Thread Connection

A look at tools for thread measurement

When it comes to measuring threads, there is no “one fits all” solution. There are so many different features of a thread that can affect its effectiveness. Threads themselves come in two different forms, either internal or external. In terms of traditional screws, which you would see in bolt/nut applications, general fastening, or some automotive applications, there are many elements that make this piece work.

The most important question to start off with is: what are you looking for when it comes to thread gauging? What do you really need to inspect? Here is a sample of some common gauging solutions for thread inspection applications:

Non-variable gauges

A common gauge for dealing with both internal and external threads is the go/no-go gauge. This comes in the form of thread plugs for internal threads and ring gauges for external threads. It is all about assuring proper assembly, not performance.

“When it comes to typical threads, what they want to know is if it’s okay or not okay, go or no-go,” says David Chang, business manager (measurement and automation products) for Renishaw Canada. A go gauge ensures the pitch diameter falls below the maximum requirement, whereas the no-go gauge would verify that the pitch is above the minimum requirement. A thread can have damages to it like nicks and dents but as long as it does not affect the part from fitting, there is no cause for rejection.

Non-variable gauges are a quick and easy way to determine pass/fail attributes of the thread. They are also a relatively inexpensive tool.

However, there are some challenges when it comes to working with this type of gauge. “The negative is that you need a go/no-go for every size of thread that you are measuring, and you have to maintain those,” says Peter Detmers, vice president of sales for Mitutoyo Canada. “Yes it is the lowest cost, but you need a one to one relationship.” So as much as these are a lower cost option, if you have a large family of screw threads, you will need to purchase a large number of gauges.

“If you have a family of parts where there are 50 different pieces with small variances between a few of them, that’s enough to require new gauges,” explains Sean Taylor, applications engineer at Hexagon Metrology, Inc. “That’s just one situation where a flexible gauging solution can help.”

The other element that is often checked with a non-variable gauge is whether or not there are enough internal threads in a hole. An operator will put a thread gauge in and see how far down it goes. “Typically there is a line on the thread gauge the will show the tolerance, as long as you can go beyond the line, it’s good,” says Chang.

This is a great system if you are able to scrap the failed product, but if internal or external threads are constantly failing, go/no-go gauges can provide little information as to why the problem occurs. For this information, there are gauging systems that can be used to determine various features that could cause a fail result.

Variable gauges

When more information than just go or no-go is needed, there are several different tools available. First, it is important to determine what measurements and features need to be known in order to choose the right tool. Does only one feature, like thread pitch, need to be measured? If the answer is yes, contacting a metrology or gauging manufacturer is the first step in finding the right option.

A thread pitch gauge, or screw pitch gauge, is just one easy tool for determining the number of threads per inch on both internal and external threads. In metric connections, the thread pitch gauge is used to determine the distance between threads (the distance on the crest of one thread to the crest of another).

These easy to use tools come in a kit featuring a wide range of leaves for different pitch measurements. The leaf is placed on the threads until the fit is snug. The leaf will feature a number or code that corresponds to a measurement on a chart, which is matched to determine the measurement.

Another method is the three-wire thread measuring system, which is a highly accurate way for determining the pitch of the thread. This method uses two wires on one side and one wire on the opposing side of the bolt or threaded area. You then measure the distance across the wires, usually with a micrometer, and then there is a calculation that is completed in order to give the determined variable.

“Normally the three-wire method can be used to inspect the master plug, for setting the ring gauge, it is used to measure male thread,” explains Detmers. This system can be used for a variety of different applications making it a flexible option on the shop floor.

However, another option that is equally qualified is the thread micrometer. This hand tool requires zero manual calculations and offers speedy results. Thread micrometers use a similar approach to a thread wire, except you are using v-anvils and pointed spindles. These systems allow for a variety of screw thread types to be measured; using a 55 degree V-anvil as opposed to the traditional 60 degree v-anvil, widens the applications this device can be used for. The v-anvils are designed to fit within the thread forms on the pitch line if you are doing a direct measurement without the need for calculations.

The main difference between the two tends to involve speed over precision accuracy. “The thread micrometer is faster for getting a measurement, the three-wire method, some would say tends to be a bit more accurate. It’s a standard methodology and has been for several years,” says Detmers.

Another challenge comes when measuring the internal thread. These two systems are used primarily for external threads. The measurement of internal threads requires several extra processes in order to get an accurate measurement. Obviously the part geometry causes a problem when trying to access internal features.

However, if a custom solution is not an option, another way to measure an internal feature on a thread would be to take a mold, similar to a dental cast, to create an external replica that can be measured by the aforementioned gauging systems.

Flexible Gauging Solutions

When primarily measuring one element or feature, a dedicated gauge or device is a great way to go. Measuring the pitch diameter might yield a favorable result; however, there is more to an internal and external thread than just this measurement. If you are looking for multiple features, a flexible gauging system might be the way to go.

When you want to explore the form of an external thread, using an optical comparator can work to highlight variables like the major or minor diameter, pitch diameter, thread or helix angle and depth of thread. Imperfections are also easily seen on an optical comparator, which magnifies the part. Optical comparators are noncontact solutions, which can be a significant advantage when dealing with delicate or very small parts that are hard to manually handle.

“A traditional way of doing it would be to take the [external] thread and put it on a profile comparator/optical comparator, and actually optically look and measure the angle of the thread,” Detmers explains.

When it comes to specific industry applications, tolerance can differ. “What I see, which is majorly automotive and some aerospace, an external thread generally has a more direct function than just holding something together. It will generally have a tighter tolerance,” says Taylor. This would require more than just a non-variable gauge to pass/fail the part. Using flexible gauging solutions will yield a variety of measurements and allow for changes to be made to part processing if a correction is needed.

Another option for a wide variety of variables is a contour system or tracer. This type of device allows for more comprehensive measuring of threads. The purpose of a contour tracer is to use a stylus that follows the profile of the thread itself. That information is transferred into a computer as digital data. Once it’s in the computer, software analyzes angles, radius and other features, and performs nominal/actual comparisons.

With this type of thread measuring system, you can really access a wide range of measurements on both internal and external threads. “The tip radius [on the stylus] can be very small,” explains Detmers. “It can get into finer thread pitch and can get a much greater pitch density, so you can get a lot higher number of points, with a higher degree of accuracy, scanning in two axes.” It’s all about checking the form of the thread, making sure there are no minor defects that can affect the overall performance.

One of the biggest questions you need to ask yourself when it comes to measuring threads is, “what do I need to know?” In today’s day and age, quality control is significant in the manufacturing process. Adding any number of gauging tools to the shop floor can always enhance overall quality concerns.

About the Author
Canadian Metalworking / Canadian Fabricating & Welding

Lindsay Luminoso

Associate Editor

1154 Warden Avenue

Toronto, M1R 0A1 Canada

Lindsay Luminoso, associate editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.