Imperial screwcutting on Colchester bantam metric lathe
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  1. #1
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    Default Imperial screwcutting on Colchester bantam metric lathe

    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.
    Attached Thumbnails Attached Thumbnails bantam-screwcutting.jpg  

  2. #2
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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

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    Lightbulb 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!

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    Gears are too cheap to 3-D print

    Peter

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    Default

    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.


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