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Imperial screwcutting on Colchester bantam metric lathe

john11668

Aluminum
Joined
Oct 25, 2009
Location
Cumbria, UK
Hi Folks
I apologise that it is some time since I have visited .
I have a current problem in that I wish to cut a 26 tpi thread (internal actually but I know that doesnt matter ) on my metric bantam lathe . Leadscrew is 6mm pitch .
I have the 127 change gear and followed the instructions in my manual (I attach the relevant section )
Lever positions B so y=2, and 5 so z =8 and compound is 127/120 so according to my calcs
driver should be 28 and Driven 55,
giving a tpi of 25.984 which as they say is close enough for government work.

But after a test cut my thread gauge tells me I have 30 tpi so clearly i am doing something wrong
The manual to me is not really very clear so can anyone help me find what my mistake is.
 

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Can't read page. Might help to post WHAT mm thread is cut in those selected positions/gears WITHOUT the 127/120 compound?

Then the lead of the Imperial wanted will be in a direct 1.058333 to 1 relationship to the lead on that metric pitch

An interesting aside is SB's use of a 127/135 compound to effect the Imperial pitch of a metric lead screw - an entirely differing ratio from the above
 
Helping out

Hello John,

The colchester manual is very misleading in how they show the formulae for calculating the ratios. If you leave the 127/120 out of the calculation then the maths work out. It appears to be impossible to cut a 26TPI with the change gears supplied by colchester however there are some possible combinations that would produce a 26TPI and also a 13TPI which is another common thread which is not catered for by the normal set of gears. By my calculations a 48 T and a 39 T gear with the levers on T and 1 (and obviously the 127/120 compound) would produce a 26TPI thread. I did not have a full set of change gears for my metric bantam and was unable to find some of them anywhere so I recently began 3D printing gears with amazing success so far. It occurred to me that I could potentially print any number of teeth to allow me to cut practically any pitch (within certain limits). Maybe you or someone you know with a printer could print you off a couple of gears. I might try it myself, I would like to be able to cut 1/2 13 UNC etc. Best of Luck!
 
It’s going backwards but I know if 120/127 were backward in my Hendey when I cut metric they were off. Flipped it over correctly and did fine.
 
Helping out

Hello John,

The colchester manual is very misleading in how they show the formulae for calculating the ratios. If you leave the 127/120 out of the calculation then the maths work out. It appears to be impossible to cut a 26TPI with the change gears supplied by colchester however there are some possible combinations that would produce a 26TPI and also a 13TPI which is another common thread which is not catered for by the normal set of gears. By my calculations a 48 T and a 39 T gear with the levers on T and 1 (and obviously the 127/120 compound) would produce a 26TPI thread. I did not have a full set of change gears for my metric bantam and was unable to find some of them anywhere so I recently began 3D printing gears with amazing success so far. It occurred to me that I could potentially print any number of teeth to allow me to cut practically any pitch (within certain limits). Maybe you or someone you know with a printer could print you off a couple of gears. I might try it myself, I would like to be able to cut 1/2 13 UNC etc. Best of Luck!
Hello! Dit you have any luck with printing gears? I was thinking about trying it as well.
 








 
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