How to install leblond regal roundhead transposing gears

I'm looking to adapt my 13" regal to cut metric threads. I've found fantastic information on this forum, but I don't understand how the quarter geared would be changed. I've taken apart my lathe in the past so I know it well, but it's more about the how transposing gears work. Would this be a second large quarter gear (127 tooth) added to the 2x 32 and 1x 120t gear that currently transmits power to the gear changer? Do I stack them? Or would it replace the 120t? Of its both, does the 127t go before the 120t gear to speed things up, or after it to slow down? How big is the 127t gear, without specs I don't know what to buy or make!

Anyways, thank you all for your patience with my questions!

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A 127 and 100 STACKED would be a 1.27 to 1 compound. The other gears, such as "stud" (upper) and "screw" (lower) have to accommodate this STACKED compound gear. Lathe makers generally provide SPACERS to enable this DOUBLE PLANE set up

Full transposing sets also depend on a stack of additional "stud" gears to provide additional metrication. The 1.27 to 1 (or whatever other compound) compound by itself can only do SOME of the the metric threads. A "close" stand in for 127/100 is a 47/37 compound

Here is South Bend's little diagram from the old days - illustrating the two planes when using compound gearing

Thank you. My screw and stud gears come with spacers, so that won't be a problem.

Should the smaller or larger gear of the stack engage the stud? From what I understand, it's the larger gear that should engage the stud in order to speed up the screw, correct?

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Actually you are "slowing down" the screw. the stud gear will turn 1.27 times faster than the screw gear.(assuming that ratio compound - and assuming stud and screw gears are the same tooth count)

Not mentioned yet is there may be a need for idler gear to be between smaller gear of compound and screw gear - this may require a different quadrant or "quarter" than you have at the present (idler gears can be any workable tooth count - they have no effect on overall ratio) See thumbnail for SB's transposing set of over 80 years ago when the set included different quadrant casting - and incidentally shows the idler gear mentioned. Do note the large number of differing stud gears in play here on the chart.

Anyhoo - its easy to "guess" what metric lead the set up produces. Say you were set up to cut 5 TPI. The lead of that is .200". .200 divided by 1.27 is .15748 - which is 4mm. Changing the set up to 10 TPI gets you 2mm and 20 TPI gets you 1mm

makinthings said:

Thank you. My screw and stud gears come with spacers, so that won't be a problem.

Should the smaller or larger gear of the stack engage the stud? From what I understand, it's the larger gear that should engage the stud in order to speed up the screw, correct?

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Again, thank you for all the clarifications. My current quadrant gear is on a banjo that I can move in an arc and the quadrant gear can slide up and down on it, so it might even be okay! (wishful thinking here). Since leblond recommends 120/127 and includes conversion values for the gear box, that's what I'll go with at first. With how close the tooth count is, do you think it would be possible to have the two gears the same size and have to tooth pitch off by just a bit?

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do you think it would be possible to have the two gears the same size and have to tooth pitch off by just a bit?

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No way. "Off a bit" sure will not be kind to any gear that has to mate with the the "off a bit" gear.

Different pitch altogether? - for sure - if gears that have to work with each other share that same pitch

Does Running a Regal have any input on this subject?

R. K. LeBlond Machine Tool Co. - Publication Reprints | VintageMachinery.org

Doesn't seem like it unfortunately.

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127/120 (1.0583333 to 1 compound) works good with such as

48 TPI = 0.5mm
32 TPI = 0.75mm
24 TPI = 1mm
16 TPI = 1.5mm
12 TPI = 2mm
8 TPI = 3mm
6 TPI = 4mm
4 TPI = 6mm

Example of calc:

8 TPI = .125" lead
.125 / 1.0583333 = .11811 = 3mm

johnoder said:

127/120 (1.0583333 to 1 compound) works good with such as

48 TPI = 0.5mm
32 TPI = 0.75mm
24 TPI = 1mm
16 TPI = 1.5mm
12 TPI = 2mm
8 TPI = 3mm
6 TPI = 4mm

Example of calc:

8 TPI = .125" lead
.125 / 1.0583333 = .11811 = 3mm

Click to expand...

People on these forums are so great. Thank you for all the help. I hope I can pass it forward in time as I learn more.

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makinthings said:

People on these forums are so great. Thank you for all the help. I hope I can pass it forward in time as I learn more.

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You are entirely welcome.