Metric Threadcutting gears for antique South Bend lathe

[bobneumann]

I figured out a way to make perfectly acceptable (Something above 99.5% accurate) metric threads on an old South Bend lathe, just by making a series of 5 different stud gears. The idea should work for almost any imperial lathe. The required gears may vary some from lathe to lathe, the attached Excel spreadsheet can help you figure out your own.

See the video for details.

The Excel spreadsheet is designed to simply display the resulting metric thread for every possible combination of stud gear size and gearbox setting. You simply visually find which stud gear and gearbox combination will produce the closest approximation to your desired metric thread. In my case I was able to make a set of 5 replacement stud gears that could produce all the common metric bolts, from the tiny .35M up to the large 5M, all with above 99.5% accuracy to the desired thread. (Most with above 99.9% accuracy)



The only variables for your lathe are, "what is your "native" stud gear size?", and "what exact tpi settings are available on your QC gearbox?" Enter those in the spreadsheet and it will display the rest for you.
stud gear metric settings.xls

Here is the resulting chart for my "single tumbler" South Bend lathe:

[Daturat100r]

You didn't try the 21 tooth gear trick for metric cutting on imperial lathe?

[bobneumann]

You didn't try the 21 tooth gear trick for metric cutting on imperial lathe?

I'm not familiar with that... By my chart, for this particular lathe, 21 teeth doesn't hit very near anything.

[Daturat100r]

It a dodge detailed in L C Masons awesome lathe work book "using the small lathe" will dig out my copy and pick out relevant info,he says it way more eloquently than I can

[bobneumann]

doing a little googling, it looks like that trick applies only to mini lathes with a 16 tpi leadscrew. My lathe (South Bend Heavy 10) has a quickchange gearbox, and I believe, though I'm not certain, an 8 tpi leadscrew. If you open the attached spreadsheet you'll see that, at least for my lathe, a 21 tooth stud gear would turn a perfectly fine 2.0 and 1.0 thread, but that's about it.

Bob

[Wmrra13]

Nice videos - I really like your homemade indexer.

[Daturat100r]

Sorry clean forgot to look up info on 21 tooth dodge surely if u have a 8tpi the dodge will work as it does on my myford which has an 8tpi leadscrew,maybe yours is a 6 tpi

[Daturat100r]

Stolen from myford site with 8tpi leadscrew the errors of cutting with 21 tooth gearwheel dodge are exceedingly small

[bobneumann]

Here's how the spreadsheet works:

1. You need to know the toothcount for your factory stud gear. Mine is 24.
2. You need to be able to read the chart on your QC gearbox, becuase you'll need to know the full list of threads that your lathe is capable of.

Instructions:
1. In field "F1" enter the number of teeth on your factory stud gear.
Example: Single Tumbler Heavy 10 stud gear is 24 teeth.

2. In Fields G3 down to G46 (or farther down as necessary) enter the exact TPI values that your gearbox is capable of
Example: Single-Tumbler Heavy 10 can do 42 unique TPI settings ranging from 4 TPI to 224 TPI
3. For a given metric pitch, visually search in the green "resulting metric pitch" field for the value nearest to the desired metric pitch, then pick the closest match for the replacement stud gears that you have to work with.
Example: For a desired metric pitch of "7", the nearest stud gear/qc setting would be a 33Tooth stud gear with a QC Setting of "5", resulting in a perfectly acceptable metric pitch of 6.985

4. Fill out the chart for all the values for your lathe, selecting for the fewest number of gears that need to be made.
As downloaded, the chart is pre-filled out for a lathe with a 24T stud gear, and a SouthBend single tumbler gearbox.
The best selection of stud gears for that 24T/single tumbler lathe turns out to be 19,26,33,34, and 37 tooth stud gears, which can create all common metric fasteners under about 50mm.

[Toolmaker51]

I don't see these so much a dodge, but extending capability using almost simple mechanics. It's juggling 'as built' components to reach or approximate other ratios. No reason to avoid incorporating any potential new ability.

Despite metric/ imperial established so long ago, VERY FEW lathes encompass the full range of both. Not practical design or marketing, with thread pitches and feed rates coupled to diameter - a 6" lathe would rarely be called to generate 2 pitch, nor a 24" doing 32 tpi. The large machine has what appear fine pitches resulting from accommodating feed rates. The reverse for small lathe isn't so.
For any involved with equipment repair, producing any pitch isn't required lets say 15 or 17, just the real pitches like 8-10-12 etc; of course extends same way into metric.