What is Thread Milling?
Thread milling produces threads with the circular ramping movement of a rotating tool. The lateral movement of the tool in one revolution creates the thread pitch.
Although not as widely used as thread turning, thread milling achieves high productivity in certain applications.
Thread milling is a method for cutting threads into a part or workpiece, but with a tool that has a limited contact area with the workpiece.
For internal threads inside holes, the thread milling cutting tool has a smaller diameter than the hole and rotates at a relatively high speed as it moves in a circular path around the circumference of the hole.
In this way, thread milling is different from tapping, as a tapping tool has a diameter that matches the hole to be threaded, and is in contact with the workpiece along its entire circumference while tapping the hole.
The function of thread milling is to cut a thread into a workpiece. It is typically used for cutting threads inside holes, but can also be applied to cutting threads on the outside of parts.
A thread milling tool has a set thread pitch (number of thread crests per inch) but can be used to create left-hand or right-hand threads, and for various flank angles.
Thread milling is used mainly for cutting threads in materials that do not have good machinability. For softer materials or materials that are easily machined, conventional tapping would usually be used to cut threads into holes.
Thread milling is also used for threading large diameter holes, or for threading to the bottom of a blind hole. These are applications in which tapping has weaknesses or is unable to be used. Thread milling has the flexibility to be used to thread the outside surface of an item as well.
What is Tapping?
At first glance, a tap looks quite similar to a bolt or screw, albeit one with grooves running down the sides. These grooves are used to conduct chips up and out of the hole during machining, just as the sharp edges on the end and periphery are used to cut the threads.
Taps work much like any other rotary tool, in that they’re gripped in a chuck, collet, or special “floating” toolholder (more on this shortly) and then driven into the workpiece at a specific feedrate.
The two caveats to this are as follows: there must be a hole slightly larger than the thread’s minor diameter (taps are not drill bits and only cut on the tool’s outer edges), and the feedrate must be precisely equal to the thread’s pitch—a 1/4″-20 tap, for example, must advance 0.05″ per revolution (or 20 threads per inch) to produce a good thread.
Just as there are many different types of threads, so does a wide variety of taps exist. Plug taps are generally used to thread “through-holes” while bottom taps are as their name describes, able to produce threads up to the very bottom (almost) of blind holes.
Spiral point taps tend to drive chips forward, whereas those with spiral flutes direct them upwards, out of the hole.
There are also forming or roll taps, which produce no chips at all. Instead, they displace material, much like the thread-rolling process mentioned in the introduction.
These follow the same basic rules as cut taps insofar as feedrate and tool geometry, but require a slightly larger pilot hole to allow for the displaced metal.
However, form taps are limited to ductile materials like aluminum, stainless steel, and superalloys, and should not be used with cast iron or hardened steels.
Thread Milling vs. Tapping
The main difference between these two milling methods is how they create the threads.
In the table below, you’ll find a more analytical explanation of their exact differences in all the important categories.
| Factors | Thread Milling | Tapping |
| Life span | Long-lasting (thousands of holes), especially when made from durable materials like carbide | Made from high-speed steel which can wear out faster, particularly when used in tough materials, lasts a few hundred holes |
| Milling | Both internal and external threads | Mainly internal threads |
| Flexibility | High — you can adjust the thread size through programming | Lower — Tapping is fixed to the size of the tap |
| Accuracy | High, offers more precise control over thread dimensions | Less accurate than thread milling — any slight misalignment or material inconsistencies can affect the accuracy |
| Thread size | No size limitation | Best for very small threads in standard sizes, struggles with large threads |
| Power | May need more power because of the complex movement and high speeds depending on the material and size of the thread | Requires less spindle power since it’s a straightforward cutting process |
| Thread quality | Produces higher quality threads with a better surface finish, especially in tough materials | Might not match the same quality or level of finish, especially in harder materials |
| Speed | Slower (i.e, 8–10 seconds for a 1/4″-20 thread) | Faster (i.e, 4–5 seconds for a 1/4″-20 thread) |
| Chips | Creates chips that are smaller and easier to manage | Sometimes creates longer, stringy chips that are harder to control |
| Applications | Custom and large threads (like pipe threads) and large holes, precision, thin-walled, asymmetric, or non-rotating parts, accurate threads with tight tolerances, right-hand and left-hand threads | More practical for small, standard threads, screws, bolts, and other parts for automotive, aerospace, electronics, medical, and construction |
| Materials | Aluminum, stainless steel, titanium, high-temperature alloys, plastics, and composites | Softer metals like aluminum, brass, and mild steel, can also be used on plastics |
| Cutting process | Carves out the threads by moving the tool in a spiral pattern | Cuts threads by following the shape of the tap |
| Internal threads process | A CNC machine guides the tool in a spiral, or corkscrew, pattern into the material | The tap has to match the thread size and shape, so it simply follows the hole’s path to create threads |
| External threads process | A CNC machine guides the tool, moving it around the outside of the material | Not typically used for external threads |
| Cost | High initial investment, slower process but does last longer and does not need additional tools | Less expensive initially (taps are typically cheaper and the process faster) but may need more regular replacement, plus more than one is needed for different hole sizes |
Thread milling vs tapping: advantages and disadvantages
Speed
Thread milling is a process that can take a long time, and while it produces a higher quality thread than tapping, tapping is a lot faster. If you are working on a project that doesn’t require extremely precise and high-quality thread, then tapping is your best choice.
