Thread milling cuts threads using a rotating cutter, rather than a tap. The tool moves in a helical path around the hole. This creates internal or external threads with excellent precision. You get more flexibility than tapping or single-point threading. A single thread mill can produce multiple thread sizes. The process works on hard materials without breaking tools.
CNC thread milling makes machining complex threads easier. You can cut left-hand or right-hand threads quickly. Thread milling tools last longer because cutting forces stay low. This guide provides comprehensive coverage of everything you need to know about thread milling. We’ll examine various types of thread milling cutters and tools, among others.
What is Thread Milling?
Thread Milling Tool
Thread milling is a machining process that cuts threads with rotating cutters. The cutter spins whilst moving in a circular helical path. This combination creates the thread form inside or outside a workpiece.
The process differs completely from tapping or thread turning. The spindle rotates the cutter at high speed. Meanwhile, the machine moves the tool in programmed circular interpolation. The helical motion forms each thread as the tool advances.
Thread milling is compatible with any CNC machine that has interpolation capability. You need a minimum of three-axis movement for basic operations. The cutter enters the hole and spirals around the wall. Each complete circle cuts one thread pitch as it moves vertically.
How Thread Milling Works
Making threads follows a series or process. Each step relies on previous steps to get the desired results. Knowing all these steps is valuable for your production setup.
The starting point is to position the machines so that the rotating cutter reaches the start location in the hole, and to also position the cutter at the proper radial distance from the hole center. The start position will dictate the final diameter of the thread.
The cutter will begin cutting helical circular interpolation around the hole. Once the cutter has cut the helical circular interpolation, it will perform a linear move (vertical) of one pitch. The helical circular interpolation generates the shape of the thread into the hole wall. In this step, it is not finished until the thread depth has been cut.
- Step 1 is to position the cutter at the start depth of the thread.Â
- Step 2 is to start the spindle at the programmed speed.Â
- Step 3 is to position the cutter radially.Â
- Step 4 is to begin cutting circular interpolation and feed vertically simultaneously.Â
- Step 5 involves performing vertical circular interpolation until the thread has been entirely cut to the desired depth.Â
- Step 6 is to retract the cutter after the final thread has been cut.
As long as you specify the thread size, pitch, and hole diameter, the machine will perform the helical interpolation calculations for you, eliminating the need for manual programming.
Types of Thread Mills
Different cutter designs serve various purposes. Each design works efficiently for a specific type of threading: the more you know which one to use, the more time and money you save.
Some cutters have a single tooth, while others have multiple teeth. This impacts both the speed of cutting correct threads and the number of various thread sizes the same tool can produce.
Your selection is based on the ideal threads you engage: the thread size, material type, and volume produced. An appropriate cutter allows for faster and more reliable threading.
Single Point Thread Mills
Single Profile Thread Mill Cutter
Single-point cutters have only one cutting tooth, which is shaped to match your thread profile. You will make several passes around the hole to get to full depth.
A single cutter can accommodate a wide range of thread sizes and pitches, providing maximum versatility in your shop. They also allow for both left-hand and right-hand threads to be cut very easily.
Multi-Form Thread Mills
Carbide Multi-Form Thread Mill Tool
Multi-form cutters have several teeth with a shape that matches the thread profile. Each tooth removes material as the cutter is moved around the hole. This cuts thread faster than single-point thread mills.
Multi-form shaped cutters are helpful when you are making a single type of thread size multiple times in a row. The tool is a higher investment cost than single-point mills, but allows for a lower cycle time. You will need to purchase a separate cutter for each type of thread.
Full Form Thread Mills
Full form cutters have the complete profile thread shape ground into them already. The tool will match your depth as well as your profile. You simply make one pass around the hole to achieve the complete thread.
This allows for the fastest threading cycle times available. Production shops prefer full-form threading tools to produce vast amounts of threads uniformly. The limitation of this type of cutter is that one cutter is required per profile thread size.
Standard CNC Thread Milling Techniques
In CNC thread milling, the cutter is moved in routes that will produce accurate threads. All the complicated calculations are done for you in modern CNC controls.
