Help me understand what going on here with my lathe. I’ve had a problem with the lathe cutting a taper (by mistake) when work is held in a 3 jaw chuck with a dead center.
Kind of tough to do what without seeing the machine, but here's my thoughts.
A little background. It’s a Logan 10” model 815 lathe that is fairly new to me. Headstock is non adjustable for alignment, except by scraping. I have the lathe bolted to a concrete floor with leveling pads under it. I’ve leveled the lathe with a .0005 over 10” level. It’s dead center across the vee ways when measured at both the chuck end and the tailstock end. Also measured at midpoint- dead level.
This is mostly a starting point. To use the level as an absolute answer, you'd have to make and custom fit (scrape in) a bridge that held the level on the actual sliding surface of the ways where the carriage rides, and another to fit the actual sliding surfaces where the tailstock rides. That is not the top where you'd rest a level across it, or lay a parallel, or whatever other method you may have used.
I can eventually get it to cut without a taper ,with work in chuck and dead center, by adjusting the tailstock screws. But, if the next workpiece is a different length then it cuts a taper until I readjusted the tailstock again for that spot on the lathe bed. So, I have to adjust the tailstock to a different alignment at different spots along the bed in order to cut straight.
You have three points at play there. The chuck, the tailstock, and the carriage. The chuck is what it is. The carriage, on it's ways, is a whole set of it's own variables. The tailstock on it's ways, is a whole set of it's own variables. And depending on the diameter of the work you're turning, the cross slide and/or compound each are their own set of their own variables.
Is this a sign of bed wear and if so, is there much I can do about it?
It certainly could be. It's very real. Even on a brand new lathe, right out of the box, if you assume it was perfectly made and perfectly set up, after the first cut, there is "some" wear. It's gonna happen. You've got to manage it more so than worry about it. (Unless it's horrible).
Could it just be a worn tailstock that perhaps I could scrape back into alignment. I could do the physical scraping.
You don't need to. You moved the tailstock with the screws. Scraping is going to do the same thing, except that it will also lower it in the process. You don't want that without a good reason. That would be another managable error, but there's no reason to introduce it if there's no benefit.
Once the bed is "leveled", that's the step in the process where you get the test bar. The final step in "leveling" a lathe is almost always to lock the level up in another room, and forget completely about it. You've got one adjustment on the tailstock to shift it left to right, and NONE of the sliding ways on any motion of the lathe can be adjusted with any practicality. The next step is to dial the tailstock in dead center in it's adjustment screws. This is best done with a center (any pointy thing) clamped in any chuck you like. Only centered enough to get a pointy end cut on it. That cut end (at any angle you like) will be, by the power of geometry, dead nutz on centerline, at whatever length you cut it to. The closer to the chuck, (or spindle nose) the better. That's a datum you can trust.
Next, you'd bring the tailstock, with a center (preferably a dead center) all the way up to the chuck. Extend the quill some, but not a lot (kind of where you'd prefer it to be when you're cutting). That's another possible source of error, so bring it out to where you'll use it most, and go with that... Lock it in place, and bring the centers together. Adjust the tailstock screws until the two points are lined up dead nutz in the X direction (the cross slide direction). Up and down does NOT matter at this time. That just puts it centered at the same point in space as the center you just cut in the chuck. Then slide the tailstock back, and don't touch the screws again. (For now). You're keeping the "measurement". Don't touch the screws.
Take your test bar, set it up between centers (proper ones now, not the one you just cut, unless you cut it to proper dimensions, then it's fine). Cut the end of the test bar at the chuck. Set up a dial indicator on the cross slide, tool rest, somewhere, anywhere on top of the carriage, and dial it in to zero. Flip the test bar end for end, and set it back up. Crank the carriage back to the tailstock where the fresh cut end of the test bar is. Check your reading on the dial indicator. This is your taper in that distance.
How long is the test bar? How much taper did you get?
Assuming it's not a gross error, your next step in "leveling", after the level has been locked away, is going to be to dial enough "twist" back into the lathe to make those two dial indicator readings be identical. Not measuring, but indicating and flipping the bar is a huge tool. There is no tool pressure, there is no anything really to add more variables than what need to be added.
In the end, you will probably find some points on the lathe where the error isn't totally linear. But if you dial it in this way in the areas that you work the most, odds are you won't care about any deviations at the extreme's of the range.
If you can dial it in to a half a thousandth or a thousandth over five or six inches, you've got a good working machine. If you're making nasa parts, or just pushing limits for your own satisfaction, you can do WAY better. In practice, tool pressures, different materials, different shaped parts, different diameter parts are ALL going to put "some" degree of deflection in the machine. You'll always have to be careful about hitting dimensions, and managing errors. it's just the name of the game. But if you can hit that accuracy, you're into press fits/bearing fits, running journals, and generally getting fussy fits inside of reasonable tolerances, and you're going to be more worried about work deflection than you are about the lathe deflection and errors combined.
