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Improving Manufacturability by Properly Applying GD&T as an Effective Design Tool

When creating or reviewing a design, here are some pitfalls and red flags to consider. A fairly common one is features being unnecessarily constrained to datums. Often a feature will be constrained to datums A-B-C, when the relationship of the feature to a particular datum or datums has no impact on the function of the part. This arbitrary constraint serves to tighten the tolerance and increase the difficulty of manufacture, which raises the cost of the part. In short, datum reference frames should reflect the function of the feature.

Features of size, whose positions are defined as Regardless of Feature Size (RFS), also often unnecessarily constrict the design and increase the difficulty and cost of manufacture, without improving the functionality of the part. These may be holes, pins, slots or other features of size which may or may not be datum features. All these features will have a size tolerance. If dimensioned properly, most often the manufacturing process may be allowed to take advantage of the size above or below the maximum or minimum material condition, without adversely affection part function or quality. MMC and LMC, (maximum and minimum material conditions), offer a very good opportunity to improve manufacturability and reduce cost.

GD&T provides many opportunities under the Y14.5M 2009 rules. Here’s a short list:

  1. Use datums of sufficient size to define critical functional features to which other features are related.
  2. Avoid defining radii of less than 180 degrees of arc as a size and position; rather, use profile tolerance.
  3. Use profile tolerance to define collections of arcs, lines, surfaces, etc., that are functionally one feature.
  4. Avoid constraining features to datums unnecessarily.
  5. When possible, utilize material condition modifiers, MMC and LMC on features of size, including datum features of size.

 

So, you have discussed these opportunities with your design team, and they have taken advantage of the GD&T potential to design a manufacturable part, while maintaining design intent and optimum quality. You have completed a major step toward improving the manufacturability of your product and increasing your profit, and maintained the quality of the product at the same time. Win/win/win, right?

Now your manufacturing team produces parts to the new improved design and delivers them to the quality department for verification. Here is where things often go off track.

Quality inspects the part on a CMM, the preferred method in most modern facilities these days, and rejects the part for being out of tolerance. Manufacturing is quite sure that the part is good and rechecks the part on a surface plate using conventional methods such as height gage, indicators, etc. The part appears to be in tolerance. Confronted with the evidence, the quality department recalibrates the CMM. In the meantime, manufacturing reviews their machine tool program, fixturing, tooling, etc. After calibration of the CMM is complete, quality re-measures the part, and it still checks out of tolerance, but the evidence of the height gage and indicator indicates the part is good. And on and on it goes. This scenario is played out countless times in factories everywhere, maybe even yours. The result is time lost in second guessing, recalibrating, adjusting processes and often the creation of an adversarial relationship between manufacturing and quality. What’s going on here?

4 COMMENTS

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Cyrus H. Bearden

01/07/2013 09:42 PM
Joe's Article is SPOT ON!!!...While reading this Article, I realized just how much time is wasted trying to second quess my results....SmartProfile is the Way to go!!!!

Thanks Joe!!!!
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