Torque and thread cutting or rolling screws
June 10, 2025
By Bob Lund | Engineering Manager
By Bob Lund | Engineering Manager
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Fastenal has torque tables for Bolts and Hex Cap Screws, Socket Head Cap Screws and more. These cover a variety of sizes and grades in both inch and metric product. We get asked often why we don’t publish a torque value for a thread cutting or thread rolling screw.
Torque is a force multiplied by a distance. That force is used to overcome friction during installation, for stretching a bolt and provide a clamping force and for cutting or forming threads. All these require torque to overcome the resisting forces and install the bolt. Let’s look further. When tightening a bolt and nut together, there is friction to overcome in the threads and under the head of the bolt or the bearing surface of the nut, whichever you are turning. In addition, there is an axial force involved that stretches the bolt to a desired clamp load. Note that stretching the bolt (or hex cap screw) is how we create the clamping force required on most bolted joints. When tightening a thread cutting or rolling screw there is certainly torque involved. You start with a pilot hole, the ASME B18.6.3 standard gives us tables for suggested starting points for drill bit or formed hole sizes. These fasteners are generally case hardened with a very hard shell and a softer center. Thing about an M&M or a skittle. This hardened surface is what cuts or forms threads in the mating material. Before we approach the torque, let’s look at the job of these screws. Case hardened products are primarily used as place holder fasteners. We do not want to stretch a case hardened fastener either during installation or during the loading seen by the product on which it is used. Picture what would happen to an M&M if you put a significant tensile load on it, it would crack. Thus, care must be taken to use this type of fastener in the appropriate applications. The standard for this type of part (ASTM B18.6.3) tells us that these types of screws are for applications where you have at least one diameter of thread engagement. This could be in a blind or through hole. The standard gives a bit of guidance as to hole sizes but suggests testing to find the best mix of hole size and installation torque for the application. There are a lot of ways to make a hole. We can drill, laser cut, stamp… or a hole could be formed in a casting etc. These methods come with different tolerances they can hold. If you take the same exact sized screw and install it in a hole at the lower end of the tolerance zone and then another at the upper side of the tolerance zone, you will see different installation torque loads were required. The smaller hole size would require a higher install torque to form threads since a larger amount of screw’s threads will be engaged with the smaller hole diameter. Material thickness in through holes or hole depth in blind holes plays a part in finding the right installation torque as well. Most of the thread formation will be done by the first few threads on the screw, but since the threads formed in the hole are essentially the same size as the threads on the screw that formed them, there will be more thread friction than in the bolt and nut world where you purposely leave a small gap for ease of assembly. The torque required for thread formation by the first few threads of the screw in a pilot hole is larger than the torque for friction in the trailing threads, but both need to be a part of our testing. Material hardness also plays a large part in the torque needed to assembly a thread forming screw. Is the material the threads will be formed in hardened? As long as the base material is a bit softer than the case hardness of the thread forming screws (HRc mid 40’s and up), the screw will be able to form threads in the mating material. Even if the materials aren’t hardened, materials like aluminum, brass and zinc are a lot softer than steel plate. The lower the hardness of the mating material, the less torque it takes to create threads. These and more reasons dictate that you can’t just assign an installation torque to a thread forming screw, so we test at each different location. To keep it simple, use the actual mating part, or perhaps a scrap piece of that material. Drill or try to use the same forming method as will be seen in production if possible, the same hole in 5 or 10 locations. Install the screw with a torque tool until the underside of the head is seated flush with the mating part. Note the max torque used to install to that level then continue to tighten until there is a failure. This could be the screw breaking into two pieces or the threads of the screw or mating part stripping. Your target tightening torque should sit somewhere close to the middle of the seating and failure torque values. Make sure this test is done wherever any of the variables change. You may also like:Vertical Divider
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