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Gear for threading dial


Ultra Member
So...I've gone head first down the rabbit hole. Bail out now if you don't want to follow!

I kinda knew how a thread dial was supposed to work but I ended up doing a spread sheet to understand better. The thread dial is just to help us know that the cutter will be lined up with the cuts we have already made. Not off by, say, 1/2 a thread.

Note that if you are cutting a thread that is an even multiple of your lead screw, you don't need the threading dial at all. If your lead screw is 8 tpi and the thread to cut is 8 tpi, 16 tpi, 24 tpi, etc, then the lead screw CANNOT get out of sync with the thread. You can open and close the half-nuts whenever you want.

However, if you are cutting another thread pitch, though, how does the dial work? By helping you know when the lead screw has turned a certain number of times. The threading dial turns at a ratio to the lead screw. If you have a 32 tooth gear, the lead screw turns 32 times for each full revolution of the dial. With an 8 tpi lead screw, the carriage would travel 4 inches in 32 turns of the lead screw (32 turns divided by 8 threads per inch). If the threading dial has 4 divisions on it, that equates to 1 inch of travel for each division or tick mark.

If cutting 9 tpi or 10 tpi or 16 tpi (etc), then the cutter will line up exactly with the existing cut whenever the dial is exactly at one of the tick marks. It has to because there are a whole number of threads per inch.

If cutting a weird thread like 11.5 tpi, the cutter will line up exactly on each of the opposing dial tick marks because the lead screw distance will be 2 inches and there are are 23 threads (11.5 X 2) in 2 inches.

Incidentally, the 8 tpi lead screw and 32 tooth threading dial gear is a good combination since it relates 4 inches of travel to 32 turns. Nice whole numbers: each of the 4 divisions on the dial is 1 inch.

The CX-706 however comes with a 40 tooth threading dial gear. That means it has 5 inches of travel in 40 turns. Unfortunately, that means with 4 divisions on the dial, each one is 1.25 inches of travel. Say you want to cut a 10 tpi thread. After one dial division, the cutter is positioned at 12.5 threads: BAD. To get proper alignment, you can only close the half nuts on opposing tick marks (1 and 3 or 2 and 4). For odd numbers of threads per inch (11, 13, etc), the half nuts can only be closed when the original tick mark comes back around. The threading dial cannot be used with threads like 11.5 tpi. A full rotation of the threading dial, 40 turns of the lead screw, is (11.5 X 5) 57.5 threads. The cutter just won't come back to a convenient alignment. A 32 tooth threading dial gear is much more convenient.

Hope this helps somebody else.



Ultra Member
Found this on thingiverse
Since the Boxford is a Southbend clone, I assume the lead screw is 8 tpi. The gear is not shaped like a worm gear but has angled teeth just like my Southbend dial.


Ultra Member
The gear is not shaped like a worm gear but has angled teeth just like my Southbend dial.

The angle of the teeth is the lead angle of the lead screw. It should mesh very well.

The worm gear looking ones have a larger area of engagement. A nice feature which cuts down on wear, but not a requirement for a threading dial as it only shows position of the lead screw and does not transmit any power.


Ultra Member
Busybee came through!

I got a complete replacement threading dial assembly but including a 32 tooth gear rather than the prior 40 tooth. No charge. After a bit of brief testing, it appears to mesh MUCH better with the leadscrew, see following:

32T gear mesh s.jpg

The dial used to 'stutter' as the keyway on the leadscrew passed by. Now it turns smoothly. I still need to truly cut some single point (imperial) threads but I'm confident that it will work now.

The paperwork identifies the replacement threading dial assembly as part number PCX701TD in case that helps anyone else.