Tapping Basics
There are a number of cutting tools for machine work. One of them is called a Tap. Taps are used to create a screw thread or threads. Dies are used for the same thing. There actually are female and male parts. This is called a mating pair. The tap is used to cut the female portion and the Die is used for the male side. To cut the threads or the process of doing it is called Tapping. If using a Die, the process is called threading. If the thread needs to be cleaned, this is called chasing. Then, both tools maybe used.
So, the female surface is made by a cut in the inside of a hole. This is a thread made by a Tap. The new surface will function like a nut for example. There are three types that machinists use: Bottoming Tap, Taper Tap, and an Intermediate Tap.
The Bottoming Tap has almost no taper and has a cutting edge that is continuous. The normal taper is from one to one and half threads. This means a tap can cut to the very bottom of a blind hole. There are already threads that have been started. A bottom tap is way too short to be used in a hole that is not threaded.
An Intermediate Tap uses an untapped hole. It has cutting edges that are tapered which help the aligning process to start the tap. The numbers of threads range from three to five. A Taper Tap is more pronounced and less aggressive on the cutting action. The number of threads usually is eight to ten. This tap is only used if the material is prone to breakage or very hard to work with. For example, alloy steel.
In machine tapping, tapping can be accomplished by using a machine or by hand. If done by hand, there are three tapping processes. If a machine does the work (which is the most common), radial drilling machine, lathe, pillar type drill machine, bench type drill machine, vertical milling machines, VMC’s, and HMC’s are normally used. Human mistakes are eliminated and work gets done faster and is more accurate. The final tap is done with a single tap. Even though tapping is used through a machine, it is still tricky to do because of breakage and quality is inconsistent.
The reasons for tapping breakage are: using a non-standard quality tap, tap geometry is improperly used for the application, worn out taps, chips clogging the machine, not enough cutting fluid or misusing it, not limiting torque, misalignment of the hole, tap and machine feed do not match which causes breakage in compression and tension, spindle speed is improperly set, and with screw machines either there is zero or improper float usage.
Special holders for tools are now being required to minimize the chance of breakage during the tapping process. They are CNC and conventional tool holders.