By Michael Gwyn, Advanced Technology Institute, North Charleston, South Carolina and Alfred Spada, Executive Editor 

How do you spot a conversion opportunity for casting? On the floor of your manufacturing and assembly operation, you undoubtedly have seen components currently made up of several stamped, wrought or machined metal parts. Could they be redesigned to a single cast metal component for improved performance? How do you determine if the potential performance gain or cost savings would make the redesign viable?

The choice of whether a component is best manufactured as welded, assembled, machined or cast component is based on the component’s geometry, production costs and requirements in application. This article looks at these issues and provides a framework for analyzing weldments and assemblies as possible conversion candidates.

When to Look to Change

The focus for design engineers often is time to market. Because of delays elsewhere, components often are designed to be manufactured by the process(es) that will deliver them with the shortest lead time. This leads to OEMs designing and manufacturing weldments and assemblies of weldments in-house, utilizing their available capacity and labor and eliminating the demands of sourcing a component (such as design changes and pattern costs). Design engineers generally feel more comfortable designing “building blocks” of simple wrought shapes. This is the method commonly taught in structural design, and manufacturing engineering and purchasing colleagues are comfortable with bringing weldments and assemblies of weldments into production.

This D357 aluminum alloy main landing gear dock door uplock support sheet metal assembly for the Boeing 777 aircraft (shown installed at left) was converted to a one-piece casting (center) for the 767 aircraft. The conversion eliminated 27 part numbers and reduced internal manufacturing flow by 65%.

Another quick-to-market option is to hog-out (machine) a shape from stock to produce a required component, simplifying the sourcing process to a one-step machining operation. Although both approaches deliver the product to the customer in a time-efficient manner, they may not deliver it in the most cost-effective manner, particularly when volume increases.

Weldments and assemblies of weldments create a significant volume of part numbers to produce (or buy), schedule and track. They require high levels of labor to fixture (weld, bolt, align, etc.) and typically aren’t as dimensionally or structurally consistent due to the inherent variances in manufacturing.

On the other hand, hog-outs have the advantage of known wrought stock mechanical properties (making them a popular choice for critical aerospace applications) but suffer from high costs because a large percentage of the bar stock they are borne from ends up on the machining room floor. Because mechanical properties of casting alloys are not as well defined nor as widely available in properties literature (casting’s mechanical properties depend on the process and involve liquid flow and solidification gradients), some structural designers feel more comfortable with wrought metal properties. As a result, however, the opportunity to learn more about the casting structural properties is often missed, along with the prospects of lighter, stronger and more cost-effective parts.

In prototype situations when time is of the essence, weldments, assemblies and/or machining are sometimes selected over casting to bring the product to market quickly. The key is to convert these fabricated components to the most efficient manufacturing method once volume production is ready to begin.