There were a lot of comments last week to the article we posted with the title: Only 17% Percent of Companies Meet Product Cost Target
Many people complained about the dearth of knowledge of the design engineer in Design for Manufacturability. In the discussion, we also started to propose some solutions to overcome this problem. However, one comment that sparked my interest was a comment about WHY design engineers often overtolerance parts that went beyond “they don’t know any better.” The comment paraphrased was:
A big problem we have is that we are making parts directly from the CAD model. A lot of Catia based models have a general tolerance of +- .005 [in.] on the entire part .including fillet radii and edge breaks. …these features have to be penciled off with a ball end mill instead of using a standard tool with a radius on it can kill profit on a job when you miss it when quoting.
That is a fascinating observation. There is no doubt that the Product Lifecycle Management companies will be pleased as punch that people are finally taking their drum beating on “model is master” seriously. FYI – I agree that the model should be master and that drawings should be generated from the 3d master data. However, this improvement to PLM adherence highlights what happens when new technology (a tool) is foisted upon a problem without without understanding the current processes and outcomes that the incumbent tool is satisfying. In this case, the old tool is paper drawings. With the incumbent tool, there was a title standard block that for companies, and that title block would give helpful bounding constraints such as:
Unless otherwise specified:
All dimensions +/- o.o1 inches
All radius and fillets +/1 0.02 inches
That helpful and protective title block may not be there with a 3d, model onl,y strategy. All the evangelism on “tolerance by exception” goes right out the window what the CAD system now has default values that are overtoleranced by definition. The CAD system itself becomes… The Evil Robot Overtolerancer.
The good news is that the Evil Robots can be controlled, and you don’t even need anyone named Yoshimi to help you do it. However, it will require some thought, before you change the default tolerances in your CAD system. Some considerations to think about are:
- What were the default tolerances in the title block on your drawings when the drawing was master?
- Can these tolerances be reduced?
- How surgically will your CAD system allow you to set default tolerances?
- Do you need different tolerence ‘templates’ depending on the primary manufacturing process. E.G. tolerance defaults may be very different for a casting that is machined than for a piece of sheet metal.
- How will you make your design engineers aware of these new default tolerances?
Whatever decision you make, make sure all the right people are at the table to make it together, including design engineering, the drafting team (if separate from design), purchasing, and manufacturing (including suppliers, if parts are made externally). If done thoughtfully and correctly, the setting of default tolerance will bring the Evil Robot Overtolerancer under control. If these changes are made in a vacuum or carelessly, you may find that you have made the Evil Robot 10x more powerful as an agent of chaos and profit destruction.
You want to be dealing with the friendly Autobots, not the Decepticons, right?
That’s today’s Product Cost Killing Tip!
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