Thread milling creates threads on the interior of an existing hole or on the exterior surface of metal. Unlike tapping, milling must be done on a machine. Milling threads, as opposed to tapping them, enables diversity and a higher quality finished product.
Prior to milling a thread, machinists drill the hole into a piece of metal. The hole is generally larger in diameter than that of the cutting tool. A computer numerical control (CNC) milling machine is used to accurately 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 may move in a circular motion in the x and y planes while the cutting tool moves up and down on the z axis while spinning. The thread miller bed may also remain stationary while all the necessary movements are programmed into and performed by the cutting tool itself.
Thread milling generally requires either a single point or multiform 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 hole circumference while moving upward. The threads are typically completed in one pass. Programming the machine enables the tool to cut the desired number of threads per inch (tpi).
Thread tapping also requires a predrilled hole. In this instance, however, the hole is usually smaller than the cutting tool. Creating threads using this method may involve a tapping wrench or a milling machine. The bit, held in place by the chuck, performs the cutting action as it enters the hole. Resembling a screw with spiraled threads, the bit contacts all of the hole edges at once.
Once the bit passes through the bottom of the hole, reversing the action removes the cutting tool. Compared to thread tapping, thread milling is the more versatile of the two methods. While each tapping tool can thread only a single size of hole, milling operations function 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 contacts less surface area, requiring less torque from the motor. Thread milling, unlike thread tapping, allows both left and right handed threads to be produced.
Another advantage of thread milling over thread tapping is the smaller size of the metal shavings. The smaller debris particles created by thread milling leave the cutting easily and quickly. Unhindered by these metal shavings, the cutting tool is able to make smoother, straighter cuts.