Spotting Drills: The Overlooked Accuracy Tool

Spotting Drills vs Center Drills: The Right Tool for the Job

Many shops use 60-degree center drills as spotting tools because they're familiar and already in the drawer. Center drills are designed for lathe work — creating the conical seat for a live center to engage. When used for spotting drill locations on a machining center, they perform acceptably but not optimally, and the difference matters in precision work.

A dedicated spotting drill has a short, rigid body with a ground point geometry optimized for creating a precise conical indent to guide the following drill. Typical included angles are 90 degrees and 120 degrees. The 90-degree spot drill creates a starting cone that guides most standard 118-degree drills cleanly — the drill's 118-degree point seats partially in the 90-degree cone, self-centering as it enters. A 120-degree spot creates an even shallower cone that's better suited to 135-degree split-point drills.

The center drill's 60-degree included angle is narrower than most drill points. When a 118-degree drill tries to follow a 60-degree spot, it contacts the cone only at the very outer edge of the spot, rather than seating in the full cone geometry. This contact geometry allows more lateral movement of the drill tip at the start of the cut — exactly the situation spotting is supposed to prevent. The 90-degree spot drill eliminates this problem by matching the engagement geometry to the drill that follows it.

The Physics of Drill Walk and Why Spotting Stops It

Drill walk — the tendency for a drill to shift laterally from its intended starting location before it fully engages the work — is caused by lateral forces at the drill tip during the entry phase. These forces arise from asymmetries in the chisel edge engagement, from surface irregularities at the drill entry point (scale, burrs, curved surfaces), and from any small misalignment between the drill axis and the perpendicular to the work surface.

The chisel edge at the center of a conventional drill point doesn't cut — it extrudes material outward. During entry, the chisel edge is pressing against the unsupported work surface, and any asymmetry in that force is transmitted as a lateral force on the drill body. On a flat, square surface with a perfectly aligned drill, these forces balance. On any surface with irregularity, or with any misalignment, they don't — and the drill tip moves laterally in the direction of the net force.

A spotting drill eliminates this problem by pre-forming a cone in the work surface. When the main drill enters, its point seats in the cone rather than engaging an uncontrolled flat surface. The cone geometry constrains the drill tip laterally from the first moment of contact. There's nowhere for the tip to walk — it's mechanically constrained to the center of the pre-formed cone.

The effect on location accuracy is significant. In a typical setup without spotting, a sharp jobber drill will wander 0.003 to 0.015 inches from its intended start location depending on the surface condition and drill condition. With a proper spot, that wander drops to 0.001 inch or less — well within most tolerance callouts. In a 50-hole pattern, the cumulative effect of eliminating walk is the difference between a pattern that assembles easily and one that requires manual adjustment of every fastener.

Choosing the Right Spot Depth and Angle

How deep should the spot be? Deep enough to create a full conical seat for the following drill's point, but not so deep that you're removing material unnecessarily or creating stress concentrations. For most applications, the spot depth should be approximately 1/3 the drill diameter. A 3/8" drill needs a spot approximately 0.125" deep — enough to seat the drill point fully in the cone without drilling so deep that you're making a hole rather than a registration feature.

Angle selection: use 90-degree spotting drills as the default for 118-degree conventional and split-point HSS drills. Use 120-degree spots when drilling with 135-degree point geometry drills. Using a spot that's the same angle as the drill point is technically the worst match — the two cones engage only at the bottom tip with no cone wall contact, which provides no centering advantage over drilling without a spot.

Spotting drills should be solid carbide for maximum rigidity — a flexing spotting drill defeats the purpose by allowing the pre-formed cone to be off-center. Most quality spotting drill sets run 1/8" to 1/2" diameter in carbide and cost $10 to $25 each. They last virtually indefinitely in light spotting work because each use is only a fraction of a second at low chip load. The cost is trivial relative to the accuracy improvement they deliver.