Why Tubing Measurement Methods Matter
An area of potential confusion between suppliers and customers of manufactured products is which measurement methods are used to determine whether parts meet specification (conforming) or fail (non-conforming) on stainless tubing products. If the manufacturer uses the hypothetical method “A” to inspect a given feature and the customer uses method “B” to inspect, it’s important to identify the strengths and weaknesses of each measurement method, as well as normally accepted industry standards, to ensure that the companies agree on a common way to inspect parts. Without using a common inspection method, it’s possible that one party may consider parts to be conforming while the other party finds the same parts to be non-conforming. Some examples of different measurement methods and the possible conflicts follow below:
OD Measurement method: Contact vs. Non-Contact micrometers
Our standard is to measure stainless (304 AND 316) and nickel alloy tubing OD with high-speed laser digital micrometers (non-contact), which is the most accurate way to measure the OD of a tube. These highly sensitive instruments have alarm-setting functionality for the tolerances/parameters, further enhancing the accuracy and limiting human error.
Non-contact accuracy is ±.000039” whereas contact measurement (hand micrometers/calipers) is accurate at best to ±.0001” for typical instruments. Not only do non-contact methods eliminate the human error when measuring stainless and nickel alloy tubing, but they are also considerably more accurate than their contact method counterparts.
Certain tubing applications allow for contact measurement via digital calipers or hand micrometers. These methods are less accurate than non-contact laser measurements however still accomplish the job a majority of the time. One can also measure tube overall length (OAL) with these tools making them more versatile than the non-contact methods. With a thin wall tube, this type of measurement method should be utilized with extreme caution as the strength of the hand may overpower the strength of the wall and thus provide an inaccurate measurement. We have found customer utilization of hand measurement tools to evaluate OD’s is one of the most frequent reasons for measurement disagreement.
ID Measurement method: Pin (plug) gauge vs. Vision system /Optical Comparator
Our standard, and a common, efficient, and very accurate way to measure the ID of a tube is via pin (plug) gauge. They are readily available in almost any size from a multitude of reputable vendors. These fabricated “rods” are made from high-grade tool steel and can be machined down to .00004” accuracy (Class X). Our highly skilled quality inspectors insert pins into the ID (inside diameter) of stainless and nickel tubes along every step of the manufacturing process to ensure conformance post-processing. Our pins are calibrated slightly under or slightly over the nominal pin gauge size to ensure accurate measurement. In tubing metrology nomenclature, this intentional sizing is referred to as “plus” pins or “minus” pins. If an inspector encounters resistance that prevents insertion of the pin into the ID of the tube with gentle hand pressure, the diameter associated with that pin gauge would be considered “no-go”. If the tube accepts the pin, it is considered a “go”.
Also important is the class of pin gauge selected. In order of least precise to most precise, the standards are as follows, per the B89.1.5-1998 ANSI / ASME standards:
Class ZZ Pin Gages have a maximum tolerance from nominal of .0002″ (either plus or minus depending on the gauge), roundness of .0001″ and finish 10 micro-inch Ra
Class Z Pin Gages have a maximum tolerance from nominal of .0001″ (either plus or minus depending on the gauge), roundness of .00005″ and finish 8 micro-inch Ra
Class X Pin Gages have a maximum tolerance from nominal of .00004″ (either plus or minus depending on the gauge), roundness of .00002″ and finish 4 micro-inch Ra
The advantage of pin gauge measurement is that it is easy to employ throughout the manufacturing processes when producing sizes from large tube diameters down to hypodermic stainless tubing and therefore permits a generous sample size during manufacturing. An important consideration that can impact measurement is the type of gauges used (plus vs. minus pins, the tolerances associated with those pins (different classes of pin gauges), and the operator’s relative skill and experience performing pin gauging on tubing ID’s.
An alternative way to measure IDs is with our vision systems and optical comparators. With any measuring method, there are pros and cons. This method is a great measuring technique for difficult to pin small parts that also require AQL sampling or similar statistical recording. Our highly technical vision systems auto-generate custom test reports which provide an extra layer of documentation and certification to our customers.
Challenges with using vision system measurement on tubing IDs include:
- They are not easily employed throughout manufacturing since tubes have to be sectioned and brought over to specialized metrology equipment to measure. This approach is impractical during many aspects of tubing manufacturing when certain processes run continuously.
- The software uses algorithms to determine where the edge of the ID begins and ends, which can be impacted by the lighting and surface quality of the ID and other factors. Just because the software dictates an edge exists at a certain location does not mean the location is precisely accurate---it is an estimate. Measuring a long sample (for example, a 2” long tube vs a .250” long section of the same tube) will also yield different results.
- Measurement of small diameter IDs (<1/32”) has markedly worse accuracy on vision systems because the lighting is more challenging. On tubes with swaged ends, the surface finish is considerably worse (from the forming operation) than the base tube which further increases measurement error.
- Vision systems do not account for friction. For example, a .062 pin measuring exactly cannot fit into a .062 hole measuring exactly due to friction. The hole must be slightly oversized to accommodate the pin’s passage into the hole.
- A practical example is that drill bit manufacturers do not “sell” a hypothetical 1/8” drill bit, they are selling a drill bit to make a 1/8” hole. The user of the 1/8” bit desires a 1/8” hole, and drill bits are accordingly sized slightly smaller than nominal. A measurement is easily taken with a micrometer to demonstrate this point.
- For the reasons noted above, the industry standard in small diameter stainless and nickel tubing is to utilize pin gauges to measure the inside diameter of tubes.
Beyond OD and ID tubing measurement methods, other potential measurement method mismatches exist as well, depending on the particular feature being evaluated. This is especially true for aesthetic evaluations like tubing product finishes or “brightness” of the appearance of stainless steel. Under what lighting conditions, and how close to the inspector’s eyes is the hypotube placed? Magnified or normal vision? These factors matter, and as outlined above it’s important for customers to be clear in their expectations before order placement to ensure that both the manufacturer and purchaser are on the same page with respect to quality inspection methods.