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Thread dial question.

home lathe user

Plastic
Joined
Sep 20, 2020
New here, but getting back into machining after 25 yrs.
I know I will catch flak,but I bought a SB 9" C lathe.......
my only question is, why does my thread dial have 6 lines, instead of 8 ?
 
It's a homemade job. You need to remark the top (make a new one) and put on 8 lines (I suspect that it came off a lathe with a 6 TPI leadscrew). I don't know how you would work the dial with the 6 lines you currently have with an 8 TPI leadscrew. Also, definitely mark your lines.

Here's how to use a thread dial:

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I wonder if your gear even meshes with the LS.
 

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It's certainly not original to your lathe, and it looks like it probably is home made. It may work perfectly fine once you understand how it should work, and figure out how it's graduated.

First, as suggested above, you will need to check engagement with the lead screw. The "pitch" of the gear, must correspond to the pitch of the lead screw. In other words, the spacing between the teeth must be such that they engage the threads of the lead screw and the gear can roll along the screw without binding or jumping teeth.
Once that's established, you can figue out how it's graduated. WITH THE SPINDLE STOPPED, move the carriage left or right so one of the long lines on the rotating dial lines up with the mark on the bezel. Mark this line "1" with a sharpie. Next, mark the position of the carriage on the bed with a sharpie. Move the carriage (the same direction you moved it before) until the next long line comes around to the mark on the bezel. Number this line "2", and make a new mark on the bed showing the carriages new position. Then do the same for the remaining line.
By measuring the distance between the marks on the bed, you can figure out how this dial will work on your machine. A thread dial is a very simple indicator of relative movement between the lead screw and carriage position. As long as one complete revolution of the thread dial corresponds to a whole number of inches, you can mark out each inch on the thread dial. Since you lead screw is 8 TPI, the half nuts will engage every eighth of an inch, or eight places per inch.
The thread dial in the chart shown above shows four inches of movement per one complete revolution. Each of the numbered lines correspond to one inch of relative movement, each of the shorter lines is one half inch of movement. Note that on your lathe, there would three places between each line where the half nut could be engaged.

For cutting:
8TPI, or any multiple of 8: Engage on any inch or any eighth (anywhere the nuts will close)
Any number of threads divisable by 4: Engage on any inch or any quarter inch mark
Any even number thread: Engage on any inch mark or any half inch mark
Any odd number thread: Engage on any inch mark
 
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It's certainly not original to your lathe, and it looks like it probably is home made. It may work perfectly fine once you understand how it should work, and figure out how it's graduated.

First, as suggested above, you will need to check engagement with the lead screw. The "pitch" of the gear, must correspond to the pitch of the lead screw. In other words, the spacing between the teeth must be such that they engage the threads of the lead screw and the gear can roll along the screw without binding or jumping teeth.
Once that's established, you can figue out how it's graduated. WITH THE SPINDLE STOPPED, move the carriage left or right so one of the long lines on the rotating dial lines up with the mark on the bezel. Mark this line "1" with a sharpie. Next, mark the position of the carriage on the bed with a sharpie. Move the carriage (the same direction you moved it before) until the next long line comes around to the mark on the bezel. Number this line "2", and make a new mark on the bed showing the carriages new position. Then do the same for the remaining line.
By measuring the distance between the marks on the bed, you can figure out how this dial will work on your machine. A thread dial is a very simple indicator of relative movement between the lead screw and carriage position. As long as one complete revolution of the thread dial corresponds to a whole number of inches, you can mark out each inch on the thread dial. Since you lead screw is 8 TPI, the half nuts will engage every eighth of an inch, or eight places per inch.
The thread dial in the chart shown above shows four inches of movement per one complete revolution. Each of the numbered lines correspond to one inch of relative movement, each of the shorter lines is one half inch of movement. Note that on your lathe, there would three places between each line where the half nut could be engaged.

For cutting:
8TPI, or any multiple of 8: Engage on any inch or any eighth (anywhere the nuts will close)
Any number of threads divisable by 4: Engage on any inch or any quarter inch mark
Any even number thread: Engage on any inch mark or any half inch mark
Any odd number thread: Engage on any inch mark

Each long line, the carriage moves 1".
I guess I will color code the lines and use the same line for threading, until I can get a proper dial.
 
Each long line, the carriage moves 1".
I guess I will color code the lines and use the same line for threading, until I can get a proper dial.

No need to replace the dial, it will work fine. Simply number the long lines 1,2&3. These are your inch marks. Close the half nuts on any inch mark(long line) for odd number of TPI threads. Close on any line(long or short) for even number of TPI threads.
You can start on any line, or half way between any two lines for threads with a TPI divisible by 4.
8 TPI or any multiple of 8 TPI(16,24,32,40ect.), start anywhere, don't even use the thread dial.

It will only get complicated when cutting a non whole number of TPI, like 11-1/2 TPI, but these are pretty rarely used, so for most threads you're fine.
 
Another question - I just made a threading dial for my seneca falls lathe - 12 TPI lead screw, and for space reasons used a 24 tooth gear. I have four spots that I use to locate closing points, and the carriage moves 2 inches per a full turn of the dial. During test cuts with the change gears set to cut 13 tpi, it seemed I could close at any points 180 degrees apart, but for 16 tpi threading, all four closure points were allowed - every 90 degrees.

Does this make sense with the theory?
 








 
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