It’s common knowledge that parts can be combined to create ass emblies, but how many designers consider building an assembly to create a part? This approach may often yield a better, more-consistent product because it eliminates stress reactions that often result from machining a part where material removed from one side of the part is not equally offset from the other side.
In the days when manual machining was the norm, designers understood that creating a part which required a lot of unnecessary material removal meant he or she would be hearing from the poor machinist who had to make it. They, thus, limited such designs to situations where there were no other options.
However, today’s easy-to-use CAM programs have given many designers the impression that it’s best, and oftentimes easiest, to machine a part from a single block of material. Therefore, the cost of material removal is often overlooked in favor of creating easy-to-design parts. Such designs, however, may often result in unsatisfactory and expensive end products.
For example, consider the plastic part with four legs. It was originally designed to be machined out of a single block of material, with nearly 85% of the material removed from one side. The resultant part warped in two directions and the product yield was around 50%.
To address this problem, our shop redesigned the part to be made from five pieces and assembled them. We made the frame from a single rectangular piece of thin material, requiring little machining on the faces, and turned the legs from bar stock. The legs were then threaded into the frame. This approach upped the product yield to 100%, and slashed machining cost.
The same theory of creating assemblies for milled components can also apply to many turned parts. In one case, a bar-feed indexer originally called for machining two journals down to a 2-in. diameter on each end from a solid 6-in.-diameter billet of 4140 weighing 93 lb. The part included several impossible to reach areas that required additional milling operations. After our shop consulted with the engineer, it became evident that a weldment would be acceptable. In this case, our shop made the new indexer design from two parts, performed the milling operations before welding, and finish-turned the final assembly. This approach saved more than 60 lb of material and many machining hours.
The moral of the story? When designing a part, don’t be afraid to ask your machine shop for suggestions. Most shops are willing to help. Understandably, not all parts can be redesigned as assemblies. But those that can may make for a more consistent and less expensive part.