Internal and External Screw Threads Screw Thread Form Internal Threads (Taps) Tap Wrenches Producing Internal Thread External Threads (Dies) Die Stocks (Setting Die) Producing External Threads

Screw Thread Form When cutting internal and external screw threads it is necessary to prepare the material correctly to enable an accurate thread to be cut, that is of the size and correct fit. (Not to loose and not to tight) When producing an external thread the material is cut to the major diameter of the screw thread, this is illustrated in the diagram below.

Screw Thread Form When producing an internal thread the hole needs to be drilled slightly larger than the minor diameter of the screw thread, this can be seen in the diagram below. This is to leave enough material for the thread to be cut. The size of drill used to cut this hole is referred to as the Tapping Drill. This size is obtained by using reference charts as shown below. Example M10 would be 8.50mm

Internal Threads (Taps) Internal threads are produced using a cutter called a Tap. There are 3 types of Tap these are the Taper Tap, the 2nd or Intermediate Tap, and the Plug Tap. The Taper Tap is used to start the thread, the first 8 threads are tapered (hence the name). This makes the thread depth very shallow at the start increasing in depth the further along the taper you move. This gives a good lead into the hole and helps to keep the tap square when starting to cut.

Intermediate (2nd Tap) and Plug Tap The next tap used is the 2nd or Intermediate Tap. This is tapered along the first 4 threads and produces a full form of thread from then on. This is used to produce the full form of thread to a greater depth in blind holes prior to using a plug tap or can be used as a finishing tap in thin material. The last tap is the Plug Tap this only has the first one or one and a half threads tapered and is used to produce the full form of thread in blind holes (where the hole does not go right through the material or as finishing cut in thicker material.

Tap Wrenches To cut the thread the Tap is held in a Tap Wrench. Tap Wrenches come in different sizes depending on the size of Tap being used. For small Taps a ‘T’ handle or chuck Tap Wrench is commonly used, For larger Taps a standard Adjustable Tap Wrench is used. The tap is held in the Tap Wrench by inserting the square section at the top of the tap into the jaws of the Wrench as shown.

Producing Internal Thread The Tap is inserted into the hole as shown, and twisted clockwise whilst applying slight downward pressure. It is necessary to use cutting oil or compound to ensure a good clean thread is produced. It is also necessary to ensure the tap remains square at 90 degrees to the work. When the thread cut reaches around 3 threads deep the tap is rotated anti-clockwise half a turn to break the Swarf, then rotated clockwise another turn forwards. This process is repeated until the tap has gone right through the work or reached the bottom of the hole in the case of blind holes, Tap Wrench is then rotated anti-clockwise to remove the Tap.

External Threads (Dies and Stocks) External threads are produced using a cutter called a Split or Button Die. The Split Die has the size stamped on the leading face. The Split Die is held in a Die Holder or Stock during the cutting process, these come in a range of sizes depending on the size of die being used

Setting up the Die in the Stock To set the Die correctly undo screws (A and B) and insert Die into stock with leading edge facing up and the split in line with the centre screw. Tighten centre screw (B) to fully open the Die and lightly pinch outer screws (A) on either side. Using a bolt of correct size try to screw it into the Die (it should not be too tight or too loose). If it is tight undo outer screws (A) and tighten centre screw (B) until bolt fits in then pinch up outer screws (A) again. If it is loose undo centre screw (B) and tighten outer screws (A) until bolt screws in but ensure it is not too tight or too loose.

Producing External Threads The material to be threaded needs to have a chamfer cut on it to provide a lead for the Die. When set correctly the Die is placed over the material, leading face down, the stock is rotated clockwise applying light downward pressure Cutting oil or compound is used to give a good clean thread. The die needs to remain square to the material during this process. When thread is cut to a depth of 3 threads stock is rotated anti-clockwise half a turn to break the swarf, then rotated another turn clockwise. This process is repeated until the correct length of thread has been produced, then stock is rotated anti-clockwise to remove.