# 3 tries and 3 fails to make a 1/4 20 thread - part 2



## twooldvolvos (Mar 3, 2021)

Thanks to everyone who gave me pointers in my post titled "3 tries and 3 fails to make 1 1/4 20 thread".  Here is an update.

First I dealt with the issue of my South Bend 10K threading dial not engaging exactly on a line.  I shimmed the mounting rod and the problem was resolved.  The half nut now engages exactly on the lines.  I also found out my threading dial will engage at 3 positions between lines.  I have 8 lines on my dial so if my math is correct, I can engage my half nut in 32 positions.




And so I attempted another 1/4 20 thread.  It worked well enough to accept my nut.  I am making progress.  Full disclosure: I did not use the same line each time.  I just used the first line to come around.




I think I still have a ways to go though.  My problem is that the finish on the thread is not smooth.  If you take a look at the image below you will see that the threads are jagged.  You can count the individual ribs on each thread.




Here is a pic of my compound set at about 29 degrees.




And here is a pic of my cutting tool.




So now I am wondering if any of you can help me get a better finish.  One thing I need to try is to engage on the same number each time although I don't think I should have to and I am low on the patience I would need to wait for the number since I am running in back gears.  If you look at the magnified thread, the individual small steps on each thread are very uniform.  It almost seems like I don't have my cutting tool or my angles set properly.  Why is the trailing edge of the thread so stepped?  I'm thinking that each step is one pass of my lathe because the number of steps is about the number of passes I made to get the nut to screw on.  My passes were between about 15 thou and 5 thou or so.  Thanks everyone for your help.


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## ericc (Mar 3, 2021)

Watch a video on YouTube and pay particular attention to the angle of your compound compared to the videos.  It almost looks like you are selecting the complement of 29 degrees, rather than 29 degrees.


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## stupoty (Mar 3, 2021)

yeah you want the other 30 degrees , might be labeled 60.

almost everyone has one of them in their collection.



Stu


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## benmychree (Mar 3, 2021)

It is quite obvious that the compound is set incorrectly; set it straight in, that is, 90 degrees to the axis of the spindle, then rotate it 30 degrees to that setting by counting degrees, not reading the degree settings on the compound; lots of people do the 29 degree thing, or whatever, I was taught in school to use 30 degrees, and all the journeymen in the shop also did so.


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## twooldvolvos (Mar 3, 2021)

ericc said:


> Watch a video on YouTube and pay particular attention to the angle of your compound compared to the videos.  It almost looks like you are selecting the complement of 29 degrees, rather than 29 degrees.



Ericc,  I think you are on to something.  Now I know that my compound has 2 zero marks.  





The first image shows the mark on the back side of my compound.  It reads about 61 degrees.  The second image shows the zero on the headstock side.  This is the one I set to about 29 degrees.  Did I use the wrong zero mark for my compound?


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## brino (Mar 3, 2021)

@twooldvolvos 

You are making definite progress, just a couple more little things and you're golden........and don't worry this happens so often it is kinda like a "right of passage" for us hobby guys learning to cut threads.

Some lathe vendors mark zero degrees as the top-slide/compound parallel to the cross-slide (perpendicular to the lathe axis).
Some lathe vendors mark zero degrees as the top-slide/compound perpendicular to the cross-slide (parallel to the lathe axis).

Apparently, your lathe does both depending on which zero line you use!
I have never seen that before.
My lathe only has a zero mark at the back.

Here's a few other posts about it:
https://www.hobby-machinist.com/threads/threading-help.49488/post-416851
https://www.hobby-machinist.com/threads/what-am-i-doing-wrong.54207/post-449371

Notice the uneven angle of your threads; the leading edge has one angle and the trailing edge has a different angle.
Also, that ragged ring and steps in one face.
Those are the two tell-tale signs of this common mistake.

-brino


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## ttabbal (Mar 3, 2021)

I bet the comments above will get you going the right direction. As you get close, do a spring pass, same depth of cut as the previous pass. It will take a little more off. More importantly, it will help clean up burrs and chatter marks. Lightly running a file over it will help get the rest, if any. You can also use a triangular file to get down in the thread form. That's usually not needed once you get some practice though.


