Inspection 101: Defining your measurement plan

Samuel Weiss

You’ve designed your part, built the CAD, and now need to kick it off for tooling. You’ve drafted your drawing and completed your tolerance stacks. But there’s one more key step: you need to formulate an inspection plan.

At its surface, this appears to be as simple as adding inspection bubbles to your dimensioned drawing. But underneath, there is important deliberation about what, how, and how often to measure each dimension critical to your product’s function, reliability, or cosmetic appearance. This is the data that you will rely on to prove the stability of your manufacturing process and to solve problems quickly if that process drifts.

Measure what matters

There are two extremes to avoid when calling out inspection points on a drawing: underspecification and over-inspection.

Underspecification (about what to measure, where to measure, or how to measure) creates ambiguity and puts decisions about inspection out of your control — and into the hands of factory engineers or operators. The fact is that no one understands how your part integrates with the system better than you do, and your vendor won’t measure it unless you force them to. It is your responsibility as the design engineer to put a stake in the ground. This includes not just which dimensions to measure, but a full measurement plan to go along with them.

The mantra that has served me best is: measure what matters.

At the other extreme is over-inspection. As junior engineers at Apple, both Anna and I have released drawings with inspection bubbles on almost every dimension. That’s a newbie mistake, too, because measurement takes both time and money — over-inspection can delay your parts and create unnecessary fallout!

The mantra that has served me best is: measure what matters. Consider how the part fits into an assembly, its datums, its contribution to tolerance stacks, its user-facing cosmetics or function. Inspection bubbles in a drawing can apply not only to physical dimensions (distance, concentricity, roughness, etc.), but also to weight, material properties, and performance specifications. Some measurements may even require destructive testing. This is why it’s common to see inspection bubbles next to notes items on a drawing too.

Be specific

A dimension is meaningless unless you also specify how and where to measure. We’ve seen production drawings that indicate a cosmetic gap of 0.1 ± 0.05 mm. That’s great, but where on the part do you measure? (Can the vendor pick any measurement location?) And what measurement tool should be used? (OMM? Calipers? Feeler gage?)

First, define how to measure. What is the appropriate tool for the job? Your expectations should be clear to your vendor and in line with their available equipment and that equipment’s capabilities. If you have both outgoing quality control at a part supplier and incoming quality control at your assembly site, you should ensure measurement methods are consistent. If they aren’t, this may become a source of a multi-day delay in getting parts from one supplier accepted by another.

Second, clearly indicate measurement locations and datums on the drawing. If a cosmetic gap is to be measured at multiple points around the part, mark those points in the drawing, including reference dimensions between the measurement locations and part datums.

A dimension is meaningless unless you also specify how and where to measure.

Quick words of advice:

Asian vendors will know how to measure standard plan view dimensions, but if you use GD&T you may have to hold their hands through the learning process (or expect that these simply won’t get measured). And GD&T measurements like “position” (⌖) can actually make your data less useful. Instead, separate measurements from X and Y datums can give you more information when parts are “out of spec”. Keep it simple!

I also recommend that every inspection point is numbered on your drawing, including individual measurement locations for multi-location measurements. Numbering should persist through drawing revisions to help track dimension stability over time. Using the same numbers on mating parts is an advanced tip that will save confusion and headaches in the future.

Have confidence in your parts and process

Finally, you need to clearly specify how many parts to measure, how often to measure, and what are acceptable limits. Based on my experience, I would define three categories for appropriate ways to control part dimensions and performance:

SPC: Critical to Quality (CTQ) parameters are controlled by Statistical Process Control (SPC). These are dimensions that are critical to the function of your product, or that are part of a critical tolerance stack. Instead of spending exorbitant time on 100% inspection, a more efficient approach is to measure a statistically significant number of parts (see graphic), calculate appropriate summary statistics (e.g. Cp and Cpk), and set appropriate limits for each parameter (e.g. Cpk ≤ 1.33).

A sample size of at least 30 works well to qualify most well-behaved distributions. At this sample size, the sample mean is approximately normally distributed, and additional sampling offers diminishing returns in Cpk confidence.

FAI: Other important parameters are controlled by First Article of Inspection (FAI). These dimensions may be important for part fit or function, but are lower risk of deviation or impact than CTQs. They might include a stamped dimension with little variation, or a molded dimension with adequate clearance in the product. These dimensions can be inspected at smaller quantity and controlled differently. For example, you might specify that three pieces from each lot are measured, and require that all three pieces be within 66% of the tolerance limits.

AQL: Important cosmetic criteria, in particular, are often controlled by an Acceptance Quality Limit (AQL). You will define categories of defects, and set limits on the incidence rate. AQL tables indicate how many units to sample from each lot and the maximum allowable number of defects within those samples to meet overall quality goals.

Everything else: Any parameters or dimensions that aren’t measured are inherently for reference only. They should be marked as such on your drawing to avoid ambiguity. Some parameters may require a more specialized approach than the above, and that’s fine too.

Ensure that your own QA or Operations team is aware of the Critical to Quality measurements and how to measure them. And most importantly, it is critical to audit your vendor’s measurement process (either with onsite visits or measurement validation at Incoming Quality Control) to have confidence in the inspection process. Your vendor should send reports for each production batch, and they should be double-checked for any deviations.

You’ll thank yourself later

This all sounds like a lot of work, and it is. I have spent hours deliberating on my own or with my fellow engineers about how to dimension a drawing and how to control those dimensions. Carefully planning how your parts are inspected ensures that, when things go sideways, you’ll have the data you need at a quality you can depend on.

Want to go one step further? Contact us to learn more about how Instrumental helps augment your inspection plan.


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