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Role of cutting tools in acheiving CpK?

Six Sigma – iSixSigma Forums Old Forums General Role of cutting tools in acheiving CpK?

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  • #31608

    Radheshyam
    Participant

    We are tooling up machines. Machines are accepected by customer after the job is passing Cpk tests.
    From the cutting tools point of view, I think only the tool life matters.
    ie. if the tool is able to maintain its edge condition for the entire sample quantity, we can eliminate any doubt on that perticular tool.
    Is the information I have is correct?
    Are there any articles on this relationship between cutting tools and Cpk?
    Thank you,
    [email protected]

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    #83461

    Eileen
    Participant

    Radheshyam
    I think you raise a good point. My experience with cutting machines shows that more factors are involved than simply the tool. Although critical to it’s performance, other factors need to be investigated to achieve a high level of Cpk. Those factors will depend on the type of machine you are testing. Once you have selected the most appropriate tool (maximize performace and minimize cost), you may need to test various chip breakers, chip removal, speed rates, feed rates, coolant amt, etc. All of these varibles will have an effect on the capability. You need to consider for your machine what else will result in variation in the operation that will change the outcome and utimately Cpk.
    Eileen, Quality Disciplines

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    #83462

    Radheshyam
    Participant

    Hello, Eileen.
    Very rarely I come across quality people who can mention amount of coolant, chip breakers, chip removal etc. No doubt, your 20+ years of experience and work on transmissions at Ford 1984 has given you  a  lot of experience.
    I am very lucky to get a message from you.
    I have few doubts to clarify, technical in nature but not suitable for this fourm. I would be very greatful if you could give me your email ID so that I can communicate without creating problem to other members of this site.
    my email is [email protected]
    Radheshyam, Manager, OEM & Projects, India

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    #83478

    Eileen
    Participant

    RadheshyamThank you for your kind email. Feel free to send me your issues and I will do my best to help.
    I tried to send you an email directly but the email came back saying the server was not found(?).
    My email is [email protected]
    Eileen BeachellQuality Disciplines

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    #83480

    Robert Butler
    Participant

    You should check chapter 13 of Bothe’s book Measuring Process Capability. With machine tool wear you are up against the problem of measuring capability in the presence of repeating trends. You can, of course,eliminate the trends by upping the frequency of tool sharpening but if you do that then you are increasing quality and increasing cost-which isn’t the idea of process improvement.
      Your estimates of short term capability won’t differ that much from the standard Cpk computations (assuming tool wear or the effect being measured is normally distributed), however, long term capability measurements like Pp and Pk will be impacted by the mean shifts that will arise from tool wear and it is recommended that you not compute these. For the cases of a moving average there is a measure Cpk,mod which can be computed instead. You also need to check the normality assumptions for tool wear. The data that I’m dealing with in this venue does not exhibit normality.
      If you find that you are up against some of the things mentioned above then the biggest problem that you will encounter once you have sorted out all of the capability computation issues is that of acceptance. Your modified computation of capability will differ from the standard Cpk computations and you will have to deal with the small army of people who seem to think that there is one and only one way to compute capability and that that computation applies to everyone for all time.

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