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SPC for tool wear

Six Sigma – iSixSigma Forums Old Forums General SPC for tool wear

This topic contains 19 replies, has 6 voices, and was last updated by  Anonymous 12 years, 7 months ago.

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  • #45235

    Blanco
    Participant

    Why does no one answer my question?
    Why do SPC on a lathe? Is it to achieve a desired time to cut or is it some other reason?
    Anyone actually use a lathe out there?
    Blanco

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    #147065

    Monk
    Participant

    Blanco,
    Why to do SPC will depend on the purpose of doing anything …?
    The whole point is….why do you collect data? because you want to know about something !
    In thecase of lathe….you can use SPC for following measure:
    – Cycle time for each job.
    – Tool change over time.
    – Tool change frequency.
    – Rate of tool change per shift.
    etc..and so on.
    Specify your requirements & more help can be provided.
    Monk

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    #147066

    Ovidiu Contras
    Participant

    Blanco:
    To put it simple: you want to control your input (tool) in order to have predictible output (part diameter).
    Hope this helps

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    #147069

    Blanco
    Participant

    Monk,
    Thanks for the reply.
    What I picked up before is the purpose of SPC is to control tool wear. Why try to control tool wear? I mean a CNC lathe can only cut down to the stop. So why is the wear on the tool important?
    Blanco

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    #147070

    Blanco
    Participant

    Ovidiu,
    Thanks –  this is what I heard so I went to look at the lathe and asked the guy why he didn’t use SPC to control tool wear. He told me tool wear does not effect part dimensions, only the time to cut to size. So I ask again why does tool wear need to be in control?
    Blanco

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    #147083

    Marlon Brando
    Participant

    In  equation: Y=f(X)

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    #147086

    Ovidiu Contras
    Participant

    Blanco,
    Tool (as an input) can have an influence on several things (as outputs): 
    1) part-related: part dimensions , part surface finish – from this point of view, I’ll suggest to push further your investigation, look into the rework or scrap level in the lathe process, go and ask the next process if the parts are ok (not just the person doing them)…
    2) productivity: as you mentioned, speed for finishing the part
    3) batching: as you look into why we have to make big batches, you’ll get answers like: ” this is the economical thing to do…more you do, less the cost per part…”. In reality, the biggest driver for this is the long set-up times (last part A to first good part B). What drives long set-up times? From what I found – it’s adjustments … and guess what drives the adjustments?
    So, in my opinion, these are 3 good reasons to control your tool wear.

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    #147091

    Blanco
    Participant

    Ovidiu,
    Thank you for your opinion.
    Let me clarify our issue with an example we can understand.
    Have you ever seen how people make copies of keys. They use one key as a template for the other.
    Now what you seem to be suggesting is the cutting rate of the wheel should be put under SPC to control the dimensions of the key?
    This doesn’t seem to be correct. Dimensions of a part do not depend on tool wear, if it did the dimensions of parts would increase with the order of manufacture.
    Our parts do not show increasing dimensions. Their distribution is random.
    There is no scrap or rework. How could there be? When a key cutter cuts keys, how many are scrap and reworked.
    My concern is a customer has demanded SPC charts for equipment that is not subject to drifting. We cut parts until the end stop prevents any more cutting.
    As for the cutting time it is not a big dependence on tool wear – maybe two mintues. We use five counters and count each material part – i.e. steel; stainless 304, 309, 430, and 409.
    Our work instructions call for a change tools after 5,000 parts.
    We have an engineer from the customer with no knowledge of machining demanding something of no value.
    The only point you make I understand is surface finish. Surface finish can be due to tool wear, but it is not a problem because of the counters. Anyway, all our parts have a rumble so it is not a problem!
    I think too many SPC people do not understand how equipment works – not you of course or the other helpful person.
    Blanco

