Appearance
A gun drill that measures 0.02 mm oversize on diameter, has 0.03 mm/m shank straightness, and a lip height difference of 0.015 mm may still drill holes that meet specification — but just barely. The same drill with 0.02 mm lip height difference running at the upper feed rate limit will produce holes 0.05 mm oversize with a surface finish one grade rougher than normal. Dimensional inspection of gun drills is not about accepting or rejecting individual parameters in isolation — it is about understanding how each dimensional tolerance interacts with the others and with the operating parameters to determine the final hole quality. Systematic inspection at every regrind or new drill receipt converts dimensional data into predictable drilling performance.
Inspection Parameters and Tools
Critical Dimensions and Measurement Methods
| Parameter | Measurement Tool | Accuracy | Measurement Location | Acceptance Criteria | Frequency |
|---|---|---|---|---|---|
| Head diameter | Micrometer — bench micrometer | ±0.001 mm | 2–5 mm behind cutting edge — 90° to flute | Nominal + oversize — ±0.005 mm tolerance | Every drill — every regrind |
| Shank straightness | Dial indicator on V-blocks | ±0.002 mm | Shank at 20% and 80% points — 360° rotation | ≤0.03 mm/m — precision — ≤0.05 mm/m standard | Every drill — every regrind |
| Point angle | Optical comparator — toolmaker's microscope | ±0.1° | Both cutting edges — projected profile | Specified angle ±1° | Every regrind — new receipt |
| Lip height | Dial indicator — height gauge | ±0.002 mm | Cutting edge at fixed radius from center | ≤0.01 mm for small drills — ≤0.02 mm for large | Every regrind — new receipt |
| Clearance angles | Optical comparator — angle gauge | ±0.5° | Primary and secondary clearance faces | Specified ±1° primary — ±2° secondary | Every regrind — new receipt |
| Shank diameter | Micrometer | ±0.002 mm | 3 positions along shank length | Shank spec ±0.005 mm | Every drill — every regrind |
| Concentricity (head to shank) | Dial indicator — V-blocks | ±0.002 mm | Head rotated — shank on V-blocks | ≤0.01 mm TIR | Sample basis — new receipt |
Inspection Tool Selection Guide
| Tool | Cost | Accuracy | Skill Required | Measurement Speed | Best For |
|---|---|---|---|---|---|
| Outside micrometer | Low | ±0.001 mm | Low | Fast | Head and shank diameter — basic dimensional checks |
| Bench micrometer | Moderate | ±0.0005 mm | Moderate | Fast | Precision diameter measurement — certification |
| Dial indicator + V-blocks | Low | ±0.002 mm | Moderate | Moderate | Straightness — concentricity — lip height |
| Optical comparator | High | ±0.001 mm — ±0.1° | High | Moderate | Point geometry — clearance angles — profile inspection |
| Toolmaker's microscope | High | ±0.001 mm — ±0.1° | High | Slow | Detailed geometry — coolant hole position — edge condition |
| CMM | Very high | ±0.0005 mm | Very high | Slow | Full dimensional certification — audit — first-article |
Acceptance Criteria and Quality Control
Go/No-Go Criteria Reference
| Parameter | New Drill (Precision) | New Drill (Standard) | Reground (Precision) | Reground (Standard) | Action if Out of Tolerance |
|---|---|---|---|---|---|
| Head diameter — absolute | Nominal +0.005/–0.002 mm | Nominal +0.010/–0.005 mm | Nominal +0.010/–0.005 mm | Nominal +0.015/–0.010 mm | Reject — rework or scrap |
| Head diameter — ovality | ≤0.003 mm | ≤0.005 mm | ≤0.005 mm | ≤0.008 mm | Reject — head out of round |
| Shank straightness | ≤0.02 mm/m | ≤0.05 mm/m | ≤0.03 mm/m | ≤0.05 mm/m | Reject — straightening required |
| Lip height difference | ≤0.008 mm | ≤0.015 mm | ≤0.010 mm | ≤0.020 mm | Reject — regrind required |
| Point angle — inner edge | Spec ±0.5° | Spec ±1.0° | Spec ±1.0° | Spec ±1.5° | Reject — regrind required |
| Point angle — outer edge | Spec ±0.5° | Spec ±1.0° | Spec ±1.0° | Spec ±1.5° | Reject — regrind required |
| Cutting edge condition | No chips — radius ≤0.005 mm | No chips — radius ≤0.010 mm | Light hone only — no chips | Light hone only — radius ≤0.015 mm | Reject — re-hone or regrind |
| Shank OD concentricity | ≤0.005 mm TIR | ≤0.010 mm TIR | ≤0.008 mm TIR | ≤0.015 mm TIR | Reject — shank damage |
Inspection Frequency Summary
| Inspection Type | New Drills (Receiving) | After Regrind | In-Process (Mid-Life) | After Repair | Storage Return |
|---|---|---|---|---|---|
| Full dimensional (all parameters) | 100% | 100% | Sample basis — 10% | 100% | Sample — 20% |
| Diameter only | 100% | 100% | Every drill | 100% | 100% |
| Straightness only | 100% | 100% | Every 5th regrind | 100% | 100% |
| Point geometry | Sample — 20% | 100% | Not required | 100% | Not required |
| Coolant hole flow | Sample — 10% | Sample — 20% | Not required | 100% | Not required |
FAQ
How is gun drill head diameter measured correctly?
