[How do I?] Fix Lathe Runout When Turing Shafts

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gapi

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I am using a Victor 1740 to turn down a 20mm shaft to 15mm with an insert bit.

I need firm bearing slip fit on each end and am coming up with almost .022mm smaller at the chucked end.
Do I need to make a tail stock adjustment.

Any suggestions or fundamentals I need to check would be great.

Thanks
 
Sounds like you need to adjust the tail stock about .011mm towards the tool bit.
 
I hope that's all it is. A magnetic base on the ways or cross slide? I use an Interapid .005 dial.
 
It really doesn't matter where you place the indicator base as long as you can get a stable reading.
 
gapi said:
I am using a Victor 1740 to turn down a 20mm shaft to 15mm with an insert bit.

I need firm bearing slip fit on each end and am coming up with almost .022mm smaller at the chucked end.
Do I need to make a tail stock adjustment.

Any suggestions or fundamentals I need to check would be great.

Thanks
Click to expand...

Are you using a tailstock center? If so, your tailstock is likely not adjusted properly. If not, it is normal to see deflection when the length is very much greater than the diameter.

There are several thing that you can do. One is to turn your end diameters to their critical dimensions separately. Bring each close to dimension and mike the diameter. make adjustments to your cross feed accordingly. If you are saying that you have a taper of .o22mm over 150mm, that is under a thousandth which many would kill to be able to do.

If you desire to keep ultra tight tolerance over the entire shaft length, another is to use a properly adjusted tailstock with a center and a follower to help control the deflection.

It does seem to me that you are pushing the limits of the machine though. Precision shafting is usually done by centerless grinding. McMaster Carr's precision ground shafting has a tolerance spec. of -.011/+.ooo mm on a 15mm shaft.

Bob
 
gapi said:
I hope that's all it is. A magnetic base on the ways or cross slide? I use an Interapid .005 dial.
Click to expand...

I do tailstock alignment in the same way as shown in this video.
With one exception, I dont use a faceplate I just place the magnetic indicator holder directly on the chuck.
Make sure the indicator holder is sturdy, or it could easily give you a false reading due to gravity bending the holder rods slightly. (cheap ebay Noga copies are not sturdy enough)
 
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I use an indicator only horizontally at the end of the part that shows taper. Move the shaft half the measured difference in whichever direction to correct the taper. Then take another cut. If there is vertical misalignment, the remedy gets more complicated, but for simple shaft turning like you are doing, the horizontal position has a much larger effect anyway.

I've seen people put the indicator on the ts ram, but the amount you need to move is most accurately measured at the part, not the tailstock. Keep the center snug during the process, although you will need to loosen the clamp to allow the adjustment screws to do their thing. Just leave the clamp snug enough to hold pressure on the shaft. Then tighten the clamp to the bed and see what the indicator tells you you actually moved the shaft. It may take a couple of tries.
 
Your lathe is robust enough. Holding tolerances that you are asking for can be done on a lathe and is all the time, but finesse would be a good thing to have. The definitive machine would be an OD grinder for this job. Using Carbide could be problem when taking small/thin cuts. Whereas HSS would not be a problem. Using an emery stick on the last few tenths might be a good thing or burnish the last few for controlling the dimension. An indicator is not needed to adjust the tail stock. Just note the fraction of a turn of adjustment and make a test cut. Then you have an idea of how much of a turn is needed for correction if any more is needed…Good Luck, Dave.
 
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