Re: 9" Model C threading

david pennington

One more thing: your math was close, but not exact.

IF your compound is at 30 degrees, as for your compound travel calculation, the travel is 0.092" to get 0.080 cut depth.

In my experience it is maddeningly easy to cut deeper than I intended. Checking early and often is a good thing. My last several passes would be for a 0.001 l" cut.


-------- Original message --------
From: davidwpennington <davidwpennington@...>
Date: 7/10/21 21:13 (GMT-07:00)
Subject: Re: [SouthBendLathe] 9" Model C threading

SB manual...and my trainer...says to set the compound to 29 deg. The advantage is the tool is pressed against both sides of the thread, primarily cutting the left side, and shaving the right. With 30 degrees, there's a possibility of leaving "tree rings" on the right side.

Greg, you did not mention back gear. For such a coarse thread, I'd recommend using back gear and the slowest speed. It doesn't hurt to practice engaging the drive and disengaging it when the planned travel is up. 8 revolutions takes up an inch, and it may surprise you how fast it happens!


-------- Original message --------
From: "G K via" <bug_hunter2000@...>
Date: 7/10/21 18:46 (GMT-07:00)
Subject: Re: [SouthBendLathe] 9" Model C threading


Excellent description.  I'll give it a try and see where we end up.

One question"  If I move the compound (which is at 30 degrees to the work), I would have to cut deeper than the rule of thumb 0.080.  I would need to go to 0.094.  Correct?  I guess the test fitting would really be the tale of the tape.

Thanks again, and have a great Sunday.


On Saturday, July 10, 2021, 06:21:15 PM CDT, Jim_B <jim@...> wrote:

This is a subject that can get complicated real quickly. 

For any OD thread the starting OD is the thread diameter. For a 1-1/2-8 thread thats 1.5” Now that can be a few thousands under 1.5 but not over 1.5. 

For UN and Metric threads the threads are 60 degree V threads. You could calculate the depth from that but in so doing you would have a sharp internal V which reduces strength. 

UN threads are not full depth. They are truncated (flat)  at the bottom and top. The depths can be 65 t0 70 %.  This gets complicaterd. 
So here is what I use. 

While a young engineer many many years ago, I was taught by Sam, a real old time machinist: “The double depth of a 32 TPI thread is forty thousands.”

You can scale any thread from that.

For 8 tpi. 

32/8 x 0.040=0.160

Remember thats Double Depth.  You will be cutting 1/2 that number or 0.080"

So for your 1-1/2-8 thread, bring you tool in till it just touches the work, set your cross feed dial and your compound dial to zero. 
Your compound should be at 30 degrees to the work and you tool at tight angles to the work. 

With you compound 1” or more to the right of the work, Move your compound, 0.005 into the work

Engage you horizontal feed at some number. With an even thread like 8 tpi you can engage at even numbers, but I always wait for the same number to come around. 
If you dont have a thread dial, once you engage the feed do not ever disengage until you finish the thread. 

With a thread dial, back your cross feed 1 turn out at the end of the work. Stop the lathe. disengage drive, move the saddle back to 1” to the right of the work.  add 0.005” to the compound, return the cross feed to zero, wait for the number on the thread dial to come up and start feeding again. 
Every third pass should be a free pass. 

Without a thread dial, back the cross feed out and turn the lathe off. Wait for it to stop. Run the motor in reverse untill the tool is 1” to the right of the work and return the cross feed to xero and add 0.005” to the compound and start the motor in forward. 

As you approach your target, reduce feed to 0.0025 and then to 0.001"

Consider the depth number to be an approximation,  Start testing fit when you get within 0.005 of the number. 

This works for me, others have their own way. 

On Jul 10, 2021, at 6:12 PM, G K via <bug_hunter2000@...> wrote:

Can you guys school me on threading?  I understand the lathe logic ( a million youtubes on this), but am unsure where to find the data I need on the thread dimensions in the Machinist Handbook.  As a first try at threading, I am using a PVC pipe and am trying to duplicate the lathe spindle (1.5-8 tpi) and if that threads into my faceplate as a check, then try an internal thread of the same size to make a spindle protector out of PVC pipe again.

Thanks a lot,


Jim B.

Jim B

Join to automatically receive all group messages.