ISE Magazine Volume : 50 Number: 3
By Merwan Mehta
Concept can bring down costs and create better assemblies
One of the great advances that propelled the Industrial Revolution was the ability to design and manufacture interchangeable parts for commercially produced products. It is hard to imagine that during the Civil War period, if a musket needed a new trigger, the entire weapon had to be shipped back to the workshop.
Products during that time were all custom-made, which is tantamount to craftsmen and craftswomen creating unique parts and components, all masterpieces, that function as a product. This worked when demand was limited. It’s hard to visualize today that without interchangeable parts we might have to call a “crafts-plumber” who would have to take the leaking faucet to his workshop and fix it with specially crafted parts.
Initially, coordinate dimensioning was used to create designs to manufacture interchangeable parts. Coordinate dimensioning results in stacked tolerances and square tolerance zones for positioning of axes for cylindrical part features like holes and posts. Geometric dimensioning and tolerancing (GD&T) was created as a language to specify the required geometrical features and relationships for parts. GD&T has been standardized by the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO).
This has alleviated the prime deficiencies of coordinate dimensioning and created a homogenous language for creating part geometries, which helps make manufacturing and assembly easier throughout the world.
An opening to lower costs
Opening up tolerances has the effect of lowering manufacturing costs. This also allows the designer to provide all available tolerance on the part prints, negating the need by manufacturing personnel to come to the designer for a waiver or extra tolerance when parts don’t fit.
The ISO 1101:2004 standard has what is called a “reciprocity” requirement where the tolerance available for the dimension of the hole and the tolerance for the positioning of the hole are merged together, and manufacturing personnel are allowed to use the size tolerance for geometrical tolerance and, if needed, vice versa.
Go simple and standardize
Although the ISO standard through its reciprocity requirement has a means to achieve the goal of using the entire available tolerance spread and reducing manufacturing cost, the concept takes a little getting used to on the factory floor. Also, if a company is using the ANSI Y14.5M—2009 standard for GD&T, the reciprocity concept is not available in it.
For this, it is best for a company to resort to zero tolerancing at MMC and standardize its use by training its design and manufacturing personnel in how to use the concept confidently. Through this, a company stands to substantially benefit by not having to throw away their best parts, creating assemblies with the best fitting parts.
Having a standardized way to specify locating features can also cut down on chaos and unnecessary wasted time due to the need for manufacturing personnel to clarify exactly what the designers were striving to achieve in their designs.
The major push back to the use of zero tolerancing at MMC may come from manufacturing personnel, who might misunderstand that by calling out a positional tolerance of zero in the geometrical frame, the designers are asking them to produce perfect parts, which is not the case. Many forward-thinking companies are beginning to realize the strategic importance of using this concept in their design and manufacturing functions to get a clear edge on their competition.
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