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Thread milling is a method of creating threads in metal using a CNC milling machine. It allows for diversity and higher quality finished products compared to tapping. Machinists drill a hole and use a cutting tool to mill the thread, which can be done for any size hole. Thread milling puts less stress on the machine and produces smaller metal chips, resulting in smoother cuts and the ability to produce both right-hand and left-hand threads.
Thread milling creates threads within an existing hole or on the outside surface of the metal. Unlike tapping, milling must be done on a machine. Milling threads, rather than tapping them, allows for diversity and a higher quality finished product.
Before milling a thread, machinists drill the hole in a piece of metal. The hole is usually of a larger diameter than that of the cutting tool. A computer numerical control (CNC) milling machine is used to precisely mill the thread. The machine performs a combination of movements, along the x, y and z axes, cutting the spiral of the thread. The metal lying on the machine bed can move in a circular motion in the x and y planes as the cutting tool moves up and down in the z axis as it rotates. The bed of the thread mill can also remain stationary while all necessary movements are programmed and performed by the cutting tool itself.
Thread milling generally requires a single point or multi-point cutting tool, held in place by the chuck. The machine inserts the tool into the bottom of the hole and rotates the bit around the circumference of the hole as it moves up. Threads typically complete in one pass. Machine programming allows the tool to cut the desired number of threads per inch (tpi).
Tapping also requires a pre-drilled hole. In this case, however, the hole is usually smaller than the cutting tool. Creating threads using this method may involve a tap wrench or a milling machine. The tip, held by the chuck, performs the cutting action by entering the hole. Similar to a screw with a spiral thread, the bit touches all edges of the hole simultaneously.
Once the bit has passed through the bottom of the hole, reversing the action removes the cutting tool. Compared to tapping, thread milling is the more versatile of the two methods. While each tapping tool can only thread a single size of hole, milling operations work for any size hole as long as the diameter exceeds that of the cutting tool. Thread milling puts less stress on both the bit and the machine, as each cut touches less surface area, requiring less torque from the motor. Thread milling, unlike tapping, allows you to produce both right-hand and left-hand threads.
Another advantage of thread milling over tapping is the smaller metal chip size. The smallest debris particles created by thread milling leave the cut easily and quickly. Unhampered by these metal chips, the cutting tool is able to make smoother, straighter cuts.
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