Helical Interpolation
Thread Milling with Helical Interpolation
The cutter follows a circular helical path around the hole, advancing vertically one pitch per revolution. This can be used for most material types, and it produces uniform threads.
Single Point Thread Milling
A form thread mill completes the entire thread in one helical pass. This is the fastest technique for regular production threads. This is only done when the setup is optimized, and spindle power is sufficient.
Multi Pass Thread Milling
Multiple light passes will come to the thread depth. Material is taken out to limit cutting forces. This means tool life is longer, while this may introduce increased cycle time.
Taper Thread Milling
Taper Thread Mill Tungsten Steel Cutter
The cutter follows a tapered helical path, similar to a conical thread. The angle of the taper and pitch is programmed. Produce sealing threads with accuracy for pipe fittings.
Step Down Circular Interpolation
Multiple small helical passes can reach full thread depth. Each pass is slightly deeper than the previous. This function helps reduce heat and chatter, improving surface quality.
Variable Pitch Thread Milling
This allows the pitch to vary along the threaded length as programmed. The CNC cycles pitch while cutting. This is especially necessary in aerospace and medical applications.
Thread Milling vs Traditional Tapping
Tapping pushes a hardened tap through material to form threads. The tap can break easily when drilling into hard materials or when creating deep holes. Additionally, you require different taps for each thread size.
Thread milling avoids most of these problems. It utilizes a single cutter that accommodates multiple thread sizes and pitches. The tool never gets stuck because it’s constantly rotating freely.
Chips evacuate efficiently during thread milling operations. The cutter teeth throw chips outward as they cut. This avoids chip packing and significantly reduces tool breakage.
What Materials Work for Thread Milling?
Different materials require different cutting approaches and tool selections. Metals need carbide cutters, while plastics work well with HSS. The table below shows common materials suitable for thread milling operations.
| Metals | Plastics | Composites | Exotic Materials |
| Aluminium | Acetal | Carbon Fibre | Graphite |
| Mild Steel | Nylon | Glass Fibre | Ceramics |
| Stainless Steel | PEEK | Fibre Reinforced Plastics | Magnesium Alloys |
| Tool Steel | Polycarbonate | ||
| Hardened Steel | PTFE | ||
| Cast Iron | Acrylic | ||
| Titanium | PVC | ||
| Inconel | |||
| Copper | |||
| Brass | |||
| Bronze |
Design Recommendations and Context for Thread Milling
A well-designed thread will yield a more efficient thread-milling process. Consider carefully the size of the access hole, thread depth, and material properties to ensure optimal performance. By following these recommendations, you will avoid not only machining issues but also quality issues.
Hole Size Guidelines
The pilot hole should allow clearance for the thread mill cutter. Allow at least 1mm past the minor thread diameter. If the hole is too small for the cutter acquisition diameter, the tool will bind and cause damage.
Thread Depth Guidelines
Thread depth will dictate the strength and pullout strength of the thread. The depth of the thread should be equal to 1 times the diameter for most applications. For soft materials like aluminum, 1.5 diameters should be followed.
Wall Thickness Guidelines
Thin walls may flex during cutting or assembly. Minimum wall thickness should be equal to the primary diameter of the thread. Reinforce critical threads in thin areas with boss features around the threads.
Thread Class and Fit Guideline
The thread class impacts how tight or loose the fit of the external and internal threads is. Class 1 fit is class loose, class 3 fit is class-tight. You can alter the diameter of the circular path to attain the thread class you want.
Summary
Thread milling cuts threads using a rotating cutter that moves in a helical path. One tool can make multiple thread sizes, which saves money. The process works on hard materials without breaking the tool, unlike taps, which can cause damage.
The cutter spins as it moves in circles around the hole. It moves down by one thread pitch with each circle. Custom milling services handle all the calculations automatically when you enter thread specifications.
Thread milling beats tapping in several ways. Tools last much longer and never get stuck. Chips clear out easily during cutting. You get better threads in tough materials, such as hardened steel.
Three main cutter types, single-point cutters, are suitable for a wide range of thread sizes. Multi-form cutters cut faster due to their multiple teeth. Full-form cutters are the fastest, but require one tool per size.