Please Explain How A Threading Dial Works. | The Hobby-Machinist

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 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.

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

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

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.