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Five Questions to Ask When Machining Threads

Jamie Rosenberger
By Jamie Rosenberger Senior Product Manager, Allied Machine & Engineering Corp.

There are many elements to consider when threading a workpiece. When is a solid-carbide thread mill better than an indexable? How does the workpiece material behavior impact thread milling? Understanding your program as well as diagnosing issues that arise are just as important. Luckily, thread milling can be better understood by asking five specific questions.

1. When would you want to thread mill instead of tap?

There are many instances where you would want to consider using a thread mill instead of a tap. In numerous cases, this comes back to one common issue: taps break. Because the tap is the exact same size as the hole, there is a lot of pressure when you are forcing the threads into the hole—even more so in difficult-to-machine materials. Additionally, a tap’s cutting edges are constantly in the cut, thus generating more heat. A thread mill, on the other hand, has little contact with the material, and the heat generated is much lower—an added benefit in any manufacturing process. Finally, when using a tap, chips are more difficult to form and remove.

 All of these issues lend themselves to tool failure. When the tap breaks, it often results in a scrapped part, so using a tap works better in inexpensive parts. If it is a more expensive part and the tap breaks, you are faced with the challenge of trying to remove the tap and salvage your part. This is time-consuming and impacts your part’s quality and manufacturing cost.

Not only would you want to thread mill whenever the part is expensive, but you would also want to thread mill when working with a large hole diameter. Of course, a tap is just as large as the hole, so for a 4" (101.6 mm) thread diameter, you need a 4" diameter tap. Instead of buying this expensive, large piece of metal or storing taps for every thread size, you could buy an off-the-shelf thread mill and interpolate the thread into multiple thread sizes, including large diameters. Lastly, thread mills consume significantly less power from your machine in the instance of large diameters.

Other advantages of thread mills include the ability to hold tight tolerances by controlling the tool’s cutting path. As the tool shrinks slightly from wear, you can easily compensate for this at the machine by using tool diameter offsets.

Nevertheless, there are occasions where tapping may be the better choice over thread milling. For example, you would want to use a tap when machining long lengths of thread. Due to the lack of radial load, there is no concern about the tap’s stability or tool deflection. In addition, when speed is preferred over thread quality, taps are again the better choice. In many applications, a tap will have a shorter cycle time than a thread mill. However, this still risks breaking the tap.

2. When should you use solid-carbide thread mills vs. indexable thread mills?

In choosing to thread mill, you have the option of solid-carbide or indexable thread mills for your application. This choice often comes down to the needs of the application in terms of quality, repeatability and flexibility.

Solid-carbide thread mills: Quality and performance are key advantages of solid-carbide thread mills. Solid-carbide thread mills run and cut faster every time. Having a constant surface footage between two different diameters will result in a different rpm. Due to their smaller cutter diameter, solid-carbide thread mills will run at a higher rpm. In combination with typically having more flutes, this will result in a faster penetration rate (in/min or mm/min) and improved cycle time. These tools typically outperform indexable thread mills in terms of quality because threads are being ground at the same time. This improves the consistency of threads. With a smaller cutter diameter, there is less contact with the workpiece, resulting in less heat generation and deflection as well.

A thread mill in operation. (Provided by Allied Machine)

Indexable thread mills: Most users are attracted to indexable thread mills because they provide the ability to change out thread forms frequently. You can take one body and change out inserts, and the machine is up and running with different forms or pitches rather quickly. Ultimately, this makes indexable thread mills better for low-production batches as well as job-shop type work with a lot of changeover and variation in the manufacturing. This again comes back to the flexibility of the tooling. You have a one-time purchase of the body and then switch over the inserts as needed.

All in all, a thread mill is simply milling a thread form and a pitch and can usually be used for both left- and right-hand threads, internal or external, multiple start threads and various tolerances.

3. How does the material impact a thread milling application?

Material removal in threading is no different than any other manufacturing process, such as boring or turning. Consider two things:

  • How much material is being removed?
  • What is the material like to machine?

The first question can be answered by the thread’s pitch. While a fine pitch does not require much material to be removed, a coarse pitch requires a lot of material to be removed. The combination of these two questions will also help you determine whether your material can be removed in one pass or not.

Regardless of how many passes you use to remove the material, as with boring or turning, a finish pass can be used for improved quality. This is often referred to as a spring pass. If needed, you should refer to the technical section of your manufacturer’s catalog or an available thread mill programming software like InstaCode to choose the number of passes that are right for you.

4. What are the best practices for programming?

As mentioned above, a thread mill can create a variety of threads like left- or right-hand, internal or external by simply manipulating the program/toolpath. Writing a program in incremental movements instead of absolute is always preferred. In doing so, you are able to insert your code for the threading portion as a sub-program or sub-routine. This is beneficial when threading multiple holes because it provides a single place for program edits. This also allows you to quickly complete a test run above the part to prove out the program. In addition to writing this in incremental movements, an arc-on and arc-off movement will improve the quality of the thread and extend the life of the thread mill.

5. How should you diagnose issues when thread milling?

Because thread mills have radial cutting forces, deflection should always be considered. Factors mentioned previously such as how much material you are removing and what the material is like to machine can be battled by adjusting the number of passes to remove the material as well as the combination of speed and feed. Additionally, consider the toolholder being used. Because of radial forces and potential deflection, toolholders such as milling chucks, hydraulic chucks or shrink-fit holders that minimize deflection are required. These solutions are more rigid and improve the quality of the thread you are machining.

Also, if the programmed toolpath is based off of the center of the thread mill or outer diameter of the thread mill, this will affect how wear offsets should be applied in the machine.

While you may find additional challenges when machining threads, asking these five questions helps build a successful application. For additional tips, read Thread Milling Pocket Guide at or call Allied Machine’s application engineers at 330-343-4283, X 7611.

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