Chasing or Re-threading Tapped Holes
Are you trying to "chase", or clean-up existing internal threads? Use a tap designed for the job at-hand.
When we at North American Tool are informed that a tap is to be used for " re-threading", another term for this task, we always recommend at tap with 0-3 degree Rake for the cutting edge.
Different Than Threading the First Time
When re-threading, very little material, if any, is being removed from the existing threads. It's possible that none of the substrate will be touched, especially when the need is to remove heat-treat scale or other contamination from the threads. Given that possibility, a tap utilizing general-purpose geometry, or any degree of hook face, will have difficulty generating a "chip" in a cutting action. The cutting edge will be weak, and typically break down. With a Rake cutting face, the edge will be better supported, with the ability to scrape the surface of the substrate when a chip can't be formed. This also works better when using the tap to remove contaminants (dirt, paint, sealant, etc...) from existing thread.
Inconsistent Forces Create a “No Guarantee” Situation
Even when the recommended geometry for re-threading is utilized, performance of the tap cannot be guaranteed. The forces acting on the tap are not consistent, as they are when new thread is created by a continuous cutting action.
Frequently, form taps are utilized to re-shape damaged thread. When this is attempted, the operator must be aware of a possible finish size issue. Forming taps are made larger than a cutting tap creating the same Class of Thread. The purpose is to compensate for material memory when the forming tap is removed. All formable material has a tendency to "spring back" to varying degrees when forced into a new shape.
Using form taps to clear contamination from existing threads may work to some degree, but will also have a tendency to "press" that contaminant into the thread form, which is likely not the goal.
In a pinch, when clean-up is necessary, using a tap made for Cast Iron or Brass may be a ready solution, as both materials require the geometry recommended for "re-threading". If special pitch diameters are required, a "special" tap should be ordered for the job.