Continuing technical progress in machine tool technology has resulted in thread milling becoming an increasingly popular process within the global metalworking manufacturing industry. Today’s CNC machine tools provide effective cutting by helical interpolation, and it is this advantageous feature that supports the growing trend of thread generation using a milling process.
Although tapping continues to be the most commonly used internal thread-generation process, thread milling recently has ended tapping’s domination as the main method of cutting threads in relatively small-diameter holes.
Even though tapping is considered a highly productive process, it has both pros and cons. The main problem encountered when tapping is chip evacuation. A long chip can clog the flute of a tap, which may cause the tap to break in the hole, and, as a result, possibly cause an entire machined part to be scrapped. However, during thread milling, effective chip evacuation is achieved without difficulty.
High material hardness is another common impediment to efficient tapping. Although the majority of today’s taps are not suitable for the machining of hard materials, thread mills produced from solid carbide considerably expand the range of hard materials that are able to be tapped.
Thread milling also is a versatile process. For instance, a single multiple-form thread mill is able to produce screw threads with the same pitch in holes of different diameters. In addition, a single-point thread mill of a partial profile is suitable for machining threads in accordance with various standards, such as ISO metric and American National.
In comparison, the dedicated nature of a tap means that it can only be applied to a thread of a specific diameter and pitch.
Also, a thread mill is an excellent tool for making threads in blind holes.
Thread milling also enables the ability to overcome traditional tapping problems, such as tool bending, wear, and material “springiness.”
Despite the growth of thread milling, traditional tapping has important advantages, including ease of operation and minimal work for a machinist, which explain its continuing widespread use. The global industry has so widely accepted thread milling as a viable option that it is simply impossible to imagine manufacturing without mills that make threads.
The world’s cutting tool producers now offer manufacturers a choice of different thread mills, including indexable and solid-carbide tools.
An indexable tool carries replaceable carbide inserts and may have a design configuration such as a mill with shank or a shell mill with a central bore for mounting on an arbor.
The main advantage of indexable tools is their great versatility. With an insert chnage, the tool can turn into a cutter for milling external or internal threads, which meet the requirements of different standards. Moreover, this process may enable machinists to find a more effective cutting geometry, depending on the workpiece material. These tools also can have internal channels to enable the delivery of an effective coolant supply into the cutting zone.
The helical cutting edge of the inserts common in today’s tooling helps ensure an easy and light cutting action, improving the performance of the tool. In addition, inserts with smooth helical cutting edges can be mounted onto the tools to perform finish milling of square shoulders, and as a result, they can successfully compete with 90-degree milling cutters.