Lathe alignment issue

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Pete301

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I recently bought a Logan 10” lathe, model 815 and I’ve been working on getting it setup. I’m puzzled by a problem I’ve run into.
I first leveled the lathe. Then I mounted a test bar between a center in the spindle and a center in the tailstock. Adjusted the tailstock till I got it cutting over a 10” length with less than 1/2 thousandths of a taper.
Then I mounted my chuck and cut a 60 degree center held in the jaws. I measured the chuck held freshly cut center and it had no detectable runout. Without taking that center out of the jaws i then mounted my test bar with a center in the tailstock. But this time i cut a taper! Bigger by 4 thousandths at the tailstock end. So I’m getting a big difference between when a center is in the spindle versus a fresh cut center in the chuck.
I’m puzzled ( and afraid) as to what could account for this.
I don’t think either bed wear or bed twist could cause the difference because it seems like both would affect the test the same. Wouldn’t matter how the center was being held in the lathe.
So is the likely problem that the spindle bore and the spindle threads aren’t concentric? Or, am I missing a different cause.
 
Pete301 said:
I recently bought a Logan 10” lathe, model 815 and I’ve been working on getting it setup. I’m puzzled by a problem I’ve run into.
I first leveled the lathe. Then I mounted a test bar between a center in the spindle and a center in the tailstock. Adjusted the tailstock till I got it cutting over a 10” length with less than 1/2 thousandths of a taper.
Then I mounted my chuck and cut a 60 degree center held in the jaws. I measured the chuck held freshly cut center and it had no detectable runout. Without taking that center out of the jaws i then mounted my test bar with a center in the tailstock. But this time i cut a taper! Bigger by 4 thousandths at the tailstock end. So I’m getting a big difference between when a center is in the spindle versus a fresh cut center in the chuck.
I’m puzzled ( and afraid) as to what could account for this.
I don’t think either bed wear or bed twist could cause the difference because it seems like both would affect the test the same. Wouldn’t matter how the center was being held in the lathe.
So is the likely problem that the spindle bore and the spindle threads aren’t concentric? Or, am I missing a different cause.
Click to expand...
Not gonna be much help for you. Just learned how to do the 2 collar test last night and despite my lathe not being perfectly leveled I still didn’t seem to have any taper issues. It’s definitely sounding like something is up with the spindle end of things though
 
Did you check the spindle center mounting for run out before cutting the test bar? Did you move the tailstock (or unlock and re-lock) when you rearranged the test bar?
 
jcp said:
Did you check the spindle center mounting for run out before cutting the test bar? Did you move the tailstock (or unlock and re-lock) when you rearranged the test bar?
Click to expand...
I did check the spindle center with an indicator before the test. Also checked the spindle shoulder and face of the shoulder. Nothing noticeable as an issue.
But you got me thinking about the tailstock. It is in a different location on the lathe for each test. The chuck extends from the headstock further that the spindle center so the tailstock isn’t in the same spot on the lathe for each test. So wear and twist could enter in as factors? But yikes, the different tail stock location is only about 3.5” and I have a .004 taper difference. I have a smaller chuck that extends less from the headstock so I’ll retest and see what happens.
 
Seems like your chuck, jaws or chuck plate is not concentric with spindle. Just $0.02 worth cause I’m just learning.

Chuck
 
Pete,
I recently went through something like this (see my saga in the recent Unidentified Deflection posts in this subforum) with my Logan 820 and learned that tapers can result from the tailstock being off in either or both the X and Y dimensions. X is the obvious one, which you tested with the bar between centers. But, if the tailstock center is lower or higher than the spindle center you'll also get a taper. I think we have the same tailstocks. I (with teaching help the first time) disassembled it and shimmed between the two halves, hoping it was too low, and landing finally on .012" shim to bring the Y alignment almost perfect. If you do that, be sure to have a sturdy place nearby to disassemble and reassemble the tailstock. It's clumsy and just heavy enough to do some damage.
Now that I have a new MT3 center for the spindle, I'm going to use the lathe dog that came with the lathe and experiment with turning between centers, see how that works out.

Tim
 
jcp said:
I would suspect a tail stock problem before a head stock or spindle problem.
Click to expand...
I’m going to follow your advice and check out the tailstock. Perhaps my test difference is more a result the tailstock not being consistent, being in different locations, too high or to low, etc.
 
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The dead center running in the spindle will run true and concentric providing the spindle bearings are ok. Something that could cause the change in runout when switching to using the dead center in the chuck, is if there is any bellmouth issue with your chuck jaws. When the chuck jaws are bellmouthed, the tool pressure on the part will push the part away from center. This would cause cutting a taper. You may want to check the condition of the chuck jaws to make sure this isn't the problem.
Just food for thought.
Ted
 
talvare said:
The dead center running in the spindle will run true and concentric providing the spindle bearings are ok. Something that could cause the change in runout when switching to using the dead center in the chuck, is if there is any bellmouth issue with your chuck jaws. When the chuck jaws are bellmouthed, the tool pressure on the part will push the part away from center. This would cause cutting a taper. You may want to check the condition of the chuck jaws to make sure this isn't the problem.
Just food for thought.
Ted
Click to expand...
Thank you Ted. That had not occurred to me so I’ll have to check it out. The number of things that could cause this problem is growing! But issues other than the spindle itself are a welcome relief.
 
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