G8688 - engaging half nut stops lead screw?

[LEEQ]

Sometimes the detents punched for those selector levers are not in quite the right place. Perhaps you need a little more engagement than given in the forward detent. Bottom line, go through it and make all adjustments as listed making sure things are where they are supposed to be like keys on shafts ect. Sometimes things are overlooked or are just plain crap in our cheap Chinese stuff. I for one am glad of the huge amount of info for guys like us trying to make them work well. I can't afford better. Once things are all aligned and adjusted you might be in good shape. If not, I can tear into the same model to answer specific questions if you need.

 

[learner]

:thumbsup: OK, I'm pretty sure I've got this figured out. I'm back threading with a workaround.

I started out with the F/R gears and worked down. I noticed eventually that after the lead screw locked up that the gear set inside a cast iron mount (Part 48, just called a 'mount') wasn't turning freely after the lead screw lever popped to neutral. So the whole A/B/C/D change gears wouldn't turn. This lockup caused the lead screw lever to pop out of forward into neutral.

The mount was cinched in tight with two 4mm cap screws and I think that tension was binding the gear. I probably cinched the screws in extra tight when I set up the gears for 14 TPI for my threading job. Once I backed the screws out, the gear would turn freely again but the bolts were not tight and I was worried the gears would rattle apart so I put some temporary aluminum shims together to get it get the screws tight while allowing the gear to spin freely when in neutral.

Oh, and I degreased some more and oiled the gears and lead screw up for good measure. I'm sure it needed it anyway. Assuming the workaround is good, I'll fab up some proper shims after letting it run in this mode a bit.

Thanks to everyone who offered suggestions, tips and research material. It was very helpful to get me thinking.

 

[learner]

:angry: Update: it's still happening. Very frustrating to be killing threading operations when the power feed stops ...

I found a short youtube video that explains what I'm seeing. Not mine but same symptom. That one seems more frequent. I may drill out the detent as suggested here and there, but I'm going to give it more thought before doing something I can't easily undo.

 

[DAN_IN_MN]

:angry: Update: it's still happening. Very frustrating to be killing threading operations when the power feed stops ...

I found a short youtube video that explains what I'm seeing. Not mine but same symptom. That one seems more frequent. I may drill out the detent as suggested here and there, but I'm going to give it more thought before doing something I can't easily undo.

Did you check out the comments on that video you listed? They might be of some help to you. It looks to be a common problem.

 

[LEEQ]

Now that the machine is deemed in proper adjustment/alignment, you might consider setting up a temporary clamp of sorts. Adjusting engagement to see if a new detent may fix the problem, and if it will where it should be. In post#25 by BRIAN of the sticky thread Show us your 7x12 Lathe! found in the Asian import Lathes and mills forum, a front mounted lever mod is described. Just a thought, if you have to reinvent that wheel anyway, you may want to move it up front at the same time. I look forward to seeing how all this works out, Lee.

 

[learner]

Did you check out the comments on that video you listed? They might be of some help to you. It looks to be a common problem.

Thanks, Dan. I did and one contributor suggested drilling out the detent. I may ending up doing that. I'll poke around a bit first.

Now that the machine is deemed in proper adjustment/alignment, you might consider setting up a temporary clamp of sorts. Adjusting engagement to see if a new detent may fix the problem, and if it will where it should be. In post#25 by BRIAN of the sticky thread Show us your 7x12 Lathe! found in the Asian import Lathes and mills forum, a front mounted lever mod is described. Just a thought, if you have to reinvent that wheel anyway, you may want to move it up front at the same time. I look forward to seeing how all this works out, Lee.

Thanks, LEEQ. Funny, I was thinking of a front lever like the old Craftsman 109 I had. Good suggestion. I'll check out that post.

 

[rangerman]

I do not have a lathe like that but if there is a detent system employed to keep the lever firmly secured at a certain position I would assume that it uses some spring to keep it in place.
Before you drill any hole deeper, how about if you replace the spring first with one that is stronger?
It's analogous to spring detent safety clutch systems used to protect motors and other moving shaft or gear systems. The amount of holding/clutching force before automatic release would be determined by the amount of spring tension acting on the detent. That is a feature that is very useful in case of a crash or when the load creates too much stress that the gears were designed to safely handle.
If you drill the hole too deep it's possible that you could overdo it to a point that it would become too hard to release the shifter out of position when you want it to shift because the ball or point cam pressure angle could be significantly altered by the deeper hole. (think of a wheel trying to climb out of a very deep pot hole compared to a shallower hole)

Years ago I made a similar mistake of drilling too deep the half-nut lever position detent hole on my old Dunlap 109 lathe that it made it extremely difficult to move the half nut lever when I wanted to engage or disengage. I had to fill the hole by bronze brazing just to make depth just right for the ball detent to easily slide out.

 

[David Kirtley]

Another possibility is that the tumbler gear is too thick. When I upgraded mine to metal, the metal one needed a little trimming to rotate freely. Just enough to rotate freely on the shoulder bolt it mounts on.

 

[learner]

Thanks, rangerman and Dave.

I've drilled out the detent just a bit and that seems to help. I did a test piece tonight that did 20 passes at .002 without it popping out. I'll keep all y'all posted.

Your suggestions have been very helpful and encouraging!