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    #147143

    Monk
    Participant

    Blanco,
    Let us go to the Basics of the way any work area measurables are mapped. There are 4 very common measures that can be linked to anything that you measure in your work area and this applies to all the industries.
    a. Cost : In your case, it can be consumables cost, m/cing cost, labour cost, ..etc.
    b. Delivery: In your case, it is cycle time to m/c similar parts or actual vs estimated time, Qty produced- plan vs actual ..etc.
    c. Quality: In your case, it is the dimensions or surface finish – actual vs spec., etc.
    d. Morale: In your case, it can be the something that is affecting the working condition of the operator or ergonomic issue etc.
    After you identify the current or proposed measures for each of the above mentioned parameters, map the effect of ‘tool wear’ on each of the above mentioned measures. If possible give a rating on the scale of 1-5, on how severe is the impact of the tool wear on the identified metrics. once you do that, you will know, whether you have any good reason to monitor the tool wear.
    The above exercise will help you to convince yourself whether you need to do what our customer is telling you to do and also it will help you to convince the operators as well in why tool wear is to be monitored. What control chart to be used will depend on the way frequency of the data collection, type of data and the purpose of data collection.
    Go ahead and give it a try. All the best!
    Monk 

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    #147149

    Anonymous

    Monk,
    OK .. perhaps you explain how a lathe works, so I can understand how the wear on a tool can aftects the part dimensions.
    I’m not interested in the other characteristics for now.
    Blanco

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    #147151

    Monk
    Participant

    Andy
    You have a valid question and the answer for that ‘lies’ within your mind. So EXPLORE your mind and you will get answers.
    I have only provided the directions in which you can expore your mind to get answer to yourqueries. the effort have to be made by you. let me know, if you need any further help.
    All the Best!
    Monk

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    #147152

    Terry
    Member

    Forget SPC.  If you really want to have some fun, try to determine when a tool should be changed.  This is one of my favorite types of projects.  Many metal fab shops spend enormous amounts of downtime and tooling cost because they allow “opinions” to be the deciding factor.
    I have asked operators and engineers about the best time to change a tool to optimize performance.  One says, “When its blue.”  Another says, “When I start my shift.”  Another says, “When its blueish rusty-brown.”  Another says, “When I’m producing scrap.” And another says, “Right after morning break so I get another 15 minutes.”. 
    Several Six Sigma projects in this area have generated significant savings by standardizing the practice around the real facts and statistical results.
    Good Luck

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    #147153

    Anonymous

    Monk,
    I’ll accept your admonishment and give notice that in future I shall use various aliases, as do most other posters on this site.
    As someone who has actually worked on fine mechanical processes, you may want to consider some of my earlier posts. If you’ve learnt nothing from them, then you ought to arrange a visit to a machine shop where I’m sure someone will be glad to show you how they operate.
    Once you actually find out how they work and how to ‘control’ them you’ll make an invaluable contribution to furthering the cause of Six Sigma.
    Andy

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    #147154

    Monk
    Participant

    Andy,
    ‘Old habits seldom go’….you are a perfect example for this. Using different alias doesnot is in perfect sync with the norms of isix-sigma website. So you are not adding any value by highlighting that. however you are jst showing, how down you can get by making statements, that donot help or contribute to the main message.
    In future, i expect you to respond in the same manner, because I donot expect you to learn from your mistakes. So go ahead and BASH! because that is the only thing that you have learnt to do in your lifetime.
    All the best! May GOD help you to be in peace with urself !
    Monk

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    #147155

    Anonymous

    Once again I accept your admonishment.
    By the way, it’s not your fault you don’t know much about lathes, you just didn’t know you didn’t know much. Look at the good side, you ‘re now in a position to make some real progress.
    Andy
     

