Your Tolerancing Methods
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is the degree to which a specific product conforms to a design or specification.
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.
Is your goal to help your organization reach Tolerancing Heaven? Do you desire complete understanding of variation in your company's product development process? If so, allow me to inform you of a topic that you may not have considered that could significantly impact your ability to achieve your tolerancing goals.
The topic in question is measurement uncertainty. [See ASME B184.108.40.206-2001 Guidelines for Decision Rules: Considering Measurement Uncertainty in Determining Conformance to Specifications.] Measurement uncertainty always exists by nature, and controlling it may improve the quality of your products. Once you have achieved Tolerancing Heaven in an engineering environment [see ETImail, Vol. 1, Issue 8], tolerance specifications on produced pieces must be verified by inspection before they are sold to customers. Are the parts actually within the specified tolerance or are they outside of the specified tolerance range by a significant factor? Some parts that appear to exist in Tolerancing Heaven may be of low value to the customer. With some widely used methods of accounting for measurement uncertainty, parts can be measured and found to be within the specification, but in reality they exceed the specified tolerance by as much as 25-33%.
Not all sources of measurement uncertainty are easily identifiable. Measurement uncertainty includes all factors that lead to differences between an actual part distance and the measurement value obtained for that part distance. These factors may include gage accuracy, gage precision, dust, humidity, operator error, temperature, vague drawing specifications, and numerous other factors. Obtaining a specific uncertainty value for all of these is virtually impossible; however, the most significant contributing factors can be quantified. Generally in industry, there are well known methods for determining gage error, accuracy, and operator error, which usually accounts for the greatest portion of uncertainty.
2.4 of Y14.5 - Often Viewed with Two Paradigms
"To determine conformance within limits, the measured value is compared directly with the specified value and any deviation outside the specified limiting value signifies nonconformance with the limits."
This passage seems simple enough; however, as with many passages in the standard, the interpretation depends upon an individual's organizational background. The product engineer may have an entirely different view than the manufacturing engineer. We will explore how these differences can create a poor business condition.
"All dimensional limits are absolute. . ."
This leads to an understanding that the actual part produced will never physically exceed the limits specified on a drawing. When a system approach to tolerancing is used, the system's functional requirements are determined, and the allocation of tolerances in the system is facilitated by tolerance analysis. The maximum tolerance that allows the system to function as intended is specified to reduce cost and provide manufacturing with maximum flexibility. This approach is robust only when the produced parts do not exceed specified tolerance limits. If parts exceed the specified limits, the system may not function as intended.
The product engineering paradigm bases the part tolerance specification on the functional requirements of the system. The manufacturing organization determines the gage error value for the measurement system that is used to verify a part tolerance. The gage error is then subtracted from the specified limits to determine the acceptance limits. (See Figure 1.) For example, if a hole diameter specification is 9 - 10 and the gage error is 0.26 (0.13 for each end of the specification), the acceptance limits would be 9.13 - 9.87. When using this approach, product tolerance analysis, engineering analysis, and tolerance allocation practices ensure that the actual parts produced within these limits will function properly. However, reducing the functional specification limits by the gage error to establish the acceptance limits may increase manufacturing cost.
". . . the measured value is compared directly with the specified value. . . "
This leads to an understanding that gage error is additive to the specified value. Manufacturing measurement systems are generally evaluated for reproducibility and repeatability of measurement with respect to gage variation. A value is obtained that predicts-with a high degree of confidence-how much measurement error can be attributed to the gage.
The most common manufacturing practice used in industry today is to compare the gage error to the tolerance value on the part print tolerance that the gage element is designed to measure. If the gage error is within a predetermined percentage of the print tolerance, the measurement device is approved. After the gage is approved, it is used to determine acceptance or rejection of production parts. The most commonly accepted practice for % error is 25-33%. (See Figure 2.) For example, if a hole diameter specification is 9 - 10 and the gage error is 0.26 (0.13 for each end of the specification), the acceptance limits could be 8.87 - 10.13.
This approach decreases manufacturing costs; however, if a resulting measurement approaches the limits of the print specification, there is an increasing probability that the approved part is actually out of the specification limit. When using this approach, the product engineering tolerance analysis used to determine product tolerances does not reflect the tolerances produced on the finished part.
