Q: I work for a small medical device company with only four core employees—president, engineer, customer service and quality assurance (me). When we inspect incoming parts, the engineer creates the inspection sheets, identifying the critical features that are to be inspected.
This approach makes sense because the engineer is more familiar with how the instrument works and what is more critical. The technique used to measure each feature also is defined in the inspection documents created by this person. That’s what my question is about.
Sometimes, there are discrepancies between the method this person wants me to use and what I think may be a better way. Generally speaking, we have the same level of understanding when it comes to quality assurance practices.
My question is: Who decides what technique is used to measure features during an inspection: the design engineer or quality assurance?
A: My advice would be to focus on the technical situation. The short answer is that the best techniques, as determined by objective criteria and dialogue, should be used. Focus on evaluating alternatives, not on whose ideas they were in the first place.
The following are some criteria to use when comparing measurement techniques. Depending on how much detail you want to go into, a +/-/0 rating system (see Table 1) or a more complex decision matrix with weighted criteria can be used:
- Accuracy. What technique provides a measure closer to the true value?
- Precision. What measurement technique gives the most consistent result on the same part over and over?
- Sensitivity. Can differences be detected in measurements as small as one-tenth the total tolerance or process spread?
- Reproducibility. Can different operators at different times produce the same measured value?
There are other factors to consider:
- Can the measurement technique be calibrated, and how easily?
- How quickly and easily can the technique be learned and used?
- Can the measurement be automated, and does automation offer advantages?
- What are the costs of the measurement equipment and the execution of the measurement?
Details regarding the execution of a gauge repeatability and reproducibility study or attribute agreement analysis are outside the scope of this quick answer, but those are the tools you can use to better obtain decisions based on facts and data. In the end, there is no need to engage in a lengthy debate. Tools and studies can be designed that give all options a fair evaluation.
Senior manager, quality engineering and risk management
Q: If any stock quantity mismatches are found in the storage area during an internal quality audit, should this be treated as a nonconformance?
A: To answer this question, you need to understand two definitions from ISO 9000 on nonconformity (section 3.6.2) and requirement (section 3.1.2).
Nonconformity is defined as non-fulfillment of a requirement. A requirement is defined as a need or expectation that is stated, generally implied or obligatory.1 Making sure stock quantities match is generally implied if it isn’t stated in your documentation. Technically, this is a nonconformance. I also would consider the following to write a value-added nonconformance report.
- Allowance. Is the mismatch outside the allowance given by the organization on the accuracy of the stock quantity? Allowance may differ by the type of material or part. Precious, high-value material or critical equipment spares with long lead times are unlikely to have a large allowance compared with low-cost, easily available fasteners.
- Ownership. Is the mismatched stock customer-supplied material? If the mismatch is a result of lost material, then verify that the customer has been informed (section 7.5.4 of ISO 9001:2008).2 There may be a contract requiring a specific level of stock accuracy and a process for reporting discrepancies or missing quantities.
- Magnitude. Is this mismatch situation isolated or systemic? Is this mismatch across various materials? Review your historical records of cycle count reports.
- Impact. What is the impact to the business? Examples include production planning interruption, incomplete kitting sets or missing delivery commitments.
By probing the proposed questions, you’ll be equipped to write a substantive, value-added finding that is highly likely to get immediate management attention.
Director, quality assurance
San Jose, CA
- International Organization for Standardization, ISO 9000:2005—Quality management systems—Fundamentals and vocabulary.
- International Organization for Standardization, ISO 9001:2008—Quality management systems—Requirements.
Q: When writing calibration procedures, I need to specify the accuracy to which the gage should be adjusted. Is there an industry standard for this?
A: First, accuracy and precision need to be stated in the calibration procedure for the unit under test (UUT) or unit being calibrated.
The definition of calibration, according to the International Vocabulary of Metrology, is a comparison of two measurement devices or systems—one of known uncertainty (your standard) and one of unknown uncertainty (your test equipment/UUT).1
Note that adjustment, alignment and repair have nothing to do with the calibration process. "As found" readings of the standard and the UUT must be recorded, and "as left" readings must be taken if any adjustment, alignment or repair are performed.
Having said that, the accuracy—the tolerance of the UUT—is stated in the calibration procedure. This is usually derived from the manufacturer’s specifications for the UUT. You must keep in mind that the tolerances may be "loosened" or made wider by the calibration department or user. But the tolerances should never be tightened beyond what the manufacturer specified in the original specifications.
So, the simple answer is this: Calibration procedures should state the accuracy or tolerance for the UUT used during calibration. If the UUT is found to be outside those limits, then the UUT is adjusted to fall within those tolerances. Generally speaking, tolerances are taken from the manufacturer’s specifications.
Bucherview Metrology Services
De Forest, WI
- International Bureau of Weights and Measures, International Vocabulary of Metrology—Basic and General Concepts and Associated Terms, 2012.