Hi
I was reading the threads here and found this one Need some advise: Beginning SPC data collection: Metal Stamping Company (http://elsmar.com/Forums/showthread.php?t=9729). If you look at the thread and the last comment by Paul F. Jackson
This is his comment:
"I would just caution you that after you begin your process monitoring and control procedures and reveal the predictable common cause variation someone will likely require you to estimate the capability of your process to the customer specifications. If you try to do that with your continuous data you must recognize that the equations for Ppk and Cpk accept only a constant value of tolerance (USL) and if there are MMC modifiers applied to the geometric tolerance the limit is variable (with respect to feature size)."

I did not understand what he meant by variable tolerance limit. Can any one throw some light one this ("MMC modifiers applied to the geometric tolerance the limit is variable (with respect to feature size)") part of his comment..
thanks in advance
Deep

I did not understand what he meant by variable tolerance limit. Can any one throw some light one this ("MMC modifiers applied to the geometric tolerance the limit is variable (with respect to feature size)") part of his comment..
thanks in advance
Deep

Hello-

I don't know if you understand the GD&T principles regarding material condition or not, but there are some features for which the tolerance might be variable depending on the size(s) of the feature(s) in question. Thus a tolerance limit used in a Cpk calculation might no be accurate because only the nominal limit shown in the GD&T feature control frame may be used.

Hello-

I don't know if you understand the GD&T principles regarding material condition or not, but there are some features for which the tolerance might be variable depending on the size(s) of the feature(s) in question. Thus a tolerance limit used in a Cpk calculation might no be accurate because only the nominal limit shown in the GD&T feature control frame may be used.Good point. I see our original poster lists in her Profile as a student. Peach works for a metal forming company (last set of posts I recall) and may not be aware of, or familiar with, Geometric Dimensioning and Tolerancing and Maximum Material Condition to the same extent precision machinists and others who work with intricate configurations of products may be.

If that is so, Peach, a lot of the terminology and drawing notations may seem like an outer space language. We have plenty of folks who can answer your questions, though. JSW05 seems tuned in. I'll look in here from time to time to check on your progress.

Note I modified the title of the thread for easy pickup by search engines.

Peach,
If you know dimensioning and tolerancing you will understand that tolerance modifiers for maximum or least material condition make the declared geometric tolerance value "variable" with respect to feature size. Read ASME Y14.5M-1994 the GDT Standard section 5.3.

The formulas for Cp,Cpk and Pp, Ppk compare sample variation to specification limits but those spec limits are assumed to be constant values. They don't work for variable limits. When you view a graphical capability analysis histogram for a unilateral tolerance like position the USL "upper specification limit" appears as a line or as a constant limit on the graph. When that tolerance however, is variable, that line should be replaced with another histogram representing the individual total tolerance values "specified USL plus variable bonus." That distribution is identical to that of feature size it is just positioned beyond the specified USL by the average bonus amount ( depending on whether its a hole or shaft, Tol @ MMC or LMC the value might be (LSL size - mean size).

When those two distributions one for position deviation and one for size are graphed relative to one another to represent the geometric deviation and its variable tolerance the area of interference of the two distributions representing the Z value for Ppk can be predicted using the classical reliability distribution model for stress vs. strain, provided both distributions are normal. The geometric distribution is typically "less-normal" as the distribution approaches its boundary of "zero deviation" so the stress vs. strain model becomes less reliable for the prediction. A monte-carlo simulation of the dis-similar distributions can provide a better prediction.

Use of MMC modifiers in product designs is not rare. They are found on most product drawings but the variable portion of tolerance is ignored in every continuous data capability prediction. It is not ignored in so if you have attribute gaging and can verify these variable tolerances with them that is probably best.
Regards,
Paul

thanks a lot for the reply
Wes Bucey : i was a student and graduated in december. Sorry i did not update my profile..
Can anyone suggest some good websites for learning GD&T..
Thansk once again
peach

thanks a lot for the reply
Wes Bucey : i was a student and graduated in december. Sorry i did not update my profile..
Can anyone suggest some good websites for learning GD&T..
Thansk once again
peach
Why not update your Profile today? Click on User CP on the left in the streamer of links at the top of the page.

