Appearance
A 10 mm gun drill that started life with 8 mm of carbide head length and 0.5 mm of guide pad wear allowance will be scrapped after approximately 20–25 regrinds — not because the carbide is worn out, but because the head geometry has changed beyond the limits of the application specification. Each regrind removes 0.08–0.15 mm from the diameter and 0.3–0.5 mm from the head length, and at some point the drill becomes too small, too short, or too weak to continue. Tracking regrind consumption and knowing the scrap limit before the drill reaches it converts unexpected tool failures into planned replacements — reducing machine downtime and preventing scrapped workpieces from out-of-tolerance drills.
Regrind Life Factors
Key Parameters Limiting Regrind Cycles
| Parameter | New Drill Value | Wear Allowance | Loss per Regrind | Typical Regrinds Before Scrap | Impact on Scrap Decision |
|---|---|---|---|---|---|
| Carbide head diameter | Nominal + oversize (e.g., 10.050 mm) | 0.20–0.40 mm total from new to scrap | 0.05–0.15 mm (half per cutting edge) | 15–30 | Most common limit — drill becomes undersize for hole tolerance |
| Head length (tip to braze joint) | 4–12 mm depending on diameter | 3–8 mm usable length | 0.20–0.50 mm (face grinding per regrind) | 15–25 | Head becomes too short for adequate chip flute |
| Guide pad thickness | 0.8–2.0 mm radial | 0.3–0.8 mm total | 0.03–0.08 mm (diameter reduction) | 10–20 | Pads become too thin — risk of pad detachment |
| Distance to braze joint | 3–8 mm from edge to braze | 2–6 mm remaining at scrap | 0.3–0.5 mm per regrind | 10–20 | Braze joint exposure — risk of head detachment |
| Coolant hole proximity | Centered — 1–3 mm behind edge | 0.5–1.5 mm clearance to edge | 0.3–0.5 mm (face regrind) | 10–15 | Coolant hole breaches — loss of coolant pressure |
Regrind Life by Drill Diameter
| Drill Diameter (mm) | Typical Carbide Head Length (mm) | Material Removed per Regrind — Face (mm) | Material Removed per Regrind — Diameter (mm) | Typical Regrind Cycles | Scrap Criteria (most common) |
|---|---|---|---|---|---|
| 2–3 | 4–5 | 0.15–0.25 | 0.04–0.08 | 5–12 | Head length — coolant hole position |
| 4–6 | 5–7 | 0.20–0.35 | 0.06–0.10 | 10–18 | Diameter — head length |
| 8–12 | 6–9 | 0.25–0.40 | 0.08–0.12 | 15–25 | Diameter — pad thickness |
| 14–20 | 8–12 | 0.30–0.50 | 0.10–0.15 | 15–30 | Diameter — pad thickness |
| 25–40 | 10–16 | 0.40–0.60 | 0.12–0.18 | 20–35 | Diameter — braze joint proximity |
Tracking and Decision Making
Scrap Criteria Reference Table
| Parameter | Scrap Limit | Measurement Method | Consequence of Exceeding Limit |
|---|---|---|---|
| Minimum head diameter | Nominal size −0.15 to −0.30 mm (application-dependent) | Micrometer — 2–3 mm behind cutting edge | Hole undersize — out of tolerance — workpiece rejection |
| Minimum head length | 2–4 mm (depending on drill diameter) | Depth gauge — tip to braze joint | Inadequate chip flute volume — chip jamming |
| Minimum guide pad thickness | 0.3–0.5 mm radial from pad OD to body | Micrometer — measure head OD and body OD | Pad detachment — catastrophic head failure in hole |
| Minimum braze joint clearance | 1.5–2.0 mm from nearest cutting edge to braze line | Visual — optical measurement | Head separation at braze joint during drilling |
| Coolant hole edge distance | Minimum 0.5 mm from reground face to coolant hole exit | Visual — optical — pin gauge | Coolant pressure loss — uneven cooling — edge overheating |
Cost Comparison: Regrinding vs. Replacement
| Drill Diameter (mm) | New Drill Cost | Cost per Regrind | Regrinds Possible | Total Cost (regrind strategy) | Cost per Hole (regrind) | Cost per Hole (new only) | Break-Even Regrinds |
|---|---|---|---|---|---|---|---|
| 5 | $80–120 | $15–25 | 12 | $260–420 | $2.60–4.20 | $8–12 | 4–5 |
| 10 | $150–250 | $25–45 | 20 | $650–1150 | $3.25–5.75 | $15–25 | 4–6 |
| 20 | $350–500 | $50–80 | 25 | $1600–2500 | $6.40–10.00 | $35–50 | 5–7 |
| 30 | $600–900 | $80–120 | 30 | $3000–4500 | $10.00–15.00 | $60–90 | 5–8 |
FAQ
How many times can a gun drill be reground before it must be scrapped?
