Drill Runout: What It Is, How to Measure It, and What to Do About It

Drill runout is one of the most common causes of poor drill life that shops don't actively track. It's invisible during operation, doesn't produce obvious error messages, and often gets blamed on the drill when the real cause is the chuck, spindle, or tool holder.

What Runout Is

Runout is the deviation of the drill's cutting edge from the true rotational axis of the spindle. A drill with zero runout has both cutting lips rotating at exactly the same radius — they share the load equally. A drill with runout has one lip cutting slightly farther from center than the other.

The consequence: one lip does most of the cutting, wears faster, and the drill produces an oversized hole. The asymmetric load causes vibration and micro-chatter that fatigues the cutting edge. Runout above 0.002" TIR has measurable impact on tool life. Runout above 0.005" is a serious problem in most materials.

Where Runout Comes From

• Chuck jaw wear. Years of use wear the jaw faces. The jaws no longer grip the shank concentrically. A chuck delivering 0.003" runout on a 1/2" drill may deliver 0.008" on a 1/4" drill — smaller shanks make jaw wear more significant.
• Chuck runout relative to spindle. Even a new chuck can be poorly concentric with the spindle it's mounted in. Common with cheap drill press chucks and morse taper adapters that have been installed and removed repeatedly.
• Spindle bearing wear. In a worn spindle, the axis of rotation shifts under cutting load. The TIR you measure unloaded may be significantly lower than the effective runout under drilling conditions.
• Drill shank runout. A drill with a bent shank or one that's been chucked with uneven grip pressure can develop inherent runout. Check the shank separately from the point to isolate this source.
• Collet wear or contamination. A worn collet or one with chips in the bore will add runout even on a good spindle.

How to Measure It

You need a test indicator — a tenths or half-tenths indicator on a magnetic base. Place the indicator against the flute land near the drill point. Rotate the spindle by hand one full revolution. The total swing is your TIR. For a more useful reading, measure at the point AND at a point 1" back toward the shank. If TIR is the same at both locations, the runout is caused by the spindle axis. If TIR increases toward the point, the drill itself has a bent axis.

Target: under 0.002" TIR at the cutting point for general work. Under 0.001" for precision drilling or small-diameter work.

What to Do About It

The diagnostic sequence: start at the spindle taper, work outward. If the taper is clean and the spindle is good, add the chuck/collet and remeasure. If that adds runout, the holder is the problem. Install the drill and remeasure. Each step isolates a segment of the system.

If runout is at the chuck: replace it. A quality drill chuck is inexpensive relative to its impact on tool life. If at the collet: inspect for chips, damage, or wear. If at the spindle bearing: this is a machine repair issue. If in the drill shank: check multiple drills in the same holder — if one shows high runout and others don't, the drill is the issue.