Moving chuck back on spindle

GunsOfNavarone said:
I did a search for cast iron spindles with steel back plates, other than the dampening properties, I really couldn’t find any negative. (Doesn’t mean there aren’t, hence my posting here before I jump in) Yes, it has a register that I made sure was snug, tight tolerances. I moved the chuck/back plate closer to the headstock as the videos show. I definitely see positive outcomes from the less flex of the setup. I can get cast iron blanks if it comes down to that, but like many people, I hate machining cast iron.
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Sorry I had not seen the vids.

Have you checked that the wobble is bending and not just the pre-load on the bearings ?

Maybe run it for a while and see if it's the same or less once the bearings are warm.

Stu
 
I actually changed my spindle bearings to Nachi about a year ago. I have reloaded them after some wear, all is good on bearing load side. Have any of you guys done this flex text to n your (smaller) lathes. There was a good amount of people with similar. Like I said, Winkys Workshop on his Atlas lathe is what got be itching to figure out a way to move the chuck towards the headstock.
 
IIRC, you have a G0602/752 lathe. I have a 602 and I tried your test. I applied in excess of 20 lbs. to the end of a 1-1/4" bar 20" from the chuck face. I observed about .0002" of deflection.

Your hypothesis apparently is that somehow the back plate is flexing excessively because of the stickout of the chuck from the spindle nose and that moving the chuck .650" closer to the spindle nose will greatly increase rigidity. The hub on the OEM backing plate is 2.5" in diameter and the spindle diameter outside the headstock is close to 2", I would expect that any flexing would most likely occur along the length of the spindle inside the headstock, bending like a violin string. The observed deflection would be due to the front bearing acting as a pivot.

As to why the large amount of deflection you observed with the OEM back plate, I would expect to see more deflection the further away you were from the pivot point but .002" seems excessive. Another cause might be the the backplate had a looser tolerance, allowing some movement as you applied lateral force.
 
Partially when I replaced my bearing it was because of the flex/movement I was seeing, not to mention good quality bearings just made me feel better. (You are correct, I have a G0602) After replacing them and retightening after some use, I'd still say in the ballpark of .004' (.005" if I'm honest) I just accepted this until I saw Winky's video and he made good points and made a difference by demonstrating the improvements. He did not show the flex like I tried to show. I can't argue with the change from .004" down to .0007" flex. Light passes show an improvement for sure but other than that I haven't done much real work on it since. The stock back plate is a snug fit on the boss as well as the bolts are torqued down. Like I said, your compound mod helped a lot, no reason to not bark up this tree as well. If you could take your .002' of flex to under .001", would it be worth it? That's a yes for me.
I stalled on this project for a while because the only way to pull that back plate/chuck tighter to the headstock was to omit the reverse thread lock clamps. Since I have that worked out, it was worth the work, I'm pretty happy. I will put graphite on the threads and probably go this route on my other chucks, at least my independent 4 jaw as this is my 3 jaw scroll.
 
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