Question about thread dial

[ctwo]

Yes, the 127 tooth gear would be a drive train gear and is usually a compound gear to complete the conversion. Another option you might have is to just use a metric thread that is close enough to the inch thread that you want, and will work well enough...

Most often, when threading different standards than that of the lathe, the half nuts are left engaged, the tool is retracted after the cut, and the lathe is run in reverse to back the carriage for another cut.

I have not done this, but I think I recall seeing that one could use an indicator to locate the carriage in the starting position (on the tail stock side), and then use the thread dial to engage the nuts on the same mark. There may be some cases where you could engage on any mark, or on any whole number, or on only odds or evens, depending on what thread is being cut. I think if you are using a thread dial gear that orients the spindle in the same place for each mark or for each rev of the dial, etc. then this method will work.

[tornitore45]

Let's explain this by means of example:
The lathe has a 6mm leadscrew
I want to cut a 16 TPI thread

Each spindle turn the carriage must advance 25.4mm/16
How many turns of 6mm screw moves 254/160th of a mm?
Turns = N = Leadscrew Turns/Spindle turns = (254/160)/6 = 254/960 = 0.264583333 = [total product of driving teeth] / [total product of driven teeth]

If we have a 127 gear and a 480 gear we are done. The 127 goes on the spindle and the 480 gear goes down on the leadscrew.
Massaging the fraction 127/480 we find that [127 x 20]/[120 x 80] is an acceptable gear train.
20 tooth on the spindle driving 120 tooth compounded with 127 driving the 80 toot on the leadscrew.
Nelglecting the 120/127 compund for a moment is clear that the 4:1 remaining ratio gives 1/4 turn to the 6 mm screw advancing 1.5 mm nearly 1/16 of an inch. OK we get the ratio right instead of the reciprocal which would be wrong.

In order to use a treading dial we need to have an indication every 1/16 of an inch or every 0.26458333 turns of leadscrew or multiple of such number. There is no gear that can mesh with a 6mm leadscrew and and move the index 360/N every 0.2645 turns of the leadscrew.

Studies have been done to prove other methods are possible but they are greatly more impractical and error prone that the aggravation of keeping the half nut closed pale in comparison to screwing up a thread.

In short when cutting a NON native thread leave the tread dial in the drawer and the half nut closed.

[John Hasler]

Make a nut for your 6mm pitch leadscrew that is also a 120 tooth gear. Run it onto the leadscrew and secure it to the carriage so that it is free to turn. Mate a 127 tooth gear to it. When the half-nuts are disengaged the 127 tooth gear will rotate 4 times for each inch that the carriage would have advanced had the half-nuts been engaged. Adjusting the tooth count of the 120 tooth gear and/or adding reduction gears is left as an exercise. The key transposing ratio is 3:127. The 3 will be replaced with a different integer for other leadscrew pitches.

[spro]

I appreciate this information. If we combine this info to the info from the recent thread of converting an Atlas to metric, we have a small booklet of how to "work back" with both physical and mental examples of the ratios and procedures.

[tetramachine]

JH, The issue is really that the leadscrew has to move the carriage X inches per rev, and the thread dial is a minor item. Because metric threads are not simple multiples of each other as are TPI threads, the gearbox is much more complex, and that makes converting to imperial a vary complex project.
Best to use the closest metric pitch 1.25 for 20tpi, 1.5mm for 16tpi, etc, and taps and dies for the rest.

[John Hasler]

JH, The issue is really that the leadscrew has to move the carriage X inches per rev, and the thread dial is a minor item. Because metric threads are not simple multiples of each other as are TPI threads, the gearbox is much more complex, and that makes converting to imperial a vary complex project.
Best to use the closest metric pitch 1.25 for 20tpi, 1.5mm for 16tpi, etc, and taps and dies for the rest.

The threading dial design I described is for use when cutting UTS threads on a metric lathe that has transposing gears installed and so is set up to cut UTS threads. Nothing to do with cutting metric threads. A mechanical threading dial for the latter would have more gears than a Swiss watch and as many levers and settings as a QCB.