Making Threads

Dallas, Texas – August 18, 2008 — With our new Drill Bit and Tap combo kit soon to be released, we thought it might be helpful to review the basics of taps and cutting threads.
Please review the following before selecting your choice in a tap. Your local Hi-Line territory Manager will be happy to help in your selection.
A tap cuts a thread on the inside surface of a hole, creating a female surface which functions like a nut. These three taps are the basic types commonly used by most machinists:

*Bottoming Tap: (TBS prefix) Has a continuous cutting edge with no taper. This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is never used to cut threads in an unthreaded hole, as the cutting edges lack the taper required to successfully start into such a hole.

*Plug Tap: (TPS or TPP prefix) Also known as an intermediate tap has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole. Plug taps are the most commonly used type of tap.

*Taper Tap: (TTS prefix) Is similar to a plug tap but has a more pronounced taper to the cutting edges. This feature gives the taper tap a very gradual cutting action that is less aggressive than that of the plug tap. A taper tap is most often used when the material to be tapped is difficult to work (e.g., alloy steel) or the tap is of a very small diameter and thus prone to breakage.

The above taps are generally referred to as hand taps, since they are, by design, intended to be manually operated. During operation, it is necessary with a hand tap to periodically reverse rotation to break the chip formed during the cutting process, thus preventing an effect called “crowding” that may cause breakage. Periodic reversing is usually not practical when power tapping is involved, and thus has led to the development of taps suitable for continuous rotation in the cutting direction.
The most common type of power driven tap is the “spiral point” plug tap (TPT prefix) (also referred to as a “gun tap”), whose cutting edges are angularly displaced relative to the tap centerline. This feature causes the tap to continuously break the chip and eject it into the flutes, preventing crowding.
Whether manual or automatic, the processing of tapping begins with forming and slightly countersinking a hole (usually by drilling) with a diameter slightly smaller than the tap’s major diameter. The correct hole diameter may be determined by consulting a drill and tap size chart, a standard reference item found in your start-up supplies. If the hole is to be drilled, the proper diameter is called the tap drill size.
In metalworking, the use of a tap lubricant (AS33, AS33C, AS33L) is essential to achieve cleanly formed threads and to minimize friction. Failure to use the correct lubricant may result in ragged threads, as well as a substantial increase in the amount of torque required to turn the tap, possibly resulting in breakage.