Threading - always too big


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Thread: Threading - always too big

  1. #1
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    Default Threading - always too big

    I'm having trouble with the threads I'm cutting. New to this.

    I am turning the part down to the same diameter as the major diameter given in the Conversational threading tab. When I measure that before the threading cycle, it is perfectly correct. I then run the threading cycle - using 8 passes for an 1/4-20 thread in brass. When I measure after cutting, the diameter is now greater than the major diameter and the thread is too big - nut won't even fit. I'm using the threading tool and inserts from Tormach, which I'm pretty sure is a partial profile insert.

    I've tried turning the part down to 5 thou under the major diameter but the results are the same.

    I am unclear as to what is happening here and how to fix it.

    Please advise.

    Thank you

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  2. #2
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    Default Re: Threading - always too big

    Your tool is not cutting very well. Kinda hard to see in brass sometimes, but I think you are mostly beating or rubbing causing a burnishing effect. At least to some degree.
    I would slow it down some and take a little more aggressive cut on it.

    Lee


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    Default Re: Threading - always too big

    Thank you - The cuts are very good - I really don't think there's any burnishing going on. I'm cutting at 175 rpm (1/4 inch thread) - and cannot go slower as the Conversational tab won't allow that.



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    Default Re: Threading - always too big

    I had exact same issue. You'll need to cut thread deeper until the point where nut goes on smoothly. I have not tried to adjust the X-wear tolerance for the threading tool, but it should work and should be I think preferred way to deal with this. I have adjusted directly the thread depth on conversational tab.

    Dennis


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    Default Re: Threading - always too big

    Yes - it's the minor diameter that's the issue. The given minor diameter is not to the very bottom of the thread, as that is not convention. So, you need to cut it bit deeper. The actual formula is Minor diameter - (0.433 x pitch). However this yields a bolt too small. I have now empirically found that the best fit for me is 96% of the given minor diameter as you x-end.



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    Default Re: Threading - always too big

    You can sometimes get that problem if the tool is a bit high: it pushes some of the metal upwards.

    Cheers
    Roger



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    Default Re: Threading - always too big

    When threading with carbide tools, I always use the formula (.6495/pitch), so .6495 divided by 20 = 0.03247 deep

    Jeff



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    Default Re: Threading - always too big

    Jeff,

    The pitch of a 1/4-20 thread is not 20. It is 0.05



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    Default Re: Threading - always too big

    Here's the final answer:

    Thread X-end = (Maj diameter – 1.082532*Pitch) – (0.433 * Pitch)



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Threading - always too big

Threading - always too big