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Thread: Metric threading with inch based leadscrew

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    olderdan's Avatar
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    Metric threading with inch based leadscrew

    Noticing recent posts mentioning 127 tooth conversion gears I thought it may be useful to show a different method using standard gears, namely a
    54T pegged with a 44T giving a very close conversion for most thread lengths.
    This is not my idea but taken from a site called Workshop Protects which seems to no longer exist, fortunately I took screenshots of it.
    Metric threading with inch based leadscrew-screen-shot-01-22-17-11.48-am.png
    Metric threading with inch based leadscrew-screen-shot-01-22-17-11.54-am.jpg

    Hope someone finds this useful.

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    Seedtick (09-28-2017)

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    Olderdan
    Do you have any idea what the foot notes are for the three astrichs? ***

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    olderdan (09-29-2017)

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    Looks like the *** line got cut off on the screen shot.

    I have this link tucked away for when I need to cut a metric thread on my old Logan lathe.

    Metric Threading

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    olderdan (09-29-2017)

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    olderdan's Avatar
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    Quote Originally Posted by hemmjo View Post
    Looks like the *** line got cut off on the screen shot.

    I have this link tucked away for when I need to cut a metric thread on my old Logan lathe.

    Metric Threading
    Thanks Hemmjo, that is a very comprehensive article, thanks for the link.

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    olderdan's Avatar
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    I think the three asterisks indicate a slightly increased error for those pitches but I have not done the maths.

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    ncollar's Avatar
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    No, three *** mean there is more info for it.

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    I would second Nelson's recommendation: the 37/44 tooth combination shown there yields a 1.2703:1 ratio: very close to the ideal 1.2700:1 ratio. according to Logan's web site, it's only 2/100s of 1% error. the 54/44 combo yields 1:2273:1.

    Kudos to OlderDan for bringing this to the surface: an easy way for inch-based lathes to cut metric threads!

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    mklotz mklotz's Avatar
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    Quote Originally Posted by olderdan View Post
    Noticing recent posts mentioning 127 tooth conversion gears I thought it may be useful to show a different method using standard gears, namely a
    54T pegged with a 44T giving a very close conversion for most thread lengths.
    This is not my idea but taken from a site called Workshop Protects which seems to no longer exist, fortunately I took screenshots of it.
    Click image for larger version. 

Name:	Screen Shot 01-22-17 at 11.48 AM.PNG 
Views:	152 
Size:	23.1 KB 
ID:	19915
    Your text in your writeup says 54:44 gear pair

    54/44 = 1.2273

    but the text in the picture you included seems to indicate 56/44

    56/44 = 1.2727

    which is, indeed, closer to the desired 1.2700000....

    Was the "54" a typo or am I missing something here?

    Of course, 47:37 is still a lot closer...

    47/37 = 1.27027027
    ---
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    olderdan (09-29-2017)

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    It was indeed a typo, thanks for pointing it out, probably would have been better to let the chart speak for itself.
    I am sure there are better combinations but the chart is for standard SB change gears which are all of an even number.

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    mklotz mklotz's Avatar
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    Quote Originally Posted by rebuilder1954 View Post
    I would second Nelson's recommendation: the 37/44 tooth combination shown there yields a 1.2703:1 ratio: very close to the ideal 1.2700:1 ratio. according to Logan's web site, it's only 2/100s of 1% error. the 54/44 combo yields 1:2273:1.

    Kudos to OlderDan for bringing this to the surface: an easy way for inch-based lathes to cut metric threads!
    Another typo I think...

    44/37 = 1.11892

    The correct form is...

    47/37 = 1.27027027...
    ---
    Regards, Marv


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