Why Drill Point Symmetry Matters (And How to Check It)

Drill point symmetry is the most overlooked variable in hole quality. A bit with a perfectly correct included angle but asymmetric lips — where one cutting edge is longer than the other, or where the lips aren't at the same height — will produce holes larger than the drill diameter, off-center holes, and accelerated one-sided wear.

It's a subtle problem that shows up loudly in the work.

WHAT SYMMETRY MEANS EXACTLY

A symmetrical drill point has three matched characteristics across both cutting lips:

• Equal lip length — both cutting edges, measured from the center to the outer corner, are the same length
• Equal lip angle — both lips are ground to the same included half-angle relative to the drill axis
• Equal lip height — both lips are at the same axial height, meaning neither leads the other during the cut

All three must match. A drill can have equal lip length but unequal height — and still produce oversized holes because one lip does more work than the other.

HOW ASYMMETRY HAPPENS

Free-hand sharpening is the main cause. Even experienced machinists struggle to maintain perfect symmetry grinding by hand — the angle changes slightly between one lip and the next. Small differences in hand pressure, bit rotation, or wheel contact time produce asymmetric geometry.

Wear also creates asymmetry. If a bit drills at a slight angle, one lip contacts the work harder than the other and wears faster. The result is an asymmetric point that makes the problem worse in subsequent holes.

HOW TO CHECK IT

The simplest check is a drill point gauge — a tool with an engraved V-groove and degree markings that lets you visually compare each lip angle against a reference. Place the drill in the gauge, check one side, rotate 180°, check the other. They should match.

For lip height, use a surface plate and height gauge. Stand the drill vertically on the body (not the flats) and measure each cutting lip at the same radial position. A feeler gauge between the lip and a reference surface gives you the height difference.

Under magnification, asymmetric bits are often obvious. One lip has a wider shiny zone (indicating more wear or a different relief angle) than the other.

WHY MACHINE GRINDING MATTERS HERE

A WinsloMatic or similar CNC point grinder maintains geometric control over all three symmetry parameters simultaneously. The bit is fixtured, the grinding cycle is controlled, and both lips receive identical treatment. The result is a measurable, repeatable point geometry.

This is why machine-resharpened bits often outperform hand-resharpened ones — not because of the grinding wheel or the grit, but because the symmetry is reliable. A symmetric point makes on-size holes. An asymmetric one doesn't, regardless of how sharp the edge feels.

When MachinistPost returns a resharpened bit, the point symmetry has been set by the machine, not by feel. That's the difference you see in hole size consistency and tool life.

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