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

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

I'll jump in; any measured dimension or property can have SPC applied. there is nothign magical or mystical about GD&T. However, one must first differentiate between product acceptance and SPC. they are not the same. It is true that things like max material condition 'complicate' acceptance rules but they have no effect on SPC...the feature is what it is. control that.

The true position of a hole can be controlled in the x axis and y axis OR as a vector inthe circle. THINK about the physics of the process and not GD&T rules. Then it will become clear how to control the process.

How long have we been saying that spec limits aren't control limits; that process control is not product acceptance?

And process capability can also be quantified but one must think beyond cookie cutter AIAG 'standard rules'

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

Bev D said:

<snip>The true position of a hole can be controlled in the x axis and y axis OR as a vector in the circle. THINK about the physics of the process and not GD&T rules. Then it will become clear how to control the process.

Click to expand...

Good points. However, it is not that simple...

For example: The (automotive) customer (whether we like it or not) requires some sort of capability index on position. I have yet to find two (quality) engineers that agree on a valid method on how to do that. If you have followed some of the (heated) discussions here about GD&T call-out interpretations, we have a LOT of work to do...let alone agreeing on how the data should be generated, collected and analyzed to come up with some valid assessment for the purpose of statistical process control.

If it was that easy....

BTW, we must think about the GD&T rules, that's what Y14.5 requires.

Stijloor.

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

Well stated Bev!

To say that “Very few, if any, GD&T symbols are appropriate for SPC applications” denies the enormous benefit that predictive statistics has in comparing observed measurement variation to defined boundaries for geometry. GD&T and SPC are both very powerful tools that, when applied efficiently, can not only describe the boundaries of acceptable variation in terms of fit and function but also reduce the scrutiny required to characterize, monitor, and adjust the parameters of a process that generates that variation.

Just as there is great potential for these tools to complement each other when understood and employed correctly there is equivalent potential for them to incorrectly define or characterize acceptable variation when one or both are used inappropriately. Pioneers of the tools, Walter A. Shewart (SPC sub grouping, histogram, X-Bar and R-Bar charting) and Stanley Parker (GD&T, MMC, LMC and attribute gauging), created simplified practices that address the complex nature of the analysis tools.

Industry expectations of quality practices however, have changed from the time that the tools were first employed. Most customers now require that producers demonstrate predictive conformance to the product specifications in terms of process capability risks, namely Cp, Cpk, and/or Pp, Ppk ratios. These indices require continuous data analysis rather than the discreet data generated by the attribute gauging.

Unfortunately the simplified practices that normalize data by sub-grouping and use ranges to estimate control limits… as well as… reporting discreet data pass/fail statistics with regard to virtual condition limits… are generally abandoned in favor of raw measurement data to perform the capability analysis. In so doing the analysis becomes more complicated. Distribution types must be examined, normal or otherwise, and best fitting curve functions employed. Separate data for size and position of variable limit tolerances must be analyzed together to reflect the virtual condition boundaries that are equivalently built into attribute gages.

Typically these precautions are overlooked simply because people don’t understand the prerequisites of statistical analysis, or do not recognize that there are variable limits with many geometric tolerances. It is quality practitioners that fail to check the data for control (randomness), assume that all data is distributed normally, and disregard feature size as a parameter for geometric position tolerance. I don’t blame them however it is those should know the limitations and prerequisites of the analysis but do not… those that demand demonstration of capability (STA & Purchasing), those that govern/solicit statistical analysis procedures (AIAG & Software Manufacturers), and those that are regarded as experts in this stuff that do not speak out about the abuses! To be fair to them although, I have found that there are experts in SPC and experts in GD&T but to borrow Dave’s words “few if any” are experts in both.

Bev pointed out, as I have to Dave earlier that parameters for process control do not require specification limits… only checks for randomness and predictability, furthermore those parameters, if chosen and monitored properly, can help the process owner to identify characteristics of the process that may be improved with adjustment. A surface that is specified parallel to another has at least three components that can be monitored… its flatness and its pitch and roll! When the variation of pitch and roll of the median plane are nominally aligned to the datum feature plane then the resultant variation in parallelism represents the “entitlement” Cp or Pp… when they are not aligned the value represents measured capability Cpu or Ppu.

Stijloor, I hope this isn’t too much of an elaboration.

This stuff needs to be fixed if we are going to continue to demand Cpk’s of Geometric tolerances (with variable limits) from producers. Otherwise drop the capability requirements and go to near 100% attribute gauging as Dave suggests, to achieve required capability levels.

Paul

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

Hello Lee,

Attached is a presentation on GD&T. Hope it may help you.

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

I stated previously that “Very few, if any, GD&T symbols are appropriate for SPC applications” but I did not state that we could not use data generated from confirming GD &T symbols in some sort of statistical information if it is helpful.

Let’s take the positional tolerances of 10 holes as an example. We have a feature control frame with a diametrical tolerance zone of 0.3 beyond MMC referencing datums A, B (MMC) and C (MMC). Datums B and C are holes.

This means that each of the 10 holes relative to their true position (theoretical centres) are located from datums B & C, perpendicular to datum A, accrued tolerances from individual hole sizes on each of the 10 holes, shape of each hole impacting its tolerance (virtual condition boundary supersedes CL), datum holes at MMC possibly accruing additional tolerances. All are to be confirmed simultaneously.

It is appropriate to separate only 2 of the requirements and plot their location while disregarding the remaining ones? Is it possible that the resulting distribution may appear quite good but we have a couple of parts that are out of specification caused by, say, the angularity of a hole?