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## twooldvolvos (Mar 3, 2021)

twooldvolvos said:


> Ericc,  I think you are on to something.  Now I know that my compound has 2 zero marks.
> 
> View attachment 357856
> View attachment 357857
> ...



Update.  Several of you figured out correctly that I was 90 degrees off on my compound angle.  Thanks for your wisdom.  They say seventh time is the charm, right?  It a good thing because I am about out of stock.  Check out my latest thread.  1/4 20 from brass.  I used any random line on my thread counter.  They all seemed to work.


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## benmychree (Mar 3, 2021)

Success at last, looking good!


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## brino (Mar 3, 2021)

Great Work!

Now (turn off the lathe and) go get your celebratory beverage of choice.



Cheers!

-brino


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## SLK001 (Mar 3, 2021)

Your original threads looked horrible.  Your compound angle is WAY off.  

Here's the scoop on using the thread dial (both .jpg and .pdf versions):


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## Janderso (Mar 3, 2021)

Outstanding


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## martik777 (Mar 3, 2021)

Congrats!   

Now if you really want to get confused, cut an internal thread


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## Janderso (Mar 3, 2021)

martik777 said:


> Congrats!
> 
> Now if you really want to get confused, cut an internal thread


or a 2 start, 3 start?


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## DavidR8 (Mar 3, 2021)

Janderso said:


> or a 2 start, 3 start?


I do those all the time when I mess up and try to pick up the thread...


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## SLK001 (Mar 3, 2021)

I see you have two "zeros" on your compound.  I don't think the one on the side is from SB, but was added by a former owner.  I can see how you got confused by the angles.

A man with one watch always knows what time it is.  A man with two watches never knows what time it is.


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## Nigel123 (Mar 3, 2021)

SLK001 said:


> Your original threads looked horrible.  Your compound angle is WAY off.
> 
> Here's the scoop on using the thread dial (both .jpg and .pdf versions):
> 
> ...


A little confused on this chart
On my lathe Even numbered threads Have to be divided by 4 then you can use all 8 lines
Otherwise you have to use 123and 4 marks
Maybe my chart is wrong
Can anyone verify this chart works I have an 8tpi leadscrew
Thanks


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## martik777 (Mar 3, 2021)

My South bend with 8TPI screw is exactly the same. Maybe yours cannot do 1/2 or 3/4 threads so they do not mention them.


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## Nigel123 (Mar 4, 2021)

martik777 said:


> My South bend with 8TPI screw is exactly the same. Maybe yours cannot do 1/2 or 3/4 threads so they do not mention them.


They do show settings for fractional threads the chart is mounted on the dial housing
I have threaded fractional a few times they turned out okay
Maybe I will try to cut an even number of threads using any of the 8 lines and see what happens(tomorrow)
Will let you know how it goes


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## SLK001 (Mar 4, 2021)

Nigel123 said:


> A little confused on this chart
> On my lathe Even numbered threads Have to be divided by 4 then you can use all 8 lines
> Otherwise you have to use 123and 4 marks
> Maybe my chart is wrong
> ...



The chart I posted is right out of the _How to Run A Lathe _booklet from South Bend, so it will work for all SB lathes out there.  I'm not positive that it will work for other lathes from other manufacturers.  

I reworded some of the text to make it clearer (for me) to understand.

I printed out the .pdf version, took it down to Office Depot and had it laminated, then posted it above my lathe.


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## Nigel123 (Mar 4, 2021)

SLK001 said:


> The chart I posted is right out of the _How to Run A Lathe _booklet from South Bend, so it will work for all SB lathes out there.  I'm not positive that it will work for other lathes from other manufacturers.
> 
> I reworded some of the text to make it clearer (for me) to understand.
> 
> I printed out the .pdf version, took it down to Office Depot and had it laminated, then posted it above my lathe.


