What tools and techniques are used in paper gaging?

HELLO

Today I learned, FINALLY, what graphical inspection analysis is.

Paper gaging is cool. In some instances geometric position tolerances are based on a pattern by tolerancing to other positions, or mid planes are specified as datums, and it's been explained that these circumstances really require graphical inspection analysis.

Questions in two areas:

Are these operations done mathematically without making a graph paper chart and template overlay to physically analyze the picture of your positions? Q mags have written about this ~crazy thing~ and there was always some mention that CMM / CAD / inspection software is not usually capable of graphical inspection analysis. Is geometric tolerancing really so hard to handle that PC-DMIS or Calypso software isn't set up for it?

1. Basically can you do graphical inspection analysis without a paper graph?

2. Are there cases where precision graphing is required, such that drafting tools used in paper gaging need to be calibrated and their measurement uncertainty estimated?

Really a truly fantastic technique, I was hoping someone would help me advance my *meteoric* junior quality technician career with a little insight into this technique. I apologize if finding out that a quality technician just learned about graphical inspection analysis makes you a little dizzy. I feel the same way.

Truthfully

--Erik

atetsade said:

1. Basically can you do graphical inspection analysis without a paper graph?
--Erik

Click to expand...

I guess you could but then would it be graphical inspection analysis?

atetsade said:

2. Are there cases where precision graphing is required, such that drafting tools used in paper gaging need to be calibrated and their measurement uncertainty estimated?
--Erik

Click to expand...

Not that I'm aware of. This subject is covered by Foster in “Geo-Metrics III” , Meadows in “Geometric Dimensioning and Tolerancing” and the “Dimensioning and Tolerancing Handbook” By: Drake, P., Jr., 1999 McGraw-Hill . The following is quoted from the “Dimensioning and Tolerancing Handbook” and explains advantages/disadvantages, accuracy and discrimination.

“18.2 Advantages and Disadvantages to Paper Gaging
Since the optimum means for a geometric tolerancing requirement is through the use of a fixed-limit gage, the primary advantage provided by paper gaging lies in its ability to verify tolerance limits similar to those of a hard gage. Paper gaging techniques graphically represent the functional acceptance boundaries for the feature, without the high costs of design, manufacture, maintenance, and storage required for a fixed limit gage. Additionally, paper gaging does not require that any portion of the product tolerance be sacrificed for gage tolerance or wear allowance.
Paper gaging is also extremely useful in capturing dynamic tolerances found in datum features subject to size variation or feature-to-feature relationships within a pattern of holes. Neither of these can be effectively captured in a typical layout inspection. The ability to manipulate the polar coordinate overlay used in the paper gage technique gives the inspector a way to duplicate these unique tolerance effects. Since it provides a visual record of the actual produced features, paper gaging can be an extremely effective tool for evaluating process trends and identifying problems. Unlike a hard gage, which simply verifies GO/NO-GO attributes of the workpiece, the paper gage can provide the operator with a clear illustration of production problems and the precise adjustment necessary to bring the process back into control. Factors such as tooling wear and misalignment can readily be detected during production through periodic paper gaging of verified parts. Additionally, paper gages can be easily stored using minimal, lowcost space.

The primary drawback to paper gage method of verification is that it is much more labor-intensive
than use of a fixed-limit gage. Paper gaging requires a skilled inspector to extract actual measurements from the workpiece, then translate this data to the paper gage. For this reason, paper gaging is usually considered only when the quantity of parts to be verified is small, or when parts are to be verified only as a random sampling.

