Tail Stock Alignment

Thanks for you comments RJ. Makes total sense to me. That Edge bar looks very useful, thanks for link. Setting up my DTI wasn't too difficult & I learned some new things I probably should have paid better attention to. If I set the DTI ball to a slightly different position within quill ID, it was usually pretty close but could deviate 0.0005" depending on the wind :) I was similarly concerned about repeatability especially if the tailstock was further away & my Nooga arm was hanging way out, maybe influenced by gravity? The nice thing about that bar method is it uses centers which are more engaged & averaging within the socket & also over longer distance so less prone to picking up any local deviations
 
Hi Peter, yes a settling time is a good idea, then rinse n repeat! Quality lathe makers used to 'season' castings for a year or more, rough machine and leave for stresses to work out before finish machining n grinding...

I've heard that it was historically common to bury them, and that more recently it has been a practice to wrap them in black plastic and set them in a southern exposure. It's difficult for me to discern best practice from folly. Regardless, metal moves, slowly... except when it is machined, then it moves fast.
Daryl
MN
 
I left the slug in the chuck overnight & ran 4 measurements on each side & averaged. Its now 0.0013" larger on headstock side than tailstock. I swear it has a beating heart. But I also used a different micrometer this time so who knows.

I did another test. I placed DTI on bar end at outboard ~5" mark, shoe laced my fishing scale around the end & pulled on it. At something less than 10 lb it deflected 0.001" The question is, how push tool pressure would my bar have seen while cutting? I'm anxiously awaiting arrival of my new measurement goodies, maybe they will shed additional light on the issue.
 
Last measurement HS=1.0410", TS = 1.0420", so 0.001" larger on end. So my reasoning now: if its deflecting a bit on the end because its unsupported (steel is held cantilever in chuck, no tailstock center engaged) then maybe I'm pretty close to true parallel? I really want to check the bed with a good level now (sourcing). I feel like the tailstock problem was the overriding issue.

I'm going to let it rest as-is & repeat this test with a chunk of 1.25" 6061. I have some sharp inserts that impart very little cutting load & leave much better finish. The cold roll steel was a quick grab thing & probably a poor choice. It took a couple cuts before I even got stable readings which I attribute to the skin & I'm always leery about stress relieving.

So the dumbbell test bar shape with reduced diameter in the middle - I just assumed that was a time saving thing so you are not cutting continuously from end to end. Is that the case or maybe I misunderstood? My logic was cutting the whole section takes a bit more time but now I can mic along the way & see if there is any funny business going on like a bell shape or whatever.

As you know, it only takes a deflection of .0005'' to get .001'' on the diameter. The two hub turning test (with free end unsupported) is better at detecting twist in the ways than by using a level.
Repeating the test with some 6061 is a good idea.

RJ has already answered the last question.

What is the tailstock problem again? The TS is adjustable as you show above. It is common to move the TS to deliberately turn tapers between centers. The TS has to be adjusted back into alignment to turn cylindrical between centers. Must not have twist in the ways to be able to align TS properly.
 
What is the tailstock problem again? The TS is adjustable as you show above.
The problem was lack of good procedure on my part. To adjust into position:
- tailstock is locked down on lathe bed
- I unscrewed the (orange) set screws that act against what looks like a gib strip on tailstock end
- I unscrewed the (yellow) set screws that look like they lock on either side of that extended boss in the base
- lightly tap the tailstock unit into position so tailstock quill is coincident with spindle plane (yes same mode as offsetting tailstock when consciously setting up for taper turning)
- tighten yellow screws so tailstock aligned
- and now last step. I just assumed tighten orange screws to lock down position. However I still had the DTI hooked up & noticed if I turn one screw & not the other. I can actually tweak the tailstock out of position. Almost like in a rotation axis viewed from the top. So I alternated tightening them back & forth watching the dial to stay at zero. I'm still not sure if these end screws are meant to be a fine tuning adjustment. Or maybe its a bit of unwanted junk movement like when I tighten my cross carriage thumbscrews against its gib the DRO shows a smidge of movement. (Actually I cured that issue with a bearing ball & a little foot cut at same angle as dovetail, but this is what I'm wondering).

