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Pattern hole position
4

Pattern hole position

Pattern hole position

(OP)
I thought a pattern hole’s measurement can be started from any one of the hole, and the position calculation result will be the same no matter which hole was chosen as origin, but the sample shown with a different result, what did I missed on the hole location measurement and position calculation?

Season

RE: Pattern hole position

I think it's because your measurements are all based on a falsely-introduced orienting of the plate to some external edge datums. The print only shows a reference to datum A, so whichever hole you call the zero, the next hole to be measured is just a straight-line connection, not both X and Y.

In the second scenario where hole #3 is the origin, there is no way all of the other three holes would have deviation in both the X and Y directions, because you can only build the X-Y system as you progress around the 4 holes (actually just the first two to be measured would be enough to square up the X-Y system). There is no perpendicular and parallel until after the second hole is measured for location.

For instance, if I start with #3 and then measure down to #4 as my next hole, the Actual Location would be 0 in the X direction and -21.0238 in the Y direction. (This comes from the -1 and -21, but using the Pythagorean Theorem to make a straight line between those two centers.) Thus, there is no deviation in two directions because it's just a straight shot center-to-center.

I haven't figured out all the numbers to really see if it all works out, but that is where I believe the issue is.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Pattern hole position

SeasonLee,
In the second attachment, why are you still using one hole as an origin? The origin of measurements is in the bottom left corner of the part.

Additionally, I would say that the drawing (in the second attachment) is incorrect. There is no basic distance in both directions between pattern of holes and datums B and C.

RE: Pattern hole position

Yes, if you introduce B and C on the outside edges, then basic dims from those edges would be needed, and then the data given in the table would not start from any hole as the 0,0 point.

So the only reason you could start with any one of the four holes as the zero point is if A is the only datum. And if that is the case, then my previous post would explain why the data seems to be different.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Pattern hole position

Actually, even with A as the only datum, I would say that one should not start with any one of the four holes. They are equally important and they all must fall within their positional tolerance zones simultaneously.

Making one of them as an origin of measurements may lead to false insepction results in certain situations -- for example, pattern that does not meet the positional requirement may be assessed positively.

RE: Pattern hole position

But pmarc, think of a CMM -- where would it establish a "zero" origin? The answer is anywhere.

Take a simpler analogy: If I want to measure the length of a board, I can hook a tape measure on the left side and measure across. But I could also hook my tape measure on the right side and measure the other way. It doesn't matter, because we're only looking at a local length; there is no false result. And the same applies here: if we are only looking at the local position of the holes, you can "hook your tape measure" onto any one of them. Then you would measure to the next hole. The 3rd and 4th holes would then follow from there, eventually forming the four cylindrical zones (there is still tolerance around the first hole, and that first zone should remain floatable so that its tolerance doesn't get bunched up elsewhere).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Pattern hole position

J-P,
Have a look at attached picture. This is what I meant in this case. For simplicity, I drew only 2 holes and considered their relationship in horizontal direction. I think it can be noticed that pattern in the upper picture (that satisfies drawing requirement) will be assessed as out-of-spec when one of the holes serves as an origin of measurements (the bottom picture).

http://files.engineering.com/getfile.aspx?folder=3...

RE: Pattern hole position

But that second picture would not be correct, because it fails to include the tolerance on the first hole. Notice how my post stated that the first hole would still have to have a "floatable" tolerance zone. Or in your case, since there's only two holes that first hole's tolerance would be bunched up into the second hole's tolerance.

So if it's inspected correctly, there is no issue.

If that still doesn't make sense, then just think about my first question in the previous post: where in your second picture is the origin?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Pattern hole position

Quote:

But that second picture would not be correct, because it fails to include the tolerance on the first hole.

Of course, the second picture is not correct exactly because of the reason you mentioned. But this is what happens when one of the hole in the pattern is selected as the origin. And this is how most of CMM operators I have met try to deal with such type of callouts -- in order to find the origin one of the holes is arbitrarily picked and the rest is measured relative to the selected one.

That is wrong, but this is exactly what seems to happen in SeasonLee's pictures.

RE: Pattern hole position

Yes, we agree then. My point was that if you insist that the first hole is not an origin, then you imply that something outside of the 4-hole pattern is the origin. And that is introducing a false datum (i.e., the edges of the plate).

SeasonLee, I know that might not be the answer to the initial question you were looking for, but I still think the situation you ask about is based on that "false datum" issue.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Pattern hole position

(OP)
pmarc
Your comments also caught my points, would you mind to guide us on how to measure the pattern holes by CMM correctly ( in case A is the only datum ).