18.3 Discrimination Provided By a Paper Gage
With paper gaging, the coordinate grid and polar overlay are developed proportionately relative to one another and do not necessarily represent a specific measured value. Because they are generic in nature, the technique may be used with virtually any measurement discrimination. The spacing between the lines of the coordinate grid may represent .1 inch for verification of one part, and .0001 inch for another. A typical inspection shop may only need to develop and maintain three or four paper gage masters. Each master set would represent a maximum tolerance range capability for that particular paper gage. The difference between them would be the number of grid lines per inch used for the coordinate grid. More grid lines per inch on the coordinate grid allow a wider range of tolerance to be effectively verified by the paper gage. However, an increase in the range of the paper gage lowers the overall accuracy of the plotted data. The inspector should always select an appropriate grid spacing that best represents the range of tolerance being verified.

18.4 Paper Gage Accuracy
A certain amount of error is inherent in any measurement method, and paper gages are no exception. The overall accuracy of a paper gage may be affected by factors such as error in the layout of the lines that make up the graphs, coefficient of expansion of the material used for the graphs or overlays, and the reliability of the inspection data. Most papers tend to expand with an increase in the humidity levels and, therefore, make a poor selection for grid layouts where fine precision is required. Where improved accuracy is required, Mylar is usually the material of choice since it remains relatively stable under normal changes in temperature and humidity.
By amplifying (enlarging) the grid scale, we can reduce the effects of layout error in the paper gage. Most grid layout methods will provide approximately a .010 inch error in the positioning of grid lines. From this, the apparent error provided by the grid as a result of the line positioning error of the layout may be calculated as follows:

Line Position Error/ Scale Factor = Apparent Layout Error

For example, if a 10 ´ 10 to-the-inch grid is selected, with each line of the grid representing .001, a scale factor of 100-to-1 is provided, resulting in an apparent layout error for the grid of .0001 inch. However, if a 5 ´ 5 to-the-inch grid is selected, with each line of the grid representing .001, a scale factor of 200-to-1 is provided, resulting in an apparent layout error for the grid of only .00005 inch.


18.5 Plotting Paper Gage Data Points
It is extremely important for all users to plot data points on the coordinate grid of a paper “

Thank you very much. That's precisely the information I was THIRSTING for.

It's amazing--this technique neatly handles highly complex geometric tolerances of position.

It's curious that this tremendous technique isn't codified into software. Are there too many variables?

Maybe I'm a moron..

After reading this thread I went to my reference library, and looked for this technique in the 'Handbook of Dimensional Inspection' and found nothing. From looking through 'The Quality Technician's Handbook' I found a chapter on the topic. The topic shows standard layout techniques being used(height gage, micrometer, then some pictures of an old old CMM, so old it's little more then a two axis DRO).

From the description of the technique I'm getting so far, this is just the predecessor to modern CMM programs to give you an easy way to see if something is an acceptable fit at MMC, etc but is a bit unnecessary with more modern programs.

This quote backs me up: 'Computer programs can be written to process of X and Y coordinate measurements and tolerance from inspection data and provide the required rotation/translation of the data to determine functional acceptance of parts. The computer can print out data indicating acceptance or rejection, or produce the data in graphical form, resembling manually prepared graphical inspection analysis.'

And, an example of I believe what they are talking about from my Zeiss.

There is very little information or even use of "paper gaging" or graphical inspection analysis. It's covered in most courses on dimensional inspection and geometric dimensioning & tolerancing.

What I gather from your .pdf, hawat, is that like most CMM software, Calypso does compute most geometrical tolerances.

The application of paper gaging measurement is most useful for datums that are features of size, i.e. dynamic, and also position tolerances dually dimensioned to the datums and to other positions.

It's hard to explain and I just found out what it really is. Basically you create, on graph paper, tolerance zones for the part. Then you create a transparent overlay that contains the actual positions and variance of the datums and see if it fits in the tolerance zones.

It's nothing a computer would ever do and apparently it's nothing 98% of engineering ever worries about because it's almost completely a lost art?

Calypso's capabilities in regard GD&T would be very interesting to read about. Unfortunately I can't, right now, describe exactly what I wonder if it will do.

Will the software calculate the true position of a hole based on ALL THREE datums, not just two basic dimensions but the perpendicular fit of the hole, i.e. virtual condition?

