Editor’s Note: PMPA Speaking of Precision would like to thank CR4 Engineering Forum, geanorm,and PJ Sikorsky for contributing this blog entry which originally appeared here.
|I can’t begin to remember how many times over the past 30 years I’ve felt the bile rise in my system as I’ve heard design engineers suggest that it doesn’t matter what material(s) are used to build parts and components. I suppose it’s natural for any of us to feel unloved and underappreciated in our professional endeavors. As a materials engineer working for two different major manufacturers over my career I often felt that materials selection was not given as much attention during the design process as it deserved. There are 5 repetitive problems I have observed related to material(s) selection over my career:1. Material Selection after the part/component is ‘designed’.
2. Material Selections not reviewed over time.
3. Materials copied from old designs.
4. Material substitutions fail.
5. Bad material selections – the gift that keeps on taking.In the remainder of this blog post I’ll briefly describe each of these problems. I’ll devote subsequent posts to each of these issues individually in more detail.
1. Material Selection after the part/component is ‘designed’.
Many times a design engineer comes to the materials engineer after lines are committed to paper and the part is ‘designed’ – the question often is ‘Okay, what should we make this out of?’ The shortcoming of this approach is that once parts are designed, materials options are limited. Done right, Material Selection is integral with the design process and parts are designed around specific material characteristics.
2. Material Selections are not reviewed over time.
Even when Material Selection is done right, the materials world is a dynamic place and what makes sense today, may not make sense tomorrow. For many years copper was less expensive per pound than aluminum (for most of my career, copper was $0.65/pound while aluminum was around $1.00/pound), today copper is 4.5 times more expensive than aluminum per pound – does copper still make sense for heat exchanger tubing or headers or connecting tubing now that aluminum is so much less expensive? Maybe, maybe not, but it is obviously a different question now than it was before.
3. Materials copied from old designs
This is similar to #2 above, but it also can create problems for other reasons: What is the right material for a one pound widget may not be the right material for an ‘identical’ 10 pound widget – material properties do not necessarily scale with the geometry of a part. Where a part is manufactured may impact the material selection – what makes sense for a part made in the US, may not be appropriate for a part made in China.
4. Materials substitutions fail.
Whether we try to convert parts from metal to plastic, or from metal castings to powdered metal, materials substitutions fail. The primary reason is that when materials change, part design needs to change. A plastic gear, even though it may serve the same function as a metal gear, if designed properly, should look different than a plastic gear. If we simply try to drop in a plastic material into the same geometry as a part that was originally made from metal we are almost assured of failure.
5. Bad Material Selection – the gift that keeps on taking
Oftentimes during the development process, problems are discovered too late in the game to ‘redesign’ parts before going into production. Instead, we call on some materials ‘magic’ and we implement special coatings, or heat treatments or exotic materials into the existing designs just to get us into production. Our intentions are always good – we say we’ll come back later and redesign the parts so that we can revert to more standard/affordable materials – but the reality is that we rarely have the time to go back and redesign and instead we pay for the premium material throughout the life of the component design, and in those cases where we simply copy material selections from old designs into new designs, we pay the premium forever.
With raw material costs accounting for 50-80% of piece part cost it seems intuitive that choosing the ‘right’ material is critical to managing cost, yet we often treat this part of the design process as an afterthought. – PJ Sikorsky
Thanks, PJ for this excellent post.