How to Read a Drill Point: What the Tip Geometry Tells You About Your Bit's Condition

Most machinists know a dull drill when they feel it — the machine bogs, the chip color changes, or the bit starts walking. But the tip of a drill bit is actually a diagnostic tool. The geometry at the point records exactly how the bit failed and what it was doing when it did. Learning to read it takes maybe five minutes of practice, and it changes how you maintain tooling.

Here are the four things to look at and what each one tells you.

The Chisel Edge

The chisel edge is the short, flat cutting line right at the center of the tip — the section that bridges the two cutting lips. On a freshly ground bit, it's narrow and sharp. On a worn bit, it goes wide and rounded.

A wide chisel edge is the single biggest cause of drill thrust. That center section doesn't cut — it scrapes and pushes. When the chisel edge is worn, your spindle works harder, your machine deflects more, and your holes wander off center. If you're breaking small drills in cross-drilled setups, a worn chisel edge is usually the culprit.

What it tells you: the bit has been used past the point where the cutting edges were sharp, and the center of the point has been rubbed flat against the workpiece. This is normal wear — it's fixable with a regrind.

Lip Relief Angle

The lip relief angle is the clearance ground behind each cutting lip. It's what lets the edge bite in rather than rub. On most jobber-length HSS bits, proper relief is somewhere between 8 and 12 degrees depending on material and diameter.

When the relief angle is gone — either through regrinding too flat or through heat-induced collapse of the edge — the bit rubs instead of cuts. Symptoms: squealing, blue chips, rapid heat buildup at the tip, hole oversizing. The edge technically exists but has no rake to enter the material.

What it tells you: too many regrinds without proper relief restoration, or the bit was run too fast in a hard material and the edge softened. A competent regrind fixes insufficient relief. If the web has thickened too much from repeated shortening, the bit may be getting close to retirement.

Lip Height Equality

Hold the bit at eye level with the point facing you and look straight down the axis. Both cutting lips should be symmetrical — same length, same angle, same height. Any visual difference means one lip is cutting more than the other.

Unequal lips are the primary cause of oversized holes. The longer lip takes a heavier cut and pushes the bit off center on every revolution. A drill that should cut a 3/8" hole will consistently cut 0.005" to 0.015" oversize if the lips are mismatched. In close-tolerance work, this is a reject condition.

What it tells you: the bit was reground by hand without a guide, or it was ground on equipment that isn't properly set up. A machine regrind on a drill grinder corrects this immediately and repeatably. If you're seeing chronic oversized holes, check your lip height before blaming the spindle.

Split Point Wear

Split points — the four-facet geometry common on better HSS and cobalt bits — eliminate the chisel edge and reduce thrust dramatically. But they wear differently than standard points. The secondary cutting edges created by the split can round over, and the web thinning that creates the split can collapse under heat or abuse.

A worn split point often looks fine to a casual glance because the overall point shape is preserved. But under magnification or close inspection, the secondary edges are rounded and the bit starts behaving like a standard dull bit — increased thrust, wandering entry, poor chip formation.

What it tells you: if a split point bit is giving you entry problems and the main lips look okay, look closely at the secondary geometry. This can be restored with a proper regrind on equipment that handles split points specifically — a basic drill grinder that only does the main lips won't fix it.

What's Fixable vs. What's Retire-the-Bit

Fixable with a regrind: worn chisel edge, lost lip relief, unequal lips, worn split point geometry. These are geometry problems caused by use and dull edges. A proper machine regrind restores the original geometry.

Retire the bit: cracks in the flute or body, heat discoloration that runs up the shank (indicates the steel was overheated past its tempering point and has lost hardness), major chipping at the cutting edge from a catastrophic event, or a bit so short from repeated grinding that the flute depth is insufficient to clear chips.

The rule of thumb is this: if the geometry is worn, regrind it. If the steel is compromised, replace it.