# Please Explain How A Threading Dial Works.



## AR. Hillbilly

I've been watching some videos on threading. I saw a guy make a cut then move the carriage back set the depth of the next cut and wait for the threading dial to hit 1 and then engaged the half nuts. He did it every pass. I don't see how that guarantees the cutter will cut in the same thread as the last cut. What if the carriage was back a little farther than the previous cut? I'd like to know where to set the carriage and when to kick in the half nuts for each pass and keep cutting in the same groove. I'd like to find some plastic of some sort to practice on.


----------



## JimDawson

The thread dial is what insures that the part and the carriage are synced up so you enter the same groove each time.  Because the threading dial is driven by the lead screw and is attached to the carriage it is able to tell you where the lead screw is at.  The lead screw is geared directly to the spindle so that relationship never changes during the threading operation.  The position of the carriage is not important.  If you move the carriage, the position of the threading dial changes.

I hope this makes sense.


----------



## Bob Korves

The threading dial helps you engage the half nuts in the same phase that it was in before.  The threading dial stays engaged with the lead screw all the time, keeping track of position while the carriage is disengaged from it.  There are various rules on which lines on the threading dials can be used with which threads, read your manual.  You can always use the same line each time.  The threading dial on an inch lathe will not work with cutting metric threads.


----------



## AR. Hillbilly

So if I was to set the carriage anywhere on the bed and kick in the half nuts on the same line on the threading dial it will cut in the same thread? Why 29 degrees on the compound? I haven't found a manual for my lathe yet. It's a 1944 Sheldon.


----------



## CraigB1960

The leadscrew and half nut of the machine will determine when they can be engaged.  Because of their accurate form and dimension the pair can only be engaged (come together) at certain points.  So depending on whether you are cutting even, odd, 1/2 thread, or multiple of leadscrew (typically 8 TPI) will determine when to engage the half nut based on the dial indicator.

A good reference for this is: How to use a lathe

Even number of threads:  Engage at any graduation on the dial
Odd number of threads:  Engage at any Main division
Fractional number of threads:  This would be like 11 1/2 TPI.  Engage at any other main division 1 & 3, or 2 & 4.  For other fractional threads, engage at same division every time.
Threads that are a multiple of the number of the TPI of the Leadscrew:  Engage at any time that the split nut meshes.

The compound should be set at 29 degrees for 60 degree thread.  "Cutting threads with the compound rest at this angle allows for the left side of the tool bit to do most of the cutting, thus relieving some strain and producing a free curling chip. The direction is controlled by setting the compound rest at the 29° angle before adjusting the cutter bit perpendicular to the workpiece axis. The depth of cut is then controlled by the compound rest feed handle."

A good reference book is "The Care and Operation of a Lathe" by Sheldon  and "How to Run a Lathe: For the Beginner"  South Bend  (This is available in our download section here)

There are manuals on the Sheldon Users group on Yahoo  (I encourage you to join that group), however the manual will not go into the operation of the lathe, only parts.


----------



## JimDawson

AR. Hillbilly said:


> So if I was to set the carriage anywhere on the bed and kick in the half nuts on the same line on the threading dial it will cut in the same thread?



Yup, it's that simple



AR. Hillbilly said:


> Why 29 degrees on the compound?



30° is 1/2 the angle of a standard 60° thread form.  The 1° offset is to clean up the back of the cut.  There are a number of folks that set the compound at 0° and it still cuts threads.  But I prefer the offset.  Also setting the compound at 29 or 30° allows you to use a tool that is ground at more than 60° and it will still cut perfect threads.  Sometimes I use a 65° tool

EDIT:  OOPS! that should have been 55° tool

Then you set the compound at 30°, with the tool set 2.5° off perpendicular then you get a 60° thread


----------



## Bob Korves

Be careful about using other manufacturer's literature on your lathe.  You may have a different pitch lead screw than a South Bend, which could make the threading rules different on your lathe.  Have you tried to find a manual for your lathe on the web?


----------



## CraigB1960

Bob Korves said:


> Be careful about using other manufacturer's literature on your lathe.  You may have a different pitch lead screw than a South Bend, which could make the threading rules different on your lathe.  Have you tried to find a manual for your lathe on the web?



Bob, he has a Sheldon, similar to mine.  That is why I recommended the "The Care and Operation of  a Lathe" with link for him.  The South Bend book is applicable as well.  

However, I have never read any other way to use a thread dial indicator than how I referenced it.   It would be interesting to hear if different from what I've learned and used over the years.  I cannot imagine an odd TPI leadscrew,  say 9 TPI, that would make it difficult to cut common pitches, cause you would always need something like a 2/3 ratio in the gear train.  Common leadscrew TPI are 8, 6, and 4.  So as long as it is an even TPI, it should work like the references I provided.


----------



## Charles Spencer

Here is a 1965 Army manual on a Sheldon 10" lathe:

http://vintagemachinery.org/pubs/2133/3556.pdf

Here are other Sheldon manuals:

http://vintagemachinery.org/mfgindex/detail.aspx?id=2133&tab=3


And here are the two manuals that most people use regardless of the brand of their lathe:

South Bend "How to Run a Lathe"
http://www.hobby-machinist.com/resources/sb-how-to-run-a-lathe-1966-27th-edition-56-pdf.2909/

Atlas "Manual of Lathe Operations"
http://vintagemachinery.org/pubs/detail.aspx?id=13891


----------



## RJSakowski

Be aware that the points of engagement on the threading dial depend on pitch of the lead screw and the number of teeth on the threading dial gear. 

