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# Metric threading on my old Sidney lathe?

#### 99Panhard

##### Stainless
The prospect, albeit remote, of getting a very early French car has got me thinking about metric threading.There was a thread about 10 years ago where member Finegrain seems to answer the question...

Metric threading on Sidney lathe

So I'd thought I'd ask just to confirm I understand it correctly. This is the gear train on my lathe...

#### Attachments

• Lathe gear train.jpg
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#### enginebill

##### Stainless
If you don't have room for a 100/127 gear set on your lathe, you might be able to 3d print gears that can get you very close to the pitch that you need, I have done that in the past.

#### 99Panhard

##### Stainless
Thanks gentlemen...
Out of curiosity, where does the 127/120 combination come from? Until now I've only seen reference to 127/100 and 127/50. I confess that the math confuses me. It was never a good subject for me and there are times when I think I must by dyslexic - at least I have to be very careful writing numbers down because I often get them in the wrong order...

In any case, if I could compute the transposing gear ratio that would turn the lead screw 4 times and move the saddle 20mm rather than 25.4 I have worked out the combinations that would give me the metric threads...20 TPI would be a pitch of 1 etc...

enginebill...I've looked at your thread on the 3D gear you made several times. I thought it was very clever but I'd actually have an easier time making the gears on the mill than on a 3D printer - since I don't have one or the proper software to use one if I did.

#### Froneck

##### Titanium
Thanks gentlemen...
Out of curiosity, where does the 127/120 combination come from? Until now I've only seen reference to 127/100 and 127/50.

I've owned various brands of lathes, My LeBlond had a chart on the gear cover for metric gearing, it was 127/120. Owners Manual for my Hendey #1, Pratt & Whitney and L&S Model X list the combination 127/120 to convert to metric. Currently my Hendey is set up with 127/120. If I recall Machinist Handbook listed 127/120. I have seen other ratios but none when computing get as near to perfect as 127/120 especially with lead screw pitches like 4 or 5. I have been cutting .75mm, .7mm and .6mm on my Hendey #1 with 36 driving 127/120. Some time ago when trying to determine the best ratio needed to cut fine metric threads I calculated other combinations, 127/120 was best. I have an old Atlas lathe manual that lists 44/52 as the change gears for metric conversion.

#### 99Panhard

##### Stainless
Thanks. That is good to know because I think that 127/120 will fit into the available space even better than 127/100.

#### enginebill

##### Stainless
If you take the imperial pitch, say 8tpi = .125" pitch divided by 1.27 (127/100) you get a pitch of .098425", times 25.4 = 2.5mm pitch. So if you set your change gears to 8 tpi you will get 2.5mm pitch with the 127/100 set. The 127/120 works too, it is just slightly less accurate but you would get a 3 mm pitch instead of 2.5 mm.

#### johnoder

##### Diamond
it was 127/120

This is 1.0583 to 1

works with 12 pitch or .0833 lead (.0833 divided by 1.0583 = .078739 or 2mm)

127 and 100 is 1.27 to 1

which works with such as 20 tpi or .050 lead (.050 divided by 1.27 = .03937 or 1mm)

All this can change depending on tooth count of stud gear

The first thing you notice is there are MULTIPLE routes to get to the same metric lead

Here is one from Monarch showing usefulness of differing stud gears. Note another ratio - 127 and 75

Note also the several completely useless metric leads

127 and 75 are brothers to the 1.0583 to 1 - except 127 and 75 do the "halfsies" - such as 6 tpi or.166666 divided by 1.69333 = .098424 or 2.5 mm

