# Thread (TPI) Calculation



## bug_hunter (Jan 20, 2022)

Hey All,

I need to cut a 13 TPI thread on my SB9c.  According to the change gear diagram, I need a 32 tooth, a 52 tooth, and an 80 tooth.  Unfortunately, I am missing the 52 tooth change gear.  Can anyone out there school me on another gear arrangement to achieve the 13 TPI.  I have the following change gears: 16, 24, 32, 32, 36, 40, 44, 46, 48, 54, 56, 60, and 80.  An 80 tooth idler.  A 54/18 compound, and a 72/18 compound.

I guess worst case, I'll have to overpay up on the fleabay for a 52 tooth change gear.

Any help/insight would be appreciated.

Greg


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## Kevin T (Jan 20, 2022)

Is there a TPI calculator that you can use online?
30 bucks isn't too bad for a lathe part IMO.








						South bend lathe 9 52T change gear 9/16" bore  | eBay
					

As shown in pictures.



					www.ebay.com


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## bug_hunter (Jan 21, 2022)

Got this one figured out.  

"Ride the Gear Train"

Thanks, and I hope you all have a great weekend!


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## silverhawk (Jan 21, 2022)

I had an online gear TPI calculator at http://www.silverhawk.net/2015/12/gearing-for-my-south-bend-junior-lathe.html. The basic formula is 

TPI = screw gear teeth * 8 / stud gear teeth

Sent from my SM-T500 using Tapatalk


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## SLK001 (Jan 22, 2022)

bug_hunter said:


> Got this one figured out.
> 
> "Ride the Gear Train"


What does that mean?  What did you end up doing?


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## IamNotImportant (Jan 22, 2022)

SLK001 said:


> What does that mean?  What did you end up doing?







__





						Ride The Gear Train software by Evan Lewis
					





					ridethegeartrain.com


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## bug_hunter (Jan 22, 2022)

I will take a look at the online TPI Calculator noted above. 

I used the "Ride the Gear Train" as suggested by a friend of mine.  And got a 13.09 TPI Thread.  An error of ~0.07% off of the 13 TPI.  I used the following gears> Stud: 44 tooth; Compound: 54/18; Idler: 80 tooth; Lead Screw: 24 tooth.

The first time I tried it I used the threading dial on numbered lines (odd TPI) and adjusted the depth with the compound screw.  This did not work out well.  The second time, I engaged the half nut and left it engaged and ran the motor alternating forward and backward.  I adjusted the cross slide screw in and out on each respective pass.  The thread turned out quite nice, and accepted a commercial 1/2-13 nut smoothly.


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## SLK001 (Jan 22, 2022)

bug_hunter said:


> The first time I tried it I used the threading dial on numbered lines (odd TPI) and adjusted the depth with the compound screw.  This did not work out well.  The second time, I engaged the half nut and left it engaged and ran the motor alternating forward and backward.  I adjusted the cross slide screw in and out on each respective pass.  The thread turned out quite nice, and accepted a commercial 1/2-13 nut smoothly.



That should have worked.  Did you investigate what went wrong with the first procedure?  Do you have a lot of wear on your carriage feed lead screws?


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## bug_hunter (Jan 23, 2022)

SLK,

I'll do a bit of investigation tomorrow and report back.


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## bug_hunter (Jan 24, 2022)

OK Guys,

I'm at a bit of a loss.  I tried threading the the 13 TPI approximation with the standard method of adjusting depth with the compound screw and the half-nut engaged on a whole number on the thread dial for threading, and disengaging the half nut for return.  And it does not produce an acceptable thread.  The thread separation looks good after the first pass, but the second pass the thread is cut slightly towards the tailstock.  The error repeats itself for the next thread, but the separation is offset from the second thread.

Not a lot of wear (although wear is relative for an 80 year old lathe).  However I wouldn't think wear should be the issue as the backlash should always be taken out of the lead screw, the cross-slide screw, and the compound screw at the setting of the depth and the movement of the carriage.

