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

Bev D · Jul 7, 2008

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

Again - we must separate product acceptance from process control. This is where this discussion is going off track. They are not the same thing nor are they mutually exclusive; although effective process control (that includes driving process improvements) will lead to virtually 100% product conformance with minimal acceptance inspection.

we are in the 21st century now...

even tho I realize many of our community are suppliers to the automotive industry, we do not have to be hostage to only doing what we think they are dictating. After all, they are not stellar examples of a good business an dhopefully we will be around long after their demise or hopefully resurrection... We control our destiny. We are supposed to leaders of improvement; not the guardians of the (long) past norm of inspect and scrap. No one can afford that. And AIAG, no matter how misguided in execution, was well intended to drive us toward process control and improvement and away from merely inspecting and rejecting.

justncredible · Jul 7, 2008

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

Maybe in the 10 years since I had to deal with hard gages they have gotten better. I have never thought of them as anything but a waste. CMMs used for SPC are very common in large manufactures.

Bev you tell'em. No reason to ever have to sort if you are useing SPC to monitor and control the process. :yes:

My day is over...... :biglaugh:

bobdoering · Jul 7, 2008

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

justncredible said:
CMMs used for SPC are very common in large manufactures.

OK...that may be true. But let's not suffer from self-reference criterion. Small companies do not always have the means to shuffle over to Wal-Mart and grab a CMM or two. Therefore, they may not be as common in that realm. Does that mean they should walk away from any part with GD&T on the print? I doubt it.

Paul F. Jackson · Jul 7, 2008

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

Dave,

Let’s try again, hopefully simpler this time. SPC never was about verifying conformance to specifications… it was about variation reduction and process improvement! Lately the acronym morphed to include both but it was never intended to do so at the onset. You should know this; you have accrued some years of experience and are you teaching this stuff!

In your 10X hole pattern example… how would you measure the hole’s “in process” parameters to give meaningful feedback to the operator, tool setter, whistle blower, yada, yada, yada,… to signal that something has changed from what was expected…or to reveal opportunities for process improvement.?

First you would look at the process!

If the holes are installed using a rotary table 1x1 or 2x2 or 5x5 it would be intuitive to choose polar coordinates as process control monitoring parameters. If the secondary datum feature |B| was fixtured coaxial with the axis of the rotary table then its position deviation relative to the 10X pattern (as a group) might indicate (half) of how far off the center of the rotary table is to the drilling spindle’s X0,Y0 axis. If the greatest variation was due to indexing then identifying which sets of features belong to the 2x2 or 5x5 would be critical to understanding the index error.

If the holes are installed on a CNC machining center or with a dedicated 10X machining head then it would be intuitive that the individual X, Y, coordinates may be more significant. The 10X pattern’s average position relative to the dedicated head would indicate how far off it was from the fixture that captures |B|, and if |B| was installed in the same operation as the 10X pattern (CNC) the errors would likely reveal the precision of the CNC machine itself.

Notice that we have not talked about B @ MMC or any tertiary datum feature @ MMC this is SPC… namely process control. If one wanted to dumb things down for the operator to an attribute check for process control then the best alternative would be as Bob Doering suggested by making the attribute gage with “less tolerance” than is given in the specification. In doing so the operator can distinguish when the operation is deteriorating and if it remains unadjusted it will likely produce defective product.

Say that we have done all of these fancy things “in process” … our process is in control, and we have set control limits for adjustment to truncate our variation but we cannot reduce the variation, and only 31/32nds with 100% inspection of our product pass the final attribute gauge that you designed. Our capability sucks even if we estimate our capability from our pass/fail performance. The problem is simply that tolerances are too tight to meet capability targets!!! What do we do?

Oh the tolerance is @ MMC

?

How can that help us?

Old tool makers, maybe older than us, knew that when the parts didn’t fit the gages that they could increase the size of the holes to pass the gauge… but sure enough someone is going to demand the same lofty capability on feature size as well. We’re screwed!

Dave, see a previous discussions to address this concern!
https://elsmar.com/Forums/showthread.php?t=16607

The point is, as Justincredible, Bev, Bobdoering pointed out… attribute gages “in process” reveal little or nothing for the operator to monitor, adjust, or improve the process and as a final inspection weapon when a part fails you have got big problems to meet the capability targets required these days!!!

Your buddy.

