Lathe is turning a taper

what method are you using to align the bed ? if you can find 2 4ft. straight edges you can clamp them to the lathe bed next to the headstock and at the tailstock then check for level across the ends. the twist if any magnified 4 ft away will show any misalignment. if you need to true up the jaws of the chuck its not too difficult with a tool post grinder.several people have utube videos on the subject. the first thing i would look at is tailstock alignment.
 
Many thanks for the loads of info and suggestions here. I will be away for the next two days, so I won't begin the more thorough investigation until the end of the week.

Thanks again to all.

Regards
 
Lots of good advice. Your TIR on the test is very linear with distance, so the rod looks good at this position. I assume the max min in the runout occurred at the same phase of chuck rotation? So, another test suggestion. After cleaning, but before reworking the chuck I would do a simple test to check the chuck jaws. 1) Measure the run out as a function of rotation angle phase, not just TIR. Unclamp the bar and rotate it ~ 120 degrees or so in a 3 jaw chuck. Keeping track of the phase angle of the chuck as well as the phase of the bar. Remeasure. Are the max points and min points at the same phase angle or did they change with the jaw clamping? Repeat at other angles to be sure. In addition to uneven jaws, I would think that small amounts of debris at the jaw can make it clamp at an angle other than on axis. Likewise, a bar which has a bump, debris, or bend at the jaws.

2) If you have not done so recheck your lathe bed for level. Really the first think to do. I do not expect my PM1440GT to remain level over 2 years. My concrete floor sets on clay and as it gets wet and dry it moves!

Good luck.
 
Within the last year I had the same problem with my lathe. To that point I had never accurately leveled or checked my lathes ways for parallelism. The way that I went about these things probably wasn’t the most efficient way but it worked to get me cutting straight.

To be more accurate though your problem could definitely be in the chuck. Or like many other people have described possibly a chip inside of it or on your workpiece or jaws. Also to turn the most parallel pieces it may be beneficial to turn between centers( opinions may differ on this ).

Bed twist was my problem but I prefer to describe it as parallelism of the ways. I measured the ways length wise at several points and and leveled them using the adjustable feet of the stand to the floor the best I could. Then measured across them using 1-2-3 blocks to raise the level above the V ways. I didn’t get get everything perfect but as good as I could using shims between the bed and the stand it’s on. This took several attempts. In the end I was cutting .001 per foot and I’m happy with that.

Best of luck with your problems. I think it’s worth mentioning here that it will make your life easier ( in my opinion ) to tackle one alignment issue at a time starting with the bed. Of course a good cleaning of the chuck is quick and easy. And checking things before any adjustments is a must. There’s a lot of things that can be causing the problem you’re describing and finding the culprit is the next step.


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