Want to cut metric threads on SB 13

[Metalman21]

I would like to cut metric threads on my 1948 South Bend 13" lathe. It has a single tumbler QC gear box and I suspect that make some differences opposed to the double tumbler set up?
I've searched and read of a 127/120 gear plus various other gears required to make proper metric threads?
What do I need and where can I get them?

 

[kitno455]

The south bend way would be to replace the end gears, banjo and gear cover with parts that look somewhat like those from a double tumbler, but are not identical. You would have a few stud gears to swap around for different pitches.

There is a less expensive alternative. Many metric pitches can be closely approximated by just swapping the stud gear only. With a bit more info about your lathe, I could work up a list for you. I would need to know the pitch of your leadscrew, the list of pitches your current gearbox will cut, and how many teeth are on the stud gear you have now.

allan

 

[Metalman21]

Allen, looking to learn what I can about it.
The pitch of the lead screw is 6TPI. The photo shows the list on the gearbox.
To be honest I,m not sure which gear to consider the stud gear, there is a small gear attached to a larger (left of the "S",see the photo of the gear train).
The last lathe I had was a change gear and I had to get creative with a couple of missing gears, the gear box spoiled me and made me forget all the ratio calculations.

 

[SBLatheman]

The stud gear should be 18 tooth.
Ted

 

[Metalman21]

I think the small one was 18, not near the lathe right now.

 

[moya034]

I have the same lathe and am intersted in this as well. If you don't get in front of your lathe soon I'll post the data from mine

 

[Metalman21]

I have the same lathe and am intersted in this as well. If you don't get in front of your lathe soon I'll post the data from mine

What? Thought I had the only one of those left! Ha ha.
I won't be back to double check until late tomorrow morning so be my guest and count yours if its handy.

 

[Metalman21]

Stud gear is definitely 18 tooth, any other info needed?

 

[kitno455]

Sorry for the delay.

For your single tumbler 13, the real trick is to use a 17 tooth stud gear instead of 18. That makes your 6 tpi screw act like a 6.353 tpi screw, which happens to be 3.999 mm per turn. That will let you cut 11 common metric pitches with less than 0.05% error (1 part per 2000). A smattering of other pitches can be made with a few other gears, with slightly worse error. All of these are more than good enough for making fasteners and the like. A couple pitches cannot be made by this method and are left blank. Long story short you can do pretty well if you can get 17, 23, 25 and 35 tooth gears. The usual warning about leaving the half-nuts engaged applies with all of these, BTW.

MMpt Gear Gearbox Error%
6.5 23 5 0.14
6 17 4 0.05
5.5 35 9 0.22
5 23 6.5 0.14
4.5 35 11 0.22
4 17 6 0.05
3.5
3 17 8 0.05
2.5 23 13 0.14
2 17 12 0.05
1.75
1.5 17 16 0.05
1.25 23 26 0.14
1 17 24 0.05
0.9 23 36 0.17
0.9 25 44 0.22
0.75 17 32 0.05
0.6 17 40 0.05
0.5 17 48 0.05
0.45 23 72 0.17
0.4 25 88 0.22
0.35
0.3 17 80 0.05
0.25 17 96 0.05

hope this helps.

allan

 

[Metalman21]

Allan
Very, very interesting and thank you.
Just to make sure I'm reading your list correctly, take 1.5mm pitch:
17 tooth stud gear, gear box on 16TPI cuts 1.5mm with a 0.05% error?
3.5, 1.75, and .35mm are the blanks that can't be cut?
Do you know of a good source for the gears? Or would they be standard from an outfit like Boston Gear?

 

[moya034]

Allan, thanks for taking the time to post that info! Very useful indeed.

 

[kitno455]

Allan
Just to make sure I'm reading your list correctly, take 1.5mm pitch:
17 tooth stud gear, gear box on 16TPI cuts 1.5mm with a 0.05% error?

You read correctly.

3.5, 1.75, and .35mm are the blanks that can't be cut?

Correct again.

Do you know of a good source for the gears? Or would they be standard from an outfit like Boston Gear?

That depends- what is the DP of the gears? I don't think Boston or Martin make 14 DP gears anymore, if that is what they are. In that case, you can make them yourself...

allan

 

[iwananew10K]

are you familiar with the "button method" of making a gear cutter?
this chart is for 20PA gears but the method is the same-a little digging on HSM will produce the chart for 14.5PA
GEAR CUTTING

 

[Metalman21]

I want to start with the 17 tooth spur gear as that will produce the metric threads I need to cut now. I am not familiar with gear cutting so if I can't buy it my solution may be to have one cut on EDM equipment.

 

[Charlesc]

Old thread I know. I have a similar problem with metric threads. South Bend 13 inch x 7 ft bed with QC and dual tumblers. My booklet "How to Run A Lathe" from South Bend has a section transposing gears but finding it a bit confusing and looks like I would need a bag full of gears which won't be easy to find a probably very expensive. Google search brought me here.

Picture of the gear train.

TIA for any input.

 

[kitno455]

So, were you planning to tell us which metric threads you want to cut?

allan

 

[kitno455]

Oh, and if that's a picture of your lathe, you have a single tumbler machine.

 

[Joe21]

There are a number of ways to achieve your objectives, depending on your needs.

Last December I looked at FarmBoySales.Com (no relation or interests) for a 3D printed transposing gear for our SB13. There are a number of others that also do these.
Since this thread was started in 2013 a lot has changed in the 3D world...

 

[Charlesc]

No it’s double tumbler

 

[Charlesc]

So, were you planning to tell us which metric threads you want to cut?

allan

Smaller threads. Certainly not over 12 mm