Metric Threading on Antique Imperial Lathe

I had to make some metric tubing nuts for an antique French engine and none of my lathes has metric threading. Luckily my L Robbins lathe has loose change gears so I can play with ratios to get what I need. I had to cut a 1.5 mm pitch and the diameter was not a standard size. 1.5 mm equals 16.932 threads per inch which is what I was shooting for but of course the standard change gears would not do that and I do not have room for a 100/120 tooth gear set.

I figured that I could use a 72 tooth gear which I had on the 4 tpi lead screw and I would need a 17 tooth gear on the stud, which I did not have. That ratio would give me 16.941 tpi which is within 99.9 percent of what I needed which I thought would work fine. Then I go and model a 17 tooth 10DP gear in Fusion 360 and send it to my 3D printer and print out the gear that I need. It worked out great. I can also do 1.75 mm and 2 mm with that same 17 tooth gear.

Not long ago a home shop machinist who goes by the Joy of Precision on the internet did the exact same thing using a Rivett 608 me thinks. Nevertheless he needed a metric thread for a project he was doing and worked out the ratio and came real close with gears he had on hand. On a short thread the pitch errors would not be an issue.

Warren said:

Not long ago a home shop machinist who goes by the Joy of Precision on the internet did the exact same thing using a Rivett 608 me thinks. Nevertheless he needed a metric thread for a project he was doing and worked out the ratio and came real close with gears he had on hand. On a short thread the pitch errors would not be an issue.

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This thread carries me back to my third year of apprenticship (1964), at tech college,this problem was posed by many machines of the time with a set of change wheels, it was easy if the machine had a 127 tooth "translation" gear (25.4 x 5 = 127), without this the method described was applied, the problem was compounded when the instructor posed the situation of a 3 percent pitch increase(threads in say plastic moulds), that problem was solved by the application of "continued fractions".
By now I think I would struggle with this without a reference book. CNC can be just too easy!!

"By now I think I would struggle with this without a reference book. "

Many years ago, I undertook the restoration of a radial drill with horribly worn bevel gears. They were very large and of an unusual pitch and count, so no chance finding anything off the shelf. I went to the local gear shop who wanted $1000 each to replicate them with a Gleason. They were different counts on each, so basically I was paying two setup fees.

In the end, I found a 1924 Colvin and Stanley book that explained how to make bevel gears on a horizontal mill and gave all the calculations for roll in, offset, etc... I ground a form tool and set up on a shaper. It worked and the old radial drill was indeed repaired.

While sitting there for hours one evening doing all the calculations, I was rather annoyed to have to run all the math. I have dyscalculia, mathematic dyslexia, so running long formulas takes me forever because I have to repeat the steps a few times to avoid errors. As I was doing this, it occurred to me that I was using a calculator, those guys would have been doing it longhand! Similar to my grandfather's Audel's Homebuilder's book set that explains in great detail how to properly use a hand saw for cabinetry work!

The 17 tooth stud gear is also metric magic on the 13" south bend lathe with the single tumbler gear box.It is substituted for an 18 tooth.

Continued fractions

Continued fractions are a handy method of quickly finding a close approximation with the gears you have at hand. Here is the relevant page by W. A. Tulpin in General Engineering Workshop Practice, Odhams, London (no publication date is shown, say about 1950).



I used the method to find a suitable set of gears for cutting worms on my Hendey lathe, which called for a ratio of 1:pi/2, using pi = 3.141592. Here is my working:



The fraction 344/219 = (96 x 86)/(72 x 73) gives a convenient combination of gears, with a pitch error of 1 in 125000, or 1 thou in 125 inches.

David.