A significant majority of the threads produced in manufacturing today are machined with cutting tools: typically taps and thread-mills for the internal threads, and threading dies and thread-mills for the external threads. These tools are the historical standard for this purpose and a familiar go-to for machinists. As machined materials and applications change with customer demand, alternatives for tooling follow suit. Although both cutting tools and forming tools produce essentially the same thread, and are gaged in the same manner, the requirements for their use, and results achieved, are in many ways different.
Historically, internal threads were created by cutting taps. These are tools that are designed to remove material from the hole leaving a finished internal thread form in the geometry intended. These tools utilize a feature, known as the chamfer, to achieve a gradual cutting action as the tap enters the hole, and flutes to allow room for the chips created by this cutting action and coolant or lubricant to assist in the process of cutting and removal of material from the flutes.
Forming taps create thread by displacement of material within the hole. The formed material assumes the shape of the thread form of the tap without the creation of chips. Flutes are replaced by lubrication grooves to provide some relief from the friction created during the process. Other features of the cutting tap, like chamfer, are altered to address the forces required to cause the material being threaded to assume a new form. Tap Lands separated by flutes are replaced by high areas called lobes. These lobes are relieved to reduce surface contact with the work material. Friction reduction is a major objective in successfully forming threads. They are also a likely solution to the challenges of tapping “blind” holes: as forming taps produce no chips, there are no issues with chip interference and removal.
Forming taps require a slightly larger hole, as the material being threaded flows into, as well as away from, the threads of the tap. The finished minor diameter of the hole will be smaller than the unthreaded hole when the process is complete. It should also be understood the Forming taps run better at speeds 1-1/2 to 2 times faster than a cut tap, and require much more attention to lubrication of the tool, as friction between the tap and the workpiece can create problems with generated heat and torque forces imposed on the entire set-up.
It is universally accepted that formed threads are stronger than cut threads. When threads are formed, the grain of the material is compressed, particularly at the root and crest of the thread form. While a cut material’s grain structure has been “fractured” by the cutting action of the tool.
Forming taps are limited in use to those materials malleable enough for the process, like non-ferrous metals, aluminum, and soft steel. They also produce a thread-form that is slightly different in appearance when compared to thread produced by a cutting tool.
This type of tap is also a likely solution to the challenges of tapping “blind” holes: as forming taps produce no chips, creating no issues with chip interference and removal.
Cutting taps are more versatile, as the geometry of the tool can be altered to match the characteristics of the material being tapped. They are not limited to the materials suiting a form tap. Materials like Cast Iron and Phenolic Plastics, which are not candidates for forming, can be addressed with a Cutting Tap. Cutting taps may be a better choice for “through-holes” as they won’t require a separate operation to repair the distortion at entry and exit caused by the forming tap. Cutting taps require less machine horsepower than a forming tap, a major consideration as tap diameter increases.
We’ve touched on just a few of the basic differences between these types of taps. Each style has its benefits and limitations. The decision on which to use is usually determined by the limitations of the job, but often influenced by the experiences of the machinist.