Cutting metric threads on an imperial lathe

O

oldscouser

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Recently I had a need to cut a metric thread on my old imperial lathe. Obviously the best way is to use a 127 tooth changegear, but a very good alternative is to use a 80/63 compound gear set, which approximates very closely to 1.27. Based on the use of a 80/63 compound gear and an 8TPI leadscrew I derived a threading chart.

Folks may find the thread chart below useful for selecting change gears when cutting metric threads on an imperial lathe. The driver gear is the output from the tumbler set. The second compound gear set with a ratio of 2 avoids the use of a 100 tooth gear driving the leadscrew and is optional.

Pitch O’all Comp Fract Driver Comp1 Comp2 Leadscrew
mm Red Red 1 Red 2 t t t t

0.7 4.5357 1.27 25/7 28 80/63 70/35 50
0.75 4.2333 1.27 10/3 30 80/63 70/35 50
0.8 3.9688 1.27 25/8 32 80/63 70/35 50
1.0 3.1750 1.27 5/2 30 80/63 idler 75
1.25 2.5400 1.27 2/1 30 80/63 idler 60
1.5 2.1167 1.27 5/3 30 80/63 idler 50
1.75 1.8143 1.27 10/7 35 80/63 idler 50
2.0 1.5875 1.27 5/4 40 80/63 idler 50
2.5 1.2700 1.27 1/1 55 80/63 idler 55
3.0 1.0583 1.27 5/6 60 80/63 idler 50

P.S. I can see that when posted all the formatting is lost, but in edit it re-appears. Sorry.
 
Yes, both compound sets look practical in the real world. Compared with 1.270000

80/63 error is 0.0125%
and
47/37 error is 0.0213%

I have no gearbox on my old lathe, just good old change gears. Sigh.
 
On a Myford ML7 (and probably many other lathes) with a quick change gearbox, an alternative option has previously been put forward involving simply changing the standard 24 tooth tumbler cluster sleeve gear with a larger or smaller tooth count. In conjunction with the gearbox you can then get sufficiently accurate approximations of metric threads. Specifically, by manufacturing (or purchasing) suitable 33 and 34 tooth gears then between the three gear options (24, 33 and 34 tooth) most standard metric threads can be cut to within a fraction of a percent error in pitch. Over a short length of thread such an error would be practically unnoticeable.

294744

294746

Furthermore, with the addition of other gear sizes some of these errors can be further reduced and almost any strange pitch (British Association Threads, TPIs not provided for by the gearbox, etc) could be achieved.

It should be noted that the ‘official’ Myford solution to cutting metric threading on an imperial Myford ML7 (or ML-7R or Super 7) lathe with the Myford Quick Change gearbox fitted is to purchase the Myford Metric Conversion Set (Part No 1481/1). But this set also only gives approximate metric threads as it does not involve the use of a 127 tooth gear required to make exact metric threads on a lathe with an imperial (8 TPI) leadscrew.
 
Say, I'd like to play with this 80/63 option. Where would one buy the gears?
 
I’m not making NASA parts, just a hobby shop. When I need to cut a metric thread on my lathe that doesn’t have metric capability. I have found that it can get close enough (ie: for a nut, short distance) in most cases using inch gearing. One of these days I will get a small China lathe and dedicate it to metric threading. And not the 127 gear stuff. On my lathe, that’s too much hassle playing around with gears. Until then, I will fudge the Metric thread cutting with inch gears…Dave
 
I'll try posting the metric thread chart as a pdf. And hope it can be read.
 

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Karl_T said:
Say, I'd like to play with this 80/63 option. Where would one buy the gears?
Click to expand...
Boston Gear makes them. I bought mine off Amazon. First you need to know the DP and Pressure angle of the gears in your lathe.
 
I ordered this gear:
Boston Gear GB63B Plain Change Gear, 14.5 Degree Pressure Angle, 16 Pitch, 0.750" Bore, 63 Teeth, Cast Iron

will play with it on my 10EE to see what it can do.
 
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