# Cutting metric threads on an imperial lathe



## oldscouser (May 16, 2019)

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.


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## Karl_T (May 16, 2019)

I use the 37/47 transposing gears on my 10EE. Learned about it here:


			Metric Threading on Logan Lathes
		


80/63 = 1.2698
47/37 = 1.2702
127/100 = 1.27   => perfect


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## oldscouser (May 16, 2019)

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.


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## Ulma Doctor (May 16, 2019)

thanks for the info guys, great stuff!!!


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## MozamPete (May 17, 2019)

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.







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.


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## Karl_T (May 17, 2019)

Say, I'd like to play with this 80/63 option. Where would one buy the gears?


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## chips&more (May 17, 2019)

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


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## oldscouser (May 17, 2019)

I'll try posting the metric thread chart as a pdf.  And hope it can be read.


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## MrWhoopee (May 17, 2019)

Karl_T said:


> Say, I'd like to play with this 80/63 option. Where would one buy the gears?


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.


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## Karl_T (May 19, 2019)

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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## MrWhoopee (May 19, 2019)

Karl_T said:


> 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.*


Here's the 80 tooth
https://www.amazon.com/gp/product/B004NYAJ2S/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

Somehow I found it for $14
I need to get back on this project, I don't yet have the other gears required for my SB Heavy 10L.


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## Karl_T (May 20, 2019)

Thanks for the link.

I already have the 47/37 which is essentially the same.

This is just a guess, but I'm thinking 40/63  (I have a 40) will give some more usable thread combinations. The 10EE has a huge number of thread possiblities.

I plan to repeat my <very old> work on the 10EE with 47/37. This time get the math right. Then repeat with 40/63. If this works out the way I think it will, may look for a 60 and/or a 120 tooth. Make this decision once experimental results and the math line up.


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## oldscouser (May 20, 2019)

If it helps this is how I did the maths for the metric thread chart.

Recalling that if an 8tpi leadscrew rotates at the same speed as the lathe spindle then you will cut an 8tpi thread on the work piece.  If you want to cut a 16tpi thread then the speed of the leadscrew is reduced by 2, and for cutting a 24tpi thread the leadscrew speed is reduced by a factor of 3.

For metric threads the overall gear reduction ratio between the lathe spindle and leadscrew is given by:

Overall gear reduction = (25.4 / 8) x (1 / required metric pitch)

For a pitch of 1mm the overall gear reduction is simply:

O'all Red = 25.4 / 8 = 3.175 exactly.

If a compound gear set that approximates to 1.27 is used in the change gears (a set such as 47/37 or 80/63) then the second gear reduction ratio is:

Red2 = 3.175 / 1.27  = 2.5 exactly.  

As a fraction 2.5 is 5/2.  So gear combos of 50/20 or 75/30 or 100/40 would give that fractional ratio.  So for example a 30 tooth gear on the tumbler output and a 75 tooth gear on the leadscrew gives the required fractional reduction.

From this single fraction for the 1mm pitch all the other fractions for various metric pitches can be derived.  For a 2mm pitch thread the 1mm fractional ratio is reduced by 2 and becomes 5/4.  For a 3mm pitch the 1mm fractional ratio is reduced by 3 and becomes 5/6.

A pitch of 1.5mm is mid way between pitches 1mm and 2mm and so the fractional ratio is mid way between 5/2 and 5/4, which is 5/3.  A pitch of 2.5mm is mid way between pitches 2mm and 3mm and so the fractional ratio is 5/5.

A pitch of 1.5mm is twice 0.75mm and hence the fractional ratio for a pitch of 0.75mm is 2 x 5/3, i.e. 10/3.

And so on for the other pitches I listed previously.  It is the fractional ratio that allows rapid selection of appropriate gears.


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## Karl_T (May 20, 2019)

You're hired! (to help me with the maths on the 10EE)


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## Karl_T (May 24, 2019)

OK, got the 63 tooth gear. DAMN it won't quite fit - a 60 tooth will.