Gun drill head diameter is measured using a micrometer or bench micrometer at a position 2–5 mm behind the cutting edges, with the micrometer measuring across the cutting edges (not across the guide pads or relief areas). The measurement should be taken at the point where the drill head reaches its full diameter — typically at the outer corner of the cutting edges where the margin transitions to the relief. The drill should be at room temperature (20°C ±2°C) and the micrometer should be calibrated before use. Three measurements should be taken at 120° rotation intervals and the average recorded. The diameter reading should be compared to the specified nominal diameter plus the drill's oversize class (A, B, C, etc.) — a Class C 10 mm drill, for example, measures approximately 10.010 mm diameter when new. The measurement position should be documented in the inspection log along with the actual reading.
What is the acceptable straightness tolerance for gun drill shanks?
The acceptable straightness tolerance for gun drill shanks is 0.03 mm per meter of shank length for precision applications and 0.05 mm per meter for standard applications. Measurement is performed by supporting the shank on two precision V-blocks positioned at approximately 20% and 80% of the shank length, rotating the drill through 360° while a dial indicator contacts the shank surface at the midpoint between the V-blocks. The total indicator reading (TIR) divided by the distance between the V-blocks (in meters) gives the straightness in mm/m. For example, a 400 mm shank with a TIR of 0.015 mm measured on V-blocks spaced 300 mm apart has a straightness of 0.015/0.3 = 0.05 mm/m. Shanks exceeding the straightness tolerance should be straightened or rejected — using a bent shank guarantees hole position error and accelerated drill wear.
How is lip height difference measured on a gun drill?
Lip height difference on a gun drill is measured using a dial indicator mounted on a height stand or a dedicated lip height gauge. The drill shank is clamped in a V-block or collet fixture with the cutting edges oriented horizontally. The dial indicator stylus contacts the cutting edge at a specific radius from the center — typically 50–80% of the drill radius. The drill is rotated to bring the first cutting edge under the stylus, the indicator is zeroed, and the drill is rotated 180° to bring the second cutting edge under the same stylus position. The difference between the two readings is the lip height difference. The second measurement should be repeated with the drill rotated 90° to verify axes. Lip height difference should not exceed 0.008–0.020 mm depending on drill size and application tolerance — excessive lip height difference creates unbalanced cutting forces that cause hole oversize and drill vibration.
What point geometry inspections are required for gun drills?
The required point geometry inspections for gun drills are: point angle (the angle at which each cutting edge meets the drill axis — measured from the drill axis to the cutting edge, typically 30–40° for the inner edge and 20–30° for the outer edge in standard gun drill design), clearance angles (primary and secondary relief angles on each cutting edge — measured on the clearance face behind the cutting edge), lip height (the height difference between the two cutting edges at the same radius from center), web thickness (the thickness of the drill web at the point — affects chip flute volume and drill rigidity), and chisel edge geometry (the width and angle of the chisel edge at the drill point center). All geometry parameters should be measured using an optical comparator or toolmaker's microscope with appropriate magnification (20–50×). The geometry should be verified against the drill manufacturer's specification and documented in the inspection log.
What documentation should accompany gun drill dimensional inspection?
Documentation for gun drill dimensional inspection should include: drill identification (unique serial number or engraving), drill type and size (diameter, overall length, shank type), manufacturer and date of manufacture, total regrind count (for reground drills) and cumulative diameter reduction, individual measurement values for each checked parameter (not just pass/fail), inspection date and inspector identification/initials, measurement equipment identification and calibration status, and a clear pass/fail determination with any conditional notes (e.g., "passed with marginal straightness — monitor hole position"). Electronic records in a drill tracking database enable trend analysis — identifying drills that show accelerated wear patterns, drills approaching scrap limits, and process control issues in the regrind operation. Paper records should be retained for at least the life of the drill.
Disclaimer: The dimensional inspection criteria and measurement procedures provided in this article are general guidelines based on industry-standard practices for gun drill inspection. Actual inspection requirements vary by drill size, application, and quality system requirements. All inspection equipment should be calibrated to national or international standards and traceable to certified reference 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.