J-P
The 2nd post (with DRF fully defined) more or less revealed the designer intent, i.e. use the edges as datum features, and I believe the CMM operator took the edges as datum too.

Season

RE: Pattern hole position

Most CMM software packages have pattern fitting routines where rotation and translation "if permitted by the specification" are iterated to solve for the size of the tolerance zones that contain the feature's attribute (aurface, axis, median plane, etc.) deviation. The presentation attached shows the typical open layout progressions chosen to fit rotation of a simple pattern.
Paul

RE: Pattern hole position

SeasonLee,
As shown by Paul, this would have to be an iterative, best-fit, CMM software analysis.

Paul,
I have a question. Page 4 of your document. If the features shown are shafts, as stated on page 2, could you clarify how the values of "Allowable Individual Feature Position Tolerance" for Features #1, #2 and #4 are calculated?

RE: Pattern hole position

Quote:

could you clarify how the values of "Allowable Individual Feature Position Tolerance" for Features #1, #2 and #4 are calculated?
PMARC,
It is the difference between the individual feature’s MMC size and its pattern constrained oriented actual mating size as the collective pattern is simultaneously expanded/contracted among the pattern features until expansion is limited by at least one feature, then sequentially expanded/contracted among the remaining features until all remaining pattern rotations have been eliminated, then verified that no individual feature local sizes violate their LMC limits.
Or
It is some abbreviation of that process that makes… sometimes intelligent… sometimes ignorant assumptions about each individual feature’s form and orientation then computes the difference in measured size from the MMC size.
Paul

RE: Pattern hole position

(OP)
Paul / pmarc
Thanks for the post, If we are using an old CMM software and the package don’t have that feature, then how to start to measure the pattern holes?

Season

RE: Pattern hole position

Thanks, Paul.
What I don't get on page 4 is the numbers.

Take Feature #3:
If the size of its actual mating envelope is 9.04, you have 9.4-9.04 = 0.36 of bonus.
0.36+0.36 gives 0.72, not 0.52.

Features #2 and #4 look OK, but there seems to be the same issue with Feature #1.

What am I missing here?

RE: Pattern hole position

Yes you are correct the sizes of #1 and #3 are switched around.
Sorry I never caught that mistake thank-you.
Paul

RE: Pattern hole position

No problem, Paul. I find your presentation valuable because it nicely emphasizes importance of the problem and shows how to deal with such kind of situations. I wish more and more CMM operators were aware of the issue you described.

I would say the numbers are less important aspect in this case - I just wanted to make sure that I got everything right.

Besides, I believe there is no need to provoke other picky bastards like me smile

RE: Pattern hole position

(OP)
Well, let's back to my question, how will you start to measure the pattern holes(in case A is the only datum)?

Season

RE: Pattern hole position

In my opinion, the pattern can be measured from any point in space as long as that point is not derived from one of the features in the pattern. If you decide otherwise, that is, to measure 3 holes from the fourth one, like it is shown in your attachments, you have to be aware that you are loosing some portion of positional tolerance that could in the end help your pattern to pass the inspection.

But regardless of approach chosen, I am afraid this positional callout with only A referenced may require some kind of best-fit algorithm. So if your CMM is not capable to handle the data, you will not get to anything satisfactory.

I am just wondering, perhaps you could combine your CMM data with a CAD software in order to simulate the best fit procedure and obtain PASS/FAIL assessment.

RE: Pattern hole position

Hi All,

Very interesting thread. The determination of actual values for multi-feature systems is not currently well defined in the standards (but we are working on it). Different approaches have been used in industry, particularly when CMM's are involved. There are a lot of techniques being used that do not necessarily achieve the optimal result.

Because only datum feature A is referenced in the FCF, the DRF is not constrained in rotation about the Z axis and in translation in the X and Y directions. In the language of Y14.5.1M-1994, there are multiple "candidate DRF's", and for each one we can calculate actual Position values for each hole in the pattern. The sets of values that SeasonLee calculated are candidate actual values, for different candidate DRF's. Each set is valid, and equally correct. So the actual Position value for any given hole in the pattern depends on which DRF was used. For example, Hole 1 has a Position value of 0 in one DRF and 2.828 in the other. Many people find this concept counter-intuitive, but it represents reality. The Position requirement is like a template, that controls the relative position of the holes.