Paul

justncredible · Jul 8, 2008

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

bobdoering said:
OK...that may be true. But let's not suffer from self-reference criterion. Small companies do not always have the means to shuffle over to Wal-Mart and grab a CMM or two. Therefore, they may not be as common in that realm. Does that mean they should walk away from any part with GD&T on the print? I doubt it.

A QC rep should be involved and have the needed knowledge to ensure the print and all supporting documentation can be done per customer agreements.

It is not very good if a salesman gets a job that can not be checked, or gets a job that reqs SPC and there is not the capibility to meet the contract.

GD&T can be checked with more than a CMM, and SPC can be done on GD&T callouts not on a CMM. "The rock never lies", the majority of the time I will recheck anything out on the cmm with hand gages. As a matter of fact I have a part running right now next to me that I did not accept the paraelleism reading on the CMM so I rechecked with a mic and accepted the parts. I will do the same if the any of the features read out on the CMM. There is nothing that a CMM checks that can not be done with hand gages.

David DeLong · Jul 8, 2008

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

Paul:

I did not say that SPC could not be used on positional tolerances. I said that it was not practical since it does not cover all the simultaneous requirements.

If it is helpful to plot centres on the 10 holes in RFS relative to datums in RFS, go for it. It is costly though. Imagine, once an hour taking a part to the CMM room, tearing down whatever is on the table to check centres. We would have a sub-group size of 10 but would require two (2) charts - one for the X direction and one for the Y direction. There would not be a correlation to the specification though.

I remember when we started utilizing SPC on the shop floor in the early 80s, we had to come up with methods so the Operator would take no more than 3-4 minutes to measure and plot the chart since they are paid to make product. The "critical" characteristics were checked on a variable checking fixture with a set block. It was paramount to keep the inspection time low. Should the Operator take the part to the CMM room? Maybe we should have the Tech take it? If we had the Operator wait to have the holes measured, we would lose our time cycle and Operator efficiency.

Statistically plotting 10 centres can absolutely be done and I would consider the characteristic a process characteristic rather than a product characteristic.

Is it possible the we could have the centres in statistical control but maybe 1 hole is quite a bit off 90 degrees and the part does not conform to the requirements? Is it possible that a Customer rejected a shipment of product since some product would not assemble and we did not confirm that it met all the simultaneous requirements included in the positional tolerances?

If a company desires to statistically plot centres, I would suggest it also confirm that the positional tolerances with an attribute checking fixture produced as per ASME Y14.43 - 2003. This might be the best of both worlds.

Respectfully,

bobdoering · Jul 8, 2008

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

justncredible said:
A QC rep should be involved and have the needed knowledge to ensure the print and all supporting documentation can be done per customer agreements.

I agree. Throw a CMM in the quote-no problem.

justncredible said:
It is not very good if a salesman gets a job that can not be checked, or gets a job that reqs SPC and there is not the capability to meet the contract.

I agree. If there is a contractual agreement to do SPC, then it should be done (even if it is attribute). And, yes, the job should be able to be checked as a part of the contract review. Checking may be with a hard gage.

justncredible said:
GD&T can be checked with more than a CMM, and SPC can be done on GD&T callouts not on a CMM. "The rock never lies", the majority of the time I will recheck anything out on the cmm with hand gages. As a matter of fact I have a part running right now next to me that I did not accept the parallelism reading on the CMM so I rechecked with a mic and accepted the parts. I will do the same if the any of the features read out on the CMM. There is nothing that a CMM checks that can not be done with hand gages.

I agree. If the operator you can lay out the dimension on a rock next to the machine within the cycle time - go for it. Heck, I always said if you want quality to determine the measurement requirements, check 100% of the dimensions 100% of the time. Plot them all, too. If you pass that you can get sleep at night that no parts are coming back. But, there seems to be some resistance to that approach. I never said that a hard gage was the only way or preferred way, but there are still many cases where it is sufficient (cost effective?) and can assure that bad parts are not made (tighter tolerance gage). Those cases may not be the elegant example of a bolt pattern in space, but more like a true position of one diameter to another to ensure fit.

By the way, I have been many places where the CMM is not trusted by the operators because it does not duplicate layout results. Makes for great conversations on the shop floor. Great example!!

AS9100Dbag · Feb 13, 2024

Paul F. Jackson said:
Lee,

I've got a couple of questions. What is the tolerance for feature size and what is the geometric tolerance for position? Is the position tolerance constant or variable (does it have a MMC modifier attached to the tolerance)?

