Missing critical dimensions and tolerances is one of the major complaints against MBD in production, especially among suppliers. Some machine shops dismiss MBD as “irresponsible laziness” because their clients provided only 3D models with no 2D drawings, 3D dimensions or tolerances in the hope that machinists can measure models in SOLIDWORKS or 3D PDF directly.
However in reality, shop floors often times still need explicit critical dimensions and tolerances to machine and inspect, so some even end up creating 2D drawings themselves based on 3D models or 3D PDFs just to add and present necessary manufacturing information. Consequently, machine shops not only didn’t see any MBD benefits, they even felt disserved: “Maybe our clients saved some time because they didn’t have to generate 2D drawings, but the workload was simply thrown over the wall to us.”
Worse still, when the self-generated 2D drawings by suppliers mismatch 3D design intentions by clients, the finger-pointing games start: “You didn’t measure the model!” “You didn’t specify dimensions and tolerances!” It’s not uncommon to even go to court. But the truth is, whether we like it or not, machine shops’ business interests favor design interpretations that can simplify manufacturing, which may not be in the best interest of overall product function or quality. Therefore, to avoid conflicts of interests, it’s really vital to set the game rules straight. Calling out product and manufacturing information (PMI) explicitly is one of the key measures here. Let’s drill down further piece by piece to its detailed benefits and problems avoided.
1. It is the design team who knows best about product use cases, design intention, important features, and technical requirements, therefore, they should be the authority to convey these without any ambiguity. If these important callouts are missing and left free to other’s interpretations, then misunderstandings and even conflicts are prone to happen. Plus, it’s in production or suppliers’ best interest to interpret requirements in favor of manufacturing, but not necessarily the ultimate product quality.
2. Calling out PMI at Design is an opportunity not just to communicate, but also to review, check, and improve design and manufacturability. Skipping this step means missing this opportunity.
3. There could a be substantial amount of manufacturing information, even just critical ones, in a design. If downstream staffs always have to manually measure to get key ones each and every time, it not only takes time and effort, but also is easy to miss design requirements and manufacturing errors, because key information is hidden in the model. The consequences are jeopardized qualities, prolonged cycles, and increased costs. For example, suppliers would definitely bump up their quotes to compensate this operational overhead. On the other hand, key callouts not only ease the reading of each and every requirement, but also present a complete check list as a visual reminder.
4. Model geometries provide dimensions, but not necessarily tolerances. The lack of tolerances obviously is prone to misreading. The best case is shop floors can follow undocumented conventions to machine and inspect. Even so, real tolerance requirements are nowhere to confirm: maybe general tolerances are fine or maybe tighter requirements get simply ignored leading to rework or scrap. The worst case is even general tolerances are unknown, leaving shop floor clueless or complete freedom. Another possibility is design team defines both dimensions and tolerances in sketches or features. If so, we might as well just expose these callouts in 3D to avoid recreation and manual repeated look-ups.
Let’s take a break here before this blog gets too long. Next, Part 2 of this same topic will discuss additional benefits besides these above four, also the PMI schemas and use cases recommended in Military-Standard-31000A. To learn more about SOLIDWORKS MBD, please visit its product page. Also welcome to discuss with me at Twitter (@OboeWu) or LinkedIn (OboeWu).