See if your boss will spring for the Standard ASME Y14.5M-1994 available from the website of the American Society for Mechanical Engineering www.asme.org/ (http://www.asme.org/)

Good luck in your new career!

Why not update your Profile today? Click on User CP on the left in the streamer of links at the top of the page.

See if your boss will spring for the Standard ASME Y14.5M-1994 available from the website of the American Society for Mechanical Engineering www.asme.org/ (http://www.asme.org/)

Good luck in your new career!

Thanks for the suggestion, I changed my profile.
One of my friends has ASME Y14.5, i can borrow from him for couple of days.
thanks once again
:thanx:

----------------------
"Geometric dimensioning and tolerancing is a modern engineering drawing system which provides the means for stating on the drawing necessary dimensional and tolerance requirements not otherwise covered by implication or standard interpretation. It is a method of specifying tolerances of geometric form, position and orientation with respect to the actual function or relation of part features which can be economically produced.
Objectives
1. To introduce the methodology, symbols and principles in the American Drawing Standards ANSI Y14.5M.
2. To describe and interpret the use of geometric dimensioning and tolerancing on engineering drawings.
3. To assist the effective application of geometrics as a communication and technical tool in industry.
"
------------------------

----------------------------
"
History
Geometric Dimensioning and Tolerancing (G.D.T.) is a language of symbols. As the demand for parts manufactured around the world grew, so did the need for accuracy. Accuracy became more critical because of competition for parts and assemblies. The idea of positional tolerancing, which provided a means for locating round features within a round tolerance zone rather than the traditional square tolerance zone, was introduced in 1956 by American Military. Later, ANSI published a complete system of symbology for geometric form and positional tolerances called “Dimensioning and Tolerancing.”

WHAT IS GEOMETRIC DIMENSIONING & TOLERANCING ?
In particular, it is a means of dimensioning and tolerancing a drawing with respect to the actual function or relationship of part features which can be most economically produced. Function and relationship are the key words. In general, it is a system of building blocks for good drawing practice which provides the means of stating necessary dimensional or tolerance requirements on the drawing not otherwise covered by implication or standard interpretation.

WHY GDT?
G.D.T (Geometric Dimensioning & Tolerancing) adds clarity and contributes many advantages to our coordinate system for dimensioning. In current high technology world and the transfer of parts around the world, we must tolerate parts in a global technical language to understand, interpret and finally manufacture engineering parts.

ADVANTAGES
There are many reasons for specifying geometric tolerancing wherever design integrity must be controlled and communicated completely to others. Two key principles for applying G.D.T. are the function and the relationship of parts in an assembly. Probably, the most advantageous part of G.D.T. is the method of specifying feature location. Also, advantages of G.D.T. include international uniformity in describing designer’s intent.



FUNDAMENTAL RULES
Dimensioning and Tolerancing should clearly define engineering intent and should conform to the following.

1) Each dimension should have a tolerance, except for those dimensions specifically identified as reference, max., min., or stock.
2) Dimensioning and Tolerancing should be complete so there is a full understanding of the characteristics of each feature.
3) Each necessary dimension of an end product should be shown.
4) Dimensions should be selected and arranged to suit the function mating relationship of a part and shouldn’t be subject to more than one interpretation.
5) The drawing should define a part without specifying manufacturing methods.
6) Dimensions should be arranged to provide required information for optimum readability.
7) Dimensions and Tolerances apply only at the drawing level where they are specified.
"


Comments:
1. These are my interview notes, if in case someone asks me, I need to know at least this much. I am not sure where I picked it up from but my guess is isixsigma.com as a reference. I hope noone minds for putting up information from other website.
2. I will appreciate if someone can give me a real world example and show how useful/important it is. Thanks a lot.

Harsh Kumar

It will make you look a lot more knowledgeable if you have actually SEEN GD&T symbols and how they are interpreted.

Go to this site http://www.efunda.com/DesignStandards/gdt/introduction.cfm
and spend some time looking through the pages. (Navigation bar on left side of page.)

