Please Explain How A Threading Dial Works.

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AR. Hillbilly

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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.
 
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.
 
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.
 
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.
 
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.
 
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?
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Yup, it's that simple:)

AR. Hillbilly said:
Why 29 degrees on the compound?
Click to expand...

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:confused:

Then you set the compound at 30°, with the tool set 2.5° off perpendicular then you get a 60° thread
 
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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 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?
Click to expand...

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.
 
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
 
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.
 
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