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I wish to contribute what I can to the clarification of what happens when an imperial lathe is used to cut metric threads.
For my dissertation I will assume the lathe has a 8TPI lead screw and a QCGB with several tpi threads.
To cut metric threads we need a means to convert the movement from the 8TPI lead screw to something more suitable for the metric world. The 120/127 gear ratio will do this and effectively converts your 8TPI lead screw to a 3 mm pitch. 3 mm pitch is finer than 8TPI so we need the conversion gears to decrease the speed of the lead screw, for example spindle pinion meshes with 127 gear and 120 gear meshes with GCGB pinion.
With the conversion gears mounted, the spindle pinion equal to the QCGB pinion and the QCGB set to straight through the lathe will cut a 3mm pitch thread. This is exactly the same configuration as would be used to cut 8TPI if the 120/127 conversion gears were not included.
Suppose your imperial lathe has an imperial thread table you could draw up a metric table beginning with the 8TPI position entered as 3mm. Now 3x8=24 and for all the other positions you need only divide the TPI value into 24. For example set the QCGB to 16TPI, 24/16=1.5 so enter 1.5mm in your metric thread table. Note that this only holds true so long as the spindle pinion and the QCGB pinions are equal.
You imperial gear box will inevitably throw up a lot of metric threads that are not according to common use.
But there are still plenty of standard metric threads that can be cut on your imperial lathe:
The first number is the required metric thread and TPI number is the setting you would use on your imperial lathe when you add 120/127 gears.
0.2mm, 120TPI
0.25mm,96TPI
0.3mm,80TPI
0.4mm,60TPI
0.45mm,53TPI
0.5mm,48TPI
0.6mm,40TPI
0.75mm,32TPI
0.8mm,30TPI
1mm,24TPI
1.5mm,16TPI
2mm,12TPI
3mm,8TPI
4mm,6TPI
6mm,4TPI
I believe this is all the standard metric fine threads (as per ISO 261) with the exception of 1.25mm. There are no approximations in the above list.
...this is a good but not entirely complete range of metric threads and includes some that might be impossible on your imperial lathe (0.45mm for example).
There is also the possibility that your imperial lathe can cut threads that are not any standard metric thread when using the 120/127 conversion gears An example is 19TPI which will cut a 1.26mm thread which might be close enough to 1.25mm depending on the application and if so would fill out the entire list of fine metric threads.
There is a whole 'nother world of threads you could cut by tweaking the sizes of the spindle and the QCGB pinions including I expect an accurate 1.25mm thread.
As always E&OE.
For my dissertation I will assume the lathe has a 8TPI lead screw and a QCGB with several tpi threads.
To cut metric threads we need a means to convert the movement from the 8TPI lead screw to something more suitable for the metric world. The 120/127 gear ratio will do this and effectively converts your 8TPI lead screw to a 3 mm pitch. 3 mm pitch is finer than 8TPI so we need the conversion gears to decrease the speed of the lead screw, for example spindle pinion meshes with 127 gear and 120 gear meshes with GCGB pinion.
With the conversion gears mounted, the spindle pinion equal to the QCGB pinion and the QCGB set to straight through the lathe will cut a 3mm pitch thread. This is exactly the same configuration as would be used to cut 8TPI if the 120/127 conversion gears were not included.
Suppose your imperial lathe has an imperial thread table you could draw up a metric table beginning with the 8TPI position entered as 3mm. Now 3x8=24 and for all the other positions you need only divide the TPI value into 24. For example set the QCGB to 16TPI, 24/16=1.5 so enter 1.5mm in your metric thread table. Note that this only holds true so long as the spindle pinion and the QCGB pinions are equal.
You imperial gear box will inevitably throw up a lot of metric threads that are not according to common use.
But there are still plenty of standard metric threads that can be cut on your imperial lathe:
The first number is the required metric thread and TPI number is the setting you would use on your imperial lathe when you add 120/127 gears.
0.2mm, 120TPI
0.25mm,96TPI
0.3mm,80TPI
0.4mm,60TPI
0.45mm,53TPI
0.5mm,48TPI
0.6mm,40TPI
0.75mm,32TPI
0.8mm,30TPI
1mm,24TPI
1.5mm,16TPI
2mm,12TPI
3mm,8TPI
4mm,6TPI
6mm,4TPI
I believe this is all the standard metric fine threads (as per ISO 261) with the exception of 1.25mm. There are no approximations in the above list.
...this is a good but not entirely complete range of metric threads and includes some that might be impossible on your imperial lathe (0.45mm for example).
There is also the possibility that your imperial lathe can cut threads that are not any standard metric thread when using the 120/127 conversion gears An example is 19TPI which will cut a 1.26mm thread which might be close enough to 1.25mm depending on the application and if so would fill out the entire list of fine metric threads.
There is a whole 'nother world of threads you could cut by tweaking the sizes of the spindle and the QCGB pinions including I expect an accurate 1.25mm thread.
As always E&OE.
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