# Lead Screw not turning consistently



## LTRDavid (Dec 11, 2020)

While threading a barrel tenon my L




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eadscrew quit turning. Fortunately I pulled out before damaging the threads beyond the point of being able to clean them up.  One observation is this happens only in US Threads setting.  In the metric thread setting the Leadscrew works fine. It appears the shear pin is still intact as the coupler and leadscrew turn together. I am trying to gather information before before I drain the QC Box and pull the cover off, so any comments would be appreciate.
Thanks


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## macardoso (Dec 11, 2020)

It looks like the clutch or engagement lever to switch between threading and feeding is loose, worn, or damaged.


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## Jim F (Dec 11, 2020)

I would be looking at the shear pins.


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## LTRDavid (Dec 11, 2020)

Yes, I think you are correct, @macardoso. Thanks for your quick reply.  I spoke with Mike at PM earlier and we are all three in consensus as to where the problem resides. He was very helpful with what information I could share.  Plus, I learned the front cover can be removed without removing the knobs!  Next step, drain the QC gear box.  I’ll take pics to share with the next guy here who may have this issue. Thanks agai.


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## brino (Dec 11, 2020)

@LTRDavid ,

Welcome to the group!

-brino


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## JimDawson (Dec 11, 2020)

Looks like the lead screw/feed selector is bouncing between feed and lead screw.  Maybe make sure it's firmly engaged one way or the other.


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## Ulma Doctor (Dec 11, 2020)

generally,
either the leadscrew OR the apron drive shaft will turn at a time.
if they are both turning, that may indicate a shear pin has been sheared and providing friction (enough to drive the opposing shaft)
a worn shifting fork may also prevent a sliding gear from completely disengaging drive.

i'll be following to see what the actual problem(s) are


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## Jim F (Dec 11, 2020)

Ulma Doctor said:


> generally,
> either the leadscrew OR the apron drive shaft will turn at a time.
> if they are both turning, that may indicate a shear pin has been sheared and providing friction (enough to drive the opposing shaft)
> a worn shifting fork may also prevent a sliding gear from completely disengaging drive.
> ...


Just the way the leadscrew is turning tells me shear pin.


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## jbolt (Dec 11, 2020)

Jim F said:


> Just the way the leadscrew is turning tells me shear pin.


I believe the OP has a PM1440GS which has a selector knob for inch or metric threads and says it works fine in the metric position which would rule out the shear pin.


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## LTRDavid (Dec 11, 2020)

Follow-up; drained the oil, carefully removed the front cover after making a picture of the knob settings. Next performed was a general check for any obvious problems, e.g. broken shear pin(s), wear marks, etc. and nothing was determined. With the obvious ruled out, an attempt was made to re-create the symptom. During this process it seemed holding left-to-right pressure on the cam the leadscrew turned consistently. This made me wonder if the interface between gears 112 & 141 were not meshing properly. So, I replaced the cover to test this and sure enough the lead screw now turned consistently. I wish I could be more specific about a solution, but that’s the facts as best I can relay them. Here’s a short video of holding pressure on the cam And a pic of the two gears 112 & 141.  
Thanks to all who responded. 








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## Jim F (Dec 11, 2020)

jbolt said:


> I believe the OP has a PM1440GS which has a selector knob for inch or metric threads and says it works fine in the metric position which would rule out the shear pin.


Not familiar with these new machines, my lathe was built in 1947.
Just the way the shaft was turning made me think shear pin, being a mechanic for 40+ yrs.


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## LTRDavid (Dec 12, 2020)

Well the lead screw turns consistently as described in my last post, but not in time.  I have it set to cut 16 TPI but I’m getting something greater.


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## LTRDavid (Dec 12, 2020)

I was able to measure the threads at 28 TPI.  Then I saw on the thread chart 28 TPI is the #8 position. So I took the front cover back off and repositioned the dial from #1 to #8.  Put the front cover back on & turned the knob from #8 to #1. Reblued the tenon & did another scratch cut.










	

		
			
		

		
	
 I’m back at 16TPI!


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## LTRDavid (Dec 12, 2020)

Well, I still managed to get the tenon completed today. Next comes chambering.


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## jbolt (Dec 12, 2020)

LTRDavid said:


> Well, I still managed to get the tenon completed today. Next comes chambering.


Glad to see you got it sorted out.


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