My Grizzly 602 has a 12 tpi lead screw and 48 teeth on the thread dial gear.  There are twelve divisions on the dial.  To cut thread pitches divisible by 3, you can engage on any number; for thread pitches that are odd multiples of 1/2 (e.g. 11-1/2 tpi) you must engage on every sixth number (e.g. 1 & 7, 2 & 8, etc.).  For all other thread pitches, must engage on every third number (e.g. 1,4,7.11 or 2,5,8,12, etc.).  

My Atlas 6 x 18 has a 16 tpi lead screw and 32 teeth on the thread dial gear.  It has 4 marks on the thread dial.  To cut even multiple threads, you can engage on any mark.  For odd thread pitches, you engage on every other mark, and for odd multiples of 1/2  you must engage on the same mark each time.

This will be different for an 8 tpi lead screw.  The best advice is to follow the manufacturer's procedure.


----------



## 12bolts

Gunrunner you need to order a set of these for the Z axis hand crank. Bill did you just spit coffee on your monitor? :rofl:

View attachment 253356


----------



## Tozguy

AR. Hillbilly said:


> So if I was to set the carriage anywhere on the bed and kick in the half nuts on the same line on the threading dial it will cut in the same thread? Why 29 degrees on the compound? I haven't found a manual for my lathe yet. It's a 1944 Sheldon.



This video gave me the best explanation for why 29 deg.




Basically to cut on one flank of the groove, the compound needs to be set at 30 deg. but not a second more. However the angle can be less than 30 and not cause a problem. Because of the potential inaccuracy of a protractor on a compound, it is recommended to set it at 29 deg. to err on the side of safety.


----------



## AR. Hillbilly

I just watched that video and learned a lot. I really appreciate this. I'm going to save this video for future use. A huge thank you to each of you.


----------



## AR. Hillbilly

Charles Spencer said:


> Here is a 1965 Army manual on a Sheldon 10" lathe:
> 
> http://vintagemachinery.org/pubs/2133/3556.pdf
> 
> Here are other Sheldon manuals:
> 
> http://vintagemachinery.org/mfgindex/detail.aspx?id=2133&tab=3
> 
> 
> And here are the two manuals that most people use regardless of the brand of their lathe:
> 
> South Bend "How to Run a Lathe"
> http://www.hobby-machinist.com/resources/sb-how-to-run-a-lathe-1966-27th-edition-56-pdf.2909/
> 
> Atlas "Manual of Lathe Operations"
> http://vintagemachinery.org/pubs/detail.aspx?id=13891


I belong to the Sheldon Yahoo group and have made friends with John. I also bought the book care and operation of a lathe. It has been a great help. On the links you just posted I found a manual very similar to mine and now have that saved. I do plan on buying the South Bend book how to run a lathe. I'm very much a beginner at all this but I'm also amazed at all I've learned since joining this forum. A huge Thank You to all who help others like myself.


----------



## higgite

This one gives a better explanation of why to set the compound to 29 degrees.

Tom


----------



## AR. Hillbilly

Thanks for that video. That is just what I'm looking for. I like to learn why I need to do something. It seems to soak in better.


----------



## T Bredehoft

AR. Hillbilly said:


> It seems to soak in better



Yes, that's the purpose of experience, too. Don't be afraid to make mistakes. They teach us far more than perfect parts. We've all crashed, many don't like to admit it.


----------



## Tozguy

I won't admit that I set the compound at 30 deg. from the spindle axis instead of from perpendicular to it. Nope.


----------



## rontlz

Bob Korves said:


> The threading dial helps you engage the half nuts in the same phase that it was in before.  The threading dial stays engaged with the lead screw all the time, keeping track of position while the carriage is disengaged from it.  There are various rules on which lines on the threading dials can be used with which threads, read your manual.  You can always use the same line each time.  The threading dial on an inch lathe will not work with cutting metric threads.


Hi Bob,
if cutting metric threads on an inch lathe and i cant use the threading dial, can i just engage the half nuts at the same line each time
or do i need to leave the half nut engaged and reverse it out?
Ron.


----------



## RJSakowski

rontlz said:


> Hi Bob,
> if cutting metric threads on an inch lathe and i cant use the threading dial, can i just engage the half nuts at the same line each time
> or do i need to leave the half nut engaged and reverse it out?
> Ron.


Different Bob, but no you positively cannot use an inch threading dial for metric threads.  All inch threads repeat every inch for whole number tpi and every two inches for 1/2 integral tpi.  You would have to run 127 threads on an inch lead screw to be in sync again for a metric thread.


----------



## RJSakowski

Ron, take a look at Tom Lipton's (OxTool) video on metric threading on an inch lathe.  He does a good job of explaining the process.


----------



## GunsOfNavarone

Listen, your gonna get a lot mumbly jumbly from the eggheads here. As a newb here myself, I can explain this much easier.... there is this this troll that lives under the apron....engage the half nut, it pokes him in the @ss. Disengage and it pokes him in the eye. After that I'm a bit hazy as well....


----------



## Surprman

When I first got my lathe and attempted threading I was using the reversing gears to move the carriage back between cutting  passes (because I didn’t have a thread dial and knew I should not disengage the split nut). Things didn’t work out well sometimes.  Turns out disengaging ANY part of the gear train will cause the lathe to ruin the threads - who knew! (Insert eye roll emoji here).   Once I started threading by simply reversing the motor between passes (after brining the tool away from the work) everything worked fine (slow but fine).  At that time I did t think I needed a thread dial because I was not doing production work and the extra time was not a big deal.  I recently got and used a thread dial for the first time and have to admit, it works pretty nicely.  Threading is much more enjoyable.

Rick


----------

