Successful threadmaking requires the right gages

Thread cutting is as close to artistic expression as one typically gets in a machine shop. Watching a machinist or toolmaker peel off perfectly formed chips with a precisely ground tool is mesmerizing. Although I understand the geometric relationships that make thread forms functional, I am still amazed that someone had the ingenuity and creativity to utilize them in a way that literally holds our lives together.

This indicator-type gage is used to measure ID threads, such as those
in large-diameter nuts for large gas turbines. Image courtesy of C. Tate.

Threads do not seem complex when simply assembling two components. However, when someone must measure threads and reproduce parts that are interchangeable with parts manufactured somewhere else in the world, threading can become the bane of one’s existence. Successful threadmaking requires proper gaging and a clear understanding of the thread requirements.

Measuring threads can be successfully performed in many ways. However, errors can create costly problems, so selecting the best measurement method is important.

Gage Selection

Selecting the proper gage requires knowing the specified thread form. Most threads conform to a standard, such as the unified thread form, ISO thread form or to an industry-specific standard like the one from the American Petroleum Institute. Threads that conform to a standard will have specific requirements for shape, size and geometric relationships that define the threads; gages will be made accordingly.

In addition to measuring ID threads, Mitsubishi Hitachi Power Systems Americas will use this set
of thread mics to develop a manufacturing process for some gas turbine hardware. Image courtesy of C. Tate.

Hard gages, like plug and ring gages, also known as GO/NO-GO gages, are the most common, especially in production environments. These gages let the user determine if the threaded element is good or bad. The two-part gages have one element that is slightly smaller than the minimum thread size and another that is slightly larger than the maximum size. When the thread is sized correctly, the GO gage will fit the threaded part but the NO-GO will not. These gages are fast, accurate and easy to use.

Unfortunately, GO/NO-GO gages do not provide any numeric values, which can make setup and adjustment of a machine tool difficult. In cases where numeric values are needed, another type of thread gage must be used.

In terms of simplicity, GO/NO-GO systems that utilize an indicator are probably the next best thing to plug and ring gages. Like hard gages, indicator-based gages allow a user to quickly determine good from bad. However, they also provide a numerical deviation from the ideal, so the user can make necessary adjustments to the machine tool. Indicator-based gages are set with master gages, which define upper limits, lower limits or nominal thread sizes. Using upper- and lower-limit masters to set the gage is the most desirable method because it eliminates the possibility of errors.

Indicator-style gages and the masters that go with them are expensive compared to other thread gages. Fortunately, they can measure a range of sizes, which makes their cost a little more palatable.

Indicator-style gages, like hard gages, are suitable for production environments. However, they require more training and adherence to proper setting procedures to ensure reliability.