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#### Froneck

##### Titanium
If you take the imperial pitch, say 8tpi = .125" pitch divided by 1.27 (127/100) you get a pitch of .098425", times 25.4 = 2.5mm pitch. So if you set your change gears to 8 tpi you will get 2.5mm pitch with the 127/100 set. The 127/120 works too, it is just slightly less accurate but you would get a 3 mm pitch instead of 2.5 mm.
That's not less accurate! The lathe is set up to cut 3mm! On my chart for the #1 Hendey I can change 4 gears to get all the metric pitches. All done by changing 1 gear. The key gear is the 127 tooth, it is 25.4 X 5 = 127 the smallest gear so as to have a complete tooth. What do you do when you need 3mm? If my memory is correct my #1 Hendey has 5TPI lead screw. I really don't have to know because I have the chart! Gets harder to do when metric threads get smaller. Another reason to use 120 rather than 100, 50 or anything else is that the gear being driven by the 100 or smaller has to be larger. Difference in diameter between 127 and 120 is not much and as size gets smaller the gear on the quick change gets larger therefor the gear driving the 127 must get larger also.If my memory is correct 36 and 40, maybe 42 on the stud driving the 127 will give me just about all the smaller metric threads to about 3 or 4mm maybe 5.

#### enginebill

##### Stainless
Sorry, I guess it is accurate, I was rounding.

I didn't realize it until now, a 127 with many gears over 50T will give a metric pitch (50,60,70) etc.

#### Froneck

##### Titanium
.5mm is quite a bit for metric threads! First turn .5mm, second turn 1.0mm and 3rd turn 1.5mm?? 3mm pitch screw will not enter 2.5mm nut one turn! .05mm +/- might make one turn.

#### Jim Christie

##### Titanium
I had a copy of either a Home Shop Machinist or Live Steam magazine from the early 1980s where someone had written an article about cutting metric threads with a 9" South Bend Lathe.
As I remember he found that buying or using a 127 tooth gear was not working for him so he explained how he got around the problem using some smaller gears with a ratio that was very close to the 110/127 used in the more common conversion sets.
I can't find the magazine or the article at the moment so I may no longer have it .
As John Oder mentioned there are likely a number of combinations that will bring you to a close enough so that the error is minimal enough to allow you to fit most threaded parts together if the threads are not engaged for too great a distance .
I don't know if this will be of any help or just complicate things more but I think this is close to what I remember reading in the article .
Metric Change Gears for SB 9A | The Hobby-Machinist
This link that he posted may be a version of the magazine article that I remember since I think there was a Conrad Hoffman that used to write in the magazines back then.
Jim

#### Froneck

##### Titanium
There are a lot of combinations but some are better tan others. Some will have very small error so they too will work. Some time ago I had a LeBlond with quick change but no chart. I also was doing a lot of metric threading. I did a lot of calculating to determine what ratio I could get the most metric threads from by using the quick change and the least number of gears to change with the lead screw I had. 127/120 worked best! Now all the lathes I have either in the manual or on the lathe a chart to use for cutting metric threads. The Chart for my Hendey #1 has 3 gears changes along with the quick change will thread all the metric threads but I needed .75mm,.7mm and .6mm so I added a 36 tooth gear to the list.

#### 99Panhard

##### Stainless
Thanks for all the input. I think I'm beginning to understand what is needed here. Here is the gear train on the Sidney from the front...

I have plenty of room but I'll have to make another input gear for the gear box in order to offset it. I've room for either the 100 or 120 tooth options. I'll have to figure out how to compute the metric pitches that correspond to the settings on the gear box in order to decide which to use.

#### johnoder

##### Diamond
Here is the gear train on the Sidney from the front...

And here is handy stuff from SB about compounding - or not

#### fciron

##### Stainless
I’m doing the same thing with my Sheldon using a 3-d printed 47t/37t gear from eBay. I realize it’s a rough approximation, but the numbers came out pretty good.

I made a spreadsheet that calculates the resulting pitch in mm from the compound gear train. (My Sheldon had a quick change gear box.) Most of them come out to gibberish, but it shows me the settings for metric threads from .40-4.00mm. At least it does when I tell it to round the the results to two decimal places.

I tell you the real reason I’d prefer a 100/127 compound gear. It could go in place of the idler and save me from building this a second arm for the banjo bracket.

#### 99Panhard

##### Stainless
Thanks again. This has all been very helpful. I now know what I'll need if the situation arises...

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