Here's a great white paper from our friends at Gagemaker called "Back to the Basics:Thread Diameters"
We often get questions about the differences between all the various diameters of threads, so here's a quick overview of some of the more common terms:
- Functional Size- the size at which two threads will screw together, but not necessarily with the highest integrity of fit. While not actually a true diameter, it is a measure of the actual maximum amount of material that makes up a threaded part. Basically, functional size is the sum of all thread elements: lead,flank angle, taper, and roundness (or ovality). Threaded ring and plug gages can detect variations in functional size but CANNOT "assure dimensional conformance" as required by ASME/ANSI. In other words, ring and plugs will tell you if something screws together but cannot ensure a proper fit.
- Pitch Diameter (or Effective Diameter)- the size at which two threads will screw together within product tolerances. It is the diameter of an imaginary cylinder that intersects the thread tooth and thread groove at the point where the two are the same dimension. It is also the minimum amount of material (minimum material limit) that controls a connection's strength. Unlike Functional Size, Pitch Diameter is not affected by lead or flank angle. However, it does determine thread size. According to ASME/ANSI, ring and plug gages CANNOT inspect pitch diameter.
- Minor Diameter- the smallest nominal diameter of a thread. On internal threads this would be the crest diameter, while on an external thread this would be a root diameter. A Minor Diameter has a maximum and minimum value determined by allowable tolerances.
- Major Diameter- the largest nominal diameter of a thread. For internal threads this would be the root diameter, on an external thread this would be a crest diameter. A Major Diameter also has a maximum and minimum value determined by allowable tolerances.
- Crest Diameter- the Major Diameter of an external thread or the Minor Diameter of an internal thread.
- Ovality - how circular the imaginary cylinder of a threaded part is or simply a numerical measurement for the roundness of a threaded part. Ovality is determined by the difference between the minimum and maximum diameters