Improving E-Learning Outcomes
This feature article explains how to realize the full impact of e-learning by establishing the way in which learners digest information through this relatively new medium.
Virtual Measurement There's a new line of thinking that says it's not enough to know that you're producing the correct part when all is said and done. Nowadays, manufacturers and customers want to understand more about the entire process, find out where the snags are, where measurements errors take place, learn the limitations of the process and how to work around them.
Shawn Ryan looks at this trend in manufacturing in the NDX.com article archives.
Advanced Concepts of GD&T Textbook
GD&T Trainer Makes Learning Fun
GD&T Instructor's Kit Goes Digital
ETI Web Special
New Fundamentals Class Begins Jan. 2003
Effective Training is offering a Fundamentals of GD&T course at their Westland, Michigan, location. This evening class begins the last week of January, 2003.
ETI Offers On-Site Training
ETI's Employment Opportunities Board
Stay up to date on the latest industry news with the ETI Tech Calendar. Click here
If the process is right, the results will take care of themselves.
--Takashi Osada, from The 5 S's: Five Keys to a Total Quality Environment. (Tokyo: Asian Productivity Organization, 1993)
ETImail is a regular online publication devoted to Geometric Dimensioning & Tolerancing. Each edition features a host of GD&T resources and links, as well as dimensioning tips by noted GD&T author and ETI founder, Alex Krulikowski. We also invite you to visit our website, etinews.com. To view past issues of ETImail, see the archives.
This five-part article covers my experiences, thoughts, and beliefs on tolerancing. It is based on observing how many organizations around the world currently handle tolerancing and how I believe tolerancing can be handled in a far more successful way in industry. I believe that using the tolerancing methods discussed in this article can save as much as 30% of part costs.
Engineers often believe the tolerance should be related to the functional requirements of the part, and the manufacturing community often believes that the amount of tolerance should represent their process capabilities. This results in another tolerancing controversy.
At the heart of this controversy, is one simple question: Should tolerance values be based on manufacturing capability or functional requirements? There are stalwart believers on both sides of the argument. Some even believe a combination of both methods should be used to establish part tolerances. (Looks like a "peace-keeping" compromise to me; the kind that will ultimately lead to failure!)
I discourage compromising tolerances, but input from manufacturing is encouraged. Part of the SACD philosophy is to structure component tolerances to optimize the system. Functional requirements usually translate into system or subassembly tolerances, within which the individual component tolerances can be derived.
As an example, let's look at the flatness tolerance on two parts with nominally planar surfaces. The parts mate together with a gasket between them to create a joint that has a function to seal. The gasket thickness and its compressibility are major factors in establishing the amount of flatness tolerance that the parts may have to ensure the gasket will be compressed when the parts are at their extremes. (The method to calculate the flatness tolerance values is shown in Chapter 12 of my book, Advanced Concepts of GD&T.) The analysis of the functional requirements of the gasket joint results in a total amount of flatness tolerance that can be distributed between both parts. The flatness "budget" could be allocated between the two parts according to the process capabilities, so that there is input from manufacturing without a compromise.
The Ultimate Goal
Cell I shows products that have a low cost to manufacture, but do not function well. In this case, the part manufacturing may be profitable, but the customers will not be satisfied with the product, so sales will diminish to the point of failure for the company.
Cell II shows products that have a high cost to manufacture, but do not function well. In this case, the amount of customers will diminish. They will not pay a higher cost for a poor product. The company will struggle in Tolerancing Hell and eventually fail. (For a definition of "Tolerancing Hell," see Part I of the Tao of Tolerancing.)
Cell III shows products that function well but have high manufacturing costs. Customers will love the way the product works, but sales profits will be poor because the product manufacturing costs are too high. The company will struggle in Tolerancing Hell and will eventually fail.
Cell IV shows products that are a good value to the customer. I think we all agree that if a company manufactures a product that functions well, and has low manufacturing costs, customers will want the product and the company will prosper. The customers will love the product and sales will be strong because the product will be a good value.
When tolerances are based on manufacturing, one is constrained to the manufacturing method from which the tolerance is derived. When tolerances are based on functional requirements, flexibility is available to select different manufacturing methods.
Tolerance Values Based on Function or Processes?
