Screw cutting

Screw cutting

The screw thread is a very important detail in engineering. It is used to hold parts together. (e.g. bolt & nut) and to transmit power (e.g. vice screw).

Internal threads

To achieve an internal screw thread, a hole has to be drilled first and then a tool called a
TAP is used to cut a thread within the hole. TAPS are made from high speed steel (HSS). The top of the tap is square which enables the tap to be held securely in a TAP WRENCH, which can be seen below. Taps are generally available in sets of three and are used in the following order:-

  1. Taper Tap
  2. Intermediate tap
  3. Plug Tap
Tap Wrench

As has been mentioned, taps are used in sequential order. As can be seen in the taps above, the Taper tap has much smaller teeth at the bottom than the Intermediate or the Plug taps. This allows the taper tap to get started by making a shallower thread cut. The taper cut is followed by the intermediate tap which has slightly more teeth. Finally, the Plug tap is used which will make the full thread cut. The plug tap also allows full untapered threads to be cut successfully in blind holes.

Remember that it is important to use a cutting compound to make sure that a clean, well finished thread is achieved.

Hole preparation

When tapping a thread in an internal hole the actual diameter of the hole to be drilled must  be smaller than the actual overall size of the thread to be cut. An explanation of this is shown in the sketch below.

The drawing above shows that if a hole was drilled which was the same size as the threaded bar, the bar would just fall through. The hole which must be drilled must therefore be smaller in diameter so as to allow the TAP to cut the threads.

The table bellow shows the diameter of hole which would be required to be drilled prior
to tapping. E.g. if an M5 (Metric 5mm) thread has to be cut, the size of hole to be drilled will be 4.2mm.

Blind Hole

A blind hole is a hole which has a bottom to it. If a blind hole is to be threaded it is very important to ensure that the depth of the hole is established before commencing to thread the hole. If this is not established it would be very easy to break the taps. A piece of tape attached to the tap indicating the depth is an ideal way of avoiding the tap from being broken by being forced into the bottom of the hole.

External Threads

In order to cut an external thread on a metal rod a tool called a DIE is used.  Most dies that you will encounter will be ‘Split Dies’.

Circular Split Dies

The picture below shows a split die, this is the most common type of die used in the school workshop. These are used for cutting external threads. The die is made from high
speed steel (HSS). To assist in starting the thread cut, the split die has a split which enables the die to be opened slightly thus cutting a shallower cut. This can then be adjusted to cut the thread deeper as required to suit the job.

Die Stock (Holder)

The circular split die fits into the die stock with the tapered side of the thread (shown by the writing on the die) facing outwards. The split in the die fits opposite the centre screw to allow the  opening and closing of the die. The two screws at the side hold the die in the stock To ensure the die can start to create a thread on the rod the rod must firstly be tapered or chamfered at the end.

To use the die stock properly the surface of the die that has the text on it should be presented to the end of the metal rod and the die stock turned clockwise to begin the process of cutting the thread.  It is important to keep the die stock arms at 90 degrees to the rod otherwise a ‘drunken thread’ will be produced.  The process of cutting the thread involves turning the handles clockwise for a half revolution, then back (anticlockwise) by a quarter revolution in order to break away the cuttings that are produced before repeating the process.

As with using a tap and tap wrench it is important that a suitable cutting compound is used to achieve a clean accurate thread.