ETImail is an online publication devoted to geometric dimensioning & tolerancing featuring a host of GD&T resources and links, as well as dimensioning tips by noted GD&T author and ETI founder, Alex Krulikowski.
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Alex's Tech Tip
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Recognizing the need is the primary condition for design.
When you specify electrical components on a drawing, you use electrical specifications. Mechanical components need their own specifications (or specs) using a system that's repeatable, clear, and not overly restrictive.
ASME Y14.5-2009 is the preferred dimensioning system for mechanical components used by most OEMs. Two reasons for this preference are:
An International Language
GD&T consists of a well-defined set of symbols, rules, definitions, and conventions that can be used to describe the size, form, orientation, and location of part features. GD&T is an exact language that enables designers to "say what they mean" on a drawing, thus improving product designs. Production uses the language to interpret the design intent, and inspection looks to the language to determine setup.
By providing uniformity in drawing specification and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the manufacturing process.
Consequently, they may be reluctant to fund GD&T training programs. The benefits of using GD&T to specify mechanical parts can readily be demonstrated when we compare GD&T to coordinate dimensioning, which has been used for well over a century.
Here are seven advantages that add up to significant savings in production:
#1. Design Philosophy
#2. GD&T Allows Round Tolerance Zones
#3. "Bonus" Tolerances = Hidden Savings
#4. The Datum System
#5. GD&T Reduces Assembly Problems
With GD&T, the inspection process ensures that parts will assemble properly, which means that assembly methods no longer need to be addressed on the assembly line. OEMs can use competitive sourcing for the same part, which results in increased profitability.#6. Statistical Process Controls
#7. GD&T is Supported by International Standards
Producing parts to GD&T's documented standards assures the OEM that parts will be accepted by the customer. Fewer replacement parts will be needed and recalls can be avoided, saving time and money.
Do you know if ASME plan to incorporate target holes in the new version of ASME Y14.5M-200x? We have target area, line & point. Some companies use target hole.
I have not heard any discussion on target holes at the Y14.5 meetings. Typically, targets are considered gage elements that are used as datum simulators to establish datums. Datum targets are often located with basic dimensions to ensure gage to gage repeatability. The point lines and areas can all be used on cylindrical shapes (including holes).
If you feel there is a need for a new type of datum target, I encourage you to write a proposal and send it to ASME for consideration by the Y14.5 Committee. When submitting a proposal to ASME, it may take 3 to 10 years for the new concept to appear in the standard.
Also, if you submit your proposal to ETI's Linkedin User Group, you could get comments from knowledgeable people from other companies, resulting in quicker feedback and potentially gain additional support from industry pros. You may want to use this method to refine your proposal before submitting it to the ASME Committee.
ALEX'S TECH TIP
There are two common — but conflicting — approaches used in industry: the manufacturing/inspection approach; and, the functional approach.
In the manufacturing/inspection approach, engineers believe the largest part surface should be the primary datum, the second largest surface should be the second datum feature, and the third largest surface should be the tertiary datum feature.
In the functional approach, engineers believe that datum features should be functional. Here are my thoughts on how to use the functional approach to select datum features on a part.
Using the functional approach, I categorize all datum features into two types: mounting datum features; and, performing datum features. Mounting datum features are the datum features that orient and locate the part in an assembly. Performing datum features are the part features that are used to define functional relationships.
Using mounting datum features to establish the principle datum reference frame ensures that the part is measured the way it is held in the assembly. As a result, the dimensional measurements are related to the way the part functions in the assembly.
The Mounting Datum Feature Rule:
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to "The 9 Rules of Composite Position Tolerancing"
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