Threading is the process of creating screw threads for fastening things together. Threaded parts are incredibly common, and for good reason: threads allow parts to be joined together easily and at a low cost. They can be created using a variety of methods, and are used with dozens of different types of fasteners. In this post we’ll cover multiple processes used to cut threads, how to add threaded inserts to certain materials, how to prevent taps from breaking, and more.
Cutting Threads: How it Works
A common method of creating threads is to cut them with a tap or die. Taps are used to cut internal threads, like those in a nut, while dies are used to cut external threads, like those on a bolt. Cutting threads with a tap is called “tapping” and cutting threads with a die is called “threading”. Both of these processes can be done by hand with a tap or die handle.
(From left) Dies, Tap Handle, Die Handle, Taps. Source
CNC machines can cut threads as well, although it isn’t like drilling a through-hole, where you can just run the spindle and peck your way down. Tapping requires a good amount of torque and precise motor control to move both the spindle and Z-axis precisely. The spindle also needs to run in reverse to remove the tap. Unfortunately, the task is not a simple one: taps break easily due to lack of chip clearing, high speeds, or poor angle of entry into a hole.
Broken tap. Source
Breaking a tap in hole can be a real problem: if it breaks above the material, it may be possible to back it out with pliers, but they typically break at the material or below it. Consider the challenge of removing a broken tap from beneath the top of the material without damaging the rest of the part or the threads. Due to the severity of this, businesses have been built upon drill and tap removal services.
The type of tap used is critical in ensuring it lasts for many holes. Tapered taps have cutting teeth that gradually interact with the work, which makes them ideal for starting holes with good alignment. Bottoming taps have a flat bottom for getting to the deepest section of any blind hole. There are many other types of taps, including those that are more severely tapered for cutting pipe threads and some with spiral flutes that assist with chip removal.
Thread milling. Source
For threading with a CNC mill, instead of using a die, a type of cutting tool called a thread mill is ideal. Unlike taps and dies, which are made for a specific thread size and pitch, thread mills can be used to mill nearly any type of thread in any direction (left or right-handed) thanks to computer control. They can also be used to cut internal threads, which is especially handy when the part requires a hole too large for a tap of the correct size to fit in the machine. The thread mill’s versatility means that a CNC mill can create threads in any size hole, as long as the part can fit in the machine.
Material Considerations for Threads
Cutting threads doesn’t work well in all materials, though. It’s advised against in woodworking, due to wood’s lack of toughness to support and retain threads, especially if the threads are small. Cutting fine threads in plastics is sometimes worth avoiding for similar reasons. Your chance of success is based mainly on the size of the hole, though: large, coarse threads may work fine. Other soft materials like HDPE or foam may hold threads initially, but likely won’t stand up to much use over time.
Tee nut. Source
When using bolts and other threaded fasteners with wood, hammered-in threaded inserts called “tee nuts” work well. With just a clearance hole and a hammer blow, the nut is installed, gripping the wood with little teeth to prevent rotation. They’re self-tightening, too: as a mating fastener is tightened on the other size, the nut is forced even further into the wood, preventing loosening. A common application for tee nuts is holding rock climbing holds onto a gym wall.
When working with plastics, there are multiple options, including helicoils, press-in, heat-set and molded-in threaded inserts. Helicoils are wound stainless steel wire with a diamond cross section - at a glance, they may appear to be overly-compressed springs. They require a starting hole that’s quite a bit larger than the final threaded hole, making them ideal for a quick re-do if you need to repair a tapped hole. They’re also easy to install, affordable, and a great option for adding threaded holes to plastic parts.
Heat-set inserts. Source
Press-in inserts are more expensive, but easier to install. They’re made out of hard material and have ridges that keep them retained in plastic, which they deform as they’re installed in with an arbor press. Heat-set and molded-in inserts provide the same end result, but with different installation methods. Heat-set inserts are easy to gently press in with the tip of a soldering iron, making them ideal for 3D printed prototypes. Molded-in inserts work better in injection molded applications, where they can be placed in a mold prior to it being filled with molten material.
Captive nuts are pressed into sheet metal. Source
Adding threads to sheet metal parts isn’t trivial either: thin material is very common in sheet metal work, due to the ease with which it bends, but will not hold more than a thread or two. In order to add more thickness, captive nuts and inserts can be pressed into the sheet metal. Like the press-in inserts for plastics, these too have ridges that force the material to deform and flow around them as they’re set.
Preventing Breakage in Threads
Selecting the ideal threaded fastener isn’t always easy: nuts and bolts come in dozens of different types, with a multitude of materials, coatings, head styles and size options. However, one potential problem that can come up is galling, or “cold welding”, which can occur when a nut and bolt or screw and threaded hole undergo extensive pressure. Stainless steel fasteners are particularly prone to this, where two points of contact may be welded together. Lubricants can be used to reduce the friction and reduce the chance of galling, although using a coarser thread will also help.
Over-tightening of a screw or bolt may cause the head to break off. This is particularly common with softer fasteners like brass screws. In order to prevent this (and galling) from occurring, a tool with torque control should be used. In fact, torque tools are ubiquitous in factories everywhere. They prevent the fasteners in planes, trains, cars, and everyday consumer products from galling or breaking.
Galling certainly isn’t ideal for most applications, but sometimes you may want to prevent things from coming loose during operation while still having control over when they come apart. Vibration can cause screws to back themselves out of holes, thanks to the gap between the mating threads of each component. If the threads were lined up exactly, they would be impossible to assemble, so a bit of slop is necessary. Unfortunately, vibration will cause a screw to vigorously shake itself back and forth, eventually out of the hole.
This is where thread locking products can be used to control the toughness of the lock: some liquid products will only hold a screw in a threaded hole until a bit of torque is applied, while others will not yield until high heat and torque are applied. Threadlocking compound even comes pre-applied to some fasteners, making them ideal for factory settings where an operator cannot apply too much or too little and simply drive in the screw.
Apply threadlocker to a bolt. Source
There are many other methods of keeping screws from backing out, including applying a nylon insert to the inside of a nut to prevent it from loosening (commonly abbreviated to “nylock nut”). On the other end of the fastener, a split lock washer can be used between the head of the screw and the material to physically push them apart. This force nudges the screw up, pushing the top of its threads into the bottom of the threads of the tapped hole. Now when the system vibrates, the screw is held in place.
Threads in CAD Programs
When it comes to 3D CAD programs, threaded holes are portrayed in different ways. Some programs include features that automatically model in accurate threads. Most, however, just use a through-hole instead to save the program from the computational power required model in accurate threads. Before creating a part that has tapped holes, the typical method is to model in the tap drill size in preparation for manufacturing.
If you’re working with us and would like a hole to be tapped, you need to model that in. It’s best to use the Hole Wizard to specify the exact thread specifications, in this case. Even though it may not be cosmetically correct, this is how the Plethora add-in will notice the threaded hole. If you don’t want to use the hole wizard and spec the tapped hole in another way, just send along a PDF with the tapped hole called out.
Threading is a reliable way to keep parts together in an inexpensive and removable manner. Due to this, threaded parts are prevalent in mechanical design, and should be familiar to all engineers. At Plethora, we're working hard to make it painless to specify threads in your design. We want you to have the satisfaction of utilizing precision-cut threads with a great fit, so let us know if there's anything we can do to improve the process.