Measuring the AR-15
Measuring is one of the two subsets of metrology that we use when working with the AR.
Measuring involves the quantification of a physical product’s attributes. The output of measuring is a numeric value that can then be compared against an acceptable range for that attribute.
A scale or balance is an example of an instrument used for measuring. An object is placed on the scale, and the scale reads a numerical weight. That weight can then be compared to the specified range to determine if it passes or fails this specification.
Instrument Readability or Resolution
Resolution describes the degree of change that can be detected by an instrument. The resolution of the instrument is very important when measuring against a specification.
If the specification is reported to 4 decimal places, we recommend that you use an instrument capable of reading to 5 decimal places, if possible. The last decimal place is the least significant figure and has the least confidence, so we try not to use it to determine conformance.
If the specification is reported to 4 decimal places and you use an instrument that reads to 3 decimal places, you cannot confidently assess conformance to the specification near the limits. To make a robust decision in this scenario, you would need to tighten the specification by 0.001 on each end of the range to have sufficient confidence near the specification limits.
Calibration and Calibration Verification
Instruments will drift out of calibration over time, especially with use. It is important to periodically verify calibration and, if necessary, to recalibrate your precision instruments.
Calibration verification can be performed using a known standard. If the reading obtained is not within an acceptable deviation from the known value, the instrument should be calibrated or replaced.
Some instruments are user-calibratable. Others can be sent to a calibration lab for verification/calibration.
To Round or Not To Round
When evaluating a measurement against a specification, you may face a situation where you have the option to round a value. If you are faced with a scenario where rounding makes a difference in the outcome, we ask you to consider the following:
Consider a part that has a lower specification limit of 0.1000″ (i.e. it may not be less than 0.1000″), specified to 4 decimal places. You measure the part using a micrometer with a resolution out to 5 decimal places (0.00001″). You measure 3 samples and get the following readings:
There are a couple of rounding options/scenarios here:
- As Read/No Rounding: If absolute reading is outside of the specified range (disregarding the significant figures of the specification), the part does not pass. This option is a bit purist, as it rejects product that could legitimately be passed.
- Rounding to Significant Figures: You could round your reading to the same decimal place as your specification. Conventional “half-up” rounding should be used (if the the digit to the right of the to-be-rounded value is less than 5, round down; if 5 or higher, round up). In our example, we would round the gauge reading to 4 decimal places.
- Selective Rounding/Rounding into Compliance: This option is for the unscrupulous or the oblivious. You could manipulate the number of decimal places in the specification to influence rounding of your measurement. In our example, if you were to treat the 0.1000 specification as if it was 0.1, you could round your reading to one decimal place (any reading over 0.05000″ would pass). This is obviously the wrong way to round.
The outcomes of these three scenarios can be summarized as follows:
You can see the correct outcomes in green. The yellow outcome represents the unnecessary rejection in Scenario 1. The red outcome represents the incorrect acceptance of a non-conformant part due to manipulation of the stated specification.
If measuring components against a specification, be sure to understand the implications of rounding practices on outcomes and decisions.
Recommended Instruments
Micrometer
We use a micrometer for very precise and resolved measurements (e.g. bolt tail diameter). A micrometer is a great tool for QC and troubleshooting. We recommend digital micrometers, because they take the reading error out of the equation. We recommend an instrument with resolution out to one hundred thousandth of an inch (0.00001″).
Vernier Calipers
We use vernier calipers to check lengths and diameters, where the precision and resolution of a micrometer is not warranted. We recommend an instrument with resolution out to one ten thousandth of an inch (0.0001″). Look for calipers that can measure internal (e.g the diameter of a hole) and external (e.g. the diameter of a shaft) dimensions. If you are measuring the length of bolt carriers, you will need larger calipers that can handle 7″ measurements (most only go to 6″).
Depth Gauge
We use a depth gauge to measure things like bolt face depth and firing pin protrusion. We recommend an instrument with resolution out to one ten thousandth of an inch (0.0001″). We also recommend threaded attachments to fit a variety of applications.
Trigger Pull Gauge
We don’t routinely check trigger pull weight. But some may want to. We recommend a digital gauge to eliminate the reading error of an analog instrument.
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