# G8688 - engaging half nut stops lead screw?



## learner (Dec 27, 2013)

Hi all,

I ran into a problem yesterday that I'd like your opinion on. I was trying to cut 7/8-14 threads on my Grizzly 8688 and a problem started where the lead screw would stop when I engaged the half nut. 

I was giving the lever a good throw to engage the half nut but it also does it when I gently engage too. Also, it doesn't matter whether or not I'm cutting the workpiece sometimes. Also, sometimes it does this when the bit hits the work.

I assume the gears are probably out of adjustment or there's not enough tension on the lever that engages the gears. The gears themselves look good and the lathe is relatively new.

Any ideas? Thanks!


----------



## MikeWhy (Dec 27, 2013)

I would first check that the carriage lock completely disengaged. Failing that, I'd be scratching my head too...


----------



## xalky (Dec 27, 2013)

It sounds to me like the lead screw is disengaged from the gear box at the headstock end somewhere. a broken shear pin maybe? Here's aparts blow up :http://cdn0.grizzly.com/partslists/g8688_pl.pdf


----------



## learner (Dec 27, 2013)

Thanks guys. I'll give those suggestions a look.


----------



## tripletap3 (Dec 27, 2013)

Double check your change gear clearance as well. This same situation happened to me on on a Grizzly at work where someone else changed the gears and left them loose. Believe it or not they didn't appear to be loose enough to slip but they did. Also amazing was that they were not damaged in the process.


----------



## learner (Dec 27, 2013)

Another good thought. Thanks!


----------



## LEEQ (Dec 27, 2013)

Over at little machine shop, they have knowledge as well as goodies. They have a guide for adjusting the mini lathe that is a must read. You could learn a lot quite quickly and make a few fast adjustments that will improve the machine operation considerably. I would adjust your lead screw and half nuts for alignment, engagement, and such. Then try it out. They take this in a logical order and if you follow along it's a snap. Best of all it's free.


----------



## learner (Dec 28, 2013)

Another good suggestion. Thanks! It's been awhile since I cruised over there. It's a good resource.

Update: this appears to be happening when the lathe is powered up now. With the lathe off, I move the leadscrew F/N/R lever to the Forward position, then turn on the lathe and the leadscrew lever pops right out to Neutral.

I'll check out the LMS documentation to see if that helps. Thanks for the help so far.


----------



## Hawkeye (Dec 28, 2013)

A similar thing happened when I was getting used to my 'new' Swedish lathe. The feed screw stopped turning, but the gears were still moving. Turns out the original owner had left the woodruff key out of the last gear in the train. Just had nut tension to keep it together.

Did you confirm that the gears are still turning. If so, you'd see whether the shaft is stopped. If the gears are _not_ turning, a belt may be slipping. In that case, does that lathe have a separate clutch for moving the carriage when you're not cutting threads? If both the clutch and the half-nuts are engaged, the carriage will be locked, stopping the gear train.


----------



## learner (Dec 28, 2013)

Thanks for the reply.

This lathe was NIB so I trust the lead screw lever assembly with gears is good. It appears intact, and 'to spec.' The gears turn when the lead screw engages in reverse. Lead screw does not disengage when the half nut is engaged in reverse.

The problem seems to be that the gear associated with the forward lead screw lever pops out when forward is engaged. The gear seems fine - no wear, burrs, etc. And it spins freely when the lever is engaging the reverse gear (putting the leadscrew in reverse.)

I'll read and poke about some more ...




Hawkeye said:


> A similar thing happened when I was getting used to my 'new' Swedish lathe. The feed screw stopped turning, but the gears were still moving. Turns out the original owner had left the woodruff key out of the last gear in the train. Just had nut tension to keep it together.
> 
> Did you confirm that the gears are still turning. If so, you'd see whether the shaft is stopped. If the gears are _not_ turning, a belt may be slipping. In that case, does that lathe have a separate clutch for moving the carriage when you're not cutting threads? If both the clutch and the half-nuts are engaged, the carriage will be locked, stopping the gear train.


----------



## LEEQ (Dec 29, 2013)

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 (Dec 30, 2013)

: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 (Jan 1, 2014)

: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 (Jan 1, 2014)

learner said:


> :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 (Jan 1, 2014)

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 (Jan 1, 2014)

DAN_IN_MN said:


> 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.



LEEQ said:


> 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 (Jan 2, 2014)

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 (Jan 2, 2014)

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 (Jan 2, 2014)

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!


----------

