January 2012 Newsletter- Thread Diameter Overview
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External Functional Roll Gage
External Functional Roll Gage
Measuring Functional Size
MRP-1000-on-USS-Buttress-on-back-ground-
MRP-1000 measuring Crest Diameter and Ovality on 2 3/8″ Tubing
Step-5
PD-8000-RSC Measuring the Pitch Diameter of
a Rotary Shouldered Connection
Back to the Basics:
Thread Diameters
Confused by Thread Diameter terms?
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
Look out for further explanations coming soon!
Functional Thread Size Gages
Pitch Diameter Size Gages
Functional Rolls
Pitch Diameter Ball Contacts
Crest Diameter Gages
Internal Functional Roll Gage PG-6000
Internal Functional Roll Gage Measuring on a Machine
RG-7000-on-Part-Left- mrponexttubing
RG-7000 Measuring Functional Size MRP-1000 Measuring Crest Diameter and Ovality on External Tubing
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Gagemaker, LP develops and manufactures innovative precision mechanical inspection equipment for the Oilfield, Automotive and General Machining industries. Recognized as the leading provider of Tapered & Straight Thread Measurement Gages; API Products, Tubing and Casing Gages, Engineering Software and Calibration Software & Equipment, Gagemaker products are used in countless facilities around the world.