Taper

[140mower]

Perfect. Thanks Mike.

As an aside, I really enjoy your use of the English language to make clear consise statements about very complicated things using relatively few words. It certainly works for me. My answer to that same question would have been 4 pages long and added nothing else. So, I'm jealous of your talent.

Edit - and then I see you added to it and essentially doubled it plus added a photo...... LMAO!

I think you might actually have one of my many faults too. I write something and then realize that not all readers are the same and Somebody who thinks differently might miss my meaning. Then the words get added. Then I worry I might have insulted the original reader. Then I realize I need to fix more. Next thing you know, I have 4 pages of mostly unnecessary verbia.

Not to worry, we all have moments of verbal diarrhea. I am just happy to see someone going through the effort of helping those of us in need.

 

[Susquatch]

but then when he cut stock - even short stock - he does. well, one sure way for that to happen is for the stock to be held by the chuck in a way that isn't concentric to the axis of rotation.

For the most part I think we loudly agree too. But not on the particular point I quoted above. Which is where I stumbled before.

I'm gunna, try to take a page out of @Mcgyver's book and be short and suscinct.

It doesn't matter how out of alignment a part is held in the chuck jaws. When it is subsequently turned, the turned part will be concentric to the spindle axis.

This assumes good bearings and a spindle with no perceptible run-out.

To create a taper on a short part, the spindle axis MUST be out of alignment with the ways (bent, worn, damaged, twisted, etc).

This assumes that the cross-slide is not slipping and the cutter is on center.

You have a nice lathe and I bet it is reasonably well aligned. Try it!

Use your 4-jaw or a big shim to mount a 5 inch part say 2" in diameter with a 3" stick out that is deliberately mounted as you described.

in a way that isn't concentric to the axis of rotation.

Use unequal shims to make it obcenely out of whack. Then cut it until the cutter cuts clean all the way around for all 3 inches of stickout. The final part will be concentric with the axis of the lathe. It has to be.

 

[mbond]

I think we are talking across purposes again. Ignoring all the bad stuff that could happen with a severely compromised machine, any cut will be concentric with the spindle axis. Exactly as you say, it has to be. But when you take that part out of the lathe, and measure it you won't get 'conformity' between the sections that were chucked and turned