially suggesting). In my head, if I buy some precision ground bar, the only way to use that to align the tailstock would be to centre drill it, mount between centres and measure with an indicator whilst sweeping down the z axis. This however wouldn’t work unless the tailstock was perfectly aligned when drilling the centres. I can now however see how mounting a large diameter piece of ground bar in just the headstock (well, in a chuck) would allow me to align the headstock.

Does what I’m saying make sense?


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RJSakowski said:
I don't use the tailstock when doing the two collar test. I use a rigid enough bar tha deflection is minimal and make very light skim cuts.
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I use a piece of 3"sch 40 aluminum pipe about 14 inches long for the 2 collar test. It should sag way less than a solid bar and with a sharp HSS cutter I don't get any resonance.
 
This post is nearly a hundred replies long. Scanning over the information, it looks like maybe we’re after some sort of upper level precision fiddling.

If you’re new: Before you go down some rabbit hole of chasing ultra (unnecessary) levels of lathe setup perfection, you might just turn a 4 inch long stub of metal in your lathe and measure both ends. Then decide if ultra overboard and time consuming lathe setup is even something you need to learn.

I doubt knowing most of this is necessary for 98 percent of us.

Good enough is good enough.

And since this was supposed to just be “do I need an ultra master 5000 super precision bubble in a bottle”. The answer is still no.
 
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