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    #147156

    Paul Gibbons
    Participant

    Monk,
    As an ex, time-served machinist who has spent too many hours standing at lathes I feel experienced enough to attempt an answer to your question. Although having said that, my skill is more as one of a machinist than SPC expert.
    On an automatic lathe the cutting position is controlled either electronically by a servo motor, or mechanically by dead stops. In both cases the variation over time will come from tool wear of some sort, assuming there is no adjustment made by the machine or operator.
    The benefit of SPC then would show you a trend in the tool wear which should be constant and predictably adjustable unless there is a special cause.
    Special causes on a lathe may come from things such as a roughing out tool breaking and the finishing tool is then left to carry out the work making it cut to a different size, or break as well. Another special cause could be a lack of coolant where the machine has literally run out of coolant or the coolant pipe has been moved by the swarf.
    I hoipe this has helped.
    Good luck
    Paul
     

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    #147158

    Monk
    Participant

    Andy
    I am confident enough that I have enough mechanical knowledge that I can walk down any m/c shop and drive improvment project.
    BTW for your information, I had worked on Lathe as well as Turret m/c during my training days and done production. So I am not new to m/cing process. Nonetheless, I am always open to learning things and constructive criticism.
    I can appreciate if you can be very specific in what you want to communicate rather then making ‘gross’ remarks.
    I still wish that the ‘Almighty’ should help you to provide peace and happiness at this tender age.
    Monk

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    #147160

    Anonymous

    Monk,
    Thank you for your kind blessings.
    If I’ve misjusdged you then I apologise unrervedly. I have no problem admiting when I’m in the wrong.
    The difficulty I’m having is correlating my own experience to the experience of other in this forum. For example, as I mentioned I’e work on several fine mechanical components, none of which showed any symptoms of dimensional drift.
    If you consider the information the other poster gave, and his reference to ‘end stops,’ it seems clear that the wear on end stops would be very slow indeed, and irrelevant if there was regular preventative maintenance.
    Therefore, in my experience I do not see a requirement to use SPC charts, and certainly no reason to use a special type tp calculate a drift.
    I can only apologise for my deceiption, but after having been called a deceiver by another I thought I should join in. But as you can see, I’m only an amateur and failed miserably.
    If I’ve added to your suffering, then I apologise again.
    All the best,
    Andy 

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    #147161

    Monk
    Participant

    Andy,
    I agree with you that in the case of machines, for which capability is well established, there is no need of having any control chart. But this is something that I would like the concerned person to take a decision on. I believe in the ‘philosophy’ of providing pointers, which can help the other person to take decision, instead of feeding with our own knowledge.
    If you read my response carefully, you will see that I have asked the person to find identify the need of doing anything. To my knoweldge, the fact that customer person is present, gives me an impression that there is a lot of inconsistency in the process. So how does the customer benefit, if the tool wear is reduced ? There is a lot of ambiguity in the information provided and so it i best for the person to decide.
    It might be good to start using the SPC to understand the capability of the lathe machine and lateron after the capability is established, they might discontinue. There is no harm in using it…but it doesnot make any sense to do it for the sake of doing it. For this it is important that the need of using SPC have to understood and that can be done if the impact of the tool wear is looked into….which might be due to –
    – material properties
    – collant qty or properties
    – speed of the operation
    – tol properties
    – operator training
    – process seqence ..and so on
    Now these are the X’s that can lead to tool wear, which in effect can be the cause of increase in the no. of tool changes, ……etc.
    I know there are a lot of things which are commonsense related but….my experience is ‘ Commnsense is most uncommon’.
    Monk

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    #147162

    Anonymous

    Monk.
    OK ..I understand why one might take this course of action during a ‘characterisation,’ or even when setting a first off in relation to previous runs. But as you know it is not uncommon to run thousands of components at a time, notwithstanding ‘Just-in-time!’
    You must be aware also that operators multiplex between several machines and does not give much time for plotting all these charts by hand – especially when companies have to compete against much lower labour costs in the Far East.
    Now if it could be shown that the dimensions of parts increase in the order of production, we’d obviously have no problem trying to find the source of variation using SPC.
    Cheers,
    Andy
     

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