Imperial threads on a metric lathe...

[John Hill]

One of the many things I have learned here on Madmodders is that the orthadox way of cutting imperial threads on a metric lathe requires the spindle to be stopped at the end of each pass and the carriage motored back to the start position by reversing the lathe.

Having an active mind and all that I was wondering how else this could be done?  Supposing for example you cannot easily reverse your lathe!  Obviously if the carriage was disengaged and manually wound back to the start the spindle could be allowed to continue to spin and eventually a synchronisation point would come around again and which point the half nuts could be re-engaged and the second cut commenced.  The question is how long to wait?  So I did the calculations:


This is a list of inch thread pitches from 10 through 60 threads per inch, the equivalent metric pitch and the number of revolutions of a 3mm pitch metric feed screw between one synchronisation point and the next.


10Tpi = 2.540000000mm 50 turns
11Tpi = 2.309090901mm 55 turns
12Tpi = 2.116666667mm 540 turns
13Tpi = 1.953846155mm 195 turns
14Tpi = 1.814285711mm 210 turns
15Tpi = 1.693333333mm 225 turns
16Tpi = 1.587500000mm 80 turns
17Tpi = 1.494117642mm 85 turns
18Tpi = 1.411111111mm 90 turns
19Tpi = 1.336842106mm 95 turns
20Tpi = 1.270000000mm 100 turns
21Tpi = 1.209523801mm 315 turns
22Tpi = 1.154545455mm 110 turns
23Tpi = 1.104347826mm 345 turns
24Tpi = 1.058333333mm 1080 turns
25Tpi = 1.016000000mm 125 turns
26Tpi = 0.976923077mm 390 turns
27Tpi = 0.940740741mm 675 turns
28Tpi = 0.907142857mm 420 turns
29Tpi = 0.875862067mm 145 turns
30Tpi = 0.846666667mm 450 turns
31Tpi = 0.819354837mm 155 turns
32Tpi = 0.793750000mm 160 turns
33Tpi = 0.769696967mm 165 turns
34Tpi = 0.747058822mm 170 turns
35Tpi = 0.725714286mm 1575 turns
36Tpi = 0.705555556mm 180 turns
37Tpi = 0.686486486mm 185 turns
38Tpi = 0.668421059mm 190 turns
39Tpi = 0.651282051mm 585 turns
40Tpi = 0.635000000mm 200 turns
41Tpi = 0.619512191mm 205 turns
42Tpi = 0.604761905mm 630 turns
43Tpi = 0.590697675mm 215 turns
44Tpi = 0.577272727mm 220 turns
45Tpi = 0.564444444mm 225 turns
46Tpi = 0.552173918mm 690 turns
47Tpi = 0.540425534mm 705 turns
48Tpi = 0.529166667mm 2160 turns
49Tpi = 0.518367346mm 2205 turns
50Tpi = 0.508000000mm 250 turns
51Tpi = 0.498039215mm 255 turns
52Tpi = 0.488461538mm 780 turns
53Tpi = 0.479245288mm 265 turns
54Tpi = 0.470370377mm 1350 turns
55Tpi = 0.461818182mm 275 turns
56Tpi = 0.453571429mm 840 turns
57Tpi = 0.445614039mm 285 turns
58Tpi = 0.437931039mm 290 turns
59Tpi = 0.430508471mm 885 turns
60Tpi = 0.423333333mm 900 turns



The first thing to notice is that although there are som rather long intervals there are a suprising number around 100-200. The 49TPI interval is about 20 feet and the much more common(?) 48TPI is almost as long.

The challenge now is to devise some form of counter or other indicator device to indicate when the synchronisation point is coming up.

[LATER] Unfortunately it seems my logic was all wrong....

[Tinker]

how about the other way around for my dad's old South Bend?

[John Hill]

Tinker, do you know the feed screw pitch?

[John Hill]

From discussion on another site it appears my logic may be all to heck and that the answer is really 127 turns in every case!

[sbwhart]

Hi John

Great bit of calculation as for a counter how about hooking up a claculator as per your idea, I bet you've got that one sorted already,  you little Bodger.

 

Nice one

Have fun

Stew

[John Hill]

Stew, a counter would be really easy,  figuring out where I went wrong is more difficult!

It appears that 127 turns of the metric leadscrew is the right point to drop in the half nuts when cutting imperial threads on a metric machine using the 120/127 compound gears. 

All you need is a thread indicator of about 5 inch diameter!

[Tinker]

Tinker, do you know the feed screw pitch?

I woke him up & asked...he says 8...

Then I read the rest of the posts...

hhhmmm...
 

[John Hill]

Tinker, 8 turns to the inch, not much different to the 3mm metric feeds then..

Cutting inches on a metric lathe seem to be a whole different kettle of fish compared to cutting metric on an inch lathe.  All inch whole-number inch threads will divide perfectly into 127 turns of the metric lead screw, which is 15 inches or 381mm.

But I am still trying to figure out the other way!