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Tailstock alignment questions

Wanted to circle back to this topic.

After inspecting my MT2 dead center for my tailstock it seemed 'off' or 'not concentric'. This is the MT2 dead center that came with my lathe. I saw new Busybee MT2 dead centers were 10 bucks so last time I was at busybee I picked one up.

Alignment now seems much better.



IMG_2409.webp
IMG_2411.webp
 
So I spent some time working a piece of 1" round bar with about 8" of stickout from my 3 jaw chuck (no dead/live center support) and examining the taper.

Initial taper was .030 over ~7", with the chuck end being the smaller dimension.

I started adding brass shim stock under the front right pad ( when looking from front) on the bed and taking another skim pass. 2-3 hours of this rinse and repeat and my taper is somewhere around .0005 over said 7-8" of bar.

EDIT: Added pics

IMG_2412.JPG

IMG_2413.JPG


I lost track of how much shim stock there is under that foot. I'm going to remove it tomorrow and measure it and then source some large pieces of the right thickness and cut them to fit the entire foot and not just the bolt boss area. I *think* I can improve on the .0005, but I suspect once I rearrange my shop and move the lathe that I'll need to redo the 'levelling' process.
 
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Going in the right direction & getting closer at least. There is a limit to seeking a perfect cylinder when cutting because the material is cantilevered, which of course it has to be for this test. So there is going to be slight material deflection on right side as the tool both cutting and deflecting the material ever so slightly. Some people advocate using aluminum because cutting forces are lower, reasonably stiff, temperature stabilizes faster, possibly cheaper as you turn it into swarf. I know it works but mostly I didn't have decent steel like maybe 12L14 of diameter I wanted to beaver away at. What I think is important is to dial in the exact same, reasonable depth of cut every pass & lock the cross feed. ie whatever is happening at the cutting edge we can only control so much, but at least by repeating we aim to be consistent.

I'm working on some graphics for lathe twist discussion which I'll post at some point. There's actually a lot of interesting things that are happening behind the scenes. Might not be able to quantify things, but maybe (hopefully) have a better understanding of what we are dealing with.

ps I always have to ask this question, especially because I don't know your particular lathe. Is the headstock 'settable' to the bed in any way? We beat this topic to death in another post but I'll just ask the question.
 
PeterT said:
ps I always have to ask this question, especially because I don't know your particular lathe. Is the headstock 'settable' to the bed in any way? We beat this topic to death in another post but I'll just ask the question.
Click to expand...

No, it is not. The headstock casting has a dovetail that matches the bed. Not sure dovetail is the correct term?
 
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OK, well that eliminates one source of misalignment & actually makes life easier. Many of the Asian lathes are assembled with headstock bolted to bed way & factory positioned with a form of set screw or shim wedge. If that gets out of whack & spindle points north or south, then it needs to be dealt with before bed twist adjustment is addressed.
 
calgaryguy said:
So I spent some time working a piece of 1" round bar with about 8" of stickout from my 3 jaw chuck (no dead/live center support) and examining the taper.

Initial taper was .030 over ~7", with the chuck end being the smaller dimension.

I started adding brass shim stock under the front right pad ( when looking from front) on the bed and taking another skim pass. 2-3 hours of this rinse and repeat and my taper is somewhere around .0005 over said 7-8" of bar.

EDIT: Added pics

View attachment 22945
View attachment 22946

I lost track of how much shim stock there is under that foot. I'm going to remove it tomorrow and measure it and then source some large pieces of the right thickness and cut them to fit the entire foot and not just the bolt boss area. I *think* I can improve on the .0005, but I suspect once I rearrange my shop and move the lathe that I'll need to redo the 'levelling' process.
Click to expand...

I agree with what @PeterT has said.

When I first did mine I reasoned that a steel pipe with a fatter section at each end was a better way to do it. Pipe has a better bending performance for the size AND WEIGHT than a piece of solid steel has.

Recently, I've been looking at what I could do to improve on the original design. I got sidetracked with milling projects, but my overall thinking is that it's good to have a bit of length to improve the measuring accuracy but not so long that it bends from its own weight. It's good to evaluate the bending and factor that into your analysis of the alignment. You can simply measure the deflection by hanging a weight at the end of the pipe. That is half the weight of the pipe itself. That measured additional deflection will be roughly the same as the bending of the pipe under its own weight.

