The use of Thread Gages is the fundamental means to verify whether a threaded part is accurate. There are many different types of Thread Gages, usually constructed as a threaded ring gage or plug gage, and are often used as GO/NO-GO type gages to check the acceptance of a wide variety of externally threaded parts such as bolts and screws or internally threaded parts such as nuts.
As with any measuring standard, Thread Gages need to be periodically inspected and calibrated to ensure that their accuracy remains within their original stated specifications. The calibration interval should be based on the amount of usage, part and gage materials, and the required specifications.
Reference standard ANSI/ASME B1.2 ‘Gages and Gaging for Unified Inch Screw Threads’ contains information on the use of the different types of Thread Gages and includes the dimensions, specifications and measurement forces for the calibration of the various Thread Gages and measuring equipment.
Thread Gages need to be calibrated by trained technicians using the proper reference standards in a temperature-controlled environment. The most widely used reference standards to calibrate Thread Gages are Universal Length Measuring Machines (ULM).
ULM’s are highly accurate measuring devices for a variety of internal and external measurements. They consist of a fixed and moveable jaw where the item to be measured is placed between them. e2b Calibration has recently acquired a multipurpose ULM and can perform high accuracy measurements on Thread Gages and many other types of length measuring instruments.
Thread Gage Measurements
The two most common dimensional characteristics that are calibrated on Thread Gages are the major diameter and the pitch diameter. The major diameter is usually calibrated by placing the Thread Gage between the jaws of the reference standard or by using plain ring or plug gages to determine the diameter of the gage.
The pitch diameter is calibrated by using a ‘three-wire method’. The three-wire method involves placing three small pin gages of a known diameter within the threads of the gage, two on one side and one on the other, and measuring across them with the reference standard jaws. A mathematical formula or programmed software is then used to calculate the pitch diameter of the gage.
Thread Gage Out-Of-Tolerance Conditions
There are two main out-of-tolerance conditions that occur in Thread Gages, being undersized due to gage wear and being oversized due to gage growth.
Thread Gage wear is the result of metal chips or other abrasive debris that becomes lodged on the Thread Gage surfaces and grinds down the surfaces during use. It only takes a small amount of localized wear to have a significant impact on the measurement of the pitch diameter. GO member gages tend to wear down quicker with normal use.
The composition and hardening process of the hardened steel that is typically used for Thread Gages has the tendency to grow in size. Within the first year after manufacture, the molecules in the steel will relax and cause an expansion of the material. Frequent temperature variations during the use and storage of the gage during this time will increase this expansion. The growth is most noticeable on larger Thread Gage sizes.
Galling is also a condition that can cause a Thread Gage to appear to grow. Galling occurs when small amounts of material cut into the surface of the gage and actually weld themselves to the gage surface, similar to plating, thus causing the gage to grow. Burrs can also be created by stray debris that damages the threads leading to an oversized measurement.
Thread Gage Care
The best ways to ensure that your Thread Gages measure correctly for the longest period of time is to make sure that they are properly cleaned before and after use, recoated with rust preventative or an easy to peel oiled-based wax coating before storage and properly store the gages to protect them from exposure to excessive temperature variations, moisture and corrosive chemicals.
If you have any questions on the calibration of Thread Gages or are interested in learning more about our thread and length measurement capabilities, contact e2b calibration for more information.