If you are interested in engineering and manufacturing, you may have heard of thread and plug gages, but weren't really sure what they were used for. While you may have been eager to create products using CAD/CAM devices, these gages are used to make physical inspections for fasteners (e.g. screws, nuts, bolts). You can use gages to make sure that these fasteners fit nicely together. Take a look at how these tools are used and how tolerant they are to wear and tear.
Thread gages—also known as pitch gages—are used to check the pitch of a thread (the helical structure) on a screw or a working piece's hole. However, the word "pitch" can be a little misleading. Instead of referring to the height of these threads, the pitch is referring to the the number of threads per inch or centimeter and how close they are to one another.
There are different thread gages on the market: some look like rods while others look almost like pocket knives. The ones that look like rods are sometimes called go-no-go instruments. That's because one end of the gauge will have a GO end and a NO-GO end. The GO end should fit cleanly into a nut, while the NO GO end should be stopped by the nut.
The ones that look like pocket knives have various blades. Each blade has a triangular serration that will correspond with differences in pitch.
Although both tools don't give you exact measurements, they will allow you to roughly figure out the right pitch and see if a screw will fit fairly cleanly with another threaded material.
While thread gages check the helical structures of a working part, plug gages check whether or not a given dimension (like a hole) is too large or small. These tools look like rods and also have a GO end and a NO-GO end. Again, if the hole has correct dimensions, then the GO end should enter easily while the NO-GO end should be stopped.
Tolerances of Both
While Gages are often made of steel, that doesn't mean they are impenetrable. You may want to look for products that have tungsten-carbide coats or extra plating to make sure they are more durable.
Even with increased durability, threaded gages should never be forced into a part with too much rotational force—a couple of turns should be sufficient. If the GO end isn't fitting into the hole, clearly the pitch isn't correct. If you use too much rotational force, you could damage the gage.
Also, both threaded gages and plug gages are subject to a phenomenon called galling, or col-welding, where surfaces are placed under pressure and began to expand, causing a nut and bolt to permanently weld together! You can prevent galling from occurring by making sure that your gages are well lubricated with oil. You should also look for triple-cold stabilization, which means that the gages have been frozen and thawed and can withstand galling. Lastly, look for any debris that has stuck to your gages. Tiny chips from burred materials can actually adhere to the surface of gages and cause warping and galling. For more information, contact companies like WESTport.