The cost of a dull drill is rarely calculated. Most shops track tooling spend as a line item — what they paid for drill bits that quarter. Some track scrap attributed to bad holes. Almost none track the compounding time cost that dull tooling adds to every cycle where it is used. That is where the real money goes, and it is invisible unless you are looking for it.
How a Dull Drill Adds Cycle Time
A dull drill does not just cut slower in a linear sense. It changes the process in several ways that each add time independently.
Slower programmed feed rates. When a bit is running dull, operators and programmers compensate by backing off the feed. Sometimes this is a formal decision — the feed in the CNC program gets reduced. More often it is informal: the machinist drops the feed override to 80% because the machine sounds unhappy. Either way, you have just extended every drilling cycle by 20-25% without acknowledging it in your accounting.
More peck cycles. A dull drill requires more peck cycles to manage chips and heat because it generates more heat per unit depth than a sharp drill does. In a program running ten holes at 2x diameter depth, adding one extra peck per hole might add 3-4 seconds per hole. Across a 500-part run, that is 25-35 minutes of added cycle time that did not need to exist.
Spot drilling or center drilling revisits. When bit wander becomes a problem — which it does as the chisel edge wears — shops sometimes add a spot drilling pass they would not have needed with sharp tooling. That is another tool change and another pass per hole.
Rejects and rework. Oversized holes, rough walls, delamination in composites, or work-hardened surfaces in titanium all require either rework time or scrapped parts. Rework time per part is almost always higher than people estimate when they account for setup, measurement, and the labor cost of fixing rather than making.
Calculating Cost Per Hole
The starting point for any honest tooling economics analysis is cost per hole, not cost per bit.
Take a simple example. You are running 1/4" HSS jobber drills in 1018 steel, 3/4" depth, through-hole. A new drill costs $4.50. You get 300 acceptable holes from it before the bit is pushing chips instead of cutting them cleanly. That is $0.015 per hole in tooling cost — seems trivial.
But that drill over its 300-hole life is running progressively slower. In the first 100 holes it is at full programmed feed. In the second 100 holes the operator has dropped the override to 85%. In the last 100 holes it is at 70% — the operator knows the drill is due for a change but there are no spares in the cabinet and the job needs to finish today. The average cycle time across that bit's life is maybe 15% above optimal. If each hole takes 8 seconds at full feed, the average over a worn bit's life is closer to 9.2 seconds. That 1.2 seconds per hole, over 300 holes, is 6 minutes of added spindle time per drill.
At a shop rate of $85/hour, that 6 minutes costs $8.50 in machine time — nearly twice the cost of the drill itself. And this is a conservative example. In harder materials, the degradation is steeper.
The Breakeven Math on Resharpening
A resharpen from a mail-in service like MachinistPost runs $3 to $5 for a standard jobber bit depending on diameter, with batch pricing available for larger quantities. Compare that against the cost of a new equivalent bit. For a quality name-brand HSS bit in 3/8" or larger, the new bit cost is $6 to $20 depending on brand and diameter. Resharpening is typically 30-50% of replacement cost.
But the more important comparison is not resharpen vs. new bit — it is resharpen frequently vs. run to failure. A bit that is resharpened at 75% of its life (before the degradation curve steepens) and returned to full sharpness holds tighter cycle times across its whole usable life. You get the same number of holes but spend less total cycle time making them. The productive life of the bit, measured in acceptable holes per hour of spindle time, is higher with regular resharpening than with run-to-failure and replace.
The breakeven calculation for a shop running 500 parts per month with 4 drilled holes per part looks like this:
- 2,000 holes per month
- Average cycle time at sharp: 8 seconds per hole = 16,000 seconds = 267 minutes of drill time
- Average cycle time at worn (10% degradation): 8.8 seconds = 17,600 seconds = 293 minutes
- Added cycle time from dull tooling: 26 minutes per month
- At $85/hour shop rate: $37/month in wasted spindle time
- Resharpening cost to maintain sharpness: 3-4 bit regrinds per month at $4 each = $12-16
The resharpening program pays for itself in recovered cycle time before you even account for improved hole quality and reduced scrap.
Real Shop Math: A Production Example
A job shop running a weekly batch of 200 aluminum housings. Each housing has 12 holes: eight 1/4" clearance holes and four 3/8" tapped holes. At full speed — sharp tooling, full feed rates — the drilling cycle per part runs 4.5 minutes. Total drilling time for the week's batch: 900 minutes, or 15 hours.
The shop is using the same 1/4" drills across the full week without resharpening. By midweek, the operator has the feed override at 80%. By Thursday the override is at 65% and the operator is pecking where the program does not call for it. Friday the drilling cycle per part is running 6.2 minutes. The weekly average across all 200 parts ends up around 5.1 minutes per part — 1020 minutes of drilling instead of 900. That is 120 minutes of spindle time lost to tooling condition, every week.
At $75/hour shop rate (this is a smaller shop with lower overhead): $150/week in recovered time available if tooling condition is maintained. Annually that is $7,800. The resharpening program to maintain those bits would run a few hundred dollars per year. The ROI is not close.
Making the Change
The obstacle is usually not economics — the math almost always favors tool maintenance. The obstacle is workflow. Sending bits out for resharpening requires having a process: designated bit holders or envelopes, a way to track which bits are out, a par stock so work does not stop during a resharpen cycle.
The practical approach is to build a two-set system for any drill that sees regular production use. Set A is in service. Set B is at the resharpener or in the cabinet as fresh-sharpened spares. When Set A comes out of service, it ships to the resharpener and Set B goes to work. Turnaround from MachinistPost is a few business days with standard service. The float between sets covers the lead time with room to spare.
The shops that have done this math do not go back to run-to-failure and replace. The numbers are too clear.