Question About Headstock Alignment

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ddickey

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I bought a MT3 precision alignment bar a while back. Decided to try it out today. I ran an indicator along the 13" length and showed .003" taper. I had a .075" shim under my far TS foot. I reduced it to .045" and the indicator now showed zero taper. On the vertical plane it was .00039" (.01mm). I think I'm good there as I'm sure there is some sag in the bar.

Anyway I remember reading an article about Rollie's dad's method so gave it a try. Let me know if I did this correct. I zeroed my indicator at the low point and got .001" movement at the HS. Did the same at the TS and got .0055". After doing the calculation I get a delta of .0025". Figured I'd try to shim my HS like the instructions say but in the process I found a couple small jack screws used to align my HS. I decided to abandon the shim and just make a HS adjustment. No matter which way I moved the HS the delta on the indicator didn't change. I did get a nice big taper though which I aligned back out.

Any thoughts on what I did? Does shimming the HS do something else rather than just aligning the HS?
 
Dicky,

Without knowing the overall condition of your bed in means of wear, I would leave well enough alone. Adding the shims as you have indicated leave me to believe, you have serious wear issues in the major components of your lathe including the bed. Not familiar with your lathe, but common with all lathes, they wear after years of use regardless how well you take care of the lathe.
What you have to do is start from the foundation and work up until the problems are solved. So starting with the foundation, the bed, do you have a straight edge of known accuracy that can be used to determine flatness of each bearing surface or shear. Feeler gages can be used for this exercise.

Don't get me wrong, you are doing a good job determining areas of wear such as under the tailstock, and that's good. The headstock should be the last thing that should not have any wear under it. It should be correct as it left the factory. I think it has told you there is wear on the bed, the 0.003" you're chasing, wanting to add shim under the headstock and don't do.
 
When I tweak something to try and fix a problem and it makes things worse, I try to go back to basics to find out where I went wrong or to find the root cause of a problem.

The fact you measured a taper means something is out, could be due to wear, could be twist in the bed, etc.

I would recheck the lathe level. A good video by Keith Rucker. Try and mount the level on the carriage.


Keith shows machining a short test adapter which is exactly the same OD as the quill, then mount the adapter in the chuck and test the height from the carriage with an indicator at the headstock, then the top of the quill. I thought this was a good way to check alignment.

If someone added shims in the past you need to try and assess what issue they were trying to fix. You may want to try and confirm the axis of the headstock and tailstock are parallel to the lathe bed. Can you mount the MT3 test bar in the headstock and separately the tailstock, then run an indicator down the bar? The bar may have some sag, but at this point it is worth trying to find out the present state of the machine before doing any more tweaks.
 
Yeah that's what I did. There were no shims when I bought the lathe. At first I tried taking the twist out by using leveling casters which worked some but when I realized I only had 2 of the 6 casters touching the ground I knew I needed to try something else. I decided to shim under the feet of the lathe on the TS end with .075" shim and got all the taper out. This was confirmed by using the two collar test bar method. That was a while ago so checked again and saw the taper. This time using a MT3 test bar. I reduced the shim to .045" and then the indicator showed no taper over 13". I did not take any cuts however. Since I had the alignment bar in the HS tried Rollie's dad's method. According to what I read my spindle is misaligned .0025"
 
After some reading I will abandon the RDM method. Will put things back together. I have a project to do.
 
Hello Dickey.

The RD method of lathe alignment is very powerful. You can use the method to align the headstock in both horizontal and vertical planes. And then the tailstock with its ram retracted and then the TS with its ram extended, again in both planes.

As you know, for a constant diameter test bar, the RDM calculation uses high and low gauge readings at 2 locations (1 and 2).

The misalignment is given by

dA = (G21 + G22)/2 - (G11 + G12)/2

Once the dial indicator has been set it at one position it should not be reset at the second.

So, for example, say your gauge shows +0.0005" and -0.0005" at position 1 (the headstock) and +0.0025" and -0.0025" at position 2 (the tailstock) then

dA = (+0.0025 - 0.0025)/2 - (+0.0005 - 0.0005)/2

i.e. dA = 0

the headstock is perfectly aligned in the plane you are measuring.

Hope this helps.
 
I monkeyed around with it some more but gave up as I could not get the difference <.0018".
I think I did it wrong as I was adding the two then dividing not thinking one number is a negative. Oh well.
 
A way to avoid negative numbers is to zero the dial indicator at the lowest position and then the other 3 readings will all be positive.
 
Good idea. Was just not sure if I was supposed it leave it alone.
 
That's not how I do the RDM method.

RDM compares the averages of the two measurements. For the headstock measurement I'll adjust the DTI so the needle moves the same amount above and below zero. Then I'll move the DTI to the tailstock end. If the needle travels the same amount above and above zero (even if it's a lot more movement) then the headstock is aligned. If the needle travels further on one side of zero, then the headstock is out of alignment by half of the difference. Honestly, I'm not sure how I'd be able to do it by setting zero to so all travel is positive.
 
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