But what if we want there to be only one answer? As Paul illustrated in his presentation, rotation and translation can be optimized to achieve the best result. So we must define what we mean by "best". The optimization approach that I believe makes the most sense for conformance assessment could be described as "make the worst point in the system as good as possible". This is a generalization of how actual values work for single-feature characteristics like Flatness. We try to get all of the points on the feature (extremities and all) to fit into the smallest zone, as opposed to getting the most points within tolerance or near nominal.

The optimal candidate DRF for SeasonLee's example is the one that makes all 4 holes in the pattern conform to the smallest possible Position tolerance. To achieve this, we must make the worst axis endpoint(s) as good as possible. Using the terminology from Paul's presentation, this equates to maximizing the minimum residual tolerance.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

(OP)
pmarc and Evan, Thanks for the valuable comments.

Actually, the concept of what I said earlier “a pattern hole’s measurement can be started from any one of the hole, and the position result will be the same no matter which hole was chosen as a origin” is from Al Neumann’s workbook page 9.4~9.7.

The attached two pages is an example of paper gage analysis on pattern hole’s position, you will notice hole #1 was chosen as the origin to collect data and the data is recorded in the chart, may I ask what is the differences in between the different measuring method? Or what did I missed?

Season

RE: Pattern hole position

SeasonLee,
Attached is 1) your pattern un-fitted (referenced from the center of the pattern)and 2) your pattern hand fitted (by eye) by rotating and translating the basic pattern and its known individual tolerance zones over a map of your measurements. Of course all of the tolerance zones are set to the minimum variable tolerance because you did not provide the oriented actual mating sizes of the four holes.

This is from a simple spreadsheet that I used to make the presentation above.
Paul

RE: Pattern hole position

SeasonLee,

Wow, that is confusing. The statement "a pattern hole's measurement can be started from any one hole, and the position result will be the same no matter which hole was chosen as an origin" is misleading when read outside of the context of Al's paper gaging example. Here's how I would try to explain it:
-The values from the Position Tolerance Actual column would be different, if a different hole was chosen as the origin
-The Paper Gage Evaluation and ACC or REJ columns would be the same, even if a different hole was chosen as the origin
-This is because the paper gaging technique allows the four measured points to be rotated/translated as a group, to optimize their relationship to the four tolerance zones (to bring the initially nonconforming holes 3 and 4 into conformance, at the expense of making the position values of holes 1 and 2 a bit worse).

So the post-optimization position result will be the same no matter which hole was chosen as an origin.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

Paul,

I did a further optimization of the data. Try a rotation of 2.446657 degrees, an X translation of 0.419371 and a Y translation of 0.015349. This should give three of the holes a Position value of 2.152, and the fourth hole a value of 2.093.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

Paul,

Hold the phone. That last optimization wasn't fully optimized - I've had too much coffee, and my hands weren't steady enough with the paper gage ;^). Try these numbers:

Rotation of 2.462486 degrees
X translation of 0.429652
Y translation of 0.009233

This results in a Position value of 2.148759 for all four holes, and should be the best possible fit.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

Paul,

Thanks. The devil is in the details, as they say. We throw around statements like "optimize to get the best actual value" but doing that in practice can be very challenging. Inspectors are often left to fend for themselves with tools and training that are less than optimal (pun intended). You and I (and many others) have obsessed over this stuff for years and we still have difficulty.

This is one of the challenges of GD&T. All of the degrees of freedom that are available to get parts to conform during pass/fail gaging must be simultaneously optimized if a unique set of numerical values is desired.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

(OP)
Thanks for all of your comments.

After position calculation, if we know all holes are within the tolerance, why do we still need to do the time consuming optimization study, what is the purpose of this work?

Season

RE: Pattern hole position

SeasonLee,

Good question! If the objective of the measurement is just to verify conformance, then doing a full optimization may not be worth the effort. Typically, the additional analysis is done on nonconforming or borderline parts, to try to get them to pass.

Some quality systems require numerical measurements for all characteristics on the drawing, for a sample of parts. Statistical values such as Cpk are then calculated for each characteristic to establish capability. If non-optimized values were used in these calculations, this appears as measurement variation and affects the Cpk values. This makes the production process appear to have more variation than it really does.

It may also be useful to know how close the part really is to being out of tolerance. Optimized actual values will be a good indicator of this, and non-optimized values will be more conservative.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Pattern hole position

(OP)
Got it, Thanks, Evan.
So, the function is same as paper gage analysis.

Season

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