II also think that aluminium collars on the two ends of the pipe VERY LIGHTLY cut with a very sharp HSS tool minimizes the resulting cutting pressures and side deflection.

I also think none of this is really necessary. But it is helpful to have a good understanding of what causes what when you are assessing the alignment of your lathe.
 
I think that's a great idea. It should leave a better surface to measure on too!
 
RobinHood said:
Just thinking out loud: what about using a tool post grinder (if you have one) instead of a cutting tool for the test? Might reduce cutting forces even more.
Click to expand...
Yes, but then it wouldn't be replicating a real world situation of the cutting forces involved during normal turning.
 
An alternative way to check for bed twist is with "Rollie's Dad's Method of Lathe Alignment" (RDM):


Briefly, this method calculates where the actual centre of rotation is in relation to the bed by simply taking a measurement on a (straight-ish) piece of stock held in the chuck. First one spot and then rotating the piece 180 degrees to the opposite side. The average tells you the distance to the centre of rotation. Do that at the near end and the far end, and the difference between the two averages tells you how much the centre of rotation has changed relative to the bed ways.

Note that you can check both the vertical alignment as well as the horizontal alignment just by re-orienting your dial indicator.

Once the bed twist is under control, you can use the normal methods to align the tailstock.

Craig
 
thestelster said:
Yes, but then it wouldn't be replicating a real world situation of the cutting forces involved during normal turning.
Click to expand...

Damn you @thestelster ! Another excellent point! The question is "Which approach is better?"

I will have to give this some thought and perhaps some research.

It also begs the age old question of how much is too much.

I always like to establish what my gut says before I dive in too deeply. Failing to do it right away, despite the risks, robs one of the opportunity to do it all.

At first blush, I think it's better to start as close to zero as possible and then factor in the various real world effects. Since the real world varies so much based on a plethora of factors (speed, feed, depth of cut, material, etc etc) factoring them in too early then requires a double change to factor them out and then factor the new ones in. Better to establish a zero and only factor in once. Aligning a headstock is not something we want to do whenever something changes.

But perhaps it's something that should be evaluated and quantified before worrying about it.

Lastly, how much fussing is warranted? Perfection for its own sake is more-or-less a waste of time. Does it really matter?

I don't mind thinking about it and even doing a few tests to evaluate it, but after all that is done, I still need to be convinced that it matters. If not, I prefer to move on to things that do. In this regard, my views are biased.
 
Susquatch said:
Since the real world varies so much based on a plethora of factors (speed, feed, depth of cut, material, etc etc)
Click to expand...
Yes, I agree, so I would think that you would do the test according to what your final operation will be. If you will be finish turning, rough turning, grinding, test under the same circumstances.
 
Interesting conversation. I do not have the depth of knowledge that a lot the posters have when it come to machining or this topic but here is my thoughts on the the matter. In the end what you want is repeatability and hitting the tolerances that are required when making parts. Skill level and knowing your machine as well as machine accuracy go hand in hand to achieve this. When you are starting out machine accuracy goes a long ways to achieving good results, just don't kill yourself trying to achieve perfection. I spend hours aligning my lathe and leveling it. I achieved very good in the first couple of hours and the rest of the time I was chasing my tail trying to achieve perfect. In my opinion it was not worth it. Just my two cents.
 
RobinHood said:
Just thinking out loud: what about using a tool post grinder (if you have one) instead of a cutting tool for the test? Might reduce cutting forces even more.
Click to expand...
You are killing me RH! My 'toys' budget is depleted as it is! ;) Seriously tho, I do not have a TPG. I'm keeping an eye out for one however.
 
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As a general response to the 'perfection has its limits' responses above, I agree 100%.

.030 taper over 8-ish inches was ridiculously bad.
.0005 taper over 8-ish inches is, for now at least, acceptable and something I can work with.

Further, so much of my 'playing' and experimenting at this point is learning this lathe; its nunaces and limitations. I havent operated a lathe since high School ~35 years ago, so this is a much needed refresher. All those mental checks many of you like take for granted arent imprinted on me yet (is my tool post locked, is my feed in the right direction, is my TS locked, is my feed rate set right, did I remove the chuck key). For me this comes with repetition and practice.
 
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