Posted April 22, 2021

Threading 101: An Introduction to Threading

The threading process of a metal workpiece
Threading is the process of creating screw threads for fastening things together. Threaded parts are common, and for a good reason: threads allow parts to be joined together easily and at a low cost. Manufacturers create them using various methods for use 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 specific materials, how to prevent taps from breaking, and more.

Cutting Threads: How it Works

A standard method of creating threads is to cut them with a tap or die. “Taps” cut internal threads, like those in a nut, while “dies” cut external threads, like those on a bolt. “Tapping” refers to cutting threads with a tap and “threading” to cutting threads with a die. One can perform both of these processes by hand with a tap or die handle.

Some taps used for cutting threads along with a manual tap handle

Tap handles and taps used to cut threads manually

CNC machines can cut threads as well, although it isn’t like drilling a through-hole, where one can just run the spindle and peck the way down. Tapping requires a good amount of torque and precise motor control to move both the spindle and Z-axis. 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.

Difficulties arise when breaking a tap in a hole: 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 threads. Due to the severity of this, some have established businesses 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, making 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 more severely tapered for cutting pipe threads and some with spiral flutes that assist with chip removal.

The threading process of using a tap on a lathe

Tapping a hole with a lathe

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—made for specific thread size and pitch—thread mills can cut nearly any type of thread in any direction (left or right-handed) thanks to computer control. Machine shops also use them 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. The chance of success is based mainly on the hole’s size, 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.

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 tightens on the other size, the nut is forced even further into the wood, preventing loosening. For example, one 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 like 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 one needs to repair a tapped hole. They’re also easy to install, affordable, and an excellent option for adding threaded holes to plastic parts.
  • 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 when installed with an arbor press.
  • Heat-set and molded-in inserts provide the same 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 one can place them in a mold before filling it with molten material.

Bronze threaded inserts

Threaded inserts are used when machined materials are not suitable for tapping.

Adding threads to sheet metal parts isn’t trivial either: thin material is prevalent in sheet metal work due to the ease with which it bends but will not hold more than a thread or two. One can press captive nuts and inserts into the sheet metal to add more thickness. Like the press-in inserts for plastics, these too have ridges that force the material to deform and flow around them as they set.

Preventing Breakage in Threads 

Selecting the ideal threaded fastener isn’t always easy: nuts and bolts come in dozens of different types, with many 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 a machinist may weld two points of contact together. Lubricants can reduce the friction and 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 issue is pervasive with softer fasteners like brass screws. A machinist should use a tool with torque control to prevent this (and galling) from occurring. Torque tools are ubiquitous in factories everywhere. They prevent the fasteners in planes, trains, cars, and everyday consumer products from galling or breaking.

Threadlock

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 when they come apart. Vibration can cause screws to back themselves out of holes, thanks to the gap between each component’s mating threads. If the threads 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 situation is where one should use thread locking products 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 exerting high heat and torque. 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.

Using threadlock to secure a bolt

Applying threadlocker to a bolt

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, one can use a split lock washer between the head of the screw and the material to push them apart physically. 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

3D CAD programs portray threaded holes 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 involves modeling the tap drill size in preparation for manufacturing.

If you’re working with us at Plethora and would like a hole tapped, you need to include that in the model. It’s best to use the Hole Wizard to specify the exact thread specifications. 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 alternatively, 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. We are an ISO 9001 certified machine shop, and our primary goal is to manufacture your parts to the highest level of quality. Our online DFM and quoting systems are ready to receive your data so we can immediately begin working with you on your project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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Topics: materials, processes, Manufacturing, finishing, CNC machining, Quality, Prototyping, threading

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