Threading Help

I'm working on threading for the first time and am having trouble getting the correct threads per inch. My lathe is a 10k and is missing the gearbox threading plate. I received a picture of one that says B and third one over for 20 tpi. I'm getting more than 36 per inch. I checked the stud gear which I think is the small hear attached to bottom of reversing bracket and it has 20 teeth. Please help if you can. Thanks!

My 10K plate agrees with your pic - B on the left tumbler lever and 3 from the left on the right tumbler lever for 20tpi. 36tpi would be C on the left and 3 from the left on the right one. Are you counting from the left on the ABCDE tumbler? The top three holes are A, C, and E. The bottom two are B and D. The 20 tooth stud gear is correct. Hope this helps.

According to what you say I am in the correct tumbler arrangement. It doesn't change anything being in backgear does it? Is there a way to make sure the gears on the side are correct? Reason I ask is that it came with an xtra set of gears. I'll count them all tomorrow and see what I have.

Dale Lusby said:

According to what you say I am in the correct tumbler arrangement. It doesn't change anything being in backgear does it? Is there a way to make sure the gears on the side are correct? Reason I ask is that it came with an xtra set of gears. I'll count them all tomorrow and see what I have.

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If the Lathe is not in back gear then that is your problem it most have the back gear engaged.

Having the lathe in a different gear doesn't change the tpi. You can thread in any gear if you are fast enough. Kenny

Easy check for correct drop gear train with a QC box is to set the same TPI as the lead screw (8 tpi I think on a 10K), pull the chuck round by hand the correct number of turns to give an inch of travel. If the travel measures anything other than an inch it will give you a clue as to what's wrong in the drop gear train. Disengage the back gear pin so the spindle turns freely when doing the turn by hand thing.

Are you sure that you are using the correct idler gear on the banjo and gearbox input driver? Your 20 to 36 (ish) error sounds about right for the idler and gearbox driver gears being transposed. The idler is always bigger.

Clive

dannyrocci said:

If the Lathe is not in back gear then that is your problem it most have the back gear engaged.

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Backgear has nothing to do with threading or tpi.

If you have extra gears it's probable that someone has installed the wrong ones. The gears should be as follows: Stud gear, 20t - large banjo (intermediate) gear, 80t - lead screw gear, 56t. All SB gears are marked as to how many teeth are on them, so you don't really have to count the teeth.

So I checked and the gear arrangement is as follows. 20t stud, 80t large gear, then down to a lower gear below on the lower bango arm that is 32t which goes to 56t on the lead screw. I took out the 32t to get the arrangement mentioned and it still is threading wrong. Not sure what to do.

Dale Lusby said:

So I checked and the gear arrangement is as follows. 20t stud, 80t large gear, then down to a lower gear below on the lower bango arm that is 32t which goes to 56t on the lead screw. I took out the 32t to get the arrangement mentioned and it still is threading wrong. Not sure what to do.

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The 20t. stud gear should directly engage the 80t. gear, and the 80t. gear should directly engage the 56t. gear on the lead screw, with no other gears in between - no gear at all on the lower banjo arm, 3 gears only in the train. If this is what you have, I dunno what's going on - maybe someone else can help.

Dale

Do the "set to lead screw pitch and rotate spindle by hand the right number of turns to get 1 inch travel" test I mentioned previously. Report back as to what the measured saddle travel actually is with 20 on the stud and 56 on the gearbox input. Then we can sort out what gear pair you need to give the correct drop train ratio.

That said 20 to 56 is pretty normal for SouthBend machines. The 80 tooth idler and, for that matter, the 32 you had on the lower arm are simply there to span the distance between the stud and gearbox input. They don't actually affect the drive ratio. They do change the direction of rotation tho'. One idler makes the gearbox driver turn the same way as the stud gear, two reverses the gearbox driver relative to the stud gear. So you use two idlers for left hand threads, which is presumably what your original set-up was for although why anyone would intentionally do an explicit left hand threading train on a machine with a perfectly good tumbler reverse beats me.

Clive

Just a thought - I don't know much about 10k's but I noticed that there were 2 versions of the older single lever gearboxes when trying to work something out for my 13" - is there a chance that there were 2 versions of the twin lever gearboxes and you just have the wrong diagram?

Another possibility - is the 80t compounded with something (smaller gear on the same shaft) or is it definitely the 80t that meshes with both the stud (20t) and leadscrew gear (56t). If it's a compound gear and both gears are meshed, then it's not acting as an idler.

I guess if everything is good on the open gears the options are that someone has rebuilt the gearbox in the wrong order (or a custom order) or just possibly a metric leadscrew, no - you would have worked that out by now because your crossfeed would also be metric.