Did a little investigating this morning and found out SB lathes move the carriage 4 ins per revolution of the dial
My lathe only moves 3 ins
Thought they were created equal was wrong


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## Nigel123 (Mar 4, 2021)

Nigel123 said:


> They do show settings for fractional threads the chart is mounted on the dial housing
> I have threaded fractional a few times they turned out okay
> Maybe I will try to cut an even number of threads using any of the 8 lines and see what happens(tomorrow)
> Will let you know how it goes


Does not work different travel per revolution of dial indicator


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## ErichKeane (Mar 4, 2021)

Heh, when you confirmed in the other thread that you'd had the compound at the wrong angle, I was quite surprised!  A bad compound angle almost ALWAYS looks like the threads in this thread, not as a double-start.  The last thread you likely had the compound right, and were just not starting consistently in the same engagement.

Note that with an 8TPI lead screw, every OTHER engagement works for 20 TPI.  So I was positive in the other thread that it was the problem  

There is a couple of people who do pretty good videos on starting threading that explain the 'why' of the compound angle that makes it quick to visualize whether your angle is right.  It is just-shy-of 30 degrees from 'straight' as the operator stands.  

For some reason the Chinese lathes have the wrong 0, which I don't really understand.


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## twooldvolvos (Mar 4, 2021)

ErichKeane said:


> Heh, when you confirmed in the other thread that you'd had the compound at the wrong angle, I was quite surprised!  A bad compound angle almost ALWAYS looks like the threads in this thread, not as a double-start.  The last thread you likely had the compound right, and were just not starting consistently in the same engagement.
> 
> Note that with an 8TPI lead screw, every OTHER engagement works for 20 TPI.  So I was positive in the other thread that it was the problem
> 
> ...


ErichKeane.  Thanks for your input.  Correct me if I am wrong but I am using a South Bend 10k for which I measured about 8 threads per inch on the lead screw.  So using the information from the "How To Run A Lathe" book, I can engage my half nut on any of the 8 lines or 1/8 revolution for all even TPI.  Your reply seems to say that I should only engage on every other line for a 20 TPI which is even.  Can you clarify?


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## ErichKeane (Mar 4, 2021)

twooldvolvos said:


> ErichKeane.  Thanks for your input.  Correct me if I am wrong but I am using a South Bend 10k for which I measured about 8 threads per inch on the lead screw.  So using the information from the "How To Run A Lathe" book, I can engage my half nut on any of the 8 lines or 1/8 revolution for all even TPI.  Your reply seems to say that I should only engage on every other line for a 20 TPI which is even.  Can you clarify?



I don't know much about your thread dial/leadscrew, but what I'm saying is you can engage every other 'engagement point' on the leadscrew (as you discovered, you can engage between lines as well!).

The relationship between that and the lines on the leadscrew are going to depend no the machine you have, but you can likely count how many leadscrew rotations it takes to get the thread dial to go around 1 time, and reverse it from there.


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## ErichKeane (Mar 4, 2021)

Note, I found this manual for a 10k (no idea how old your machine is, so no idea if that is too new) that shows: https://cdn0.grizzly.com/manuals/sb1002_m.pdf on page 50 that every other 'line' works for 20 TPI.  You likely need to find that chart for your machine, or simply use the same line everytime.


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## DavidR8 (Mar 4, 2021)

From the "How to Run a Lathe" book:


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## twooldvolvos (Mar 4, 2021)

ErichKeane said:


> Note, I found this manual for a 10k (no idea how old your machine is, so no idea if that is too new) that shows: https://cdn0.grizzly.com/manuals/sb1002_m.pdf on page 50 that every other 'line' works for 20 TPI.  You likely need to find that chart for your machine, or simply use the same line everytime.


Thanks, ErichKeane.  That helps.  My lathe is from 1973.  The engagement points for laths seem to vary widely depending on which lathe you have.  Even laths that are named the same vary greatly.  My 1973 South Bend 10K seems to be much different than a new South Bend 10K.


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## SLK001 (Mar 4, 2021)

twooldvolvos said:


> My 1973 South Bend is 10K seems to be much different than a new South Bend 10K.



New 10K?  If you're referring to the one Grizzly sells, it is just a Taiwanese model with a South Bend sticker slapped on.  As for true SB 10K's, the basic operation was the same from day 1 until the end of production.  Every SB lathe produced from the 1920's on used the threading dial information that I posted earlier.  Now OTHER BRAND lathes may or may not conform to those standards.


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