A tap can break inside a drilled hole during a machining task, and a broken tap creates a dificult situation for machinists. A tap often break because the pilot hole are too small for the tap to function corectly. If the pilot hole is to small, the tap must exert a high amount of pressure to cut the threads, and this high pressure cause the tap to snap.
Conversely, if the pilot hole is to large, the threads will not have enough material to grip, and the screw will not hold secure in the material. A person must balance the size of the pilot hole to ensure the screw has enough thread engagement. Thread engagement refer to the amount of contact between the screw threads and the material.
How to Choose the Right Pilot Hole
Many machinists aim for 75% thread engagement because 75% thread engagement provide sufficient strength for a load while also reducing the stress on the tap. To calculate the correct pilot hole size, a person must consider the major diameter of the screw and the number of threads per inch. However, the specific material being used will change how a person choose the pilot hole size.
Stainless steels is a material that is prone to work hardening, and work hardening means the stainless steel becomes harder when a tool touches the stainless steel. Because stainless steel becomes harder during the process, a person may need to use a larger drill bit to prevent the tap from breaking. Aluminum is a soft material, and because aluminum is soft, a person may choose a slightly smaller pilot hole to ensure the screw has enough thread depth.
Therefore, the material type determine the specific pilot hole size that a person should of use. A person must distinguish between a clearance hole and a tapped hole. A clearance hole is a hole that is larger than the screw so that the screw can pass through the hole without touching the sides of the hole.
A tapped hole is a hole that have threads cut into the walls so that the screw can grip the walls of the tapped hole. For example, if a person is joining two metal plate, the top plate requires a clearance hole, but the bottom plate requires a tapped hole. If a person drills a clearance hole where a tapped hole should be, the screw will not grip the material, and the assembly will be loose.
Technique is important when a person is drilling a pilot hole. A person should use a center punch to create a small dimple in the metal before drilling, because a center punch prevent the drill bit from wandering across the surface. If the drill bit wanders, the pilot hole will be crooked, and a crooked pilot hole will result in slanted thread.
Additionally, a person must use cutting fluid during the tapping process. Tapping create high levels of friction, and friction create heat. Because heat can cause a tap to seize and snap, a person must use cutting fluid to reduce heat and reduce friction.
When a person works with a blind hole, the person must be careful about the depth of the hole. A blind hole is a hole that does not go all the way through the material. A person must drill a blind hole deeper than the intended depth of the screw because the tap need extra space to clear out metal chips.
If the tap reaches the bottom of a blind hole before the tap has finished cutting the threads, the tap will jam and the tap will break. Thus, drilling a deeper blind hole allow the tap to clear the metal chips and prevents the tap from jamming.
