View Full Version : Metric = 127/120 or 127/100

03-21-2005, 09:26 PM
I’m confused.
Which is it 127/100 or 127/120?
I have a heavy 10” South Bend lathe with the extended quick-change gear box, 70 therads capability including 27 TPI and I would like to cut metric threads. To do this, it was my understanding I need transposing gears at a ratio of 127/120. Except as I read and investigate it seamed that I keep hearing about the 127/100 ratio. So, now I’m confused.
Can some knowledgeable person shed some light on this dilemma I have, and point me in the right direction?
I will be eternally grateful, not to mention a lot less confused.

03-21-2005, 10:40 PM
There is 25.4 mm per inch (exact)
25.4 X 5 =127
20 X 5 = 100
is the way I have always heard it explained.


03-21-2005, 10:49 PM
Edited to apologize for lack of thought put into my original post. Doh!

[This message has been edited by vinito (edited 03-22-2005).]

Paul Alciatore
03-21-2005, 11:10 PM
The 127 tooth gear is the basis for the conversion between English and Metric. But it is usually used as part of a compound gear and the other side of the compound needs to be a number that is compatible with the other gears in the set-up. That includes both the gears inside and outside of the gear box. Different numbers of teeth on that gear will allow different sets of metric threads. The choice is for the best such set. Some compounds use 100, some 50, some 120, and other values that are generally multiples of simple numbers like 5, 2, etc. Some sets of change gears are in multiples of 5 while others, like SB's, are in multiples of 4. Additional gears are added to these basic series for special purposes, hence the 42 tooth used for 27 TPI.

I've been meaning to do the comparison of 100 vs 120. Could you post a list of the threads available on your gearbox? I assume it's a two lever box. A, B, C, D, E and 1 - 8? Or is it the single lever box?

If I could have a complete list of the English threads you can cut with the box, I can determine the metric threads possible with both compounds.

Paul A.

03-21-2005, 11:14 PM
According to How To Run a Lathe, it is 127/100.

03-21-2005, 11:43 PM
According to "Screwcutting in the Lathe" by Martin Cleeve, the transposing gears are explained as follows: There is exactly 2.54 centimeters to an inch. Mutiplying by 100 gives 254 centimeters equals 100 inches. Dividing both by 2 gives 127 and 50, these are the correct transposing gears (127 is a prime number and cannot be reduced further). The South Bend chart shows 127 and 100 being used, but the other gears in the gear train must be taken into account. For example, to cut a 1mm pitch thread you would use a 40 tooth stud gear driving the 127 tooth gear, which is keyed on the same axis to the 100 tooth gear, which is driving another 100 tooth gear on the lead screw. The two 100 tooth gears are a 1 to 1 ratio so the overall effective ratio is 127/40 = 3.175 times 8 threads per inch of the leadscrew = 25.4, the number of mm to the inch.

Paul Alciatore
03-22-2005, 03:42 AM
I have several spreadsheets set up to find all possible threads from a given set of gears on a lathe. One of these was using the standard SB-9 two arm gearbox and additional stud gears ranging from 16 to 48 tooth. This sheet was set up for a 127/100 compound. I altered it to find all possible combinations with a 127/120 compound and the same set of additional stud gears.

My results show that it is possible to cut 22 different metric threads with pitches ranging from 0.5 to 6 mm with the 127/100 compound. Some of the finer threads require the use of a second standard SB compound (1:3, 1:4, or 1:6).

The sheet with the 127/120 compound showed only 11 different metric threads were possible, ranging from .25 to 5 mm pitch. The use of additional compounds did not increase this list.

The use of additional, non standard stud gears will, of course, add additional values to both of the lists.

Both compounds will work but it would appear that the 127/100 compound is more useful for the SB-9As. This will not necessairly be the case with other lathes. Each set of lathe change gears and quick change box will have to be evaluated to find the best metric compound. I would need your specific information to find a definitive answer to your question.

Paul A.

03-22-2005, 08:44 AM
The key is the fact that 25.4mm = 1 inch.
Somehow or other you need a gear ratio that takes that conversion ratio into account. You can jigger the ratio around with other gearing to take into account gear ratios built into the QC gearbox and such, but at some point it has to involve the irreducible prime number 127.

For an exact conversion, that is. As was discussed a few days ago, other ratios such as 47/37, are "close enough" for nearly all occasions, and handier to deal with.

Historical note: Back in the first half of the 20th century, there were three "standard" inch-to-metric conversions: the English, the Canadian, and the U.S. One of them was 2.53999-something, one was 2.54 exactly, and one was 2.54000+ -something. They finally got together and standardized on 2.54.

03-22-2005, 10:34 AM
[If I could have a complete list of the English threads you can cut with the box, I can determine the metric threads possible with both compounds.

Paul A. [/B][/QUOTE]


This is a list of threads from my gearbox:
A-4,4 1/2,5,5 1/2,5 3/4,6,6 1/2,6 3/4,7,7 1/2
B-8,9,10,11,11 1/2,12,12,13 1/2,14,15

Please let me know what you find.


03-22-2005, 11:41 AM
With such a wide range of feeds available, you can probably come close enough to most metric threads to make the need for transposing gears a moot point.

Paul Alciatore
03-22-2005, 12:28 PM

Wow, that box has a lot more settings than the one for the 9s. I will try to run some charts tonight. I am curious as to how it will come out.

Oh, I also need to know what sise stud gear or gears you normally use with each range of settings.

Paul A.

03-22-2005, 01:16 PM
How many teeth on the spindle gear, stud gear, and the gear on the back of the quick change box. Also how many threads per inch is your lead screw.


03-22-2005, 01:16 PM
JCD: Did you figure out how you are going to index your gear blanks to make you transponding gears yet? I could easily draw up a circle with 127 divisions and one with 100 divisions it that would help. Do you have the 16DP milling cutter you will need?

Gary P. Hansen

03-22-2005, 02:04 PM
[QUOTE]Originally posted by garyphansen:
[B]JCD: Did you figure out how you are going to index your gear blanks to make you transponding gears yet?


With compound indexing on my dividing head I can achieve 127 teeth. I finely figured out how, with the help of several people on this sight.

I purchased the correct cutter for the 16Dp gear.



03-22-2005, 08:07 PM
Heavy 10" SB with Q/C gearbox is equiped for normal threading (US) as follows:
Stud = 40T
Screw Gear (Ggearbox input shaft) = 50T
Center Gear = 80T


Bill Cook
03-22-2005, 08:37 PM
These are the metric and inch charts on a Sheldon.
The only part of the metric setup that came with it was the chart.