G8688 - engaging half nut stops lead screw?

L

learner

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