Taps & Dies

Hand Taps 

Three taps constitute a set –

  1. Taper – use this tap to start the thread.
  2. Intermediate – use this tap continue the thread.
  3. Plug – use this tap to thread to the bottom of a blind hole.

See the image below Fig1 showing a full set – look at them closely to understand why its important taps are used in the correct sequence. For example if try to start a thread using the plug tap first its clear that there is no lead-in “Taper” to establish the thread. Remember threads need to conform to specific standards for example BS3643.  So the consequence of using a plug tap to start a thread could be –

  • Misaligned  thread “Drunk thread”
  • Poor quality thread.
  • Broken tap
  • Work-piece  scrapped as broken taps almost always cannot be removed.
  • Injury to self or others as taps shatter when they break.

 

Fig 1.

Sometimes you may need to use a Intermediate tap to start a thread. Why? For example the Bench fitting tap wrench project  requires a M-6  thread into a blind hole with a thread depth of 7mm, the hole is around 9mm deep. If a taper tap were to be used the bottom of the tap would contact the bottom of the hole without full thread form being achieved due to the length of the taper. The operator may not realise the tap had hit the bottom and continue to rotate the tap thus stripping the partially formed thread away.

So in the case of tapping a blind hole relatively shallow, use the Intermediate tap to start then the Plug tap to finish. Caution must be exercised when using the plug tap when approaching the bottom of the hole. Most tap breakages occur in this situation as the operator continues to rotate the tap wrench when the tap has bottomed out. Tips to avoid this include being aware of  how deep you are tapping and feel for the increasing pressure on the wrench which maybe quite subtle. Also don’t forget to follow correct tapping procedures – one rotation clockwise one rotation counter clockwise (RH Threads). Use the recommended cutting compound depending on the material being threaded. Clear the debris from the hole & tap.