Tolerance on threading inserts? How close can you get by measuring top of thread? - Page 2
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  1. #21
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    Quote Originally Posted by gregormarwick View Post
    .....

    For a typical thread that you'd cut with even a size 11 laydown insert, if you hold crest diameter close to mean sizes, then even a 10µm variation in insert geometry is unlikely to push you out of tolerance on PD.
    Name me one carbide company in the world whose threader geometry prints are 10 micron true to form total. Or even someone at 20 to 30 microns.
    Bob

  2. #22
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    Quote Originally Posted by CarbideBob View Post
    Name me one carbide company in the world whose threader geometry prints are 10 micron true to form total. Or even someone at 20 to 30 microns.
    Bob
    Bob you would likely know more about this then anybody. What kind of tolerance do you think most tooling companies are working with? Sounds like your suggesting well over 30 microns? 50 microns would be 2 thou, thats 0.004 in diameter! I was thinking might be tighter then that.

    Just had a look at the tolerances on my thread (1.75-16 2A) and major diameter is 1.7484 / 1.7390 which is a range of 0.0094. PD is 1.7078 / 1.7025 or a range of 0.0053. Major dia tolerance range is 0.0041 wider then PD so tooling manufacturers have that much extra room to work with and still be in spec. That doesn't look good for simply measuring the OD.

    So with these numbers in mind I have a PD range of 0.0053 and a worst case tolerance on insert of 0.0041 so there should be a small 0.0012 window in the center of the OD range where im guaranteed PD is within spec. And this would be assuming max allowable error on the insert. I bet in reality they would be much closer, making the window little wider, but nothings guaranteed I guess.

  3. #23
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    This is the sort of issue where I find it well worth investing in an hour or so of quality time with the CAD program so I can see exactly what is going on and explicitly see the effect of tolerance.

    I'd probably draw up sections of thread at nominal and ± max tolerances as per the drawings and overlay insert shapes at nominal and various ± variations to see how much error can be accommodated before the thread goes out of tolerance. Results of this sort of exercise can be surprising. My instincts are usually pretty good but I've learned not to trust them too much with a problem than gets me thinking.

    Objectively its the PD on a full form insert that needs to be right followed by the front projection so the tool can be set to give the correct PD by measurement. The OD doesn't really do anything so the manufacturing process will be optimised to push errors onto the OD. Which makes it a poor choice for quality control.

    As mentioned previously I just use a thread micrometer like a normal micrometer to track variations from a carefully made and verified section of thread. On cruddy alloy and other difficult materials I find the major issue is itty bitty burrs on the corners or small swarf stuck where it shouldn't. But I don't do quantity, more than 10 and customer gets a surcharge! On cruddy alloy I'd be sacrificing a die head chaser for tooling!

    Clive


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