Runout is the amount a drill tip deviates from the true spindle centerline as it rotates. A perfectly concentric drill tip traces a single point in space as it spins. A drill with 0.005" runout traces a small circle — its tip describes a circle 0.005" in diameter as it spins. When that drill contacts the work, it cuts a hole 0.005" larger than the drill nominal diameter on each revolution because the cutting edges are sweeping a larger arc than the drill size.
For 1/2" holes with a tolerance of ±0.005", runout of 0.005" at the drill tip means you are using the entire tolerance budget just from setup — there is nothing left for wear, chip packing, or material variation. For small drills (#30 and smaller), even 0.002" runout can cause breakage because the drill flexes to follow its cutting arc instead of cutting straight.
Measure runout with a dial test indicator (DTI) — a round-body dial indicator with a contact point on a swivel arm. Mount it magnetically or clamp it to the drill press column or quill housing. Contact the drill shank, not the flutes, about 0.5"–1" above the chuck jaws. Slowly rotate the chuck by hand (machine off, spindle locked if possible). Read the full indicator sweep — the difference between the highest and lowest reading is the total indicator runout (TIR).
Acceptable runout for general drilling: 0.003" TIR. For precision work: 0.001" or less. For small drills (#40 and smaller): any runout should be minimized — even 0.002" TIR is problematic at these sizes.
Measure at two positions: at the shank just above the jaws, and at the tip of the drill. If shank runout is low but tip runout is high, the problem is likely in the drill itself (bent shank, eccentric geometry). If shank runout is high, the problem is in the chuck or spindle.
Drill chucks are the most common source of runout in drill press work. Keyless chucks have more runout than keyed chucks in general because their self-centering mechanism is less precise. Both types accumulate runout as they age from jaw wear, debris in the jaw mechanism, and impact damage.
To isolate the chuck: remove the drill and install a precision ground test bar (a straight ground rod with a known diameter and concentricity). Measure TIR at the bar with the indicator. The TIR at the bar reflects chuck and spindle runout without any tool contribution.
Alternatively, rotate the drill in the chuck. Most three-jaw chucks have one position where runout is lowest (the jaws happen to grip more concentrically in that orientation). If rotating the drill 120° and remeasuring changes the runout significantly, the problem is in the chuck jaw-to-jaw variation.
Chuck fix: replace the chuck if it is excessively worn. Before replacing, try: cleaning the jaw surfaces with solvent to remove built-up chips (packed chips in the jaw tracks cause eccentric gripping), and checking the chuck arbor taper for fretting or damage. A damaged taper seat causes chuck wobble that no amount of jaw adjustment can correct.
Spindle runout is the runout of the drill press spindle itself — separate from the chuck. To measure spindle runout, remove the chuck and measure the quill spindle taper directly with a precision ground arbor fitted to the taper. If the spindle has significant runout without the chuck, the spindle bearings are worn or the spindle itself is damaged.
Spindle runout is less common than chuck runout because spindle bearings are designed to last the life of the machine under normal loads. But in a machine that has been used to drill hard materials at high thrust forces, or has taken impact damage (a drill breaking violently can shock-load spindle bearings), spindle runout can develop.
Fix: spindle bearing replacement is a machine service job. If measured spindle runout is low but combined chuck + spindle runout is high, the problem is in the chuck, not the spindle.
The drill itself can contribute runout through a bent shank, eccentric point geometry (unequal lip lengths), or an undersized shank that does not grip concentrically in the chuck jaws.
A bent shank — common in drills that have been dropped or had side loads during use — shows up as high TIR at the tip with acceptable TIR at the shank. You can sometimes see the bend by rolling the drill on a flat surface and watching for the shank to wobble.
Eccentric point geometry (one lip longer than the other) shows up as a hole that is consistently larger than nominal even with good chuck and spindle concentricity. This is a resharpening problem — the lips are not equal length or at the same angle. A properly resharpened drill should have lips within 0.001"–0.002" of equal length and within 0.5° of the same angle.
Undersized shanks grip eccentrically because the chuck jaws contact the shank at different depths. Check shank diameter with a micrometer — it should be within 0.001" of the nominal drill diameter. Undersize shanks from cheap drills are a recurring source of runout.
A drill resharpened with unequal lip lengths creates tool-induced runout even in a perfect chuck. We regrind both lips to equal length and angle — eliminating one common source of runout before the drill even goes into the chuck.
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