|
Page 5 of 10
Design of Junctions
A junction is a region in which different section shapes come together within an overall casting geometry. Simply stated, junctions are the intersection of two or more casting sections. Fig. 4 illustrates both “L” and “T” junctions among the four junction types, which also include “X” and “Y” designs.
Designing junctions is the first step to finding castable geometry via the six-faceted system for casting design. Fig. 4 illustrates that there are major differences in allowable junction geometry, depending on alloy shrinkage amount and pouring temperature. Alloy 1 allows abrupt section changes and tight geometry, while alloy 3 requires considerable adjustment of junction geometry, such as radiusing, spacing, dimpling and feeding.  Fig. 4
Considerations of Secondary Operations in Design
System-wide thinking also must include the secondary operations, such as machining, welding and joining, heat treating, painting and plating.
One aspect that affects geometry is the use of fixturing to hold the casting during machining. Frequently, the engineers who design machining fixtures for castings are not consulted by either the design engineer or the metalcasting engineer as a new casting geometry is being developed. Failure to do so can be a significant oversight that adds machining costs. If the casting geometry has been based on the four casting characteristics of the alloy, then the designer knows the likely surfaces for riser contacts and may have some idea of likely parting lines and core match lines. These surfaces and lines will be irregularities on the casting geometry and will cause problems if they contact fixturing targets.
It is best to define the casting dimensional datums as the significant installation surfaces, in order of function priority, based on how the casting is actually used. Targets for machining fixtures should be consistent with these datum principles.
There is nothing more significant in successful CNC and transfer line machining of castings than the religious application of these datum fixture and targeting principles.
Drawings and Dimensions
The tool that has had the most dramatic positive impact on the manufacture of parts that reliably fit together is geometric dimensioning and tolerancing (GD&T), as defined by ANSI Y14.5M—1994. When compared to traditional (coordinate) methods, GD&T:
• considers tolerances, feature-by-feature;
• minimizes the use of the “title block” tolerances and maximizes the application of tolerances specific to the requirement of the feature and its function;
• is a contract for inspection, rather than a recipe for manufacture. In other words, GD&T specifies the tolerances required feature-by-feature in a way that does not specify or suggest how the feature should be manufactured. This allows casting processes to be applied more creatively, often reducing costs compared to other modes of manufacture, as well as finish machining costs.
GD&T encourages the manufacturer to be creative in complying with the drawing’s dimensional specifications because the issue is compliance with tolerance, not necessarily compliance with a manufacturing method. By forcing the designer to consider tolerances feature-by-feature, GD&T often results in broader tolerances in some features, which opens up consideration of lower cost manufacturing methods, like castings. Fig. 8 illustrates GD&T principles applied to a design made as a casting. Note the use of installation surfaces as datums and the use of geometric zones of tolerance.
|
