Understanding the realities of manufacturing process variation – also known as “variational behavior”– is critical today due to the ever-increasing globalization of product development and continually tightening time-to-market pressures. With less time and opportunity to build and analyze physical prototypes, today’s web-based, globally dispersed design teams are instead using virtual prototypes to drive innovation and make informed product development decisions. Critical to this process is the design engineer’s ability to first predict how the CAD model will beaffected by manufacturing variations, and then to set tolerances accordingly.

Setting tolerances manually, however, is easier said than done. The easiest method is for the design engineer to apply standard tolerances wherever they exist. Standard tolerances are typically associated with common parts and assemblies. But this practice can lead to trouble later on, either in later design steps or when the model is turned over to manufacturing.

First, the standard tolerances simply may not fit into the final dimensions of the product-to-be. This might force the designer to make compromises as the model or assembly grows, and these might either compromise the quality – or delay the time-to-market of the product. Secondly, using standard tolerances can prove costly for manufacturing if, for instance, the standard tolerances are tighter than necessary for this particular product. Too-tight tolerances may force Manufacturing to use specialized machinery; if the designer can relax the tolerances – but still keep them within the product’s specifications– then Manufacturing can machine the part using less expensive machinery.

Instead of going with standard, generic tolerances, the designer should therefore set tolerances only as precisely as required: too tight, and you may incur higher manufacturing costs or higher scrap percentages than necessary; too loose and the product, while manufacturable, may not satisfy its end-user requirements.