Sheldon Sebastian 13" Gear Head Headstock Issue

Yes, you can set up and check for wobble in the saddle. It takes two dial indicators, I would think .001" increment is sufficient. They need to be mounted so they touch off, one at at the vee way touching the saddle at the vee, and the other at the flat touching the saddle at the flat. Rock the carriage handwheel back and forth enough to get readings on the indicators. Perfect alignment, they should read the same at all movements. If worn, both will jump around and not be consistent with each other. Check this at the chuck, midways on the bed, and at the tailend of the bed. Report back with your findings.
 
If the wobble is leadscrew related is it possible to measure the wobble while the carriage is engaged? So theoretically you could see the moment that is patterned into the work.

Did you put class three spindle bearings in your machine?
 
No, it's not leadscrew specific. You want to check this without the carriage engaged, feed or half nuts.

As for class 3 bearings, I've never had to replace any spindle bearings in the lathes I've had over the years. My dad replaced a double row tapered roller bearing on the head stock of his Axelson lathe that was a class 3 bearing. The original bearing had water damaged from when the lathe was on a navy ship that was sunken in WWII and salvaged years later. You will never notice the difference between a class 3 or 0, 00 Timken bearing on a lathe. And we used to do some tool post grinding on that lathe that left very nice finishes, too!

One other thing you can try related to the lead screw. Remove the leadscrew support on the tail end of the leadscrew. Engage the half nuts on the lead screw. Do this out on the tail end of the leadscrew. Slip the bracket back on the shaft and see how close the bolt holes line up. Should be pretty close if not right on. Also attempt this at the QCGB end of the leadscrew too. Since the lead screw will be hanging and flexing, this may be a little harder to detect alignment. If it's out any at all, it should magnify as a bunch of movement in the leadscrew when the half nut are engaged. Also, the leadscrew bracket may or may not line up very good, maybe even to the extent, that you can't get the bolts back into the holes.

Is this the Quick Change gear box you replaced the gear in? If so, this could be your culprit.
 
Ken,

I will check the alignment this afternoon. Yes it is the box I replaced the gear in, but the gearbox is tight on guide pins, I wondered if it had moved so I checked it and it can't go anywhere but straight off, no movement up or down or side to side. I also indicated the shaft at the gearbox without the lead screw and there isn't any play, maybe .001 -.0005 if I'm forcing it. I'm going to double check this again to be sure.
 
Ok so I checked the alignment, with the half nut engaged it aligns perfect on either end, when you disengage the leadscrew drops maybe .03 or less, you can easily still put the bolts in but you have to put slight upward pressure. I replaced the bronze bushings in the apron but I kept the old ones so tonight I put the old ones back in which are 1.04+ vs the new ones 1.00... no change. The bronze bushings also let the worm gear slop around more so if I had it too tight its now loose... but it still patterns. The gearbox is next, but the new gear I made was turned on the spindle thats in the gearbox so I know that gear isn't wobbling. Not saying thats not it but. Im going to check the gears in the head stock and I may go through and replace all the non spindle bearings. I assume these don't need to be class 3.

I sent timken the pictures and they are not sure if the bearing wear pictured would produce that particular pattern. I did check the preload on the other machine... when I take it out of gear and give the chuck a flick it rotates maybe 2/3 of the way around, the one I'm working on goes around maybe 1.5 - 1.75. The other machine is noticeably stiff. Not saying its not worn out but. As a note the bed on that lathe drops .08 over about 20 inches. and the ways are beat. But it cuts like a dream.
 
One other thing, I was contemplating the gears and bearings in the headstock, I also have a vibration issue, so I was wondering if that could be it... but then I remembered that I tested this with no motor, and no gears engaged, so just a drill and the carriage. Needless to say when you run it on a drill a lot of vibration goes away. But I still get the patterns.
 
I wouldn't worry about the gearing in the gear box or headstock. If the gears in the headstock was causing problems, it would show up as straight horizontal lines on the part. They would be superficial, too.

I had the same problem show up on my 9" SBL a few years ago when I was trying to turn the OD on a piece of steel about 3" in diameter. The exact pattern. Didn't do it any smaller diameters, which I cut most of the time on this lathe. What I did to fix it was take the saddle and blued the bed and did a few impressions to check for bearing on the vee's. I found the top of the vee ways was riding in the relief of the vee on the saddle. Milled it out a bit. Then rescraped and fitted the saddle to the bed again. Went back together and tried it again. Problem went away.
 
I'm planning on pulling the saddle tomorrow night and checking it. When I blue it anything in particular to look for. I don't have scraping experience, so my repair will have to be just to the groove if I find a problem.

But that brings up another test, I need to see if the pattern is the same on large and small diameter work.
 
Jahue said:
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But that brings up another test, I need to see if the pattern is the same on large and small diameter work.
Click to expand...
Give it a try and see what happens. At this point, anything is worth trying. On the saddle, mainly see where it is rubbing along with what kind of wear pattern your seeing. Once done, before going back together, as I said earlier, file down any ridges that may have started. You never know, that little bit may solve the problem. Remember, the wear ratio between harden ways and soft ways bearing against each other is like 10-20 to 1. So for every .001" lost on the bed ways is equal to .010" or more off of the ways on the saddle. Good lubrication and cleaning can reduce this to near zero.
 
Ok so tonights tests. Took the apron apart, took the saddle off, stoned and filed carefully all surfaces to make sure no burrs were standing. Blued the ways and put the saddle back on. Of course its not bearing 100% but its bearing all the way along the back side of the front way and periodically on the back, mostly near the rear. I reassembled the whole thing cleaned every surface, new way oil, even surface lubricated, and readjusted all the gibs. I also filed the relief on the v grove to make sure it wasn't riding on anything it wasn't suppose to. Ran a test... same pattern, and this time I over tightened and under tightened (loose, very loose) the gibs front and back to see if the pattern changes. No changes. I moved on to the chucks, 3, 4, and 6 jaw all have identical patterns. I was even using the same rod to verify. The pattern also shows with the gearbox in neutral and hand feeding the carriage.
 
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