Decided to go ahead and make up a 10EE threading table with the 37/47 gear set I got from Logan Lathe.

Pics of the thread gear train and thread dial attached.  I installed a 37 tooth on top and the 47 tooth in the bottom position. Set the dial to A-5tpi and measured Z travel per ten revolutions  => got 20mm.  Repeated with dial set to B-10tpi  and got 10mm per ten revolutions.

SOMEBODY PLEASE DOUBLE CHECK THIS. The math I get is 10/ US lead = metric lead in mm.  I used this to make a new thread table. See pic and attached spread sheet.  The blue numbers are pretty useless.  RED with two digits is spot on for a metric lead. RED with three decimal places is the closest to a standard metric lead this machine can do.

A couple metric leads, 0.35 and 0.40,  are actually closer with 48/24 gears in "C" range. AGAIN SOMEBODY PLEASE DOUBLE CHECK THIS.

EXTRA BONUS QUESTION for any math majors out there. I have a 48 tooth. If I replace the 47 with 48 (or even the 24) will any of the threads that do not quite match get closer to correct?


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## oldscouser (May 25, 2019)

Hello Karl.  I googled '10EE' lathe and found a clip of Keith Rucker taking delivery of a Monarch 10EE lathe into his workshop.  Wow, that lathe is definitely not your normal 10-inch swing bench lathe.  What a beast.

I have looked at your data and pics and have a couple of queries and an observation.

Looking at your change gear pic, if we call the small gear at the top gear A and the other gears B, C and D (at the lower left of the set):
* can we assume that gear A rotates at the same RPM as the lathe spindle?
* is gear D the input to the gearbox?
* are gears A and D on fixed centres?
* are the centres of gears B and C adjustable?

If so then the only way I can make sense of your thread chart is if normally (for imperial threads) the 48 tooth gear is in position A and the 24 tooth gear is in position D.  So the gearbox input runs at twice the RPM of gear A (and presumably the lathe spindle).  If you then fit the 37 tooth gear to position A and the 47 tooth gear to position D, the gear box input RPM is slower than the spindle RPM and the numbers on your chart are correct.

It would appear that there is only a limited number of recognizable metric pitches to be found using the gear box.  If you have a requirement for a specific metric thread then it might be possible to determine appropriate change gears for that one.

atb, Ian


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## Karl_T (May 26, 2019)

Played with a 37 48 combination. Worked the chart a bit more and came up with this, best that can be done with the gears on hand.


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## oldscouser (May 26, 2019)

Well that looks like a practical set of metric threads.  Job's a good un.


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## Karl_T (May 26, 2019)

I'm sorry, forgot to acknowledge your kind offer to help, above. Pretty sure I got it right.

yep glad I looked at the 37 48 combo cause .75 and 1.5 leads are pretty common. I only do hobby work, this is plenty good for my needs.


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## oldscouser (May 30, 2019)

Karl, perhaps a final thought on cutting metric threads on your lathe with a QCGB.

As well as using gears such as 37-47 and 63-80 as spur gears to give a gear reduction that closely approximates to 1.27, the two gears can be mounted on a common shaft as a compound gear to give the magic 1.27 reduction.

If you mount the compound cluster in your change gear set in the lower RH position (which looks adjustable), fit a 50 tooth gear in the bottom LH position, then mount one of 3 gears with teeth 60, 50 or 35 in the top position, then that should give you the following metric pitches (mm):

4, 3, 2.5, 2, 1.75, 1.5, 1.25, 1, 0.8, 0.75, 0.6, 0.5, 0.35

All with the accuracy of the compound set 80/63 or 47/37.

So, with a 60T gear mounted in the top change gear position, the following gearbox selector positions (ABC and TPI) give metric pitches of:

A+3 = 4mm, A+4 = 3mm, B+3 = 2mm, B+4 = 1.5mm, C+3 = 1mm, C+3 3/4 = 0.8mm, C+4 = 0.75mm and C+5 = 0.6mm

A 50T gear mounted in the top change gear position gives:
A+4 = 2.5mm, A+5 = 2mm, B+4 = 1.25mm, B+5 = 1mm and C+5 = 0.5mm

A 35T gear mounted in the top change gear position gives:
A+3 1/2 = 2mm, A+4 = 1.75mm, B+3 1/2= 1mm, B+5 = 0.7mm, C+3 1/2 = 0.5mm and C+5 = 0.35mm.

