Crazy Threads...

True, but if you set the compound at 60 degrees and thread from the compound you get a mess. I think that is what Ted is getting at.
It is not necessary to feed a thread at an angle other the 90° using the cross slide, why do people want to make it more work then needed? On the CNC lathes that I program the canned threading cycle has an "approach angle" which defaults to 29.5 °, if changed to any angle that does not cause the tool to pass through the last tool path it makes no difference as the X,Z tool positions remain the same.
 
i cant thank everyone enough for their comments.
im going to pull the apron off tonight after work/dinner. i will document along the way.
we'll get to the bottom of this... we have to! i cant afford another lathe..

edit - i will also try to take a video before i pull everything apart to show my process and the result. maybe it IS something im doing wrong... jeez, i hope it is anyways...

Before jumping headfirst into the gear box, if the machine is small enough to allow rotating the spindle by hand whilst driving the lead screw try this.
Place a dial indicator against the carriage, set the gears to a thread setting, engage the half nuts and rotate the chuck 1 revolution by hand after taking a turn or two to eliminate any lash, if for instance turning a 4 TPI thread the indicator will read .250" per revolution of the spindle, if not then something is off, I do this on manual lathes almost every time that I thread, with machines that are too large or have spindle brakes that will not allow this select the slowest spindle speed and jog it a revolution.
 
The compound angle looks correct in the pic above. It is certainly not 30 degrees off of where it is supposed to be.

Ya Bob I think you're right. I guess my internal protractor was out of whack last night !

Ted
 
It is not necessary to feed a thread at an angle other the 90° using the cross slide, why do people want to make it more work then needed? On the CNC lathes that I program the canned threading cycle has an "approach angle" which defaults to 29.5 °, if changed to any angle that does not cause the tool to pass through the last tool path it makes no difference as the X,Z tool positions remain the same.

It's just been my personal experience that when I've tried cutting a thread at 90 deg., I get a very poor quality thread. The deeper the cut the worse it gets. I think when the tool is cutting on both flanks it tends to cause chatter which results in a poor looking thread. Like I said, just my personal experience.

Ted
 
my lathe has a clutch and a half nut. the shear key is in the clutch... but the half nut is locked into the lead screw when engaged... regardless if the key was stripped, it is not used when the half nut is engaged, correct?
The lead screw must maintain position with the spindle, if for any reason there is a position error between the two the machine will not repeat consecutive moves. There is a closed gear train between the spindle and the lead screw, if there is any slippage in this train from from the screw itself or any component the position of the tool will be effected.
 
OK, at the risk of restating what may have been stated before. A little detective work:
First off, the setup for the compound and the threading tool look correct from photos 2 & 3.
Second, the OP stated that the lathe was new to him within a month but there is evidence of much use from photo #2, indicating a previous owner.
Third, From threading table in photo #4, the lathe was originally set up with an imperial lead screw.
Fourth,there appears to be an asymmetric thread as well as what looks like a double cut thread.

If the OP is using the same index mark on the thread dial, he should be cutting a proper thread, regardless of gear setup unless:
1. there is a broken shear pin (already suggested).
2. The PO had replaced the gear on the threading dial with a gear with a different tooth count.
or 3. The PO had replaced the Imperial lead screw with a metric lead screw.

I would lean toward #3 which would explain why the multiple start asymmetric threads and why it was difficult to get the half nuts to engage properly.

You can verify the lead screw by hand cranking toward the headstock and stopping when a particular index number on the thread dial is aligned with the index mark. Measure the distance from the headstock to the carriage. Now continue cranking towards the headstock until the index number is again aligned. Remeasure the distance from the headstock to the carriage. If it is an Imperial lead screw, the difference in distance will by a whole multiple of 1 inch. Most lathes are multiples of 2" but for my Grizzly 602, i6t is 3". If it converts to a nice round metric number, it is a metric lead screw.

If you have a metric lead screw, the method suggested by several above of not disengaging the half nuts but reversing the lathe to back up to make another pass will work. You should be able to cut perfect threads. They may not have the correct pitch, however. Offhand, I would not expect to be able to use the threading chart, depending upon how the change gears were set up on the lathe.
 
OK, at the risk of restating what may have been stated before. A little detective work:
First off, the setup for the compound and the threading tool look correct from photos 2 & 3.
Second, the OP stated that the lathe was new to him within a month but there is evidence of much use from photo #2, indicating a previous owner.
Third, From threading table in photo #4, the lathe was originally set up with an imperial lead screw.
Fourth,there appears to be an asymmetric thread as well as what looks like a double cut thread.

If the OP is using the same index mark on the thread dial, he should be cutting a proper thread, regardless of gear setup unless:
1. there is a broken shear pin (already suggested).
2. The PO had replaced the gear on the threading dial with a gear with a different tooth count.
or 3. The PO had replaced the Imperial lead screw with a metric lead screw.

I would lean toward #3 which would explain why the multiple start asymmetric threads and why it was difficult to get the half nuts to engage properly.

You can verify the lead screw by hand cranking toward the headstock and stopping when a particular index number on the thread dial is aligned with the index mark. Measure the distance from the headstock to the carriage. Now continue cranking towards the headstock until the index number is again aligned. Remeasure the distance from the headstock to the carriage. If it is an Imperial lead screw, the difference in distance will by a whole multiple of 1 inch. Most lathes are multiples of 2" but for my Grizzly 602, i6t is 3". If it converts to a nice round metric number, it is a metric lead screw.

If you have a metric lead screw, the method suggested by several above of not disengaging the half nuts but reversing the lathe to back up to make another pass will work. You should be able to cut perfect threads. They may not have the correct pitch, however. Offhand, I would not expect to be able to use the threading chart, depending upon how the change gears were set up on the lathe.

+1 on number 3. Verifying if the machine has an imperial or metric lead screw will either identify the problem or remove it from the equation.

Tom S.
 
The only issue with reversing the motor trick, threading in front of that shoulder will be next to impossible without crashing... i may have to just abandon this particular project for now i guess.
Stop the lath short of the shoulder and finish up to the shoulder by turning the lathe by hand. At the slower gear settings it's hard to turn the spindle by hand so I use a 3/8 extension from a ratchet set for extra leverage. You could also use the chuck key. Just stick it into the square holes used for tightening the jaws.
 
It is not necessary to feed a thread at an angle other the 90° using the cross slide, why do people want to make it more work then needed?
Well, some people do, and I was just noting that if one chooses to do so and gets mixed up and sets the angle wrong it make a mess of the threads. It's possible to get mixed up because of the two different ways that compound scales are labeled.

I agree that the OP would be well-advised to lock his topslide and feed straight in with the cross slide until he solves his problem: it eliminates some varibles. Once he has things sorted out he might want to experiment to see if feeding at an angle gives him better results.

I feed straight in for small threads but for larger ones I find that I get a better finish feeding at 29 degrees with my (small, old) lathe.
 
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