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Leadscrew Clutch Options?

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sparky961:
My lathe/mill has a parallel-face dog clutch between the spindle-driven gear train and the lead screw.  Anyone that has used a dog clutch in this application knows that under even fairly moderate loads the clutch does not want to disengage easily.  This certainly leads to some sphincter tightening moments when you're cutting up to a shoulder at a pretty aggressive rate.

My question is this: what's a simple way the parts can be modified or re-designed so that it engages/disengages smoothly and easily under load?

I've used many lathes where the action is smooth and easy regardless of load, speed, or whatever.  What mechanisms are they using?  To ensure accurate threading it wouldn't be able to slip.  I was thinking maybe making new "gears" (dogs? what's the correct term?)  with teeth at something like a 45 or 60 degree angle, likely requiring spring pressure to keep them engaged.  Any obvious flaws to this idea, or better ones?

chipenter:
My Faircut lathe had an auto knock off that worked verry well , the clutch parts were hardened and slid off smoothly , the more teeth the less presure mine had four .

velocette:
Hi Sparky
You can use a lot less steep angle than 45 degrees to  about 5 to 10 degrees no springs and use an eccentric type mechanism that is locked engaged by going just over center.
Thrust bearings will be needed on each side if the clutch dog mechanism.
Do a search for "Over Center Clutch"
Then instead of friction clutch substitute a dog clutch.
Screw cutting and and using dog clutch not be possible with all thread pitches only a very few.
Personal preference is to drive the lead screw with a 24 volt variable speed gear motor and a poly vee belt drive and use a carriage stop when cutting up to a shoulder.

Eric

Manxmodder:
Eric, I agree with you that the 45 deg angle on the drive dog is excessive and would make disengagement difficult.

On your second point about dog clutching restricting the range of thread pitches I don't see why that would be the case.

If the clutch is placed between the spindle and drive gears then engagement with a single point engagement clutch will always be accurately repeatable no matter what thread pitch is being generated.

Also I can't see how a carriage stop could be used with the leadscrew half nuts engaged as when the carriage meets the stop the leadscew/half nuts would come under destructive strain.....OZ.

sparky961:
Ok, a picture's worth at least a few words, right?  A saving grace of this machine is that it did come with a full set of exploded drawings.  It has helped many times to see how things come apart and where I can modify things without having unanticipated problems.

I was hoping to be able to improve the action without massive changes.  Looking at the attached exploded view, I was wondering about maybe replacing the "dogs" (142, 143 - lower centre) with custom versions having a different contact face angle, and also very smooth, tough and hardened (4140 HT?).  I'm definitely concerned in doing this that the clutch will have a tendency to disengage on it's own due to the pressure forcing the dogs apart.  To counteract this I was thinking to add a spring or lock of some sort to the actuating lever (135) or associated parts.

Some of you have probably read posts of mine in the past dealing with this machine and its limitations.  It is for this reason why I don't want to spend a ton of time on this.  If I had more space, I'd almost certainly have a separate full sized lathe and mill.  Who knows, maybe it just turns out to be an interesting discussion on clutches and machine design.

Regarding the angle of dog engagement, can you be more clear on whether this angle is from horizontal or vertical?  Maybe a sketch or link?

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