We also have a unilateral tolerance (round) that must been calculated on each hole. The true position (zero) is not a lower tolerance zone so how should one handle the plotted points? Maybe we could vector them (X & Y co-ordinates) but the tolerance zone is round? I have seen people try using a +/- 0.15 in place of a diametrical tolerance zone of 0.3 but that is plainly incorrect. There is a way of reflecting the plotted points using a tolerance proportion but complicated.

Now if one plots points to analyze the pattern and it might be helpful, great, go for it, but capability studies as well as ongoing SPC on the shop floor are not appropriate nor practical.

The only practical way to confirm the simultaneous requirements of positional tolerances is with an attribute checking fixture with pins of virtual condition size staked in each of the 10 holes (force of 1 finger) with the part mounted on datum A and datum holes B & C on pins of either MMC or virtual condition size. This is appropriate method of confirming the positional requirement and simulates the assembled condition.

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

Hard gages cost huge amounts of money and provide zero information, they are used for large runs and as a catch for bad parts, not to make adjustments to a process. They are pretty much junk gages. A cmm for bolt patterns will provide the numbers needed and that is what you would want to use for such a 10 bolt hole study, and ongoing long term studies. Worst case you would use a hieght stand and calc out everything. A hard gage will not tell you how to adjust to make a passable part, only that it fails.

SPC and GD&T are fine together, there is some contention about reporting the X, and Y or the callout. I report the callout and not the dims that make up the callout.

If I was doing a study on the machine I would report the X and Y, a study on the process to meet customer specs reporting only the spec callout is correct.

Now that I have thought of it it has been over 10 years since I last saw a hard gage for TP on a shop floor being used. I do find it surprising anyone uses them still. I guess there is a need if you have huge runs and no cmms.

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

Stijloor said:

Good points. However, it is not that simple...

For example: The (automotive) customer (whether we like it or not) requires some sort of capability index on position. I have yet to find two (quality) engineers that agree on a valid method on how to do that. If you have followed some of the (heated) discussions here about GD&T call-out interpretations, we have a LOT of work to do...let alone agreeing on how the data should be generated, collected and analyzed to come up with some valid assessment for the purpose of statistical process control.

If it was that easy....

BTW, we must think about the GD&T rules, that's what Y14.5 requires.

Stijloor.

Click to expand...

My experience is that it's not all that difficult once you separate the needs of process control and product acceptance. even for capability indexes. The automotive industry does allow for alternate methods of calculation when the distribution is not normal and GD&T controlled features certainly fall into this category (the use of MMC alone does this). One can use a simple defect calculation and back calculation of a capabilty index. I've never had difficulty with this as long as I talked to the supplier rep with the data in hand...

Yes you must think about GD&T rules for product acceptance, but there is no law that says that GD&T applies for controlling a physical process...In fact if you monitor your process, improve it and control it you will meet the GD&T callouts. I understand that occassionally there are concerns with measuring "too much" but in many cases the same dimensions that are measured for acceptance are also useable for process control; often it's as simple as using each dimension by itself for SPC and collectively for acceptance...

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

justncredible said:

Hard gages cost huge amounts of money and provide zero information, they are used for large runs and as a catch for bad parts, not to make adjustments to a process. They are pretty much junk gages. A hard gage will not tell you how to adjust to make a passable part, only that it fails.

Now that I have thought of it it has been over 10 years since I last saw a hard gage for TP on a shop floor being used. I do find it surprising anyone uses them still. I guess there is a need if you have huge runs and no cmms.

Click to expand...

I have seen them in operations where the time it took to get the part to a cmm, get it cued into the workload and get a report was too long or the product pricing structure did not support the overhead of a cmm. When used that way, you can - in a gross sense - set them up to tell when an operator to make an adjustment, and to a lesser degree how far. You make the gage at 75% of the zone the tolerance calls out. Of course, MMC is a critical component of even attempting to use this approach. But, it is far wiser than gages at full tolerance (only good for sorting, and complicated with "fit" issues not found on a cmm). Less helpful than variable data - and certainly not considered for SPC.

The right answer to any question: "it depends"

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

bobdoering said:

You make the gage at 75% of the zone the tolerance calls out. Of course, MMC is a critical component of even attempting to use this approach. But, it is far wiser than gages at full tolerance (only good for sorting, and complicated with "fit" issues not found on a cmm). Less helpful than variable data - and certainly not considered for SPC.

The right answer to any question: "it depends"

Click to expand...

Gauges should be made at MMC or virtual condition size using 10% of part tolerance as per ASME Y14.43-2003. One would then decide how to apply the 10% gauge tolerance. In the automotive industry, one should apply the tolerance to never accept a nonconforming product.

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

David DeLong said:

Gauges should be made at MMC or virtual condition size using 10% of part tolerance as per ASME Y14.43-2003. One would then decide how to apply the 10% gauge tolerance. In the automotive industry, one should apply the tolerance to never accept a nonconforming product.

Click to expand...

You always have the option to make a gage a tighter (within) tolerance if you wish to do so as a process control - as in 75% of the accepted zone rather than the full allowed zone. The resulting product will meet a gage made to ASME Y14.43-2003 most readily.

For final inspection purposes, you should have a gage that meets ASME Y14.43-2003 - as final inspection should be considered the equivalent of the customers dock. Having one will calm any customer auditors, and will tempt production to use the more open tolerance gage to circumvent the process control gage. Always good justification for a gage...