What do product engineers need to know about machine screw threads? It’s a surprisingly twisted subject that may spin your head around. 

Hello again and welcome. This is Gordon Styles, President and CEO of Star Rapid. I’ve been involved in rapid prototyping and new product development for over 35 years, and I cordially invite you to join me for another fascinating and informative episode of Serious Engineering for Serious Engineers.  

Nuts, bolts, screws and various other threaded fittings are used around us every day in millions of applications, but that wasn’t always the case. From the time of the ancient Greeks, screw forms were used mainly to raise water or to press wine and olives. 

In fact, it wasn’t until the 14th century that hand-made threads and bolts were used in the way that we do now – to hold two or more things together under mechanical tension and compression. 

But just because threads and screws are now commonplace that doesn’t mean they’re simple. In fact, modern thread forms come in surprisingly sophisticated geometrical shapes that took hundreds of years to figure out and standardize, and engineers are still working out which types are best for which applications. 

We think it’s about time we gave some long-overdue respect to the humble machine thread while learning some interesting and useful information that every machinist and engineer should know.  

Terms and Definitions 

To begin with, let’s define our terminology. In the most basic sense, a bolt or screw can be any shaft that has a helical groove machined or formed around at least part of its length. That’s for external threads, of course, while internal threads are the reverse image, where the thread forms are machined into a base material surrounding a central cavity. 

This helix shape converts rotary motion into linear travel while also applying holding torque along its length.  External threads are male (for reasons that escape me), while their corresponding holes or nuts have female threads. Of course, to fit together properly these mating pairs must have the same geometry, what machinists call “thread form”. If the mating threads don’t quite match and you try to jam them together anyway, life as you know it is over. 

Regardless of the type of thread – and there are many – they’re all defined by the following features: 

Crest: High point of the groove, like the mountain top 

Root: Low point of the groove, like the valley bottom.   

Thread Angle: Angle between opposing flanks.  It’s possible to have two different angles, and these are called buttress threads.  In fact, there are all sorts of really weird thread forms such as tapered threads, knuckle threads, trapezoidal, German Buttress Threads, Panzer Gewinde, and British Association threads 

Major diameter: largest diameter 

Minor diameter: smallest diameter 

Pitch: Distance between crests. For imperial threads, this is often stated as threads per inch or TPI 

Pitch Line: This is an imaginary line, running parallel to the centerline of the bolt, lying half-way between the high and low projected intersections of the angled flanks. These would be the theoretical crests and roots, were they not truncated or radiused. We’ll talk more about why they’re made this way in just a moment.  

Pitch Diameter: Measured from pitch-line to pitch-line on opposite sides of the thread. When it comes to measuring threads, this is, in my humble opinion, the most important dimension. 

Fine, Medium and Coarse Threads 

Most thread standards have some version of fine, medium and coarse threads for different applications. 

Fine threads are typically found in smaller diameters and on precision instruments where it’s necessary to make micro-adjustments. The drawback however is that they’re more easily cross-threaded, stripped, or galled. A galled thread happens when the mating threads get partially smushed together. [Smushed is not to be confused with mashed, mushed, crashed, crushed, flattened, matted or splatted.] 

Galling can be prevented with thread coatings such as an oxide or by using a lubricant. Helicoils are also used for inserting a harder thread inside of a softer surrounding base material.  

Medium threads are the most common for general-purpose assemblies.  

Coarse threads are the easiest to make, have the most resistance to pull-out and are often found on very heavy industrial machinery. Coarse threads also have the most clearance between threads for a plating or coating that may be applied later.  

Finally, we all know through experience that most screws and bolts are right-handed, meaning they’re tightened clockwise. Why would anyone want a left-handed bolt? Well, there are some moving parts, for example the left pedal on a bicycle crank that turns counterclockwise most of the time. So, you’ll want to use a left-handed bolt to keep it from coming loose during operation. 

How are threads made? 

External threads are made a couple of different ways.  First, they can be cut by removing material using a dedicated screw-cutting machine or a lathe. That’s how we’ve done it for a long time, when we weren’t meticulously cutting them by hand with a file. 

More recently, perhaps in the 1860’s, we started to cold-roll threads. Thread rolling doesn’t waste material and it can improve the strength of threads because it compresses the stock material rather than shearing it. 

There are currently four main methods for creating internal threads: 

1. Thread cutting with a tap or with a threading tool 

1. Thread forming with a thread forming tap 

1. Thrilling, which is done with a tool that both drills the hole and then cuts the thread by helical interpolation. 

1. And Punch Tapping.  You first drill the hole, then you punch a weird looking tap into the hole on a fast spiral, and then back cut on a slow spiral to form the thread. This is very popular now for fast production of engine blocks and other high-volume parts. 

Thread forms of course need to be uniform along their length, with enough clearance between flanks so that the fittings can turn smoothly while still providing the necessary grip. 

Remember we talked before about the theoretical top of the crest and the bottom of the root being sharp angles. But in practical usage they are actually rounded off a bit. If the crest is too sharp then external threads will be brittle and easily chipped, while also being too easy to gall. And a slightly rounded root provides clearance for better rotation. 

What are the standards? 

There are many standards depending upon the application, too many for us to list here, but you can easily find online resources for common North American, European, and Japanese standards. Some of them are listed in the description box below.  

There are also very useful online tools to help you convert from one standard to another, but we think the best and easiest to use is ME ThreadPal. Everything you need to find the right thread form for most hole sizes and standards is right there. 

We hope that this overview has been useful and informative. In an upcoming episode we’ll explore how to apply this information to save time and money when designing new products. Stay tuned for that, remember to ding the bell, like us and subscribe. We’ll see you next time for more Serious Engineering.