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Metric Threads on a 16" SB

  • Thread starter DanBernoulli
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DanBernoulli

Guest
Since 16" lathes seem to be the topic of the day, how practical is it to cut metric threads on one? I would like to cut metric threads (1 to 3" telescope optics) and I only have one lathe - my 1937 South Bend 16" single tumbler lathe. I talked to the South Bend people some years ago and the necessary gears were much more than $1000 and much more than my budget could afford. Is it feasible to make a set of gears to do this? Can it be done without changing the tumbler gears? If so which gears do I change and how hard is it/ how much time does it take to change the gears? There are only a few threads I need to do. Is it something one wants to do on a one-off basis or, in spite of having more time than money, am I better off getting a lathe that is set up for metric threading?
 

Finegrain

Diamond
Joined
Sep 6, 2007
Location
Seattle, Washington
Since 16" lathes seem to be the topic of the day, how practical is it to cut metric threads on one? I would like to cut metric threads (1 to 3" telescope optics) and I only have one lathe - my 1937 South Bend 16" single tumbler lathe. I talked to the South Bend people some years ago and the necessary gears were much more than $1000 and much more than my budget could afford. Is it feasible to make a set of gears to do this? Can it be done without changing the tumbler gears? If so which gears do I change and how hard is it/ how much time does it take to change the gears? There are only a few threads I need to do. Is it something one wants to do on a one-off basis or, in spite of having more time than money, am I better off getting a lathe that is set up for metric threading?

There are several gear sets that can get you common metric threads, for shorter money than $1k.

To get started you need a 127/100 transposing pair, or 80/63 or 47/37 with slight pitch error. I can cut these gears. Being a 16" lathe, the 127/100 gears would be pretty large and that means relatively expensive stock, so the 80/63 or 47/37 might be more attractive, if you can live with the slight pitch errors wihc for most applications gets lost in the thread allowance anyway, and if you are cutting both male and female threads, is utterly moot.

Once you have this transposing pair, you can cut 0.5mm, 1mm, 2mm, 1.25mm, and 2.5mm threads without any other gears, by setting up for 40, 20, 10, 24, or 12 TPI on the quick change box.

You'll need another gear to make 1.5mm and 3mm threads, and yet another to make .7mm, and yet another to make 1.75mm.

Beyond that, you will need some spacers or custom gear studs to put all the gears in the right meshing orientations.

Hope that makes sense, and contact me via PM or e-mail for more details, and pricing on the gears.

Regards.

Mike
 

kitno455

Titanium
Joined
Jul 9, 2010
Location
Virginia, USA
I've never seen the metric set for the single tumbler 16, but other single tumbler machines need a new banjo. The heavy 10 metric set also changes the endgear DP, IIRC. So things get a little complicated.

What pitches are listed on your gearbox, and what pitch is the lead screw?

allan
 
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DanBernoulli

Guest
Only really need to do 0.75 mm threads.

My lead screw is 6 tpi.

Threads on my qc tumbler are
2, 4, 8, 16, 32, 64
2-1/4, 4-1/2, 9, 18, 36, 72
2-1/2, 5, 10, 20, 40, 80
2-3/4, 5-1/2, 11, 22, 44, 88
2-7/8, 5-3/4, 11-1/2, 23, 46, 92

I've looked for threads that are close but a far as I can tell none are close enough.
 

OldMachinist

Cast Iron
Joined
Mar 28, 2010
Location
Nashville, IL USA
The metric transposing gears for the 16" single tumbler does require a different banjo.
Here's what my set looks like on the lathe.
19.jpg
 

kitno455

Titanium
Joined
Jul 9, 2010
Location
Virginia, USA
If my math is correct, you should replace the 18t gear on the reverser stud with a 17 tooth gear. Then your gearbox will turn at .944444 times its normal speed. Then your 32 tpi setting will cut 33.882 tpi. That's 0.7497 mm, for an error of 0.046%.

this assumes you have an 18 tooth stud gear, of course.

allan
 
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DanBernoulli

Guest
I apologize to the group for not replying sooner. My day job unexpectedly turned into a day and night job for a few days. As far as metric threads - I do like the idea of changing one small gear then changing a couple of large gears and needing to fabricate a new cover and perhaps banjo. If indeed a 17 tooth gear will work it sounds good to me. Eventually I will likely get a smaller lathe that is built to turn metric threads, but not anytime soon.

BTW, which gear is the 18 tooth reversing stud gear? In the picture of the gear train on my lather is it the small gear that has the gray colored shaft with two brownish dots?

Dan
 

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kitno455

Titanium
Joined
Jul 9, 2010
Location
Virginia, USA
The stud gear is the one on the pivot of the reverse lever. It has a nut on the shaft so it can be removed. Yours looks more like 36 teeth?

allan
 
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DanBernoulli

Guest
Here's what my chart looks like. And here's a better view of the gear chain with teeth count and diameters.

Dan
 

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kitno455

Titanium
Joined
Jul 9, 2010
Location
Virginia, USA
Neat! They must have swapped the 36t stud gear for an 18t on the later models with the 4 tpi box. Likely no changes to the box at all. Given how much stress those coarse feeds put on the box, and how useful really fine feeds are, you might consider doing the same, provided your banjo will swing up enough to make contact with an 18t gear. Then you could even switch to a later model chart.

Anyway- you are in luck with metric threading. Using 36,34,32 and 31 tooth stud gears, you can make all the common metric pitches, with less than 1% error. The ones using the 34 are nearly perfect. Here's a chart I worked up for you, the metric pitch, the stud gear tooth count, the gearbox setting, and the % error:

MM Stud Gearbox Error%
8 34 3 0.046
7.5 32 3 0.346
7 32 3.25 0.757
6.5 32 3.5 0.757
6 34 4 0.046
5.5 31 4 0.581
5 32 4.5 0.346
4.5 32 5 0.346
4 34 6 0.046
3.5 32 6.5 0.757
3 34 6 0.046
2.5 32 9 0.346
2 34 12 0.046
1.75 32 13 0.757
1.5 34 16 0.046
1.25 32 18 0.346
1 34 24 0.046
.9 36 28 0.794
.8 32 28 0.794
.75 34 32 0.046
.7 32 32 0.794
.6 34 40 0.046
.5 34 48 0.046
.45 36 56 0.794
.4 32 56 0.794
.35 32 64 0.794
.3 34 80 0.046
.25 34 96 0.046

Note that the 31 is only used to make the 5.5mm pitch, so you could certainly skip that one. Also note that your standard 36 tooth gear will cut some of the same fine pitches as the 32, but I left them off the chart, and only use it for .9 and .45. If you need those two threads to have a little less error, you could use a 33, but that adds another gear to the mix.

Hope this helps!

allan
 
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DanBernoulli

Guest
Allan, You really went to town on the thread combinations. Thanks for crunching the numbers. It saves me a lot of work. Now I just need to get some gears made.

Dan
 

kitno455

Titanium
Joined
Jul 9, 2010
Location
Virginia, USA
Oops

False alarm. I've hand-checked the numbers, and they are correct. In fact, it appears this system will work well for other single tumbler machines, with only minor changes.

allan
 








 
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