Hand Threading and Reaming - Taps
A Tap is a tool used to cut a threaded inside a hole.  The hole is drilled and then reamed to the correct size.  The tap is made from high speed steel and has sharp cutting edges with the required pitch and diameter to cut female or internal thread.

Hand taps
A Tap is like a bolt made from High Speed Steel (HSS) with perfect thread cut on to it.  The threads are not full because flutes have been cut into the tap length to give it a sharp edges.  The flute also allow for chips (cuttings) to be removed from the cutting area of the tap

Type of hand taps
Hand tapes are supplied in a set of three. They all have straight flutes. The three taps are normally referred to as:
  • First or taper tap - The first tap is used to start the thread cutting procedure.  The first 8 to 10 threads are ground to a taper so that at the point the diameter of the point is slightly less than the root diameter of the thread. This enable the tap to enter the hole.  The following tooth will cut progressively deeper until the tap reach the end of the taper portion.  This tap will leave a taper in the bottom of a blind hole.
  • Second, intermediate or plug tap - The second tap is to cut the taper section deeper. The first 3 to 5 threads are ground to a taper
  • Third tap. It is also referred to as a bottoming tap or finishing tap - The third tap is used to eliminate the taper section.  This allow to cut thread parallel all the way down in a blind hole. The first 1 to .5 threads are ground to a taper

 
Tap markings
Taps are marked with the nominal size, the pitch, and the appropriate thread form symbol and pitch diameter symbol. Symbols typically used are listed. All standard ground thread taps are marked with the letter "G" to designate Ground Thread.


 

The tap above heave the following markings: M10x1 HSS. This markings have the following meaning:

M - metric thread
10 is the major diameter of the thread = 10 mm
1 is the pitch of the thread. - 1 mm pitch
HSS, refer to the tap material. High speed steel.

 
Tap and drill size

Before a hole can be threaded, the hole must be drilled.  The hole size must be smaller then the major diameter of the thread as to leave enough material for the thread to be cut.  The hole diameter is normally calculated by subtracting the pitch from the major diameter.  Hole Demeter = Major diameter - Pitch

For example: M10x1

Drill diameter = Major Diameter - Pitch
                      = 10 - 1
                      = 9 mm

Course thread is for general fasteners - Bolts and Nuts
Fine threads can be more easily tapped into hard materials and thin-walled tubes. Fine threads require less torque to develop equivalent bolt preloads. Fine threads have less tendency to loosen since the thread incline is smaller and hence so is the off torque.
Tap wrenches
A tap wrench is a hand tool used to turn taps or other small tools, such as hand reamers and screw extractors.  There are two main type of tap wrenches, namely the bar type and the chuck type.

The correct size tap wrench must be used to ensure the correct 'feel'.  If the size is to large, you can easily break the tap.  If the tap wrench is to small, excessively heavy force will be required to turn the tap
   
BAR TYPE CHUCK TYPE