Hi all,
I recently aquired a lathe (MyFord ML10), and I’m reaching the point where I need to cut some thread.
I’m looking to cut a 1.25mm pitch thread. According to the chart on my lathe, this can be done with a 25, 50, 63, 40, 35& 70 gear. If my math is correct, this means a reduction of (25 / 50) * (63 / 40) * (35 / 70) = 0.39375. That is the lead screw does 0.39375 rotation for each turn of the driver.
I do not have a 63T gear, and I was looking into ordering one, I syumbled on this other chart on the Myford website : https://www.myford.co.uk/acatalog/D-10428METRICSCREW.html#SID=291
It indicates that the same 1.25 pitch can be achieved with a 25, 40, 50 & 60 gear (which I all have).
But doing the math : (25 / 40) * (40 / 50) * (50 / 60) = 0.41666666666, I get a reduction that is close, but not the same as the previous one.
Is that to be expected ? Are threading reduction giving an acceptable pitch with some tolerance? Or is there some difference between older and newer lathe in the leadscrew or spindle to driver reduction?
Or did I make a mistake in my math (can’t rule this out) ?

For context, I am trying to build a worm gear for a robotic project, I’m using an aluminium lazy suzan from amazon in which I’ve cut teeth using a M8 tap (that where I got the 1.25 originally). I was planing to drive the contraption with a M8 threaded rod, but every threaded rob I’m getting from the hardware shop is bent to some degree, making the system too hard to turn for the small Nema17 motors I’m using. So my next step is to cut my own threaded rod from a piece of brass.

In the Cat Like Tread song from Pirates of Penzance there is a line ‘here’s your crowbar, here’s your centre bit’. As it is about taking time off from piracy and doing a bit of burglary I don’t see what use what we would call a centre drill (bit) would be. Anyone got any ideas?