Carbide Drill Runout: TIR Specs and Why They Matter More Than With HSS

HSS drills are forgiving about runout. A bit of wobble in the chuck, and the drill flexes slightly and keeps cutting. It may wear faster and cut oversized holes, but it keeps going. Carbide does not flex — it fractures. Runout that an HSS drill tolerates without complaint will chip a carbide cutting edge within a few holes and potentially break the tool entirely.

If you run carbide drills, TIR (Total Indicator Reading) checks before every new setup are not optional. Here is what to measure, what the acceptable limits are, and what to do when runout is out of spec.

Why Carbide Is Different

Tungsten carbide is approximately 3× harder than HSS but significantly more brittle. Its modulus of elasticity (stiffness) is also much higher — roughly 2× that of steel. This means when a carbide drill experiences eccentric loading from runout, it cannot flex to absorb the load the way an HSS drill does. Instead, the cutting edge takes the full cyclic impact load. Above a threshold TIR, carbide edges begin to micro-chip. Above a higher threshold, catastrophic fracture becomes likely, especially in interrupted cuts or hard materials.

TIR Limits for Carbide Drills

Compare these to HSS acceptable runout limits, which are typically 2–4× looser (a 1/4" HSS drill in a good keyed chuck commonly runs at 0.002" TIR without problems). The carbide limits are tight enough that a standard keyed chuck often cannot hold them, especially as the chuck jaws wear.

How to Measure TIR

You need a dial test indicator (DTI) with 0.0001" resolution and a magnetic base or holder to position it near the drill point.

Measurement procedure

1. Mount the drill in the spindle using the holder you plan to use for cutting (collet, shrink-fit, hydraulic — not keyed chuck for carbide).
2. Set the DTI so the contact point touches the drill shank within 1" of the holder face, perpendicular to the shank axis.
3. Rotate the spindle by hand one full revolution while watching the DTI needle.
4. Record the total sweep (highest reading minus lowest reading). This is your TIR at the shank.
5. Move the DTI to the drill body near the tip and repeat. Runout amplifies toward the tip — the tip reading matters most for hole quality but the shank reading tells you if the problem is in the holder or the spindle.

Interpreting results

• Shank TIR in spec, tip TIR excessive: the drill has a bent or poorly ground shank. Replace the drill or check for shank damage.
• Shank TIR excessive: the problem is in the holder, collet, or spindle. Try a different collet or check spindle bearing.
• Both in spec but still getting chipping: check for contamination in the collet bore (chips, coolant deposits) or a worn collet that grips unevenly.

Toolholders for Carbide Drilling

The holder is the primary determinant of TIR for a given spindle. Keyed drill chucks are the worst performers — internal runout typically ranges from 0.002–0.005" even when new, well beyond carbide limits. Keyless chucks are not better in most cases.

Checking the Collet Bore

Even a precision collet fails if the bore is dirty. Coolant deposits, chip fragments, and oil residue in the collet bore prevent the collet from gripping evenly. The result is runout that varies from setup to setup even with the same drill and spindle.

Before every carbide setup: clean the collet bore with a lint-free cloth and light solvent. Clean the spindle taper as well. A one-minute wipe prevents the kind of TIR variance that looks like a bad spindle but is actually just contamination.