As a check, I turned a standard thread with the appropriate (per the SB manual) change gears, with the standard (described above) threading procedure, and I get threads to repeat on top of themselves.

Given this, unless some has another idea of what to try, it seems that the small error 13.09 TPI versus 13 TPI shows up when the half nut is disengaged.

I would be willing to try something else if there are any ideas or enlightenments out there.

Either way, the thread looks good, and threads nicely on the 13 TPI nut when the half nut us left engaged, and the spindle is reversed  for the return movement.


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## SLK001 (Jan 24, 2022)

bug_hunter said:


> Given this, unless some has another idea of what to try, it seems that the small error 13.09 TPI versus 13 TPI shows up when the half nut is disengaged.


I believe that THIS is your problem.  You aren't cutting a (real) 13TPI thread.  You're going to have to treat this thread like a metric thread and never disengage the half-nuts.


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## Flyinfool (Jan 24, 2022)

SLK001 said:


> I believe that THIS is your problem.  You aren't cutting a (real) 13TPI thread.  You're going to have to treat this thread like a metric thread and never disengage the half-nuts.


This is my first guess.

Or you may have to engage always on the same number, not just any number.


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## bug_hunter (Jan 24, 2022)

Flyinfool said:


> Or you may have to engage always on the same number, not just any number.



Tried that with the same result.  And at the slowest speed, given my rookie status, is a LONG time to wait for the same number.

Guess its just something I need to deal with if I'm going to thread without the appropriate change gear.  I think if I'm ever going to need a 13 TPI, I may still pick up a gear.  Fortunately there is no rush to run out and get one at this time.

Thanks to all!


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## SLK001 (Jan 24, 2022)

Flyinfool said:


> Or you may have to engage always on the same number, not just any number.



This won't work.  The thread dial doesn't have any correlation with the spindle with a thread with that much error, so he's going to have to keep the half-nuts closed.


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## Kevin T (Jan 24, 2022)

SLK001 said:


> This won't work.  The thread dial doesn't have any correlation with the spindle with a thread with that much error, so he's going to have to keep the half-nuts closed.


The last three threads I needed to make were 13TPI!


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## SLK001 (Jan 25, 2022)

Kevin T said:


> The last three threads I needed to make were 13TPI!



Yeah, but all you had to do was to set your QCGB to 13 TPI and go!


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## Manual Mac (Jan 25, 2022)

You have a lot of change gears. Yeah, I do to.
but I needed 40t, bit the bullet & bought one.
Buy the 52t, it’ll only hurt once.
if you lived closer I’d lend you mine.
Cheers, a fellow 9C owner.


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## Grendel (Jan 26, 2022)

I seem to recall even thread counts being on any number but odd thread counts only on odd numbers on the threading dial, also you will need to account for backlash by going past the start point so you are approaching the cut from the same direction.


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## SLK001 (Jan 26, 2022)

Grendel said:


> I seem to recall even thread counts being on any number but odd thread counts only on odd numbers on the threading dial, also you will need to account for backlash by going past the start point so you are approaching the cut from the same direction.



What you say is for the most part true.  However, where does one engage for a 13.09 TPI thread?  ANSWER:  The thread dial does not work, so leave the half-nuts engaged.  

Here's the information for using the thread dial for SB lathes with an 8 TPI leadscrew:


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## Grendel (Jan 26, 2022)

I actually have a thread dial that has a 16 tooth gear (non South Bend), which only has the numbers 1 and 2


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## Kevin T (Jan 26, 2022)

Manual Mac said:


> You have a lot of change gears. Yeah, I do to.
> but I needed 40t, bit the bullet & bought one.
> Buy the 52t, it’ll only hurt once.
> if you lived closer I’d lend you mine.
> Cheers, a fellow 9C owner.


And will help with any resale scenarios in the future.


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