I work for a small design engineering company who occasionally farms out work in order to maintain our focus on the higher end work. Last week I received an approval package from one of our suppliers for a tooling package. I usually blow these off because it’s “just tooling” but this time I decided to take a closer look at the detail. What I found prompts me to write.
The tooling is for a composite lay-up mold. Our parts are usually very large and require extremely accurate molds to lay-up in. We don’t need a closed cavity like an injection mold or die-casting mold. I began by comparing my original model file to the cavity in the tooling. The part was most definitely in the tool, which would have ended my analysis, but I decided to check out how this tool was put together. The split lines for this tool were perfectly placed on the exact split lines of the part, however being an open tool with no closed cavities it would have been easy to make a straight split line outside of the silhouette line since we’d be removing those pieces to release the part, anyway. By having a curved split line the machine shop now has to contour the flange instead of making nice flat planes. This is an example of the power of 3D CAD creating this beautiful curved mating flange that deviated from a straight line by only 1 inch. What this designer didn’t account for was how much more work it was to machine that surface. Now, this supplier has sophisticated 5-axis machine tools so it’s their own nickel on the line to produce this geometry. One thing is for sure, those sides will only go on one way, but then a well-placed dowel or notch would also make assembly a no-brainer. Matching contoured split lines is harder than matching flat split lines.
When I got to the drawings I saw another example of automation gone amok. This same part has a line of symmetry that could be used to mirror mold pieces along a diagonal line. It’s not immediately apparent but certainly there when I designed the part. So, I checked to see if opposite parts were, in fact mirrored. Nope, the mold surface was mirrored (by design) but all holes, pins, fasteners and clips were unique to each mirrored part. Most 3D CAD will maintain associability between a mirrored part and its parent. That’s good automation. Even assemblies can be mirrored to good advantage. How about creating arrays of fasteners instead of individual instances?
When memory was scarce, rendering was done overnight and 3D was a fraud we took care to conserve resources not the least of which is our time to create efficient computer models and there was a sense of pride in getting a model to fit on a floppy disk.
Have we gotten lazy or are we just not paying attention to the basics of good design practice because we have the resources at our fingertips to create huge 3D models with no intelligence or economy? I’m not talking about trivia like schematic vs. modeled threads or weld callouts vs. actual beads. I’m talking about planning up front and thinking about that guy out on the shop floor that actually has to flesh out your work. Cut them some slack and knock off the fancy curves and give them 90degree angles not 90.22 degs (I’m not kidding), straight lines and round holes.
By Marinus B. (Ben) Bosma