This morning I screwed a 11" vase held on a faceplate back on my lathe to re-turn the neck and make it a little less tacky. The unsupported neck end wobbled quite a bit and I assumed the old axiom or urban myth, "replaced work is never centered" had got me. This is about _replaced_ work and I understand that blanks originally misaligned on a faceplate can be turned to round. Like woodturners, faceplates need not be perfect to be useful.
The spindle and faceplate threads had looked ok so I hadn't cleaned them. I took a closer look and there it was, a smashed miniscule chip. I mean just a meer wisp. This time I cleaned the threads and now there was almost no wobble.
I decided to clock by eyeball the runout along a 2 X 10 maple cylinder made as smooth and accurate as I could turn it. It was held on a clean good quality face plate without tail support. Not the accurate precisely controlled (except for oversight) govt. grant experiment with stipend, travel and meal allowances, but good enough for a NASA subcontractor or this 'shadetree scientist'. Certainly not the contribution to mankind like the study that proved overeating makes you fat.
I digress, but anyway the blank ran quite concentrically at the spindle and with no significant runout as I looked along the shaft. Then I interposed various bits of debris, shavings, pieces of thread, washers that weren't machined spacers, even a bit of dried varnish. Turns out I didn't need to be funded by a govt. grant for a dial indicator electron microscope to see the runout as I neared the blank's end. It was clearly visible and significant. (however I could have used the prestige, stipend and the meal & travel allowances)
What I'm trying to say in as many words as possible is, "Don't assume that your lathe's spindle and fixings threads are clean". Wipe them each time. An unseen wisp of debris makes a wobble that will increase as the blank is longer. It's worse than some of you may think.
Turn to Safety, Arch Fortiter