When should thread milling be used instead of tapping to create threads?

Thread Content

With the development of CNC thread milling technology, particularly the advent of 3-axis machining centers, CNC thread milling has gradually gained widespread recognition in the machining industry. Furthermore, it is well known that threads can be obtained using the traditional thread machining methods we are familiar with, among which tapping is the most similar to thread milling. Because they both form threads through the relative rotational motion between the tool and the workpiece. So, when faced with different working conditions, how do we choose the appropriate approach? This article tells you their exact meaning. Conditions for CNC thread milling: 1. A machining center with 3-axis motion (or more). 2. The thread length should not exceed 3 times the length of the tool’s cutting edge. Advantages of CNC thread milling: 1. Thread mills can be used to manufacture threads of different diameters but with the same shape. For example, by using a threaded mill to change the interpolation radius to machine M15x1.0, M18x1.0, and M20x1.0 threads, the number of tools required can be reduced, tool-changing time can be saved, efficiency can be improved, and tool management can be simplified. 2. It improves the accuracy and finish of the threads. Thread milling is accomplished through the high-speed rotation of the tool and spindle interpolation. The cutting method is milling; the cutting speed is high, and the resulting threads are aesthetically pleasing ； The tap has a low cutting speed and produces long chips, which can easily damage the inner hole surface. 3. The internal thread facilitates discharge. Milling threads involves chip breaking; the chips are short, and the diameter of the cutting tool is smaller than that of the thread hole being machined, thus allowing for smooth chip removal ； During continuous cutting with a tap, the chips are long, and since the diameter of the tap is the same as that of the hole being machined, it is difficult to remove the chips. 4. If a tap is used, it is certainly possible to use electric discharge to break off the broken part, but the process will be very complex; if damage is caused to the part, there will be losses. If a threaded milling cutter is used, first of all, it is not easy to break due to the low force applied ； Even if it breaks, since the diameter of the machined hole is larger than that of the tool, the broken part can be easily removed. In terms of product output, thread milling is much higher than tap cutting. 5. It is not easy to form viscous chips. For softer materials, sticky chips are easily generated during processing, but thread milling rotates at high speed to break the chips. The tap’s cutting speed is low; the entire thread and the machined surface are involved, which makes it easy for debris to stick. 6. The machine is required to have low power consumption. 7. Due to chip breaking during thread milling, there is partial tool contact resulting in low cutting force; whereas with full thread contact of the tap, the force is high, requiring a powerful machine. 8. Damaged cutting tools are easy to handle. Firstly, thread mills experience less stress and rarely break. If this occurs, since the machining aperture is larger than the tool, the broken part can be easily removed ； The tap is subject to high stress, has poor chip removal, is prone to breaking, and results in a larger hole after breakage. It is a bit easier to handle; however, small holes present difficulties. For example, when machining ordinary threads, thread milling is not cost-effective considering the cost per unit. Plain threads are classified as those with a general hardness of less than 50HRC and a diameter of less than 38 mm, although this is not a clear dividing line. Ordinary tap screws are generally made of high-speed steel, with market prices ranging from a few dozen dollars; however, the price of thread milling cutters is more than 10 times higher, and their service life per piece is no more than 10 times that of tap screws. Secondly, the aspect ratio should not be too large; typically, an L/D ratio of 50 is required. This makes it suitable for thread milling, as the cutting process generates chips, and the local contact area with the tool is small. The cutting inserts are made of cemented carbide, which results in low wear and a long service life. Ordinary high-speed steel taps simply cannot be used for this purpose. For example, using solid cemented carbide taps is not inexpensive, with costs being similar to those of thread milling cutters. Based on our existing machining experience, the efficiency and cost-effectiveness of thread milling are definitely higher than those of taps. 2. Machining of composite holes (with chamfers) is also suitable for thread milling. Thread mills have many functions and can be integrated into threading and chamfering. 3. Thin-wall machining is suitable for thread milling; the machining force exerted by thread mills is low, resulting in minimal deformation. Additionally, the bottom hole can be made flat, and the threads can be placed close to it, thus requiring very little space. 4. For machining with high thread precision, thread milling offers a higher thread speed, excellent chip removal performance, higher thread precision, and better surface finish, making it more suitable for this type of processing. 5. The material is soft and made of titanium alloy, making it suitable for thread milling, as thread mills do not tend to generate sticky debris. For unstable cutting, the thread mill can fully adapt to such conditions, as its cutting principle is inherently intermittent milling.