Yes that's true - the part always spins on the spindle axis, but if the chuck doesn't hold the work concentric to that axis, a taper of some sort is inevitable. That's exactly what happens when you purposefully cut a taper with an offset tailstock etc.
I think this might be the first time you and I disagree. Mind you, it might also be terminology or choice of words. Let me address your comments one at a time. Please forgive any repetition. My intent is to get us both on the same page speaking the same language. But more than that, your comments provide a great opportunity to explain things in a more holistic way for the benefit of anyone else reading this. Hopefully it will be helpful and not too wordy.
but if the chuck doesn't hold the work concentric to that axis, a taper of some sort is inevitable.
You will note that I originally said:
"
the part ALWAYS spins about the axis of rotation of the spindle. Therefore, the part will get cut concentric to that axis. A taper will still be concentric, but it will have varying diameter depending on the geometric relationship between the cutter and the axis of rotation."
I added the bold this time. The important point I was trying to make here is that it doesnt really matter how the part is held as long as it is firmly held.
At any one point along the axis, a lathe tool will cut a concentric circle about the axis of the spindle
. Other points may not have the same diameter for all the reasons that you mention and more, but all of them will still be concentric. A taper is just a series of concentric circles each with a different diameter. (I'll come back to the subject of tapers and offset tailstocks a bit later.)
Perhaps you missed my comment about the diameters changing at different axial distances as outlined in the last half of the bolded quote above.
Fundamentally, my original point was that even a crappy chuck that has a deliberate offset will cut concentricly to the spindles axis. However, the diameters along its length might wander around because the cutting tool wanders around, not because the axis moves. Any lathe where the axis moves needs new spindle bearings or a trip to the junkyard.
Imagine a two inch bar that is deliberately clamped in a 4 jaw 1/2" off center. That's basically a gross example of a 3 jaw with poor jaws. When you first start cutting, only one side will cut. As the cuts progress, it will gradually make the far side cuts longer and longer until it eventually cuts all the way around. At that point, the bar will be a perfectly concentric bar that is 1.0 inch in diameter.
That's exactly what happens when you purposefully cut a taper with an offset tailstock etc.
Yup, agreed - but with some caveats. Usually, tapers cut using a tailstock offset, are cut between centers - ie with a center in the chuck, and a center in the tailstock and the part driven by a dog. In this case, the axis of rotation isn't the spindle, it's the axis between the two centers. An exception to that is for stock that is flexible enough to be held fixed in the Jaws but can still bend to reach the offset tailstock center. This latter case doesn't cut a perfect taper though. It will be more of an inverse parabolic taper with the fastest rate of change at the tailstock.
I'd bet really big bucks we don't really disagree. My real point was to explain to the OP that poor jaws (no matter how bad they are) are not the reason he is getting a taper.
A part that has been gripped unevenly by the chuck is _my_ most common error. In my case operator error, but if a chuck that has some kind of balance / alignment issue, or hasn't been mounted accurately the problem will happen too. Assuming even tool pressure, these tapers should be consistent. And when this is the problem, a tail stock will reduce the taper by more than the 1:4 ratio for tool pressure deformation
I don't understand why that would be. Balance issues are another whole kettle of fish and so is inconsistent chuck mounting. But a chuck that is poorly mounted or is misaligned will still cut a concentric bar unless the tool or the chuck actually moves during the machining.
Perhaps what you mean is that an existing concentric part that is misaligned or poorly mounted won't maintain its existing axis....... But a part that is misaligned when it is mounted will end up axially aligned after it is cut.
Worn ways are probably worn unevenly, so the taper generated by that problem probably has waves or bulges in it.
A twisted bed is probably the worst thing to deal with.
Yes. That's all about a tool moving around to different places because the rest of the machine (ways, bed, etc) is misaligned.
It should be noted that a level bed doesn't cut intrinsically better than a bed on any other consistent plane, but level is usually easier to find than any other angle, and probably has other benefits to the stability of your machine
I'm not sure what you mean by this. To the extent that a bed bends or twists the tool away from a line that is consistently parallel to the axis of rotation bad things happen. But in truth, as long as the ways remain parallel to the axis of rotation of the spindle, a lathe can do a perfectly good job inside a shop on a ship while it wanders around in the waves.
Perhaps I am misunderstanding your point here.
All in all though, I am actually quite happy to use your comments to provide an opportunity to expand on the subject of spindle axial concentricity because it's a topic quite often not well understood by many hobby machinists (and prolly a few pros too). In essence, I'm practicing with your help.
A spindle's fundamental operating principle is the fact that anything firmly attached to the spindle via a chuck or a plate or whatever will spin about the spindle's axis of rotation. Perfect cylinders are cut when the cutter moves parallel to that axis. Shapes and contours are a result of the tool being moved away from that parallel axis. At its most basic level, a lathe is a spindle rotating on its axis in space and a bunch of other hardware attached to it that are all designed to move the cutter perfectly parallel to that axis or in some other controlled relationship to create concentric tapers, contours and other shapes.
Just to take this last point one giant step further, aligning a lathe is really all about aligning all the other parts of the lathe with the lathes spindle so that the cutting tool is kept parallel to the axis of rotation instead of following bent, twisted, or worn ways. That's my inside out way of looking at it.
Hope that all helps everyone reading this. If not, please feel free to disagree.
