Geometric Tolerancing and SPC - Calculating position upper and lower control limits

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

Welcome to the cove, I found this place enlighting and I am sure you will find it too.

plot the control chart,range chart & calculate the cp,cpk,for flateness required within 0.1mm.pls. help me how can i draw the above becouse there no LSL.
Ranjeet Singh
Chandigarh

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Control Chart has nothing to do with specs, so you can draw them the standard way without any worry.

Cp can't be calculated because it takes in account both specs

Cpk can be calculated (only the upper limit <Cpku> of course) and some guys may think is a good idea, I take better Cpmk, at least it takes the target in account, if you use the Cpku as Cpk you are without knowledge taking Cpku = Cpkl, altho it doesn't exist.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

Sir pls. tell me about DOE,How can we get help in process manufacturing auto parts of casting(SG,CI),forging and machining .

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

Sir pls. tell me about DOE,How can we get help in process manufacturing auto parts of casting(SG,CI),forging and machining .

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I think you may get more answers if you post your question as a new topic, because most or the people check the topic description, and DOE is a little bit out.

Well, it a too broad question, DOE can help any process as a tool to detect how variation in some specific conditions affect the product, so you may find some conditions where you may have a more robust product (to the variations in your process), have less problems with out of specs parts or make the produt behave better (with negligible or without effects on the end product) as part of a product improvement or materials reduction program.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

poong said:

Hello sir
I want to plot the control chart,range chart & calculate the cp,cpk,for flatness required within 0.1mm.pls. help me how can i draw the above because there no LSL.
Ranjeet Singh
Chandigarh

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Actually, you do have a lower spec - 0 mm. Hard to get better than that. But, the biggest problem is that the data will not be normal. Unilateral tolerances (max flatness) with 0 as a minimum only behave normally when your process is bad. The closer it gets to 0, the curve "smashes" against the 0 limit. I believe this behavior is closer to the Weibull curve, although others may know of a better curve. Some have pretended that the distribution is half of a normal distribution, but that is a backyard estimate. I would recommend using a 75% of tolerance upper control limit. An individuals chart may make the most sense, since the variation of flatness of 3 or 5 consecutive pieces should be negligible, if you are doing precision machining. Spend the measuring time making sure you are covering the designated area as completely as possible. Set your upper range limit based on the expected range found in consecutive parts from your capability run. Anything greater than that should be considered unexpected or "special cause". Make sure you have appropriate gage R&R for that measurement, too. And, since the distribution is truly not normal, Cpk, etc., calculations do not apply (see AIAG PPAP 4th ed: 2.2.11.5 Processes with One-Sided Specifications or Non-Normal Distributions) - so do not bother with them. The key is to understand you flatness variation, and how to "control" it. Can you adjust - or "dial in" - your process for flatness? If your chart can tell you when to "adjust", it is working. If not, it may just be a report card chart,and an academic exercise at best.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

bobdoering said:

Actually, you do have a lower spec - 0 mm. Hard to get better than that.

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In a unilateral tolerance zero (0) is not a specification limit. It is the ultimate achievement or ultimate goal. If one had a flatness tolerance of .020, does that mean that the USL is .020 and LSL is 0 with a nominal value of .010? Absolutely not!

If one wanted to calculate capability of flatness, one could calculate the Ppk and Cpk but not the Pp or Cp since they require specification limits. Ppk/Cpk are calculated from the process average to the only specification limit which is .020 in this example.

Flatness is not an appropriate feature or characteristic to apply SPC unless one has a CMM and a scan mode but that is unlikely on the shop floor. We all measure it differently and most likely, set it up differently. We could get data of some sort but it is possible that one part may look statistically good but another person may measure it out-of-specification.

Very few, if any, GD&T symbols are appropriate for SPC applications.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

David DeLong said:

In a unilateral tolerance zero (0) is not a specification limit. It is the ultimate achievement or ultimate goal. If one had a flatness tolerance of .020, does that mean that the USL is .020 and LSL is 0 with a nominal value of .010? Absolutely not!

If one wanted to calculate capability of flatness, one could calculate the Ppk and Cpk but not the Pp or Cp since they require specification limits. Ppk/Cpk are calculated from the process average to the only specification limit which is .020 in this example.

Flatness is not an appropriate feature or characteristic to apply SPC unless one has a CMM and a scan mode but that is unlikely on the shop floor. We all measure it differently and most likely, set it up differently. We could get data of some sort but it is possible that one part may look statistically good but another person may measure it out-of-specification.

Very few, if any, GD&T symbols are appropriate for SPC applications.

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I agree, there would not be a nominal. Nominal is really a non-issue for non-normal distributions. Ppk/Cpk are calculations that relate to normal distributions, so they would not apply to this situation at all. (see AIAG PPAP 4th ed: 2.2.11.5 Processes with One-Sided Specifications or Non-Normal Distributions) Again, it is only normal if you are so far away from 0 that the full distribution is normal - which is typically if you have a bad process. The distribution should be skewed towards 0 if the process is of any value.

I agree, it is not as easy of a feature to control as length or diameter, but if it is truly critical to the function, it is worth generating a methodology to control it - preferably a visual one to spot special causes before a bunch of scrap is made.

As far as one person finding the feature good and another finding it bad, that is a measurement problem that needs to be resolved between the customer and the supplier in any event. An agreed upon measurement technique must be established or there will never be agreement between the parties - and indiscriminate rejections will plague the life of the part.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

David DeLong said:

Very few, if any, GD&T symbols are appropriate for SPC applications.

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Actually, when SPC for precision machining is done correctly, roundness (for circular features) or paralellism (for lengths) is perfect for SPC applications (as a measurement of range). But, that is a different discussion.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

David DeLong said:

Very few, if any, GD&T symbols are appropriate for SPC applications.

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Nonsense!

Paul F. Jackson

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

Paul F. Jackson said:

Nonsense!

Paul F. Jackson

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Paul:

Good to see that you are back.

Re: Geometric Tolerancing and SPC - Calculating position upper and lower control limi

Paul F. Jackson said:

Nonsense!

Paul F. Jackson

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Paul,

Would you be so kind and elaborate on your post?

Thank you so much.

Stijloor.