If all components of an assembly could be produced and joined with perfect repeatability and accuracy, the task of assigning tolerances would be simple. However, each manufacturing step introduces its own variability and with it, potential tolerance problems. For instance, molded-plastic part dimensions vary with processing fluctuations. Stamping and machining create part-to-part differences in metal components. Assembly steps such as positioning, guiding, indexing, fixturing, and welding present additional sources of variability. When developing part tolerances, consider the following:
Exercise discretion when assigning tolerances between the components and assembly processes. Give the tightest tolerances to the part, feature, or process that adds the least cost to the entire process. It may be more economical to loosen the tolerance on the plastic component and tighten the tolerance on the assembly procedure or mating components. Consider all the sources of variability and optimize tolerances for the lowest overall cost.
- Avoid specifying arbitrarily tight tolerances to components and the assembly process, as it can add needlessly to costs;
- Accommodate part and process variability in your design;
- Include design features such as slotted holes, alignment features, and angled lead-ins to lessen the need for tight tolerances;
- Take advantage of the ability of the injection-molding process to mold small features with excellent repeatability; and
- Avoid tight tolerances on long dimensions and on features prone to warpage and distortion.
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