Prolonging Tool Life of Deep Hole Drills by Using the Appropriate Pilot/Starter Drill

Selecting the appropriate pilot drill is critical to maximizing the performance and tool life of a succeeding deep hole or micro drill. The purpose of a pilot drill is to create a clean, consistent working surface for the succeeding drill to enter the workpiece.

Important aspects such as diameter tolerance and point angle (Figure 1) are often overlooked leading to inconsistent, unacceptable results.

A pilot drill diameter should be slightly larger than the nominal diameter, whereas the succeeding drill diameter should be smaller. This is often accomplished using positive and negative tolerances respectively (Figure 2). Selecting the incorrect pilot drill diameter can lead to premature failure of the succeeding drill as it will enter the pilot hole using its margins rather than its center-cutting point.

It is also important for a pilot drill’s point angle to be equal to or larger than that of a succeeding drill. The purpose of a larger point angle is to guide the succeeding drill to the centerline while preventing its corners from colliding with the point angle left by the pilot drill. Using a pilot drill with a smaller point angle usually results in chipped corners on the more expensive deep hole drill after drilling 10-20% of the expected tool life.

Pilot drilling
Figure 1. Three examples of a subsequent drill entering different pilot holes. The preferred situation with a pilot hole that has a larger diameter and point angle is identified with a green check mark.
Pilot drilling
Figure 2. Demonstrates one method of achieving the appropriate pilot hole diameter. The pilot drill and succeeding drill have equivalent nominal diameters, but tolerances prevent the pilot hole from being smaller than the deep hole drill diameter.

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