Thread Milling Tips for Tricky Aerospace Materials

Because this thread mill has only three teeth, there is little cutting pressure on the tool during operation, so deflection typically is not problematic. Photo credits: Allied Machine and Engineering

Thread milling is becoming more popular for a few reasons, says Jamie Rosenberger, threading tool product manager for Allied Machine and Engineering, manufacturer of holemaking and finishing tools. For one, she says, most new CNC machine tools now offer helical interpolation as a standard feature. (This simultaneous XYZ motion of a spinning thread mill into a part’s hole is required to enable the tool, which has a body diameter that is only a fraction of the hole diameter, to cut the threads.) Therefore, new machinists and programmers who have “cut their teeth” on this type of newer machine tool technology seemingly are more open to considering thread milling as a viable alternative to traditional tapping operations.

In addition, Rosenberger says thread milling is particularly well suited for challenging, expensive materials, such as tool steel, stainless steel and high-temperature alloys. In fact, Allied Machine and Engineering’s new AccuThread T3 line of thread mills was designed specifically for these applications.

She explains that during tapping operations, the tap is completely engaged with the workpiece, which results in a good bit of heat generation because the tap’s cutting edges do not get a chance to cool down and coolant has a tough time reaching them. This is particularly problematic when tapping high-temperature alloys, commonly used for aerospace and oil/gas applications, because those materials resist heat rather than absorb it. As a result, all the heat generated during tapping is directed into the tap. This, combined with the high tool pressure resulting from multiple teeth being engaged with the material, can cause the tap to wear prematurely or even break off in the hole. The latter scenario might require time-consuming rework or cause the workpiece to be scrapped, which can be costly given that threading is typically one of the final machining operations performed on a part. These considerations are what make thread milling more attractive to some shops, especially those threading expensive workpiece materials, even though thread mills are more expensive than taps.

That said, it can also be challenging to machine threads in high-temperature and hardened materials using conventional thread-milling tools that machine a complete thread in one 360-degree helical movement (for example, a thread mill that has a sufficient number of cutting edges to mill the entire thread profile into a 0.75-inch-deep hole in one helical rotation). The high cutting pressure generated because all cutting edges are simultaneously engaged with the material can cause tool deflection and poor thread finish.

This thread mill spins counterclockwise to enable it to perform climb milling as it is moved helically in a clockwise motion down into a hole when creating a right-handed thread.

Conversely, Allied Machine and Engineering’s AccuThread T3 solid carbide thread mill with proprietary, multilayer AM210 PVD coating cuts essentially one thread at a time in a continuous helical motion into a hole, which minimizes tool pressure and the risk of deflection. Although these tools have three teeth, the first performs the bulk of the thread-cutting action and the other two essentially clean the threads it creates. Therefore, there is little cutting pressure on the tool, so deflection typically is not problematic. In addition, during thread milling, the cutting edges have a chance to cool, because they are not constantly in the cut and it is easier for flood coolant to reach them.

There are two strategies for using these thread mills. As shown on the left, the tool can create a thread in a continuous helical motion into a hole. Conversely, as shown on the right, the tool can create the complete thread profile in segments, cutting three teeth at a time and repeating until the operation is complete.

What is also advantageous about the AccuThread T3 is that the tool is spun counterclockwise to enable it to perform climb milling as it is moved helically in a clockwise motion down into a hole when creating a right-handed thread. With climb milling, a tool’s cutting edge creates a “thick-to-thin” chip. That is, it forms the thickest part of the chip when it engages with the material and creates the thinner portion of the chip when it exits the cut. This generates less deflection than conventional milling (in which the tool effectively rubs on the material as it engages to create a “thin-to-thick” chip) and results in more effective chip evacuation to minimize chip recutting.

Thread Milling Tips

Given the benefits that thread milling offers, Rosenberger says Allied Machine and Engineering still gets questions about how best to leverage this technology. Here, she provides a few tips for shops that are considering thread milling:

As opposed to tapping, thread milling can provide better hole quality while minimizing the risk of scrapping parts, which is especially important when parts are large and expensive. However, it is not the best threading solution for all applications. Tapping is still typically preferred when producing threads that have length-to-diameter ratios of more than 3:1.
Think of thread milling like most other machining practices. The more stock to be removed or the more challenging the material, the more passes that may be required. For example, coarse thread pitches might require multiple passes.
Climb milling is always preferred to conventional milling due to reduced tool deflection and less generated heat.
Always use cutter compensation when thread milling. This enables you to control the precise diameter of the thread without risking scrapping the part due to creating a thread diameter that is too large.
Always use rigid toolholders. During cutting, thread mills experience radial side pressure and should be securely clamped in toolholders such as power milling chucks, hydraulic chucks, shrink-fit holders or end-mill holders. ER collets should not be used for thread milling.
Don’t spend time writing your own thread-milling routines. Many software packages are available from thread-mill manufacturers, such as Allied Machine and Engineering’s InstaCode, to save you time by providing the code to you.

Allied Machine and Engineering Answers 5 Threading Questions

When do I thread mill instead of tap? Because a tap’s cutting edges are constantly in the cut, they generate more heat, which can lead to tool failure and scrap parts. A thread mill, on the other hand, has little contact with the material; therefore, the heat generated is much lower. Thread milling is best when working with a large hole diameter and it has the ability to hold tight tolerances by controlling the tool’s cutting path. However, a tap should be used when machining long lengths of thread and when speed is preferred over thread quality.
When do I use solid carbide thread mills versus indexable thread mills? Solid carbide thread mills run and cut faster every time. They typically have more flutes, which results in a faster penetration rate (ipm) and improved cycle time. These tools also typically outperform indexable thread mills in quality because the threads are being ground at the same time. However, indexable thread mills provide the ability to change out thread forms frequently. It requires a one-time purchase of the body that enables the user to switch over the inserts as needed.
How does the material impact a thread milling application? There are always two things to consider: How much material is being removed, and what the material is like to machine. While a fine thread pitch does not require much material to be removed, a course pitch requires a lot of material to be removed. Answering these questions will also help determine if the material can be removed in one pass. Regardless of how many passes are used to remove the material, just like with boring or turning, a finish pass can be used for improved quality.
What are the best practices for programming? 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 subprogram or subroutine. This is beneficial when threading multiple holes because it provides a single place for program edits. This also enables you to quickly complete a test run above the part to prove out the program.
How should you diagnose issues when thread milling? Because thread mills have radial cutting forces, deflection should be considered. Factors mentioned previously such as how much material is being removed 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 the radial forces and potential deflection, it is necessary to use toolholders such as milling chucks, hydraulic chucks or shrink fits that minimize deflection. Ultimately, these tooling solutions are more rigid and, therefore, improve the quality of the thread being machined. It is also necessary to understand if the programmed tool path is based off of the center or outer diameter of the thread mill. This changes how wear offsets should be applied in the machine. For additional thread milling tips, check out this Thread Milling Pocket Guide.