If the product engineering paradigm is used, the product function is protected. However, manufacturing must use a measurement acceptance zone that is smaller than the print specification. When purchasing production and measurement equipment, the systems must be designed appropriately.
If the manufacturing
paradigm is used, the product function is at risk. Product engineering
must account for additional tolerance in the tolerance analysis to account
for measurement uncertainty. In order to satisfy the system function requirements,
product engineering must adjust tolerances down to account for measurement
Excerpts from the Wired Magazine Website
A HISTORY LESSON
Today, according to the National Institute of Standards and Technology, there are close to 800,000 global standards. But go back a century and a half and you find an American economy in which there were literally none. On April 21, 1864, a man named William Sellers began to change that. Sellers initiated the first successful standardization fight in history, over the humble screw. That struggle was not just about a particular standard. It was about the importance of standardization itself. To win, Sellers relied on technical savvy - as well as political connections, clever strategy, and a willingness to put progress ahead of the self-interest of his own friends and colleagues.
On that April evening, a crowd of Philadelphia engineers and machinists gathered in the lecture hall of the Franklin Institute, the professional society to which they belonged. Sellers was the institute's new president, and they were there to hear him speak publicly for the first time. In the world of these men, Sellers was a legend, the finest tool builder of his time. After starting as an apprentice machinist at 14, Sellers had his own shop by the age of 21, and a decade later he was the head of the most important machine-tool shop in Philadelphia, the city at the center of America's machine-tool industry. If Sellers was going to insist that national standards were necessary, then it was definitely an idea worth taking seriously.
The speech, "On
a Uniform System of Screw Threads," played against the backdrop of
war between North and South, which added resonance to Sellers' call for
a national standard. "In this country," Sellers noted, "no
organized attempt has as yet been made to establish any system, each manufacturer
having adopted whatever his judgment may have dictated as the best, or
as most convenient for himself." At the time, American screws, nuts,
and bolts were custom-made by machinists, and there was no guarantee that
bolts made by shops on different streets, let alone in different cities,
would be the same. "So radical a defect should exist no longer,"
Yes. This is a good question. Unfortunately, the Y14.5 standard doesn't cover this type of application. Since it is not covered in Y14.5, you may get a number of different solutions based on whom you ask. Based on the drawing in the attached sketch, here is one possible way to specify the relationship of a center drill hole to the outside diameter of a cylindrical part.
Apply a position
tolerance to a circular element of the conical surface of each center
drill hole as shown in the attached sketch. The tolerance zone for the
circular element of the cone can be used to determine the
I have described one method to handle your application. Keep in mind, there are a number of ways this application could be toleranced.
My question is about a GD&T callout on an automotive component drawing. It is for a holes axis to be perpendicularity + Ø0.036 / 25 + plane datum A."
I am not sure the meaning of the part of --/25. Is it projected to 25 mm (but no P accommodated) or per 25 mm length of axis? Where could I find the explanation regarding this?
I think this may
be an application (or misapplication) of a unit per unit callout as described
in Y14.5 in figure 5-4 on page 160. The standard shows the concept of
unit per unit length used on straightness. Some companies have expanded
the concept to other geometric controls.
THE MOST OUT OF ELECTRONIC FILES
Companies are constantly
improving their systems. MatrixOne Inc., for example, recently updated
its newly released Matrix10 with a file-collaboration server that speeds
the response of distributed file systems by reducing the amount of data
sent over the network when a user initially requests a file.
on the 'Tao of Tolerancing'
My main comment is that I got halfway through the article before you made the clear distinction between variation (the thing that actual parts do) and tolerance (the thing people include in designs to accommodate the thing that parts do). Some of my audience don't yet have a grasp of the difference.
Also, I was hoping you would include a matrix relating functional tolerances to process capability: If capability is far tighter than functional tolerances then quality is good but product costly (choose cheaper process). If capability is slightly tighter than functional tolerances then quality is good and good value (optimum situation). If functional tolerance is slightly tighter than capability then quality is slipping (value MAY still be there) and if functional tolerance is far tighter than capability then quality is poor (find new process or redesign to increase tolerance to variation).
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