Calpyso.

I could see the value if for some reason drawing out the part feature centers on graph paper would be much faster then using a CMM, and just comparing the overlay to a master sample.

It just seems that would be slower most of the time.

If for some reason you had a hole pattern and were relatively sure the holes diameter wasn't going to move, and you had a way to make a mark on paper at the centers, you could quickly mark up a sheet of paper, and compare to an overlay. Just seems unnecessary for what I do, but I guess it could let you do small production runs when building a proper fixture would be cost prohibitive.

As to the topic being covered in classes on dimensional inspection or GD&T, every time that chapter came up, we skipped it. Told it was archaic iirc.

As to Calypso's GD&T, it attempts to, for true position calculations, use the functional fit instead of Least Squares, per ASME, for a circle calculated pin, minimum circumscribed, for a hole, maximum inscribed. It will also do this for cylinders, as the inner or outer tangential calculations.

Notes on Calypso and GD&T.

I went back into my GD&T notebook and found a bit:

Here is the load down with True Position (also know as "position"). First and most important.
If you set up a base alignment to datums -A-, -B- and -C- and you also have a true position of a feature called out to
datum's -A-, -B-, and -C-, you could have different results depending on how you choose to evaluate the data. Remember
any feature measured in Calypso defaults to a "Least Squares Calculation ("Best-Fit")", but the calculation of the feature
can be changed by simply going to the "Evaluation Button" for the measured feature and selecting a different method on
how the feature is to be evaluated (i.e. "Maximum Inscribed" could be used for a functional fit analysis for a hole).
Let's assume you did not change the evaluation method for the cylinder (or circle) you have measured, so by default,
Calypso will use the Least-Squares method for the location, as well as size of the hole. Now if you calculate the
true position by hand, you will get a result based on the Least-Squares method.

Now the interesting part (this is one of many areas where you see the POWER of Calypso) in regards to the true
position characteristic in Calypso. If you select true position from the "form and location" pull-down menu and
open this characteristic and select the feature you want to evaluate, in your case a circle or cylinder, calypso
will default to the functional fit analysis (not Least-Squares) for the feature. For example, if you measure an ID,
then Calypso will default to the Maximum Inscribed Element (in this case circle/cylinder) {basically the smallest
pin that will fit thru the hole}. On the other hand, if you measure an OD, then Calypso will default to the
Minimum Circumscribed Element (in this case circle/cylinder) {basically the smallest ring gage that will fit
around the post}. You do have the option of changing the Evaluation methods, in the True Position Characteristic
for the feature, after you select the feature, just simply click on the evaluation button and choose the method you
want. Keep in mind, if you simply change the Evaluation Method, you can change the location and size of the feature
you want are evaluating.

Now let's assume you took the default method for the evaluation of the hole ("maximum inscribed element") and you are
reporting the position of this hole back to -A-, -B-, and -C-. Let's also assume you set the base alignment to
datum's -A-, -B- and -C- as stated in the first paragraph. If you don't fill in the datum reference frame (i.e.
primary, secondary and territary), then calypso will use the Least-Squares method for calculation of the datum
featues. If you fill in the datum reference frame (i.e. Primary is -A-, Secondary is -B-, and Territary is -C-),
then Calypso will default to the functional fit analysis for the datum featues (i.e. outer tangential's for planes,
max inscribed for holes or min circumscribed for posts) KEEP IN MIND YOU HAVE THE OPTION OF CHANGING THESE
EVALUATION METHODS FOR THE DATUM FEATURES TO WHAT EVER YOU WANT, JUST BY SIMPLY CLICKING ON THE EVALUATION BUTTON
FOR THE FEATURE USED IN THE PRIMARY, SECONDARY, OR TERRITARY. Once again, you could be seeing different results,
just based on your method used for the evaluation.

