Most cutting taps are available in three chamfers. Taper, plug and bottom are how they are referred to in most of the current literature. Why do we choose one over the other and what are the advantages?
Unlike a drill or center cutting end mill, taps introduce the cutting edges by means of a chamfer. Drills are cutting on the tip (axial), center cutting end mills plunge (axial) then may traverse to cut radially on the workpiece. Most end mills are cutting on the radius, but they are to leave a “straight” or “smooth” finish.
A tap must cut on the radius and leave threads in the workpiece. In order to do this a tap uses a chamfer to spread out the chip load per tooth and extend the life of the tool while providing a better surface finish to the threads in the workpiece. The primary considerations for choosing a chamfer are the type of hole (usually blind or through) and the machinability of the workpiece.
Taper Taps may be used when a through hole is being machined. This is particularly true when the material is tougher to machine. The gradual taper of the chamfer over 7 – 10 threads produces the thinnest chips of all three styles. They can be used in material that creates higher torque (tougher) because they generate the lowest amount of torque for the tool to handle. Usually, they will also provide the best finish of the three styles. The downside is there needs to be sufficient reach or clearance beyond the workpiece material for the tap to go in deep enough to reach the first full thread to the depth required.
Plug Taps are the most commonly used and provide the “Goldilocks” alternative for the two extremes. This style has a 3 – 5 thread chamfer and may be used in through holes and many blind hole applications. Many blind hole applications don’t require threading all the way to the bottom of the hole. This style produces what a machinist would call a “normal” chip and leaves a good finish. If one doesn’t know what style to choose, this would be the most logical default.
Bottom Taps are most often selected because the job requires threading to the bottom of a blind hole. With only 1 – 2 thread chamfer, there is a lot of material being removed (thicker chips) and performs better in lower torque material. Nothing is ideal when faced with a hole that has very little clearance and threads are needed all the way to the bottom of the workpiece, so the downside of this chamfer is usually a shorter life because of the greater stress on the limited number of threads that actually cut.
There is a “bonus” or fourth variation available called the Semi-bottoming Tap that is recommended when a difficult application is encountered in a blind hole. One won’t be able to reach the very bottom of the hole with threads, but this can be used in the situations that are not requiring the actual bottom tap.