Cutting threads on an older TOS lathe.
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  1. #1
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    Default Cutting threads on an older TOS lathe.

    New to the forum...
    A few yeas ago I acquired an older TOS sn45b lathe. I’ve used it quite a bit here and there making pins, bushings, tools etc. A good addition to the field line boring part of my business.
    Lately I set about to cut some threads. I have the procedure down, or so I thought. The resulting threads are beautiful but the pitch is wrong. All I have is my familiarity with this machine but I don’t have much in the way of documentation. The change gears seem right.
    Set the machine up by the table on the machine. Obviously, I’m making a mistake somewhere.
    If anyone out there have some expertise with this procedure on this machine I would appreciate any help offered.
    Thanks in advance.
    Tom

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    Measure the number of threads on your workpiece, per inch. Refer to the thread chart on the machine to see if this pitch is enumerated on the chart. You may have selected the wrong pitch on the gearbox.

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    Thanks.
    Do you have access to a TOS sn45? I was actually hoping someone would so I could explain just how I have the thing set up..
    But I did do as you suggest and where I was trying for 8 tpi I actually cut 9 1/2 tpi. Which does not show up on the chart. However I suppose it could be a metric pitch.
    I just don’t know.

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    You have a good clue. Somewhere is a wrong gear. I am not familiar at all with your lathe, but on many there is a gear driven by the spindle, at spindle speed, which drives the rest of the threading gear train, called the "stud gear". If, perchance your lathe has such a gear and it has 19 teeth or some multiple of 19, and you got with the lathe a spare gear with 16 or a multiple of 16 teeth, swapping those should cure your problem.

    The principles are simple, which is not to say always easy to apply. I would start by measuring your lead-crew to see if it is a metric or inch leadscrew. Obviously to cut 8TPI, your leadscrew needs to turn far enough to advance the carriage by 1/8" for each rev of the spindle. It is just arithmetic...if you can get at the gears to count the teeth.

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    Might want to get yourself a manual?

    tos sn40b manual - Bing

    have fun

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    According to the chart I need a change gear set with 30, 60, and 127 teeth. That’s what I have installed.
    However I measured a few things as you suggested:
    Lead screw has 4 TPI
    Set up the lathe to cut 8 TPI and rolled the spindle by hand 8 times. Carriage moved just under 15/16 inches (measured with a ruler)

    I can only conclude the change gear set is incorrect.
    Strange that they are stamped with the numbers that the machine chart says I required.
    Perhaps the machine originally had a metric lead screw?

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    Already have that.
    Doesn’t help much.
    Did you think I wouldn’t look?

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    If your leadscrew is 4TPI there should not be a 127 gear in the geartrain to cut a inch thread
    The 127gear is for going from metric to inch or v.v,

    I know the threading chart on those SN machines go bad real quick
    Perhaps they put a new chart on which might be a wrong one for a metric leadscrew
    Try to find a chart for a lathe with 4TPI (That must be one where no Z=127 gear is used for imperial threading)
    Your dials are these metric or imperial Just to verify you have a imperial machine

    Peter

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    Quote Originally Posted by Peter from Holland View Post
    If your leadscrew is 4TPI there should not be a 127 gear in the geartrain to cut a inch thread
    The 127gear is for going from metric to inch or v.v,

    Peter
    Thanks Peter.
    I’m thinking I need the 120 tooth gear. That would advance the carriage a little quicker and make up that small difference between what I’m getting and what I want.

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    Put up a pic of your gear train and the threading chart. Peter is correct, there should be no 127 tooth gear in the gear train to cut inch based threads.

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    You need the right chart if yours is the wrong one

    Peter

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    Quote Originally Posted by Peter from Holland View Post
    If your leadscrew is 4TPI there should not be a 127 gear in the geartrain to cut a inch thread
    The 127gear is for going from metric to inch or v.v,

    I know the threading chart on those SN machines go bad real quick
    Perhaps they put a new chart on which might be a wrong one for a metric leadscrew
    Try to find a chart for a lathe with 4TPI (That must be one where no Z=127 gear is used for imperial threading)
    Your dials are these metric or imperial Just to verify you have a imperial machine

    Peter
    Thanks again Peter.
    Now that I think of it the 30 tooth pinion drives through a 60 tooth idler and will require a 120 tooth gear at 1/8 the speed of the spindle. You are absolutely right and the 127 tooth gear won’t cut it. Pun completely intended..
    I’m feeling a little stupid now.....but thanks.

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    Last TOS i ran had a 3TPI leadscrew.

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    A lot of lathes use a 120/127 change gear to switch between inch and metric, having it wrong will give about 6% lead error, not the difference between 8 and 9-1/2 tpi though.

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    I own a medium size polish lathe, not a TOS though but an Andrychowska (year 1952 AFAIR). I believe they are similar in many respects.
    I have copied the manual (including a comprehensive thread chart) and will mail it if you're interested. Send me a PM!
    Regards, fusker

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    Chart is already linked in Johnoder post, 127 gear is correct for 1/4 lead screw, check if lead screw shear pin is not broken and slipping.

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    Did you set up the thread dial for inch threads?
    1.jpg2.jpg3.jpg4.jpg5.jpg
    John

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    Set correctly the TOS lathe enables the operator to disengage halfnuts when threading both metric and inch threads.
    thread-dial.jpg
    John

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    A 127 or any other gear can be used as an idler,just so as its not driver or driven,or in a compound train......and the multitude of thread pick off gears and two meshing gears is one of the charming results of metric threads being unscientific,and invented by the French.....An imperial leadscrew needs but one gear and one indicator,because inch threads are all logical.

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    Quote Originally Posted by john.k View Post
    A 127 or any other gear can be used as an idler,just so as its not driver or driven,or in a compound train......and the multitude of thread pick off gears and two meshing gears is one of the charming results of metric threads being unscientific,and invented by the French.....An imperial leadscrew needs but one gear and one indicator,because inch threads are all logical.
    If it is used as a idler you are correct that it can be part of the geartrain
    That inch threads are logical is because you guy are used to it and then try to translate metric threads into inch measurements
    For metric threads you best use metric measurement instruments With inch threads the inch units As soon as you translate it it gets complicated


    Peter
    I


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