Bore Centering

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Henryrifle

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I own a relatively inexpensive asian lathe that appears capable, based on results, of producing accurate barrels. No doubt my question, and, possibly my methodology, will underscore my lack of experience in machining and lathe work in particular.

I use the 4-jaw chuck and some aluminum shims to hold the barrel in the head stock. An outboard spider is used to support the muzzle end of the barrel. I have been using PTG range rods with appropriately sized bushings to center the bore prior to cutting and threading the tenon. Starting with a .001 gauge, I adjust both the 4-jaw and outboard spider to obtain the smallest practical runout of the range rod both close to the tenon and at the end of the range rod furthest away from the tenon. After there is less than .001 runout I switch to a .0001 dial indicator. It is a Mitutoyo 2803S-10. This is not a expensive indicator but it is a decent one--I believe. I continue adjusting the 4-jaw and spider until I get less than .0002 of runout close to the tenon and as far away as the rod allows. All of this adjusting is done with the lathe out of gear and spinning the 4-jaw by hand.

Here comes the question part: I expect that when I run the lathe under power that I will get the same runout measurements but, that is never the case. The runout is always much worse under power. Short of buying a better lathe, can anything be done about that and if anyone else is experiencing this, what workarounds are you using that you are willing to share?

I assume this is a result of inexpensive bearings and other mechanically sloppy tolerances. I have 'resolved' or worked around this issue but writing numbers on the tenon with a permanent marker and using my phone to video the lathe in operation in slow motion. I play back the video and make the necessary adjustments to get back to less than .0002 runout under power. I do always wonder, though, which measurement to believe: The ones where I turn the chuck by hand or the one under power. Given that I cut, thread and ream under power, I have chosen that that set of measurements to get 'right.'

Thank you for thoughts,
Henryrifle
 
I would say your problems are within the headstock bearings. Take the chuck off and check the spindle runout. Push and pull the spindle with a indicator on it what’s the readings. Might be as simple as the preload.
 
If I understand you: using a decent quality DTI and taking readings inside the spindle nose do you get less than .0002 total ind runout?

If so, run out will only get worse as you add more tooling or do more operations.

Also the accuracy and rigidity of the lathe bed, saddle, cross slide and tool post will impact precision.

One work around is to use piloted reamers. Or a hollow mill for outside diameters.
 
I find lifting the indicator probe off the work and giving the work a few light taps with something 'settles' the work. It can shift quite a lot. I now do this as a matter of course when setting up.
I think this movement is in the chuck jaws.
 
Thank you all for the ideas. Will start testing the spindle, spindle face and nose tomorrow. Will report back findings.

Henryrifle
 
I don't have range rods. I figure at some point I'm gonna have to turn the barrel to contour it and get it concentric with the bore so I just do it up front. On the chamber end I put the center into the bore as any damage will be machined away anyway. At the muzzle I use a centerdrilled brass insert that has about a 1.5 inch stem and is .001 over land diameter. I drive that into the bore and then turn on centers. If the muzzle is to be cut I don't bother with the brass piece. Once everything is concentric with the bore I indicate on the OD at both ends using a 4 jaw and a spider. I only built two rifles this way. Both 45 caliber smokeless muzzle loaders. The bolt action groups 1 MOA. The break open is a cheap shotgun converted to a smokeless muzzle loader by stubbing and groups 2 MOA .
 
Henryrifle said:
can anything be done about that and if anyone else is experiencing this, what workarounds are you using that you are willing to share?
Click to expand...

My experience with range rods is the same. Some of the things I did just because:

-replace the 4 jaw with a spider, gives less overhang and less amplification of inherent spindle run out
-refine my technique to get as little stress in the barrel as possible
-get a better .0001'' dti

My theory is some of the extra run out when running under power is due to dti jump. Also, clearances in the range rod bushing/rifling and bushing/rod might be a source of run out more present when running under power. Turning by hand and stopping at each jaw makes more sense to me.


I also concluded that the readings opposite a jaw taken with the barrel stationary were the most reliable. Results on the target left me wondering how on earth can one determine the difference .0001'' of run out makes.
 
Do you warm the machine up before indicating the barrel? Are you sure that the barrel is not being stressed when you are adjusting the rear spider? It is very easy to put the barrel in a bind when you adjust the rear spider. The barrel needs to be able to "gimble" at the chuck.
 
I think there are two parts to the question: how much error is due to the spindle and how much is due to everything else (carriage, bed flex, etc.)
How big is this lathe? Brand?
-Mark
 
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