Machining Tolerances
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  1. #1
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    Default Machining Tolerances

    Hi All,
    we have production parts with Tolerances of 0.00025" total.
    there are 1000's in qty different styles ,turned to 1" long to 2" long
    from 1.75hex down to 1" dia to these tight tols.

    Normally in a production circumstances I would only price 0.0008" total tol.
    or pass.

    If we invest in a new machine what would you recommend. We have had Mazak salesman here
    and he seem to think there in no problem! holding this tol. after referring to the Tech.people

    What is your opinions please
    Thanks
    GTW

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    Quote Originally Posted by Gerrythewelshman View Post
    Hi All,
    we have production parts with Tolerances of 0.00025" total.
    there are 1000's in qty different styles ,turned to 1" long to 2" long
    from 1.75hex down to 1" dia to these tight tols.

    Normally in a production circumstances I would only price 0.0008" total tol.
    or pass.

    If we invest in a new machine what would you recommend. We have had Mazak salesman here
    and he seem to think there in no problem! holding this tol. after referring to the Tech.people

    What is your opinions please
    Thanks
    GTW
    How are you going to measure these? Have a climate controlled shop (not just AC)?

    1000's of parts, in different configs with a total tol of .00025" doesn't sound like fun, but there are guys with waaayyy more experience with this type of tolerances and production.

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    This is certainly possible, but there are occasional losses so quote accordingly. My experience holding 0.0003" total tolerance on three different dia's on a 6" long shaft yeilded 94% good parts. Climate controlled room and indicating micrometers are required.
    Mazak QT-200 Nexus.

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    It can be done but you will need to 100% the tight tolerance diameters and probably need an operator giving the machine attention 24-7.

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    It can be done but like others said a very competent operator temp controlled atmosphere good tooling will all play a factor.

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    Is CpK involved here?

    Does your .00025 inch tolerance band include CpK or not... a real game changer if applied after the fact.

    Mike

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    Default Tol's

    Quote Originally Posted by mister honey View Post
    Is CpK involved here?

    Does your .00025 inch tolerance band include CpK or not... a real game changer if applied after the fact.

    Mike
    Hi Mike,
    The Capability process is the reason for a new machine quote
    The machine shall be required to produce X amount to the desired Tol's
    before we purchase.
    We shall as a Co. be doing whatever is required to
    assist this process.

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    In my experience, you should require CpK buy in from your MTB, workholding, coolant and cutting tool vendors as well as the material supplier.

    Everyone plays a part in the CpK process with tight tolerance workpieces.

    Mike

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    What aspects of the part are held to that Tolerances of 0.00025" (total).

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    When I machine tight tolerance shaft journals like that, it is a struggle to keep within the boundaries. Part of the issue is that tool wear is a continuous factor which I need to learn how often to adjust the tool offsets and when to change the insert. It's not really a machine positioning fault that is at issue.

    Also, I've seldom seen a turned surface that couldn't be smoothed by 2 tenths with about a 1 second application of a fine abrasive cloth before the final diameter settles down to what I would call a real dimension that won't change during an assembly operation (if force is involved). So there is the part that meets the tolerance, and then there's good parts. I just don't think a lathe is the proper machine to achieve final dimension with.

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    Whoaa! I disagree with Hu. I think a good Lathe is just the Machine.

    Obviously you shouldn't be looking for some syphilis ridden, 20 y/o.

    But really, we are ass-U-ming that you are talking Production, but Thousands a year is not a lot, for 1 dedicated Machine. (Depending on the rest of the part). But that means a more or less dedicated Operator to supervise it, it ain't no lights out project, running with a Feeder. IMO any decent Lathe should hold it. Any decent Operator can maintain the Tolerance goal.

    I also think the Spengineer has a good idea of Manufacturing process control. Offset values that are within reach. A single .0001" offset isn't going to throw the part out.

    R

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    Who's the customer?

    Anyone spec'ing .00025" either knows exactly what they're doing or doesn't know what they're doing.

    What did they spec for surface roughness? What's the material?

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    What material ?

    Did I miss that ?

    And hex bar stock initial interrupted cuts (do I have that right ?)

    Any hard turning ? I'm guessing not.


    __________________________________________________ ___________________________


    The NLX 2500 | 500 (short bed) is very accurate (easy / near sub micron for roundness) leaving enough "spare" to achieve 7 to 10 micron part accuracy.

    If memory serves me right it has cooling through the casting(s) so that thermal regulation is smoother.


    On the MAZAK's they don't have through casting thermal regulation and hence their thermal compensation jumps about a bit.

    The "Super Precision" Lathes from Hardinge place fans at strategic locations to more artfully smooth things out a bit more.



    I would guess that the NLX 2500| 500 will be the no-brainer minimum fuss solution for process control...

    The MAZAK I would bet would do a really good job but have higher rejection rate.

    The 'SP" Hardinge lathes might need a bit more skill and operator time + eye but in the right hands can yield very close form tolerances for cylindricity beyond what you are asking for here.


    __________________________________________________ _______________________


    I agree with what HuFlunfDung is saying about precision contact surfaces on a shaft have a bit of art to it as to what gets relieved (to create more certain contact surfaces) and what will be hardened and then possibly ground etc. …

    funny I didn't see the other posts lol

    Yeah, like Orange ---> Surface finish Ra and Rz maybe ? Roundness + cylindricity etc. ? Again material ?

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    Quote Originally Posted by litlerob1 View Post
    Whoaa! I disagree with Hu. I think a good Lathe is just the Machine.

    Obviously you shouldn't be looking for some syphilis ridden, 20 y/o.

    But really, we are ass-U-ming that you are talking Production, but Thousands a year is not a lot, for 1 dedicated Machine. (Depending on the rest of the part). But that means a more or less dedicated Operator to supervise it, it ain't no lights out project, running with a Feeder. IMO any decent Lathe should hold it. Any decent Operator can maintain the Tolerance goal.

    I also think the Spengineer has a good idea of Manufacturing process control. Offset values that are within reach. A single .0001" offset isn't going to throw the part out.

    R
    Certainly, a lathe can be made to do it. But since you're working right on the sharp edge of attainable, it's WORK to make it happen. And for thousands of parts, WORK is stressful.

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    Hi Orange vice,
    I am at home at the moment. The surface finish is ok to achieve. The material is a hex Mild Steel only 1 dia with the tol's but ok to machine no surprises.

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    Quote Originally Posted by HuFlungDung View Post
    Certainly, a lathe can be made to do it. But since you're working right on the sharp edge of attainable, it's WORK to make it happen. And for thousands of parts, WORK is stressful.
    Yeah, I hate work too.

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    It's possible, it's gonna cost a pretty penny. And as said there will need to b constant babysitting. Tools need constant monitoring as will offsets. If not a 24 hour operation everything will need to start at zero every morning and be adjusted from there.


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