Well, what I am saying is that you can trust the numbers on the chart but you have to understand what they mean.

Typically on larger lathes, you have 4 kinds of "threading" modes. SAE/English threads, metric threads, and the corresponding thing for making worms using either diametral pitch, or module pitch.

So you have 4 sections of the chart. I suspect that the first section is for module pitch worms, the second is for metric threads, the third one is for diametral pitch worms, and the 4th section which has 4 gear setups is for inch/SAE threads.

The worm pitches are defined in terms of the pitch of the gear they match with. Gears in the non-metric world are dimensioned in terms of diametral pitch, i.e. how many teeth per inch of diameter. So a 20 diametral pitch gear with pitch diameter of 1" will have 20 teeth, and a gear of the same pitch but with pitch diameter 1.5" will have 30 teeth. Gears of same diametral pitch have the same size teeth so they can mesh with each other. They can have different diameters and therefore different number of teeth, but the size of the teeth are the same.

Now, take a gear with diametral pitch of 40. It means that there are 40 teeth in a gear with diametral pitch of 1". How wide is each tooth? The pitch circumference is 1" x pi, so each tooth is (1"/40) * pi wide from base to base. So the worm pitch that will match that gear has to be the same, i.e.

(1"/40) * pi = 1" / (40/pi) = 1" / 12.73239545 = 0.07854"

But that last couple of numbers are not that important.

If you want to make a worm, it will be specified with the numbers on the chart.

If you want to make a thread, it will be specified with the numbers on the chart.

You just need to know what you want and what each section of the chart means.

I am absolutely convinced that the 4 sections mean exactly what I said. And what I am describing in my previous post about putting a DTI to measure the movement of the carriage when you do 10 turns of the spindle is just a way for you to prove to yourself what the reality is exactly.

I am convinced that if you take an hour to set up the 4 different configurations you have in there and measure the effect on the pitch, you will be 100% confident of whatever you will discover. You don't need to believe me. You just need to believe the machine. It will tell you exactly what it is doing.

I hope I made the matter a little clearer. If something is a little hazy still, come back and ask or send me a pm, we can have a chat on the phone. But do yourself a favor first, and measure the pitch as the machine will tell you. I believe it will answer all your questions better than I will ever be able to.

Good luck having that conversation with your lathe! ;-)

Jacques