Skip to content 
What Does Helical Milling Mean?
Helical milling is an innovative machining method involving the progressive cutting of the workpiece with a cutter following a programmed helical path to achieve desired dimensions. Machinists often use this process for initial drilling and hole making since it offers a range of strategies that allows you to create complex borehole geometries, such as making different borehole diameters, complex tapered holes, conical holes, and executing finishing operations without changing the tool.
Most machinists consider helical milling an ideal hole-making process since it allows excellent control over material removal, making it suitable for machining large-diameter holes. Similarly, its controlled cutting engagement makes it a perfect option for machining hard materials like Ti-alloys, Carbon Fiber Reinforced Polymers (CFRP), and composite structures. This process usually occurs on a CNC machine and requires simultaneous motion in multiple axes. It can be performed on either horizontal or vertical milling machines.
Most individuals often confuse helical milling with helical interpolation due to certain factors. However, you should understand that they are not the same. Helical milling is a broader term that includes the application of helical interpolation to achieve the desired results. Helical interpolation is a motion strategy used in CNC machining, such as milling. It is a CNC programming technique where the tool gradually enters the material through a helical path, rotating around its axis and progressively into the material to create a hole, contour, or thread.
Technical Specifics of Helical Milling
We will explore helical milling specifics to help you master this metal machining strategy.
The process begins with the end mill traveling along a helical trajectory. This means that the milling center combines the vertical Z-axis movement and the horizontal x and y-axis since the process requires simultaneous motion in the three axes. Although it makes the CNC program very complex to write manually, several CAM systems have adopted helical milling as one of the standard machining strategies.
Generated chips in helical milling often comprise two zones: the red zone created by the face of the mill and the blue zone made by the side of the end mill. Machining experts have proven that the tool and bore diameters determine the ratio between the two zones.
More so, the blue zone grows as the cutter diameter increases. Unlike the red chip, the blue chip is discontinuous and encourages vibration, adversely affecting the milling process. The surface finish of the milled parts deteriorates significantly and is less precise due to the increased vibration and poor chip formation. Also, the radial cutting forces increase as the volume removed by the side of the mill grows, bending the tool within the hole and reducing tolerance. However, larger mills exhibit more rigidity and can counter this adverse effect to a certain extent.
The red zone dominates when the milling tool is smaller, ensuring lower radial force and vibration. However, the system’s rigidity limits the decrease in tool diameter. AT-Machining professionals recommend using a larger cutting tool for roughing and smaller tools for the final cut with low depth and feed to achieve a top-quality surface finish.
Why Not Drilling?
Drilling is a generally known hole-making technique. When conventional drilling is not suitable, machinists employ helical milling. When manufacturing metal components, drilling usually takes up to 33% of the total number of machining operations and 25% of cycle time. Aside from these statistical figures, why should you consider drilling? Irrespective of the simpler kinematics, drilling exhibits enormous constraints that justify your decision to utilize the versatile helical milling method for your project.
For instance, the diameter of drill bits determines spindle speed. Drilling speed is highest at the outer point of the drill since it travels the longest distance per revolution. At the same time, there is nearly no movement in the center of the drill since the area only rotates in place without covering any distance. As such, material deformation occurs due to the pressure of the drill since there is little to no cutting action near the revolution axis. Therefore, it leads to higher force requirements and rapid tool wear.
The drill, particularly a worn drill, will bend a thin metal layer as it leaves the stock due to the axial thrust force. You will have to manually remove the resulting leftover material protruding around the drilled hole. However, the resulting leftover material is significantly smaller when you use a mill instead.
Furthermore, drilling presents poor conditions that hinder efficient chip removal since it traps the generated chip within a narrow space. The processed material only leaves the cutting zone through the drill flutes. Chip evacuation impacts the cutting temperature and surface finish of the hole. For instance, bits of metals scrape the inside of the hole and reduce the surface finish as they leave for the surface through the flutes.
Machining experts have proven that generated chips retain about 80% of cutting heat. Consequently, chip removal problems may increase the temperature of the drill and cause rapid tool wear. Nevertheless, machinists must employ discrete drilling techniques to achieve an increased chip removal rate.
Reasons Why You Should Consider Helical Milling?
Having discussed some of the notable drawbacks of the drilling method earlier in this article, we’ll explore why you should consider helical milling for your project in this section.
Manufacturers of different products embrace helical milling because it creates holes with minimal cutting forces, allows better tool life control, and optimizes machining efficiency. This promising process enables you to achieve any diameter with greater precision and surface quality without necessarily changing the cutting tool. If you have drilled a hole wider than 40 mm, you know that using only one drill is not ideal. A better approach would be to use a range of smaller drills to gradually increase the hole’s width.
For instance, you can first drill to 10 mm, then widen the diameter to 20 mm with a bigger drill, then 30 and 40 mm. Then, you can ream or countersink the hole if more precision or surface finish is necessary. The whole process requires about 4 to 6 tool changes to complete the desired hole dimensions.
Helical milling only requires one endmill to cut out the hole and finish it to the desired tolerance and surface quality with smaller feed. In helical milling, you can achieve different hole diameters with the same end mill, saving time and ensuring precision.
One of the main advantages of helical milling is better chip removal and lower cutting temperature. Chip thinning occurs in helical milling due to the unique cutting dynamics of the end mill. Likewise, the endmill does not occupy all the bore space, so you don’t have to extract the tool after plunging every 20 to 30 mm. Spraying coolant into the hole is a good way to eliminate generated chips and lower temperature during milling operation.
Unlike drilling, you can predict tool wear and implement trajectory modifications in helical milling. One of the common challenges in drilling is that tool wear is often easy to notice once the tool is entirely broken, and the drill can get stuck in the bore, particularly when drilling hard materials.
While there is a better distribution of cutting forces with helical milling, you can also predict tool wear using standard calculation methods or manufacturer-specified tool life. Also, you can even account for these changes during the milling process to modify the trajectory to maintain the desired diameter dimension and manage tool life, which is not easily attainable with drilling.
Conclusion
Helical milling is a reliable alternative to conventional drilling in certain applications requiring high precision and minimal tool wear. Although it requires a CNC machine with precise multi-axis capabilities and a longer machining time, helical milling remains invaluable in producing a flat bottom hole or making an existing hole larger. Since the cutting tool follows a programmed helical trajectory to remove material from the workpiece progressively, it minimizes defect rates, saves machining time, and optimizes the overall efficiency of the milled parts.
AT-Machining is a top CNC machining specialist with various high-performance helical milling cutters that deliver vibration-free cuts and superior surface finishes. Our skilled and experienced team of experts can help you drive innovation, unlock efficiency, and increase throughput in your manufacturing operations. Contact us today to learn more about how our expertise can help elevate your projects!