However, if the object being manufactured has very little margin for error, then thread milling is a better choice.
Effectiveness
In thread milling, a single tool is used to machine holes of various sizes. This reduces the cost of purchase of the tools and reduces the time lost in the replacement of the tool when machining a hole of a different size.
Thread milling can also help you process internal and external threads and substantial threaded holes such as pipe threads. This helps you eliminate the need to invest in large rigid taps to thread large holes.
From this, we can see that tapping requires a huge amount of different materials, and changing hole size means you also need to change tap size each time, which takes time and energy.
Thread Fit
CNC machine tools can be programmed to use thread milling to adjust the thread size and pitch to minute differences.
The flexibility of the tools helps provide tighter tolerances and smoother surfaces than tapping and also means you can thread any size hole with just one headpiece. Tapping is fast and simple but requires you to have a variety of tap sizes on hand.
Thread Quality
The thread quality of thread milling is far superior to that of tapping and is one of the main benefits of using thread milling. In addition, due to the threads of thread milling being created by a smaller headpiece, less material is lost, and the thread size can be adjusted in smaller increments.
On the other hand, tapping essentially brute forces the threads, leading to a loss of material and strength and a more jagged finished product.
Which One Should I Use?
As with so much else in the manufacturing world, the answer to “which approach is best?” depends on numerous factors.
Tapping and thread milling each have distinct pros and cons, and the choice of one over the other comes down to production quantity, material hardness or toughness, available machine tool power, accuracy requirements, and personal preference. Here are some things to consider as you weigh the two options:
Speed
As a rule, tapping is slightly faster than thread milling. Depending on the workpiece material, thread depth, and machine tool speed, the 1/4″-20 thread discussed so far might take 4-5 seconds to tap, and perhaps twice that to mill with a full-profile cutter.
Yes, single-plane thread mills with their need to make multiple passes are far slower, but this is the exception rather than the rule.
Simply put, there isn’t much difference between milling and tapping until you’re faced with thousands of workpieces.
Power
Since a tap creates a complete thread in a single pass, it requires significant torque to drive it, especially in tough materials.
For threads larger than 3/4″ or so in diameter, all but geared head, very powerful machining centers will struggle. Thread milling has no such limitations and can machine whatever size threads are needed.
Size
There’s a caveat to this last statement. For very tiny #000 and smaller threads like those used in wristwatches and some medical devices, thread mill availability becomes limited and a tap might be the only choice, particularly if the threads are more than a few diameters deep.
Tool life
Here’s another big feather in thread milling’s cap. Because most taps are made of high-speed steel (HSS) and most thread mills are of carbide, the latter enjoys longer tool life (not to mention faster spindle speeds).
And if a worn tap breaks in the hole (an all-too-common occurrence), chances are good that the workpiece will be damaged or scrapped.
Thread mills are more predictable in this respect, and if one breaks during operation, it’s much more likely that the workpiece can be fixed.
Flexibility
this is thread milling’s superpower. When tapped threads are out of tolerance, a different “H-size” tap will be needed (available in increments of 0.0005″ larger or smaller). Not so with thread milling, where a simple offset adjustment quickly brings threads back to spec.
In addition, taps are designed to produce only one size and type of thread, and often boast material-specific geometries.
A full-profile 16-pitch thread mill, on the other hand, can cut any 16-pitch thread (provided it fits in the hole) just as a 20-pitch thread mill can cut 20-pitch threads, and so on.
And single-plane thread mills can cut whatever size or pitch thread is desired by modifying the program.
This last bullet—flexibility—is also thread milling’s Achilles heel. The programming is admittedly more complex, which is why some shops have steered clear of it. But given the large number of online programming calculators and widespread CAM support for thread milling, there’s no reason to avoid it, especially considering its greater flexibility and thread quality.