Metric thread cutting tip for a lathe
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  1. #1
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    Default Metric thread cutting tip for a lathe

    Hi All

    I have never cut an external metric thread on a lathe before.

    Could you please view the attachment, and let me know if this is the correct tool for the job?

    20210302_074121-1-.jpg

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    Probably. You are going to need to find out the included angle of the tip before I can be totally sure. 60 degrees is metric and US imperial threads. 55 is whitworth.

    By eyeball it looks 60 but you should check.

    Sent from my SM-G973F using Tapatalk

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    For any standard English (US, not Whitworth) or metric vee thread the tool's angle should be 60 degrees as viewed from the top AND it should have a radius or flat at the tip that is determined by the pitch. There's a somewhat complicated drawing for both US and for metric threads to define that flat or radius, but you won't go wrong if you make it to remove about 1/10 to no more than 1/8 of the thread's (a sharp vee thread of the same pitch's) depth from the tool's tip.

    A flat is easier to grind on a tool. I suspect that the real reason why the standard drawings allow a radius is because that is what happens as the tool wears so allowing it is one way of prolonging tool life between sharpenings.

    From appearances, the tool in the photo seems to have a rather large radius, but there is no way to tell the scale of the drawing. Nor do you state the pitch of your desired thread so it could be OK or it could need resharpening.

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    Quote Originally Posted by EPAIII View Post

    A flat is easier to grind on a tool. I suspect that the real reason why the standard drawings allow a radius is because that is what happens as the tool wears so allowing it is one way of prolonging tool life between sharpenings
    A radius is FAR FAR superior, for fatigue resistance especially, but also UTS, stress corrosion resistance etc.

    Machinists often mistake a “clean” part for a strong part.
    Sharp inside corners make weak parts.

    Another fact, Rolled threads are far superior to cut threads. I’ll bet many highly stressed fasteners have failed where a rolled thread was cut a little longer on a lathe. NEVER do that to a highly stressed critical fastener.

    That is in fact unethical in my book without warnings/disclaimer. A material science high crime

    P.S. think Carrol Smith (?) cites this as one reason the Whitworth thread form is superior, with its larger root radius.


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