Any one of the methods you choose for evaluating a characteristic is correct from a programming stand point.
You may want to get together with your customer or engineer in charge of the project as to which method they
would perfer. If you choose to use the functional fit analysis for the evalution of the characteristics,
just make sure you take a good number of probing points on all features (i.e. maybe 40 points on a 30mm diameter
bore for touch triggering machines and 400+ pts on a 30mm bore for scanning machines). If you are looking for the
best repeatability then choose the least-squares method for the evaluation of all characteristics.

I am sorry this response is a little long winded, but I feel it is important for everyone to understand the different
evaluation mehtods for featues in Calypso.





I feel I have a descent understanding of metrology in general, but I will be the first to admit that my firm grasp
of the GD&T concepts start to slip a little once more than two MMCs are applied in true position calculations. I
will list out a couple thoughts about what is going on. Anyone feel free to correct any of my statements below,
because I may be wrong...

1) The coordinates used to calculate GD&T Position of a hole are not the "Best-Fit" results that appear in the
feature window in Calypso. The coordinates are based off another algorithm (I belive maximum inscribed for holes).
This better simulates the true function of the part in the real world. Using Best-Fit coordinates is not the
proper way to calculate position according to the GD&T standards.

2) When applying Maximum Materal Condition to secondary and tertiary datums, the resulting tolerance IS NOT simply
the Base Tolerance plus all of the bonus values. The "MMC"s on the datums allow the coordinate system to physically
move a bit based on the size of the datums. Adding all of the Bonuses from all of the datums may give you a general
idea of how things will work out, but is not correct.

I do know that Calypso attempts to follow the latest GD&T Standards to the letter. With that said, I have had it
blow up on GD&T position callouts just like you describe. There are two things you can do as a CMM programmer..

1) Wait on the new version. I don't know what you are using, but v 3.2 (see the many discussions below regarding v 3.2)
seems to be more solid when it comes to GD&T Position callouts with multiple modifiers. I have not noticed as many
issues with this version as I have seen with v 2.3 and prior.

2) (This usually works for me). Call the Designer/Engineer that gave you the print with the offensive callouts.
Get them to describe exactly why the dimension has multiple MMC modifiers and ask if simply applying the MMC to
the feature and primary datum is sufficient. If not, work with the designer/engineer to find out exactly what
the MMCs are controlling and find another way to verify that the part is good with Calypso's tools.

----

Shamelessly stolen from the Zeiss User's forum. (and often hopelessly Zeiss specific)

What if the datum reference is a feature of size? What if position tolerance is called out to both a feature of size datum and a set of other positions?

This is not archaic.

You saw correctly that the idea behind paper gaging is that it's cheaper and more accurate than a physical functional gage, assuming you've found the points accurately.

I wish I could explain in proper terms what I'm trying to see if Calypso does. My earlier question is based on this. Let's say the part is a square plate with a hole in the middle. Let's say a position tolerance is called out in reference to A B and C. B and C are just two sides. A is the plane of the top of the part. The hole goes in crooked. The virtual condition is the resulting cylinder that will pass through the hole, perpendicular to A. Does Calypso do virtual condition?

Ah well, it's almost quitting time and I got paid today. I'm actually going to a book store BUT I PROBABLY WON'T FIND ANYTHING ABOUT ENGINEERING!!!

Hawat, paper gaging or graphical inspection analysis is really a phenomenally awesome technique. I highly recommend looking it over, but you'll be hard pressed to get good training on it. Graphical inspection analysis handles some seriously complex geometric tolerances. It's so nifty, it reminds me of Mendeleev's work developing the periodic table. Everything fits so nicely. It's like GD&T was designed around paper gaging.

Maybe they stopped using whatever geometric tolerances were such a good fit for graphical inspection analysis, and it's only encountered on old prints. I highly doubt that. I don't know what the deal is.

I have to agree with Hawat, paper gaging is not really needed with a modern CMM. I've used it in shops with older CMM's and lately (past year or so) as a referee in instances that our CMM has failed by a small amount.