A big “Thank You” goes out to all who responded to my last info letter. From the feedback I have received, it seems that by including a drawing to really show what I was talking about, definitely helped to communicate the concept and so it was a lot easier to understand. I will keep that in mind for this and future letters. While I am talking about “you,” if any of you receiving this have any questions about anything design related, please ask away and I will possibly include it as a topic here. Of course, anything proprietary will not be disclosed.
Moving along on topics that I think are of interest to both clients and potential clients, let’s take a look at Design For Manufacturability (DFM) and a related topic, Design For Assembly (DFA). Recently some have chosen to combine them and they are now sometimes known as DFMA or DFM&A, so if you come across it, do not be intimidated by such a long acronym.
Basically, both are very simple concepts. However do not confuse simple with easy as there is a great deal of thought and work that goes into the evaluation that results in a solution that is simultaneously functional, robust and cost effective. That just gives you the part. It now has to be assembled correctly and in the correct sequence. What is the best assembly method and process, time-wise and cost-wise? How do you decide the best way to go? Can the parts be assembled incorrectly or in the wrong sequence? If so, in my opinion it is not a good design. (That is like the example of “lazy engineering” mentioned last week where the process changed from machining to investment casting a drive shaft, without changing the drawing to gain the advantages that are available with the investment casting process.)
Okay, let’s look at the DFM process first. In my opinion, one first has to have some idea of the anticipated annual volume of production of the part or final product and a general idea of the material to be used to make it. Immediately, one must consider the environment it will be operating in and the physical, thermal and chemical loads it will be subject to. Does the component or final product have an overall size envelope or weight target it must meet? How about a cost target? By the way, do not forget the various types of loads that are due to processes themselves, such as radiological or thermal sterilization procedures. Evaluate all of these simultaneously and you’ll begin to understand why I say it’s simple but not easy.
As a quick case in point there was a rather large casting that had several large webs (ribs) that ran across the width of it. The same feature had to be machined in each web. As a cost-saving change, instead of machining each web individually, it was decided to broach them all in one operation. (The broach in this case was about 30 feet long!) When this new process was first tried out, the broach wiped out all the webs, destroying the casting. After a thorough evaluation, it was determined that the best and most cost-effective solution was to thicken each web. The casting was initially designed to meet all foreseeable loads that might be expected in service, but the loads it was subjected to in the manufacturing process were greater. However, because the broaching process was so much cheaper and quicker, it was worth making the change to beef up the casting to withstand the broaching process.
I will continue this discussion in the next info letter. Let me just end off with an observation regarding this subject. Do not immediately conclude that your Design Engineer is goofing off just because he seems to be staring off into space “doing nothing.” He is most probably evaluating these very points in his mind, over and over and over again.