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Topic Setting spec limits when you dont have any?

Setting spec limits when you dont have any?

Home Forums General Forums Methodology Setting spec limits when you dont have any?

This topic contains 12 replies, has 8 voices, and was last updated by Profile photo of Chris Seider Chris Seider 2 months, 2 weeks ago.

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  • #169331 Reply

    Afternoon all,

    Some help please…..

    We have a process which doesnt have any spec limits set by the customer but we would like to set some limits to pick up poor batches. Should we just carry out SPC and use control charts to determine if we have an issue?

    We currently reject batches that appear to be perfectly good and are not outside the spec limits as there arent any, which is costing us a lot of money per annum. I need to get this proposed change through QA though so would just like some help on how to approach this,.

    Thanks in advance…

    #169333 Reply

    My suggestion would be to work with the customer to set a spec limit. Control charts are one of many good tools to monitor the health and stability of a process but should not be confused with spec limits. Your process may be statistically in control but totally unusable for the customer. Also, I would be careful not to assume that batches appearing perfectly good are batches that work well in the customer’s process. The customer may not be able to pinpoint why Batch #1 worked wonderfully with their equipment yet Batch #2 caused crash after crash on the production line… This would be a good chance to team with the customer and learn something together.

    Better understanding of their requirements might lead to future business opportunities. For example, you might be able to sell them ‘off-spec’ product at a discount so they get a good deal and you don’t have to throw it out assuming it works in their process…

    This might be a good opportunity to work with the customer to establish some spec limits. You might even be able to sell them ‘off-spec’ product at a discount.

    #169334 Reply

    Confusing post…so I will assume you have no VOC-based spec limits rather you are rejecting lots based on internally-derived spec limits that have not been validated from the customer’s perspective.

    First start with the customer and have them quantify what good looks like…you want to capture a numerical spec limit on the “critical few” product characteristics as well as clear operational definitions….also, make sure you are both measuring those “critical few” items using the same methodology/definitions.

    As for Control Charts, will simply articulate the voice of the process over time, with the CL coming from the data itself, not the customer, so I would advise against using the CL as defacto Spec Limits (SL). Use the control charts to monitor process performance over time and determine if the process output remains stable. Also, remember if your VOC-based SL fall outside the CL, you will need to fundamentally change your process to achieve them (assuming stability is present).

    If you can’t do any of the above, then you have a couple choices as I see it. First, you can set a ‘soft’ target which sets some arbitrary target / spec range based on someone other than the customer’s ” best guess”…its often done, I wouldn’t reccomend it as it is has not been validated by your customers nor has it been quantified and tested for effectiveness. Second, use something called EPO or “Expected Percentage of Occurance”. This can be used to effectively baseline your process capability when good specs aren’t available, which isn’t uncommon for service-based or internally-driven projects (eg The target value is X cause mgt said so).

    Here are the basic steps with MTB pathways in parentheses where applicable:

    1.Determine stability (Stat > Control Chart > TBD)
    2.Determine shape (Stat > Quality Tools > Individual Distribution Identification)
    3.Create the Probability Plot based on the proper distribution type (does not require normality, only a properly fitted distribution type)
    4.Determine the desired percentage (usually 80%-95%)
    5.Determine the response variable range within this “Expected Percentage of Occurrence”

    Now simply baseline your process EPO and work to improve the output accordingly. Remember to test all “improvements” for significance. Good luck.

    #169335 Reply

    Its a strange process and the customer really doesnt have a spec limit. An internal spec was set some ten years ago but a recent audit has picked up that there is no statistical approach to this setting, VOC doesnt know what the sepec is and would require lengthly and costly testing to determine the level required.

    Could you please talk me through the EPO please as im sure this is the right thing to do in this instance, Im ok with points 1 and 2 but from point 3 – 5 im confused. Could you explain more please and where these are in minitab?

    Help is much appreciated.

    #169337 Reply

    mikebuckle wrote:

    Its a strange process and the customer really doesnt have a spec limit. An internal spec was set some ten years ago but a recent audit has picked up that there is no statistical approach to this setting, VOC doesnt know what the sepec is and would require lengthly and costly testing to determine the level required.

    Could you please talk me through the EPO please as im sure this is the right thing to do in this instance, Im ok with points 1 and 2 but from point 3 – 5 im confused. Could you explain more please and where these are in minitab?

    Help is much appreciated.

    Spec limits have nothing to do with statistics from the production process (although perhaps the customer may derive them as requirements to fit some other part of a larger system).

    So, based on what you say, if you deliver anything to the customer, they will accept it happily and pay you whatever you asked? If not, then you can derive spec limits from the point where your customer complains or wants a discount/refund.

    That said, the best thing to do (as has already been suggested) is to work with the customer to develop better knowlege about the customer’s needs and therefore the specs.

    #169339 Reply

    Search this site under EPO or google it, as it should be readily found. Come back if you can’t find it. Good luck.

    #169340 Reply

    Companies can compete on price, quality, or service. A company could not compete on price or quality so they decided to provide exemplary service. By working closely with their customers they got very good at VOC and learned how they could compete on quality. This may be what your company needs to do.

    #488691 Reply
    Profile photo of John
    John
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    Actually, what the original poster asked (the theme) is extremely common in manufacturing. I run across this problem all the time at work. There are many, many measurements that need to be taken long before the client even cares to comment.

    The only way I know of to “set the spec limits” in this case is to run the process, and, set the “spec limits” outside what you see as a normal controlled process, which is producing the desired output.

    Regards,

    John

    #488692 Reply
    Profile photo of John
    John
    Participant
    Reputation - 21
    Rank - Aluminum

    It’s not. Try it yourself.

    John

    #488961 Reply

    @johnyoga Look at a process called Realistic Tolerancing. There have been articles on iSixSigma on this topic in the past.

    Good luck.

    #488997 Reply

    I even wrote one of them. @Mike-Carnell Exciting reading of course! lol

    #489011 Reply

    @cseider I tried to do a search before I answered this but I had a really crappy internet connection. I could have given a link to your article. What is it?

    #489017 Reply
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