What Causes Carbide Drills to Chip Out When Drilling for a Tapped Hole?

Carbide drills chip out at the margin or the cutting edge when the drill flexes or the exit breaks through unevenly, and both are more likely on tap-drill sizes because those holes are usually deeper relative to diameter and closer to full engagement than a clearance hole ever is. Carbide has excellent compressive strength and terrible tolerance for bending or shock. Anything that loads the edge unevenly for even a moment takes a chip out of it.

Why tap drills are harder on the tool than they look

A tap drill removes almost the full diameter of material with no room for a pilot to true up runout first. Any misalignment between the spindle axis and the hole, from a worn collet, an off-square vise stop, or interrupted material, puts a side load on the drill the moment it touches down. With HSS that side load just deflects the tool slightly. With carbide it can crack the edge on first contact, especially on drills under about 6mm, where the core diameter is small relative to the flute length.

Breakthrough is the other common failure point. As the drill exits the back of the hole, the remaining wall thins out and the cutting forces on the two flutes stop being balanced. On a drill press or a hand-fed setup this shows up as a bang at breakout. On a CNC with a fixed feed rate through the exit, it shows up as a chipped margin because the tool doesn't get to react.

Where it actually goes wrong

• Runout. Even 0.02mm of runout at the spindle nose gets amplified at the tip of a long drill and turns into an uneven load on entry.
• Peck cycles that don't clear chips. A packed flute forces the drill to push through its own chips instead of cutting, which is a compressive load carbide doesn't like either.
• Interrupted cuts. Drilling into a casting skin, a previous hole, or a keyway mid-drill loads one flute more than the other for a moment.
• Wrong point geometry for the material. A drill ground for aluminum run into stainless or titanium sees edge loads it wasn't designed to survive.
• Too much speed, not enough feed. This lets the edge rub before it bites. On carbide that reads as chipping rather than the dulling you'd see on HSS.

What actually helps

Check runout before blaming the drill. Under 0.01mm total indicator reading at the tip is a reasonable target for anything under 8mm. Use a spot drill or center drill first on anything without a pre-existing hole so the carbide drill starts true. Reduce feed rate through the last 10-15% of hole depth on blind holes and through breakout on through holes; most control software supports this as a separate feed segment. Match point geometry to material. A 140° split point handles stainless and titanium better than a standard 118° point because it centers faster and reduces initial thrust. If chipping keeps happening on the same hole size regardless of what you change, it's usually a fixturing or squareness problem. Not a tooling problem.