SB 13 metric to imperial conversion gears ?
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    Default SB 13 metric to imperial conversion gears ?

    Hello everyone,

    Looking at purchasing a nice condition 1979 SB 13. It's a metric only machine and I need to cut standard threads. I see it's frequently talked about making a standard threading lathe cut metric threads with metric transposing gears but I didn't find anything about making a metric machine cut inch threads.

    Is it possible or is it too much trouble?


    Thanks for any help in advance,
    RR

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    Seems Metric lathes were special order, as were the transposing gears for them. This set involves both 135 and 127 tooth gears and the several stud gears.

    You can see INFO for them in any recent HTRAL, but how the lathes and such things as the unusual transposing set were acquired eludes me


    Quote Originally Posted by RampedRaptor View Post
    Hello everyone,

    Looking at purchasing a nice condition 1979 SB 13. It's a metric only machine and I need to cut standard threads. I see it's frequently talked about making a standard threading lathe cut metric threads with metric transposing gears but I didn't find anything about making a metric machine cut inch threads.

    Is it possible or is it too much trouble?


    Thanks for any help in advance,
    RR

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    If this is a full on proper metric lathe, wouldn't it have a metric lead screw and different gearbox too? Someone posted a pic recently of a metric SB and the tumblers on the box were opposite the usual arrangement. If thats what he has, not sure there is an easy way to cut imperial threads on it.

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    The one I'm speaking about, is said to have a metric gear box and a metric lead screw.

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    Quote Originally Posted by gadget73 View Post
    If this is a full on proper metric lathe, wouldn't it have a metric lead screw and different gearbox too? Someone posted a pic recently of a metric SB and the tumblers on the box were opposite the usual arrangement. If thats what he has, not sure there is an easy way to cut imperial threads on it.
    Nope, it uses the same basic idea- transposing gear with a factor of 254 in it, and some stud gears.

    allan

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    Quote Originally Posted by kitno455 View Post
    Nope, it uses the same basic idea- transposing gear with a factor of 254 in it, and some stud gears.

    allan
    Kitno: Do you mind spoon feeding me a bit more on what I need to purchase in addition to the lathe itself to cut imperial threads.

    Disclaimer: this is my first lathe and I'm researching as I go.

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    Quote Originally Posted by RampedRaptor View Post
    Kitno: Do you mind spoon feeding me a bit more on what I need to purchase in addition to the lathe itself to cut imperial threads.

    Disclaimer: this is my first lathe and I'm researching as I go.
    I'm not Kitno, but I can maybe shed some light on the math for you.

    The 254 conversion is based on the fact that 1 inch = 25.4 mm by definition. This is an exact conversion as defined by the International Bureau of Weights and Measures. Most metric to imperial conversions are approximations (like 454 grams = 1 lb--that is approximate; more like 453.59237...), but the inch to mm is exact. Because of this, there has to be some conversion in the drivetrain from 1:25.4 so say 10:254 since there is no way to have fractional number of teeth on a gear. This can be reduced to say 5:127 by dividing by two, but that's as far as you can divide since 127 is a prime number. Therefore, there must be a 127 tooth gear in there somewhere if you are wanting to use a metric lathe for inch threads, or an inch lathe for metric threads.

    Hope this helps.

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    Thumbnail is from revised edition 55 of How To Run A Lathe. If you acquire that edition - or later - you can look at the left most chart #248 and see what gears are involved in inch threads on a metric lathe. As mentioned in my post above, it involves both 127 and 135 tooth gears, and an assortment of stud gears

    As can be seen, this particular scan is illegible for that purpose

    On second glance, I would suggest taking this factory info with a grain of salt - since there is zero reference to how QC box is involved. See post #5 here for a chart that DOES include QC gear box position, but this is metric from inch lead screw - not what you want

    Lathe cutting wrong thread pitch
    Attached Thumbnails Attached Thumbnails sb-metric.jpg  
    Last edited by johnoder; 07-18-2017 at 01:04 PM.

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    I don't have a copy of HTRAL that includes this info, but the stud gears could be determined from first principals- If i knew the pitch of the leadscrew, the number of teeth on the standard end gears, and the list of metric threads the machine will cut, I could back into a list of gears. The problem however, is if the 13" lathe uses 14DP gears. Those are not sold off the shelf by places like Martin or Boston Gear. They would have to be made.

    allan

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    I believe that the metric transposing gears for the SB13 are 16 DP.




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    If the lathe uses 16DP like Genepoole said in the above post. Does that make the conversion simpler or cheaper?

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    16DP gears are available from Boston gear, though they will be narrower face width than the factory transposing set. In theory this makes them weaker, but not enough to matter. It probably means you will need some washers under the nuts that secure the gears, to take up the slack. It also means that you cannot use any of the existing gears that are on the lathe, unless it has already been converted to 16DP.

    allan

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    Magic...the metric lead screw will in fact be 4.5mm pitch

    Ordinary 13" lead screw is 6 TPI - which happens to be 4.23332mm

    The 135/127 ratio is 1.062992:1

    4.23332 times 1.062992 is 4.5

    Maybe I did that right.....

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    Quote Originally Posted by johnoder View Post
    Magic...the metric lead screw will in fact be 4.5mm pitch

    Ordinary 13" lead screw is 6 TPI - which happens to be 4.23332mm

    The 135/127 ratio is 1.062992:1

    4.23332 times 1.062992 is 4.5

    Maybe I did that right.....
    The pitch of the leadscrew is 4 and the gears are 14 pitch

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    The pitch of the leadscrew is 4
    Since the lead screw is 4mm pitch, the following lash up does some interesting things

    63 stud
    drives 120
    which is connected to 127
    which drives 42 screw gear

    In the A1 position, it cuts .250" pitch like it is supposed to - and maybe all the way across the QC to the right


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