Small Gages in Metal Parts Sourcing

Barbara Osborne

The Practicality of Pin Gages for Tube ID Inspection

Here at Metal Cutting, for many applications it is not enough for us to create parts to a customer’s specified tolerances. A big part of our Quality Assurance is to also make sure we can measure the parts effectively and provide proof positive that they are, indeed, as specified. The method chosen — and the type of tool used — for determining whether a part meets requirements can make the difference between meeting specs the first time and having to do additional work, which in turn increases costs and creates delays for all.

As a producer of very small metal parts including short lengths of small diameter tubing, we often use very small gages known as pin gages to measure the inside diameter of the tubes we make. It is a simple “pass-fail” method of inspection for making sure that a tubing ID is not too small.

The X-Y-Zs of Pin Gages

There are different classes of small gages — Y, Z, ZZ, XX, XXX, and so on — with different tolerances attached to them, based on the tolerance allowed in the manufacturing process of each pin. For example, for gages ranging in size from 0.001” to 0.08250”, Class Z gages have a tolerance of no more than 0.0001”, while Class X gages have a tolerance of no more than 0.00004”.

The smaller the part, the more the tolerance of the small gage matters. A tighter tolerance pin gage (such as Class XXX) helps to ensure you have a straighter gage that is more uniform throughout the length of the pin, for more accurately checking the ID of a small diameter tube that itself has a very critical tolerance.

For most short tubing with an ID from 0.005” to 0.200”, pin gages typically yield acceptable results that can be correlated. However, it is important to remember that if you require an exact measurement of a finished part’s diameter, these small gages are not going to meet your needs. That is because a pin gage is a go/no-go method and is not designed to provide a measurement value for individual characteristics of the part. In addition, a pin gage can only measure one limit of tolerance, not both plus and minus.

The Limits of Small Gages

There also comes a certain point where even the smallest small gages may not be physically feasible for determining sizes and tolerances. For example, with a tube ID of less than 0.004”, small gages such as go/no-go pin gages are simply not practical; it would be very difficult to insert a delicate pin gage into such a small diameter tube without bending the tube or damaging the pin itself. In addition, pin gages are impractical for inspecting very long cylinders, such as small diameter tubing that is several feet in length and requires a consistent ID throughout.

In these instances where small gages are not appropriate, we may turn instead to other, more sophisticated methods for inspecting IDs, including tools such as smart scopes and various optical comparators. Generally, these methods uses a sensor to measure various points around the circumference and inside a tube to calculate whether the average ID is within tolerance.

Optical measurement methods are more precise than pin gages and can be used to measure circles, arcs, and more complex shapes. However, these methods are more costly than using a pin gage and do have drawbacks; for example, optical tools cannot verify if the ID is consistent throughout the tube, and accuracy can be affected by the surface finish of the part being measured. In addition, optical measurement cannot be used to verify IDs on very long parts, because the parts cannot be positioned so that the light will go all the way through. Rather, the method can only be used on short parts that can be placed on the glass of the smart scope. Measuring the Z-axis has its limits.

Don’t Skip the Inspection Step in Your Specs

Just as you put a lot of time and effort into determining part dimensions and tolerances, you should also carefully consider your options when deciding how you will determine whether finished part meets your specifications.

As a company that specializes in very short lengths (as short as 0.008”), when Metal Cutting Corporation looks at a project and an engineer’s drawings, we always try to ascertain whether what the customer is asking for can, in fact, be measured for its actual value — and if not, whether it can be verified through inspection using a pass-fail tool such as pin gages. Making sure we know and agree up front what will be measured, and how, is a critical step in our parts manufacturing process. In addition, we regularly calibrate the small gages and other tools we use to inspect the parts we make; the tools that we use in-house to calibrate other tools are themselves sent out to be calibrated, so that they are all NIST traceable. (You can read more about calibrated measurement devices here.)

Understanding the fundamentals of parts measurement and the use of small gages and other inspection tools is crucial to making informed decisions about your specifications.


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