External thread inspection can seem confusing and, in some cases, frustrating. There is a wide variety of inspection tools available for external threads. They may be inspected with functional thread ring gages as pass/fail inspection. They may also require some type of actual measured value that would require pitch micrometers, thread measuring wires, tri-rolls, laser, or image based measuring systems. All of these measuring tools have various strengths and weaknesses. As with all measuring tools, knowing what you need to measure and the measuring device capabilities is critical to successful and accurate inspection of the product produced.
In the United States, adjustable thread ring gages are manufactured per ASME standards. For our purposes, we will be focusing on ASME B1.2, “Gages and Gaging for Unified Inch Screw Threads” and ASME B1.16M, “Gages and Gaging for Metric M Series Screw Threads.” The blanks for these gages are manufactured per the ASME B47.1, “Gage Blanks.” Please note that solid thread ring gages are also covered in the above standards. However, they are an article unto themselves and will not be covered here.