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Interesting. I was reading this old Southbend lathe installation & levelling guide. They show a screw adjuster located above the tailstock base hold down bolt for adjusting the bed. I kind of wish my Taiwan lathe cloned that feature, maybe would take the stand out of the levelling equation?

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Not sure why I didn't stumble on an informative 2 part YouTube video earlier, but it covers lots of points discussed: Tom Lipton (Oxtool) Leveling and Setup of the metalworking lathe P1 & P2. After his lathe levelling iterations kind of got to the end of the trail, he ended up taking his the last thou taper out by headstock adjustment. I'm not there yet but starting to wonder out loud if that's where I might end up. Excerpts from my <cough> extensive manual in that regard. I'll have to snoop around corners of the lathe but this is what I'm thinking. Now whether its meant to be tweaked or that's where they wrenched it down at the factory & called it 'within spec' is another matter.

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OK, I had a productive day. This time I chucked a ~7" bar of 1.375" diameter 6061 aluminum because it was handy. I didn't do the dumbbell relief cut but took a few skim passes with a really good insert. I took 3 headstock/tailstock measurements at 3 different clock positions, average them & got 0.0031" difference over 6.5" length. Tailstock dia > headstock dia. So same direction as my steel bar. A larger difference because it was over longer length & I wasn't to confident about the CRS bar anyway. So assuming the lathe is level (not 100% sure there yet but will have to revisit that when I get my good level) this would infer the headstock axis is pointing to the rear of the lathe, tailstock side. Or counter-clockwise if viewed from the top.

I sucked up my kahoonies & loosened the 4 bolts I was 99% sure were holding the headstock in position. Well... 3 bolts. Dang. The one closest to operator, inside the pulley cover, behind a gear, up in the attic, behind a casting boss, not quite reachable with any hex wrench known, or modifiable by man... I could not get a hex wrench engaged with limited rotation to loosen properly. I maybe got a start on it with the ball end of hex but was concerned I might gal the hex socket so decided to leave it for now. I did remove the gear to get at it BTW, but I think even more extensive gear bracket disassembly would be required. Why not use a not a hex head bolt that would be so easy to reach with ordinary wrench he asks himself?

Anyway I set up 2 dial indicators on either end or bar so I could monitor lateral headstock rotation movement. I then turned in the jack screw 1/ turn or so on rear of lathe & saw the head move 0.0025 on TS side & 0.0005" on HS side. Shy of target 0.003" but thought I'd start there. The remaining hold-down bolt I couldn't loosen allowed the slight rotation movement & that's all I was hoping for. Clamped the bolts down again, re-installed gear, let the lathe warm up a bit, took another skim pass. This time my differences were 0.0008", 0.0007", 0.0009" TS>HS so call it 0.0008" average over 6.5". This definitely did some good & I suspect my issue all along. Going to leave it be for now & then start at the beginning again when level arrives.

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Now that you have rotated your headstock, you should check if you are cutting a flat face. You could mount a face plate and mount a dial indicator on the cross feed. Measure at a point near the front and mark the spot. Rotate the spindle and move the cross feed to the rear to intersect the spot again. Your indicator should read the same (some folks set their lathes up to cut a slight concave face).
 
You are one step ahead of me RJ. I JUST finished skimming a 10" faceplate a few weeks back on the old headstock setting. Once I'm happy with the lathe setup, yes will have to check the plate & likely give it a light re-surface. It wasn't an onerous job, but the cast iron powder seems to get everywhere. Going to lay down some lightly oiled paper towels or something.

So my story is still a bit open ended until my accurate level arrives to validate lathe twist status (the leg jacking rigmarole). But is the proper sequence like what Tom Lipton shows:
- adjust lathe level (twist)... but within reasonable limits
- if still cutting taper, zero lathe twist & proceed to headstock

Reason I ask is I got thinking about this bed twist issue, if done in isolation as a cure. If the dumbbell test cutting is always confined to relatively short length of the test bar, say 10". And you kept twisting the bed until taper was removed over that 10", couldn't you end up with a piece of licorice extrapolating the twist angle out the remaining 30" of bed or whatever?
 
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