I don't suppose that you are questioning about the upper and lower control limits for feature size.

Process control examination for the position tolerance (I think) is best accomplished by monitoring the X,Y,Z coordinates of the feature location separately. It provides two important benefits, It helps to distinguish whether the individual coordinates are accurate (mean centered on the basic location) or not and it helps to discover how precise each axial deviation is and how much each is contributing to process variation.

You commented about the position tolerance being non-linear scattered about 360 degrees. The position tolerance describes the zone that the axis, median plane, center, etc. of the feature must reside within. The zones can be described in a number of ways (spherical, circular, cylindrical, rectangular, square, cubic, etc.) all determined by the symbols used in (or absent from) the feature control frame and from the way the tolerance "leader lines" are depicted on the specification.

The individual coordinates can be normally distributed when examined separately but when they are combined to determine the displacement from the basic location the resultant radial separation (or doubled "diametrical deviation") is typically a skewed distribution that is 'nearer to' and 'truncated by' zero and tailed toward the USL. Monitoring the derived position deviation for process control is not good for a couple of reasons. The position deviation does not distinguish between process parameters "X, Y, & Z" that are accurate but not precise where mean-shifts are no help and ones that are precise but not accurate where mean-shifts may improve. The other reason is that the deviation is not typically a normal distribution.

The control limits for the coordinates are established by the process variability so there is no relation to the drawing specification for position. Process control can be done effectively
by monitoring the X Y & Z of geometric deviations.

To predict the process capability for the true position deviation one must first establish that the process is "in-control" and that can be done with the individual coordinates. If the geometric tolerance is constant one can predict capability by applying the appropriate distribution function that best fits the skewed true position deviations and transforming the data. If the geometric tolerance is variable you can use a method that compares the (USL plus the Mean variable tolerance "bonus tolerance" minus the Mean geometric deviation) to (three times the square root of the combined variances for size and geometric deviation). It assumes normality for both but it demonstrates prediction error margins comparable to predicting the capability of a constant tolerance with the Weibull Method.

I would not recommend using: the residual tolerance method described in "Simple Process Capability" Quality Magazine by me, the percent of tolerance method described in "Calculating MMC Cpk" by Marty Ambrose, or the adjusted true position method "Calculation of Cpk under conditions of Variable tolerances" Quality Engineering by Glen Gruner because each one of those methods uses an individual pair of variables for size and geometric deviation to produce a surrogate variable the can be compared to a constant limit. In so doing the underlying variation from the independent sources can either be amplified or moderated in the surrogate.

I will be publishing another paper soon.

I loved finding your posts referencing Glenn's paper. I've been working with him for the last 2 and a half years, until he retired last month. This paper came up in a discussion with him shortly after I started working here. I can't seem to find the copy he sent me, so I was trying Google and happened upon your posts.

I giggled when I did, because earlier today I was trying to find information on this subject, having forgotten about Glenn's paper, and I downloaded your presentation on this subject from engineeringtips.com.

I had been lamenting Glenn's retirement for as long as I'd known it was coming. He's exceedingly bright, and his experience across multiple domains of manufacturing is unparalleled.

Paul F. Jackson · Feb 27, 2024

Old problems still debated! I would have liked speaking with Marty Ambrose (% of Tolerance) and Glenn Gruner (Adjusted TP), when I first presented "Residual Tolerance" to address conformance predictions of geometric feature tolerances with a limit that is itself variable. The crux of the problem is figuring the area of intersection of two measurement distributions (once you have diligently satisfied all data integrity prerequisites). It is putting a value to that observation that will draw such a great rebuttal. All of the methods that create a surrogate statistic to compare to a constant limit modify the inherent variability of the contributing distributions and are therefore wrong. So I think it is best to just plot histograms of the two distributions on the same graph carefully positioning the MMC or LMC size limit coincident with the geometric tolerance limit and observe the whether there is intersection. Improvements may be possible either by adjusting size within its limits or reducing geometric variation to separate the potential intersection without even mentioning statistically abhorrent terms.

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

Bev D

Heretical Statistician

justncredible

bobdoering

Stop X-bar/R Madness!!

Paul F. Jackson

Quite Involved in Discussions

justncredible

David DeLong

bobdoering

Stop X-bar/R Madness!!

AS9100Dbag

Registered

Paul F. Jackson

Quite Involved in Discussions

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