A typical gun drill can be reground 15–30 times before reaching the scrap limit, depending on the drill diameter. Small drills under 6 mm diameter typically achieve 5–18 regrinds — their shorter carbide heads and smaller diameter tolerances limit regrind life. Medium drills 8–20 mm diameter achieve 15–30 regrinds — the most common range. Large drills over 25 mm can achieve 20–35 or more regrinds because they have longer carbide heads and more available material for regrinding. The actual number varies with the amount of material removed per regrind (which depends on the wear condition), the application tolerance (tighter tolerances reduce regrind life), and the operator's regrind technique (conservative regrinds extend total life). Maintaining a regrind log for each drill provides the data needed to predict scrap dates and plan replacement orders.
What determines when a gun drill should be scrapped?
A gun drill should be scrapped when any of the following limits is reached: the diameter has been reduced below the minimum acceptable size for the hole tolerance (typically 0.15–0.30 mm below the nominal drill diameter), the remaining carbide head length is insufficient to provide adequate chip flute volume (minimum 2–4 mm depending on drill size), the guide pad thickness has worn to less than 0.3–0.5 mm radial (risk of pad separation at the brazed joint), the distance from the cutting edge to the braze joint is less than 1.5–2.0 mm (risk of head separation), or the coolant hole exit is less than 0.5 mm from the reground face (risk of coolant blowout). The first parameter to reach its limit becomes the scrap criterion — in most gun drills, diameter reduction is the first limit reached.
How much material is removed per regrind?
The material removed per regrind varies by drill diameter, wear condition, and regrind practice. On the diameter, a typical regrind removes 0.04–0.18 mm total — split between the two cutting edges (0.02–0.09 mm per edge). On the face (head length), a typical regrind removes 0.15–0.60 mm from the tip face to restore the cutting edge geometry. The amount removed should be the minimum necessary to completely remove the wear zone from all cutting edges and restore the specified geometry — removing more material than necessary wastes regrind life. The regrind operator should measure the wear zone width on each cutting edge using 10× magnification and set the grinding infeed to remove the wear zone plus 0.01–0.02 mm additional to ensure complete removal.
Can the maximum regrind life be extended beyond typical limits?
The maximum regrind life can be extended beyond typical limits in several ways. Using a larger initial oversize (selecting a C or D class head instead of A class) provides additional diameter reserve — adding 3–8 regrinds depending on the oversize increment. Reducing the material removal per regrind by regrinding more frequently (at shorter intervals with less wear) can extend total regrind cycles by 15–25% because less material is needed to restore the edge. Applying wear-resistant coatings (TiAlN or AlTiN) that reduce the wear rate per hole extends the interval between regrinds but does not change the number of regrinds possible — it extends total holes per drill. Using a drill with a longer initial carbide head provides more available material for face regrinding. However, extending regrind life by using drills beyond the normal scrap criteria is not recommended — the risk of in-hole failure increases significantly when the head geometry or braze margin falls below the minimum recommended values.
What tracking system is recommended for monitoring regrind life?
The recommended tracking system for gun drill regrind life is a database or spreadsheet that records for each drill: unique identification number (engraved or etched on the shank), drill diameter and type, initial measurements (diameter, head length, pad thickness), regrind date and regrind number, measurements after each regrind (diameter, head length, pad thickness, material removed), cumulative material removed, operator ID, and current status (in service, at regrind, scrapped). Color coding systems — applying a colored band on the shank that changes with each regrind cycle — provide a quick visual reference for operators to see at a glance how many regrinds a drill has undergone. Bar coding or RFID tagging with a handheld scanner linked to the database provides the most efficient tracking for high-volume operations. The regrind log should be reviewed monthly to identify drills approaching the scrap limit and to order replacements before scrap drills impact production.
Disclaimer: The regrind life estimates, scrap criteria, and cost data provided in this article are general guidelines based on industry-standard practices for gun drill reconditioning. Actual regrind life varies significantly with application conditions, material, coolant type, regrind quality, and the specific drill design. Operators should establish their own regrind life data through systematic tracking and adjust scrap criteria based on their specific application requirements and quality standards. The authors and publisher assume no liability for any damages or losses arising from the use of this information — always follow original equipment manufacturer guidelines for your specific equipment. Content is for informational purposes only and does not constitute professional engineering advice. Verify all parameters with qualified personnel before implementation as of 2026.