Known as the "Doctor of Dimensioning," Alex Krulikowski is a noted educator, author, and expert on Geometric Dimensioning and Tolerancing (GD&T). A design manager with one of the world's largest manufacturing corporations, he has more than 30 years of industrial experience putting GD&T to practical use on the shop floor.
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In an ideal world, inspection should not be necessary, and the goal should always be to minimize the need for inspection through the continuous improvement of processes.
Achieve Total Quality, 1992
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.
ETImail is now available in a printable PDF format. To read the PDF file, you will need Adobe Acrobat Reader.
Minding Mechanical Specs Pays Off in Production
When you specify electrical components on a drawing, you use electrical specs. Mechanical components need their own spec system that's repeatable, clear, and not overly restrictive. ASME Y14.5M-1994 is fast becoming the spec system of choice for mechanical components by today's OEMs. Two reasons for this preference are:
GD&T is an international language used on drawings to accurately describe a part. The language 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 set up. By providing uniformity in drawing specification and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the manufacturing process.
#1.The design philosophy of GD&T is that of functional dimensioning, which means that a part is defined by how it functions in the final product. Instead of copying a tolerance from an existing drawing, the designer bases the tolerance on what is needed for proper function. Assigning dimensions and tolerances based on part function allows the maximum amount of tolerance to produce the part. When properly applied, functional dimensioning can often double or triple the amount of tolerance on many component dimensions, which reduces manufacturing costs.
With coordinate dimensioning,
tolerance zones are not related to functional requirements. Problems
can result when designers assign tight tolerances because they
are not focused on determining a functional tolerance.
#2. GD&T allows round tolerance zones. In Figure 1, Arrow A points to a GD&T symbol that specifies a round tolerance zone (for the mounting holes). The zones specified by coordinate dimensioning (see Figure 2, Arrows A) result in a square tolerance. Round tolerance zones allow for 57% more tolerance than square zones, resulting in more usable parts. By allowing more tolerance on parts, the process will be more capable, reducing manufacturing costs.
#3. In addition to the tolerance gained from using round zones, GD&T allows a "bonus" tolerance under certain conditions. This bonus tolerance is gained by using the MMC (Maximum Material Condition) modifier, as indicated by Arrow B in Figure 1. The MMC modifier allows a hole to have additional tolerance when it is produced larger than its minimum size. This is a win-win situation for the OEM because engineering can be assured that the part will assemble when the holes are the smallest, and manufacturing can have additional tolerance when the holes are larger than their minimum size.
In coordinate tolerancing, the tolerance zone is always fixed in size (Figure 2, Arrow A), at all hole conditions. This results in a number of functional parts being scrapped and a more stringent condition for manufacturing. With bonus tolerance, more functional parts are used, and more tolerance is allowed for production, resulting in lower operating costs.
#4. GD&T's datum system communicates clearly one set up for inspection. Datums are theoretical planes, points, or axes, and are simulated by the inspection equipment. The symbol used to specify a datum feature is shown in Figure 1, Arrows C. These symbols denote which part surfaces touch the gaging equipment assembly requirements; they are often the features that mount and locate the part in its assembly. Datum reference letters are specified (see Figure 1, Arrows D) inside the geometric controls and denote the sequence in which the part surfaces contact the gaging equipment. This sequence is needed in order to have multiple inspectors set up the part in an identical manner.
#5. GD&T reduces assembly problems. Since the inspection process with GD&T ensures that parts will assemble properly, assembly methods no longer need to be addressed by the guy on the assembly line with a two-by-four and a hammer. The inspection process with GD&T ensures that OEMs can use competitive sourcing or obtain multiple sources for the same part, resulting in increased profitability.
#6. In the area of inspection, GD&T supports the use of SPC. GD&T's datum system provides the repeatable part measurements that are necessary for making a meaningful SPC chart. With coordinate tolerancing, SPC data may include assumptions which reduce the accuracy of the data.
The use of the profile control is another example of how GD&T supports SPC. The profile control helps in two ways: it establishes a mathematically-defined tolerance zone, and it relates the measurement to datums. When coordinate tolerancing is used, the precise tolerance zone definition simply doesn't exist. For example, try to define the size and location of the radius (Arrow B) in Figure 2.
Because the datum system and profile control allow SPC data to be more accurate, needless changes in the manufacturing process are avoided, rewarding the OEM with time and cost savings.
#7. GD&T is supported by national and international standards. ASME Y14.5M-1994 and a series of ISO standards rigorously document the interpretation of each GD&T symbol and concept. On the other hand, coordinate tolerancing is like folklore; it's not well documented, even though it has been around for 150 years.
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.
Standards in the News
ETImail's Standards in the News takes a look at real-life issues where standards have failed or need improvement. This month: Open source software finds creating standards critical to success.
LACK OF STANDARDS
BLOCKING OPEN SOURCE DEVELOPMENT
COMMON STANDARDS CALLED "A CRUCIAL AND CRITICAL MOVEMENT"
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 send your proposal to the ETInews discussion board, you could get other comments from knowledgeable people from other companies. Submitting your proposal to the GD&T discussion board will probably result in quicker feedback and could gain additional support from industry. You may want to use this method to refine your proposal before submitting it to the ASME Committee.
Alex's Tech Tip
THE TEN PRINCIPLES
TO PRODUCTIVE STUDY
1. Learning occurs in
small steps. Begin here and now--not tomorrow--to study
and to solve problems.
Although these principles
can work for you, you alone can decide to commit the time and
effort it will take to apply them.
Have comments about anything you've read in ETImail? ETI will post your comments here and provide a forum for more discussion about GD&T topics.
to the March 2002 article: "The 9 Myths of GD&T"
"There is a great deal of fact to this myth. GD&T and the ASME Y14.5M-1994 standard are very complex and can be very confusing to anyone who is unfamiliar with GD&T and the ASME standard or who has been poorly trained in it. But with proper instruction by a highly qualified instructor with excellent training skills a person can understand GD&T well and have his confusion relieved." Hampton Scott Tonk
to the February 2002 article: "The 9 Rules of Composite Position
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