The Real Cost of Dull Drills in Production

The Costs Nobody Is Tracking

When a shop manager thinks about drill cost, they think about the purchase price on the invoice. $18 for a 1/2" HSS drill, $45 for a 1/2" cobalt. That's the number that shows up in the accounting system. What doesn't show up: the extra 15 seconds per hole a dull drill adds to cycle time, the 2% scrap rate increase from holes drilled with dull geometry, or the 3 hours per month a machinist spends fidgeting with problematic setups caused by tool performance issues. These costs are real. They're just invisible because they're distributed across labor, machine time, and scrap accounts rather than sitting cleanly in a "tooling" line item.

Quantifying these costs requires some measurement and some arithmetic, but the math isn't complicated. The point is to build a number that management and operators can actually use to make decisions — specifically, the decision about whether to invest in systematic reconditioning versus the current default of running drills until they fail or get lost.

Cycle Time Analysis

The clearest measurable cost of dull drills is cycle time. A dull drill requires more thrust to advance, which means either slower programmed feed or the machine working harder at the same feed — resulting in higher spindle load, more deflection, and worse hole quality. A sharp drill in mild steel at the correct feed runs clean. A drill at 75% wear running the same program takes 20 to 30% longer per hole due to reduced penetration rate and increased dwelling.

Measure it directly: time 10 holes with a known-sharp drill, time 10 more holes with the same drill after its typical run-to-replacement interval. The difference, multiplied by your machine hourly rate (including operator cost — typically $80 to $150 per hour for a CNC machining center), gives you the dollar cost of the performance degradation per hole.

Example: a part with 40 drilled holes, cycle time 8 seconds per hole sharp, 11 seconds per hole dull. Degradation per part: 40 × 3 = 120 seconds = 2 minutes. At a $100/hour machine rate: $3.33 per part in lost capacity. On a run of 500 parts: $1,665 of machine time consumed by the performance gap between a sharp drill and a worn one. That's real money — and it doesn't require any calculation to see that spending $3.50 to recondition the drill before it gets to 75% wear would have recovered most of that capacity loss.

Scrap Rate Correlation

Dull drills produce worse holes: oversize (from lip height imbalance as the drill wears asymmetrically), out-of-round (from runout-like wobble as the worn geometry becomes unbalanced), rough surface finish, and more burring at breakthrough. For parts with tight hole tolerances — clearance fits, bore-and-ream sequences, press fits — the degraded hole from a worn drill becomes scrap or rework.

Even a modest scrap rate increase is expensive. If a part costs $150 in machining time and 1% of parts are scrapped due to out-of-tolerance holes from worn drills, the scrap cost per 100 parts is $150. Improving drill management to reduce those scrap events to 0.2% saves $120 per 100 parts — much more than the cost of more frequent reconditioning.

Track scrap by cause code in your production system. "Out-of-tolerance hole" as a cause code, over several months of data, will show you whether drill condition is contributing to your scrap cost. If it shows up in the top 5 cause codes, the cost of better drill management is easy to justify. If it doesn't appear, either your drill management is working or nobody is accurately coding the scrap.

Building the Full Cost-Per-Hole Model

A complete cost-per-hole model for your shop combines all the cost elements into a single number you can use for decisions:

Cost per hole = (Tool cost per hole) + (Machine time cost per hole) + (Scrap cost per hole)

Tool cost per hole: purchase price divided by total holes produced across all reconditioning cycles. For a $15 drill that produces 250 holes before first regrind, regrinding 7 times at $3.50 each, and then retiring: total holes = 250 × 8 = 2,000. Total cost = $15 + $24.50 = $39.50. Cost per hole = $0.0198.

Machine time per hole: (cycle time in hours) × (machine rate). At 8 seconds per hole and $100/hour: 0.00222 hours × $100 = $0.222 per hole. This dominates the model — which is why cycle time degradation from dull drills is the most impactful cost driver.

Scrap cost per hole: (scrap rate) × (part cost). At 0.5% scrap rate on a $150 part: 0.005 × $150 = $0.75 per hole — but spread across all the holes in that part. For a 40-hole part: $0.75 / 40 = $0.019 per hole attributable to scrap risk from this hole.

Total cost per hole in this example: $0.020 + $0.222 + $0.019 = $0.261. Of that, only 7.6% is tool cost. The other 92.4% is machine time and quality cost. This is why optimizing tool cost — buying the cheapest drill, skimping on reconditioning — is the wrong focus. The correct focus is minimizing machine time per hole (sharp drills, correct speeds/feeds) and minimizing scrap (consistent geometry, sharp tools, good setups).