Any manufacturing company that makes "things" will expect all quality folk to be relatively conversant with GD&T, to the point of interpreting a drawing to make an inspection layout.

thanks a lot . I think i know the basics of GD&T. This variable tolerance is entirely new to me. I am still waiting for that SME book.
Thanks once again for all the comments and suggestions

Hello!
I am a student and learning GD&T. Would annbody help me how to calculate datum shift in an external feature of size when TOP callout with m modifier is used with feature control frame.
Thank you!

If you have a feature of size (pin or hole) that is external (pin) and it is reflecting in a feature control frame at MMC (M with a circle around it) here is how you can figure out the tolerance shift.

Add the tolerance (lets say 0.2 mm) shown in the feature control frame to the largest allowable size of the feature (OD of 10 +/- 0.3 for this example) is 0.2 + 10.3 equals 10.5 OD. This is called the "virtual condition size". Now subtract the actual size of the feature (let's say it is 9.95) and we get a diametrical tolerance zone of 10.5 - 9.95 = 0.55. Divide that by 2 and we now get the actual radial tolerance of 0.275.

If the pin is off position more than 0.275 from the theoretical centre, the feature is considered nonconforming.

Hope this helps.

Hello!
I am a student and learning GD&T. Would anybody help me how to calculate datum shift in an external feature of size when TOP callout with m modifier is used with feature control frame.
Thank you!

Great example of Datum Shift (http://www.tec-ease.com/tips/november-98.htm).

Stijloor.

I created the attached presentation to show how "variable limit tolerances" are typically addressed in continuous data inspections from the most rudimentary to the most rigorous.

Naturally an attribute gage is arguably the simplest way to check conformity while applying all available tolerance but many try to inspect these specifications without dedicated gauging and the results vary widely as illustrated.

Paul

8332

Hi:
I am new to this group. I have read a few posts. I am sure I am in the right
forum...

I have a question on calculating Total tolerance from individual components.

Here is an example:
let's say I am building a mini model of a building.

specifications for Block1= 1.4mm+- 0.02mm
Specifications for Block2= 3.6mm +- 0.05mm
Glue thickness= 0.40mm+- 0.01mm

I like to find the Maximum thickness of the stack of Block1+Glue+Block2.

I understand it is NOT a simple case of just adding up the "nominal" values
and then the sum of toerances..
Assuming Normal distribution for all the components, How to calculate
the maximum thickness ?

Thanks
padaki

Padaki,

Your question does not directly relate to this thread title but rather one dealing with the probability of extremes of stacked tolerances. A long long time ago a Designer/Engineer dealt with that question and published a SAE paper. His name is Arthur Bender...google him and read his paper! His recommendations have prevailed for many years in moderating the difference between arithmetic and the probable extremes of accumulated tolerances. It is only recently that his "safety factor" recommendations have been disavowed by some due to the stringent statistical controls on individual contributors of tolerance stacks.

The number of individual contributors in the statistically probable stack conclusion has always been a point of controversy, the more the better, consequently, the less the worse. Naturally the confidence in the predictions would suffer statistically if the number of contributors was reduced. Many require that there is a minimum number of tolerance contributors before a statistically stacked tolerance result can be considered trustworthy but few would venture as low as three as you described.

Paul

Paul:
Thanks a lot. Appreciate your quick response and recommendations.
Padaki

Hi:
I am new to this group. I have read a few posts. I am sure I am in the right
forum...

I have a question on calculating Total tolerance from individual components.

Here is an example:
let's say I am building a mini model of a building.

specifications for Block1= 1.4mm+- 0.02mm
Specifications for Block2= 3.6mm +- 0.05mm
Glue thickness= 0.40mm+- 0.01mm

I like to find the Maximum thickness of the stack of Block1+Glue+Block2.

I understand it is NOT a simple case of just adding up the "nominal" values
and then the sum of toerances..
Assuming Normal distribution for all the components, How to calculate
the maximum thickness ?

Thanks
padaki

Padaki,

This paper (http://adcats.et.byu.edu/Publication/91-1/DesRes_w_figs.html) may be of interest to you too.

Title: "A Survey of Research in the Application of Tolerance Analysis to the Design of Mechanical Assemblies."

Stijloor.

Stijloor:

Thanks a lot...
Interesting paper...

padaki