Gauging the AR-15
Gauging is the other of the two subsets of metrology that we use when working with the AR.
Gauging involves the use of a standard comparator to evaluate a test subject. The direct output of gauging is a pass/fail condition and decision. As such, gauging is generally faster and more objective than measuring.
An example of a gauge is a pin gauge. A pin gauge is used to verify the size of a hole. It only has one value, and we use that value to check conformance of an object to key specifications. If the pin gauge is a GO gauge, it should fit into the hole; if it does not, the hole does not meet the minimum specification and, therefore, fails.
Gauge Error and Bias
Ideally, a gauge would be exactly what you want it to be, with no error. If a gauge has a nominal value of 0.2500 inches, it should be exactly 0.2500 inches, right? The real world doesn’t work that way. Even the most precise (and expensive…) gauges and standards have error.
Because gauges help us make decisions about an object, error in a gauge means that an incorrect decision can be made.
Good quality control practices would dictate that the manufacturer assumes the risk of incorrect judgement. This means that the manufacturer is willing to reject good product to avoid passing bad product on to the consumer, undetected. This is called “producer’s risk”, “Type I error”, or “alpha risk”.
When applied to gauging, this means that we should be using gauges that favor failing good product over passing bad product on to the consumer. From the gauge error angle, you need to position the instrument error such that the error bias will not contribute to passing bad product onto the consumer. This means that the potential gauge error should be fall within the specification range.
This idea sounds good to the consumer, but we are discussing this from the perspective of the producer. So, it requires a bit more understanding than “only sell me good stuff”. And of course, it is a difficult concept to visualize.
The concept is further complicated when we consider that there is likely a minimum and a maximum specification for every feature. We want a product that is big enough, but not too big.
The concept is even further complicated by the fact that “big enough” means two different things if you are measuring a hole versus a shaft.
We will present a focused illustration of measuring a hole with a pin gauge, and a shaft with a ring gauge.
Pin gauges and ring gauges are manufactured with directional bias. We often see tolerance/error represented as plus/minus (±) a value, meaning that the actual value could be a specified amount over or under the target value. Rather than having even error distribution equally around a target, the error is biased in one direction. The bias is represented by a plus (+) or minus (-) symbol in the gauge specification, following the gauge class; for example, a Z-class gauge with a positive bias will be listed as Z+.
- Plus gauge bias means that the actual value may be any value between the target and some amount of tolerance over the target (i.e. Target -0/+X).
- Negative gauge bias means that the actual value may be any value between the target and some amount of tolerance under the target (i.e. Target -X/+0).
Determining which bias is appropriate requires us to revisit the risk of incorrect measurement. Assuming we want to only pass conforming product (at the risk of rejecting good product), we want a very specific combination of gauge bias. We will examine these for both pin gauges and ring gauges.
Pin Gauge
Principle:
- GO gauge must fit into the hole in the test subject. If the GO gauge fits, the hole is big enough and the subject passes. If the GO gauge does not fit, the hole is too small, and the subject fails.
- NO GO gauge must not fit into the hole in the test subject. If the NO GO gauge does not fit, the hole is not too big, and the subject passes. If the NO GO gauge fits, the hole is too big, and the subject fails.
Appropriate Bias (Producer’s Risk):
- GO gauge with positive bias.
- NO GO gauge with negative bias.
The following diagrams illustrate the 4 potential pairs of conditions for a GO/NO GO pair. The blue object is the subject, and the white object is the gauge. The dotted line represents the nominal (target) value of the gauge, and the red bars represent the potential gauge error. The risk for each pair is listed under each diagram.
Ring Gauge
The principle of a ring gauge is the exact opposite of the principle of a pin gauge.
Principle:
- The test subject must not fit into the GO gauge. If the subject does not fit, the shaft is big enough, and the subject passes. If the subject fits, the shaft is too small, and the subject fails.
- The test subject must fit into the GO gauge. If the subject fits, the shaft is small enough, and the subject passes. If the subject does not fit, the shaft is too big, and the subject fails.
Appropriate Bias (Producer’s Risk):
- GO gauge with negative bias.
- NO GO gauge with positive bias.
The following diagrams illustrate the 4 potential pairs of conditions for a GO/NO GO pair. The blue object is the subject, and the white object is the gauge. The dotted line represents the nominal (target) value of the gauge, and the red bars represent the potential gauge error. The risk for each pair is listed under each diagram.
Gauge Class
Gauges often come in different classes, which correspond to the degree of precision. The higher the class, the more exact the nominal value is reported and the smaller the gauge error.
Pin gauges and ring gauges come in six classes, ranging from ZZ to XX. These classes have an associated acceptable tolerance/error (based on ANSI/ASME B89.1.5), determined by the nominal gauge size. Because all of our gauges fall within the first interval of ANSI/ASME sizes (0.010″ to 0.825″), we only need to focus on one set of class values. The allowed error (tolerance) for each class is listed in the table below.
Gauge Wear and Maintenance
Gauges are machined to very tight tolerances. They are generally made of stainless steel or a steel alloy.
Gauges will wear due to use. Sliding the gauge over other metal parts will eventually affect the actual value of the gauge. We recommend periodic verification of dimensions. Any gauges that are out of specification should be replaced.
Gauges may corrode, if not properly handled and stored. We recommend using a pin gauge handle for pin gauges to minimize transfer of moisture from your hands and fingers. We recommend a light film of oil on all precision gauges to prevent rust (which will affect the gauge dimensions). Corroded or rusted gauges should be replaced.
Recommended Gauges
Pin Gauge
Pin gauges are used to verify the diameter of holes. We use pin gauges to check the bores and holes of many parts of the BCG assembly and key holes in the lower receiver. We use Z Class McMaster-Carr pin gauges.
Ring Gauge
Ring gauges are used to verify the diameter of shafts. While ring gauges are faster to use than calipers or a micrometer, we actually use a micrometer for the few components that require us to verify shaft diameter (bolt tail, cam pin shaft, gas tube, etc.). We do have the ring gauges, but we like to have a measurement value for these components, rather than a simple pass/fail result. It is also worth noting that ring gauges are very expensive, so this may also factor into your decision to use them.
If you want to use ring gauges, we use McMaster-Carr X Class ring gauges.
Throat Erosion Gauge
A throat erosion gauge is a caliber-specific gauge that is used to measure the diameter of the barrel bore at the muzzle. It is a stepped gauge, where each step corresponds to a different value. The more a barrel is used, the more the muzzle will erode. If you start off with a barrel near the upper limit of bore diameter, the barrel will quickly erode beyond the limit and you will notice a deterioration in accuracy and precision of your shots.
We use the Brownells throat erosion gauge.
Barrel Straightness Gauge
A barrel straightness gauge is a precision rod that is passed through the barrel. If the rod binds, it implies that the bore is not machined straight.
The gauge may either be a standalone rod that is dropped down the bore, or it may be threaded onto a cleaning rod and then fed through.
We recommend the Pacific Tool and Gauge Bore Straightness Gauges. The come in 0.001″ or 0.0001″ tolerance. We find that the 0.001″ tolerance works fine.
Headspace Gauge
Headspace gauges are used to verify the depth of the chamber for a particular cartridge. These are precision machined to verify that a chamber, as measured from the bolt face to a datum line on the chamber shoulder, is long enough to allow the bolt to safely close, but not long enough to allow the case to expand and burst.
For incoming QC of barrels, we check headspace with a Pacific Tool and Gauge Extension Headspace Gauges, which have a removable star nut that mimics the bolt lugs.
We use primarily Forster Headspace Gauges for checking headspace with the intended bolt.
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