Overcoming Torqueing and Stalling
Are your special taps creating too much torque, or stalling? Teeth chipping? Common machinist’s wisdom says, “when having difficulty tapping a drilled-hole, make the hole diameter larger”. This assumes that the tap, with less material to remove, will cut easier. It’s common sense at first glance, but other things need to be considered. Like a cork in a bottle, the tap “fills” the hole gradually. It has a point-diameter that is fixed in manufacturing. The point has to be smaller than the hole to allow gradual entry of the tap. The chamfer ground from that point also has a defined length; 1-2 threads for a Bottom Tap, 2-2.5 threads for a Semi or Modified Bottom, 3-5 threads for a Plug chamfer, and 7-10 for a Taper chamfer. The chamfer is ground at an angle. This angle, in concert with the helix of the thread, allows each cutting tooth in contact with the hole to remove gradually more material as the tap rotates, until it reaches full diameter.However, increasing hole size allows the chamfered section of the special tap to drop further into the hole before making contact. This reduces the available chamfer length to do the work of the tap. Even though the increase in hole size has reduced the amount of material available for removal, it also reduces the finished percentage of thread remaining. At some point, thread strength could become a concern as percentage of finished thread is reduced. Ideally, all available teeth of the chamfered portion of the tap are involved in the cut for maximum efficiency. As with most things in life, ideal conditions are rare. The goal is to do the best you can with what you have to work with.
In a nutshell, if a difficult tapping application is made easier by increasing the hole size, go with it. If the adjustment shows no improvement in the performance of the tap, maybe the point described above offers an explanation.