A single-piece job on a manual mill with one drill bit is forgiving. You can feel the bit change, you compensate, and if a hole is off you catch it before the next one. A 500-part CNC fixture run is a different situation entirely. The machine does not feel anything. The program runs identically on part 1 and part 487. The only thing that changes across that run is the condition of your tooling, and if you are not actively tracking it, you find out about drift when the QC check at the end of the day flags twenty parts.
How Drill Wear Shows Up in a Production Run
Wear in a production drilling application is gradual but directional. The failure modes that matter for fixture runs are different from one-off work.
Hole diameter growth. As cutting lips wear and the drill begins to deflect more under thrust, holes trend oversized. This is often the first measurable sign of wear. If your tolerance is ±0.002" on a hole and you start the run at +0.001" over nominal, you have one thousandth of clearance before you are making rejects. On a 500-part run that can happen in the last 200 parts if you are not watching.
Position drift. A worn chisel edge — wider, less sharp — requires more thrust force to start the hole and is less self-centering than a sharp point. In a fixture where the part is well-located and the drill entry is consistent, this matters less. In applications where the drill is entering at an angle or the entry surface is rough, a worn chisel edge wanders more. Position drift in a tight-tolerance fixture part is often not blamed on the drill, but it should be one of the first suspects.
Surface finish degradation. Hole wall quality degrades as the outer corners of the cutting lips round off. This usually appears before diameter issues become gross, and it is visible under a loop or with a bore scope. If a subsequent tapping or reaming operation depends on hole finish, surface degradation from a worn drill is a real upstream quality issue.
Burr increase at exit. Exit burr size correlates with drill sharpness and thrust force. Sharp bit, low thrust, small burr. Dull bit, high thrust, large burr that needs deburring. If your deburring time starts climbing partway through a run, look at the drill.
Checking Geometry Mid-Run
The most practical tool for mid-run geometry verification on drills is a simple visual inspection combined with a test hole and measurement. You do not need optical comparators or profilometers for this — you need a drill point checker or a careful eye, a micrometer, and a few gauged test holes in scrap material.
Set a checkpoint interval when you start a production run. For 500 parts in mild steel with a 3/8" HSS drill, a checkpoint every 100 to 150 parts is reasonable as a starting point. At each checkpoint, pull the drill from the spindle and inspect the lips. Look for: corner rounding visible to the naked eye, chisel edge width compared to a fresh reference bit, any chipping on the lips. If the lips look good, measure the last five holes for diameter and position. If everything is still in spec, put the drill back and continue. If either the visual inspection or the measurement shows drift, it is change time.
The key is establishing the baseline measurement at the start of the run with a fresh bit. Without a baseline, you do not know if the hole at part 300 is within spec because of the tolerance window or in spite of drift from a much wider nominal start point.
Resharpen Scheduling for CNC Work
Reactive resharpening — change the bit when it fails — is the wrong model for production CNC work. You are not just dealing with the cost of the bit. You are dealing with whatever scrap or rework accumulated between when the drill actually went dull and when you noticed. In a 500-part run with 20 parts made after the drift started, you have a rework problem.
Preventive resharpening is the better model. Establish your actual bit life for the specific material, diameter, and depth you are drilling — not from a catalog, from your own data. Run controlled tests if necessary. Then schedule tool changes at 80% of established life. You lose a small amount of remaining life on each bit, but you never make a bad part from a worn drill.
This approach requires a bit that cuts consistently after resharpening. Inconsistent grinds are a real problem here — if you resharpen a bit and the lip heights are unequal or the relief is wrong, you have created a new source of variation. Outsourcing resharpening to a service that provides consistent, verified geometry eliminates this variable. MachinistPost grinds to specific geometry on a dedicated machine tool, which matters in production applications where "resharpened" needs to mean "back to spec," not "sharper than it was."
Batch Resharpen Programs for CNC Shops
A CNC shop running recurring production jobs has a clear use case for batched resharpening. The workflow looks like this: maintain two sets of drills for each production size. When Set A finishes a run (or hits its scheduled change point), pull the set, label them with the job and the run count, and ship them for resharpening. Set B goes in for the next run. By the time Set B is scheduled for replacement, Set A is back from the resharpener.
Batch pricing on resharpening makes this economical. The per-bit cost drops meaningfully when you are sending five or ten bits at a time versus single bits. For a shop running two or three recurring fixture jobs, the annual resharpening volume is predictable enough to negotiate a standing arrangement with a service provider. MachinistPost handles batches — send a padded envelope with your bits and a note about what you are drilling and the geometry you want, and the turnaround is a few business days.
The per-bit economics in a batch program typically run at 30-40% of new bit cost for standard jobber drills. On 3/8" and larger bits, the savings per bit are significant. For a shop cycling through twenty 1/2" HSS bits per quarter, the difference between replace-new and resharpen-batch is several hundred dollars per year on that one size alone.
Documentation and Traceability
For any shop doing aerospace, medical, or automotive production work with quality documentation requirements, drill condition at the time of each run may need to be part of the process record. A preventive resharpening schedule with clear change points creates a simple, auditable trail: Bit set #3, resharpened [date], used for runs [job numbers], retired at [run count]. This is harder to reconstruct with a reactive approach where bits get changed when someone decides they look dull.
Even for shops without formal quality system requirements, running a simple log of resharpen dates and run counts per bit size builds the data that makes scheduling decisions better over time. After three or four cycles you have real, material-specific, machine-specific bit life data — and that is worth more than any catalog spec.