Where Do I Begin?

I think it is time to check the half nut/nuts to make sure they are engaging fully, and that the half nut shaft is not bent, or the threads damaged (which I think unlikely). It could be that a key or shear pin has half sheared, allowing disparity between the carriage position and the leadscrew, thus messing up the thread by cutting in a slightly different position on the second pass. If the half nuts are still difficult to engage and disengage, that is where I would start. If you have a manual, check to see if there is a shear pin or key.
Phil
 
I wonder if a gear on the leadscrew or spindle is slipping. Also make sure your half nuts are engaging solid, not backing out a little from the detent. If the detent backs them out slightly, that and a bent leadscrew would lead me to believe that might be your problem.
 
Thank you.

Yendor,

I am not cutting threads in both directions. The last picture I posted was done cutting thread in one direction - towards the spindle. I just kept the half nut engaged, turned off the motor, back out the cutter, reversed the motor to move the carriage back to the right, then made the 2nd pass.

Phil,

Unfortunately the only documents I have is just a few pages on the controls and starting up the lathe. I have a TravAdial blocking any view of the half nuts. It looks like I need to remove the bracket holding the lead screw and feed rod, and TravAdial. and slide the carriage all the way to the right to even inspect the half nuts. I don't know if I were to replace the half nuts that I have to completely remove the carriage out of the lathe. I've never been this road before.

Andre,

I do not know and could not tell whether the half nut lever backs out or not, I'll have to make some runs and observe. One thing I never noticed before and was not looking for it before either, but the lead screw bends down every time I push the half nut lever down to engage.

Do you guys know if the half nuts on these 70s era Taiwanese lathes are brazed on, or soldered on to their housing? Next question, if I were to replace the half nuts and no commercial ones are available, do y'all know who could machine or rebuild mine?

I dread the thought of tearing the carriage apart.
 
Found the problem, the dowel pin activating the lower half nut sheared off. It is 8mm x 16mm, a catalog item with Grizzly, but they are out of stock and their best guess for next shipment is 21 April. Local places do not have metric pins. Though 5/16 is close, it may be shy too small for a press fit. Are these hardened pins designed to shear?

Looking at the piece that came out, surface does not entirely show fresh break, crashing the cutter into the schedule 80 PVC and unlodging the workpiece off the 3J stalled the motor just finished it off.
 
Glad you found the problem, if this pin is designed to shear, make one out of aluminium, However it sounds like you mean the pin that slides in the semi circular slot to close the half nuts (assuming that is the way your lathe works) You have a lathe, make one! Good to know it hasn't done any serious damage, big sighs of relief all round;-)
Phil
 
Phil,

Thank you.

Yes, it is the pin that closes the bottom half nut. I do not know if these are designed to shear or not. I could turn one, but I have the carriage all torn apart right now. I sure hate to put it back together to turn a dowel and had to take it apart again.
 
Local machine shop? Buddy with a lathe?, local model engineers club? Yes I was being a bit daft, I should have realised you would have the apron stripped:afro:
 
McMaster-Carr sells 8mm dowel pins in multiple lengths.
 
Thank you, gents

Got some coming from Fastenal, at McMaster-Carr their largest diameter is 6mm. BTW, are these pins in this application designed in as shear pins? Looking at McM-C online, they have 3 different materials, for dowel pins. The steel alloy is hardened to minimum hardness of 52, which I would guess will shear in 2 pieces, the other 2 are SS.
 
I Don't think that these pins are designed to shear, usually in the screwcutting chain there is a shearpin in the drive to the leadscrew. On my Colchester it is in
Philone of the shafts that drive through the banjo gears, so if the leadscrew is overloaded the pin pops and disconnects the drive to the SC gearbox, thus it also protects the half nuts. This pin on yours has sheared because the loading has forced the half nuts apart. It is possible that there is a key that holds one of the gears on to the drive shaft between the lathe and the screw cutting gearbox. You could take this key out and replace it with an aluminium one( which you would have to make) in order to stop this happening again. Have a look through your manual to see if this is possible,
 
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