If product tolerances are based on the manufacturing processes, the company will always end up with the results in cell I, II or III. If the tolerances are based on manufacturing processes (optimizing manufacturing) and are looser than required for function, this will result in products that will be easy to manufacture and will function poorly. Thus, the company will have the results illustrated in cell I.
If product tolerances are based on manufacturing processes and are tighter than they need to be for product function, the parts will be difficult to manufacture and may or may not function as intended. The results are reflected in cell II or III. Surprisingly, many times a manufacturing engineer will request a tolerance based on a process that is tighter than it needs to be for product function. This is costly and does not necessarily result in a better functioning part.
If product tolerances are based on functional requirements, it does not automatically mean the company will prosper. Basing product tolerances on functional requirements can result in a company having the results in cell III or IV.
If the tolerances are based on functional requirements, but not assigned properly (due to poor processes, skills, or compromises) the organization will end up with the results in cell III. Having the results from cell III is still a state of Tolerancing Hell, and will lead to struggles and eventual failure. As the organization improves its abilities to assign tolerances, it may receive the results shown in cell IV.
If a company produces products with the results reflected in cell I, II, or III, it will struggle in Tolerancing Hell and eventually fail. As the ability to use the system approach to component design increases, the company's results will improve and reflect those in cell IV, and the company will prosper.
Optimizing Function and Cost
The optimum part tolerance is the tolerance that is not too tight or too loose. The optimum tolerance is the tolerance that protects the part function without unnecessarily restricting the manufacturing process.
When tolerances are based on manufacturing processes, it is difficult to produce good value products; however, when a company bases tolerances on the functional requirements of the product, it can produce products that are a good customer value.
I will close by summarizing the Tao Tolerancing Principles (TTPs) covered in this part of the article. (TTPs 1-4 are covered in Part I of the article. TTPs 5-9 are in Part 2.)
TTP #10 - The amount of tolerance on a part directly affects the costs to produce the part.
TTP # 11 - If part tolerances are too loose or too tight your company will fail.
TTP # 12 - It is the ultimate goal of all companies to make a profit.
TTP # 13 - The optimum tolerance is the tolerance that protects the part function without unnecessarily restricting the manufacturing process.
TTP #14- If a company bases tolerances on the functional requirements of the product, it can produce products that are a good customer value
Next Issue: Part 4 - Tolerancing Heaven: a plan for how to lift your organization from Tolerancing Hell to Tolerancing Heaven.
POPULARITY OF ROCK CLIMBING CREATES NEED FOR STANDARDS
"Compared with the countries where this sport has had a long history, China is still lag[ging] behind in terms of standard and development in general. We are some 30 years behind the pioneering Europeans," said Li Zhixin, director of the MSAC and executive vice-president of the CMA. Full Story
On page 15-3 of Advanced Concepts Of GD&T, Alex states that the basic dimensions used with datum targets have a gagemakers tolerance. My question is: where are we getting this tolerance from? If the dimension was not basic and was not toleranced, we would have to go to the unspecified
SOLVING THE PROBLEMS OF MEASUREMENT UNCERTAINTY
This standard is also recommended reading for designers and engineers. The book is only 31 pages long, but contains many useful ideas on measurement planning, such as:
This book is an important addition to your reference library.
Comments from Canada:
I only found this page on the Web purely by accident - I was browsing for some inspiration due to several hours of frustration this morning. It was spent trying to interpret a drawing in response to one of our Inspectors plea for help " What does this mean?/How do I measure this part/How do I interpret the results in relation to the GD&T? (etc etc etc). The print has quite a number of GD&T's with composite position tolerances applied to threaded holes, seating faces and a number of other "port holes" that are perpendicular to each other and meant to be parallel with the seating faces.
Three hours and 4 people later, we believe we had an idea of what the drawing intended. I am still not convinced that we got it right.
When I saw the line "Tolerancing Hell", I immediately felt at home reading the article. ASME Y14.5 is the standard that is used at this facility and we still have difficulty with practical interpretation. When you get, "I think it means this, or I reckon it means that" then there is scope for much misinterpretation.
As mentioned previously, I look forward to the next three installments of this article. I live in hope!
We would appreciate it if you'd keep us in mind when you need GD&T training, consulting, or GD&T products. Feel free to contact us by email or by phone at 734-728-0909 or 800-886-0909.
The contents of this newsletter may not be reproduced in any form without express permission from Effective Training Inc.