Regards, Ian


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## Karl_T (May 30, 2019)

I'm sure you are right.

I am plenty happy with the leads possible with just the 37 and 47 tooth setup . Only two are very far off at all 1.739 for the 1.75 or 0.6% and the 0.612 for 0.600 for 2% (I'd reject this one). All the others are closer


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## mcmental (Feb 28, 2020)

hi there, sorry for dragging an old thread up but i have a harrison l2 and i have previously managed to cut a 1.5mm thread on it but im scratching my head trying to do a 1.25mm, i have the magical 127 gear!! can anyone shed some light maybe please? my leadscrew is 4tpi, heres a few pictures of the charts on the lathe


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## Karl_T (Feb 28, 2020)

Just an educated guess.  Put your metric set in, then set the lathe to cut 8 tpi and see what you get


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## MrWhoopee (Feb 28, 2020)

Karl_T said:


> Just an educated guess.  Put your metric set in, then set the lathe to cut 8 tpi and see what you get



On my SB Heavy 10L (8 tpi leadscrew) with transposing gears, that will get a 2.5 pitch. Setting the QCGB to 16 tpi gets 1.25 mm pitch. I believe those same settings will work on the OP's lathe.


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## Downunder Bob (Feb 29, 2020)

My lathe is a full imperial machine. Imperial lead screw, norton box etc, but it also came with some change gears as well including 127 and 120 T gears so it can cut true metric threads it also has a metric thread chart, so is ready to go.

Because Australia changed from imperial to metric fairly recently, only about 50 years ago many new machines sold here are set up this way.


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## oldscouser (Feb 29, 2020)

Hello mcmental.

You need to work out the required speed reduction between the lathe spindle and the leadscrew.  For a 4tpi leadscrew and a 1.25mm pitch thread the reduction is:


Reduction = 25.4 / (1.25 x 4)  = 5.08 exactly

Looking at your thread chart I see that position A of the gearbox gives a reduction of 2.  I suggest you try to use a 40T gear on the top shaft and a 80T gear on the bottom shaft and a compound 100/127 on the intermediate.  (the 40T top gear drives the 100T gear and the 127T of the compound gear drives the 80T bottom gear)

This will give an overall reduction of 5.08:

O’all reduction = gear box red’n x 80/40 x 127/100  =  5.08

If the gears do not fit in the envelope you will need a different combination.


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## ian999 (Oct 3, 2020)

The best way to cut metric threads on an imperial machine, or any other threads is to throw away your change gears entirely!
Make yourself an Electronic Lead Screw (really a gearbox) where a microcontroller does the work of controlling leadscrew speeds.
See the thread "Asian Lathes & Mills/Electronic Lead Screw". My comments and pics are at post #457.
At times the project was very frustrating but now it gives me lots of freedom.


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## rjs44032 (Oct 3, 2020)

There is another decent combination 36/34

Error per thread for the given combinations at 2.5mm:

36/34 - 1.16 micron

47/37 - 0.53 micron

80/63 - 0.31 micron

While the last two options are closer approximations, the 36/34 is the most economical. These gears are standard and available from most vendors for fraction of the cost of the other 2 options. Additionally, the 18/17 is even less expensive than those. However, the 36/34 will be easier to compound with bushing. Moreover, the error is still less than 0.00005" per thread

One other observation I have learned doing this research is that the error for all approximate transposing combinations increases as the thread size increases. Conversely, the error decreases as the thread size decreases. All these are still acceptable for the ISO thread specs up to 7mm.

Final observation is that the combination does not have to be set up on the same axis. For example, you could place 37 on the stud and 47 on the gearbox and run the division gears as the compound.

Best Regards,
Bob


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