A gun drill is only as good as its straightness. The drill rotates at high speed inside a hole that is only 0.2–0.5 mm larger than the drill diameter. A drill that is bent by 0.05 mm will wobble, producing an oversized hole, poor surface finish, and accelerated guide pad wear. Straightness verification is a critical incoming inspection for every gun drill — new and reground.
Straightness Specifications
Industry Standards
| Drill Diameter (mm) | Maximum Straightness Deviation | Measurement Span | Typical Method |
|---|
| 1–6 | 0.01 mm per 100 mm length | Full length | V-block + dial indicator |
| 6–15 | 0.02 mm per 300 mm length | Full length | V-block + dial indicator |
| 15–25 | 0.03 mm per 300 mm length | Full length | V-block + dial indicator |
| 25–40 | 0.04 mm per 300 mm length | Full length | Roll test or V-block |
Tolerance Interpretation
| Straightness Deviation | Effect on Hole Quality | Classification |
|---|
| < 0.01 mm per 300 mm | No measurable effect | Premium quality |
| 0.01–0.02 mm per 300 mm | Minimal effect in most applications | Standard new drill |
| 0.02–0.04 mm per 300 mm | Noticeable in tight-tolerance holes | Acceptable for general use |
| 0.04–0.06 mm per 300 mm | Oversize holes, surface finish degradation | Marginal — limit to roughing |
| > 0.06 mm per 300 mm | Unacceptable for drilling | Reject — do not use |
Measurement Methods
V-Block and Dial Indicator Method
| Step | Action | Detail |
|---|
| 1 | Place two precision V-blocks on surface plate | 200–300 mm apart for short drills |
| 2 | Place drill in V-blocks with shank supported | Drill tip overhangs one V-block |
| 3 | Position dial indicator at midpoint | Contact on drill body, perpendicular to axis |
| 4 | Rotate drill slowly | Observe indicator reading |
| 5 | Record maximum TIR | Peak-to-peak reading |
| 6 | Move indicator to 3–5 positions along drill | At equal intervals |
| 7 | Record maximum TIR at each position | — |
| 8 | Compare to acceptance criteria | Per diameter and length |
Roll Test Method
| Step | Action | Detail |
|---|
| 1 | Place drill on a flat, clean surface plate | Horizontal position |
| 2 | Roll the drill slowly | Observe gap between drill and plate |
| 3 | Watch for light between drill and plate | Indicates bend |
| 4 | Mark high spots | With marker or tape |
| 5 | Repeat at 90° rotation | Check both planes |
| 6 | Estimate straightness | Gap size × approximate factor |
Tip: The roll test is the quickest straightness check and should be performed on every gun drill before it is placed in the machine. It takes 10 seconds and catches gross bends (> 0.05 mm) reliably. For quantitative measurement, use the V-block and dial indicator method.
Optical Comparator Method
| Parameter | Specification |
|---|
| Magnification | 10–50× |
| Measurement resolution | 0.001 mm |
| Best for | Small drills (< 6 mm), tip geometry verification |
| Limitations | Limited length (comparator stage capacity) |
Laser Measurement Method
| Parameter | Specification |
|---|
| Accuracy | ±0.001 mm |
| Measurement length | Full drill length (any) |
| Setup time | 5–10 minutes |
| Cost of equipment | $5,000–$20,000 |
| Best for | High-precision drills, production verification |
Runout Inspection at the Machine
In-Machine Runout Check
| Step | Action | Detail |
|---|
| 1 | Mount drill in machine spindle | Use correct collet or holder |
| 2 | Position dial indicator on drill OD | Near the bushing (entry point) |
| 3 | Rotate spindle by hand | Slow, steady rotation |
| 4 | Read runout at bushing position | TIR at drill OD |
| 5 | Move indicator to tip position | 10 mm from drill tip |
| 6 | Read runout at tip | TIR at drill tip |
| 7 | Compare readings | Increasing TIR toward tip = bend |
Acceptable In-Machine Runout
| Drill Diameter | Runout at Bushing Position | Runout at Tip Position |
|---|
| < 6 mm | < 0.015 mm | < 0.020 mm |
| 6–15 mm | < 0.010 mm | < 0.015 mm |
| 15–25 mm | < 0.008 mm | < 0.012 mm |
| > 25 mm | < 0.008 mm | < 0.010 mm |
Straightening Methods
When Straightening Is Possible
| Condition | Can Straighten? | Method | Success Rate |
|---|
| Bend < 0.02 mm over 300 mm | Yes | Light press | 80–90% |
| Bend 0.02–0.05 mm over 300 mm | Yes | Progressive press | 60–80% |
| Bend > 0.05 mm over 300 mm | Limited | Multiple passes, risk of over-correction | 30–50% |
| Bend at braze joint | No | — | 0% — brazed joint cannot survive straightening |
| Bend near tip (within 20 mm) | No | — | Tip geometry will be affected |
Straightening Procedure
| Step | Action | Detail |
|---|
| 1 | Identify bend location and direction | V-block + indicator measurement |
| 2 | Mark high side of bend | Permanent marker |
| 3 | Support drill on two V-blocks | Spanning the bend |
| 4 | Apply controlled pressure at bend apex | Arbor press or hydraulic press |
| 5 | Over-bend slightly (spring-back allowance) | Typically 2–3× the correction needed |
| 6 | Release pressure and re-measure | Check straightness |
| 7 | Repeat with decreasing correction | Iterative |
| 8 | Final measurement and documentation | — |
Warning: Gun drills can be straightened only a limited number of times. Each straightening cycle introduces residual stress that may slowly relax. A drill that has been straightened twice is at higher risk of re-bending in service. After two straightening attempts, consider retiring the drill.
Quality Verification Documentation
Inspection Record
| Field | Example |
|---|
| Drill part number | GD-10-1283 |
| Supplier | Precision Tool Co. |
| Date received | 2026-06-01 |
| Inspection date | 2026-06-05 |
| Drill diameter (nominal) | 10.000 mm |
| Drill length | 1,200 mm |
| Straightness measurement method | V-block + 0.001 mm indicator |
| Straightness result | 0.008 mm TIR over 300 mm |
| Runout at shank | 0.003 mm |
| Tip condition | No visible defects |
| Acceptance | Pass |
| Inspected by | J. Smith |
FAQ
How straight does a gun drill need to be?
For general deep hole drilling, a gun drill straightness of 0.02 mm TIR or less per 300 mm of length is acceptable. For precision applications (IT6–IT7 tolerance holes), 0.01 mm per 300 mm or better is required. The drill should be at least 2× straighter than the required hole straightness — any bend in the drill is transferred directly to the hole.
How do I check gun drill straightness without special equipment?
The simplest method is the roll test: roll the drill on a clean, flat surface plate. If light is visible between the drill and the plate at any point, the drill is bent. The roll test detects bends of approximately 0.05 mm or greater. For quantitative measurement, support the drill on two V-blocks and use a dial indicator. A surface plate and V-blocks are the minimum equipment needed.
What causes a gun drill to become bent?
The most common causes are: improper handling (dropping or striking the drill), incorrect storage (drill not fully supported, causing gravitational sag), over-tightening in the collet or holder, excessive cutting forces (feed too high or material inclusion), and thermal stress from uneven heating during drilling. The drill tip (first 50 mm) is most susceptible to bending because it is unsupported during entry.
Can a bent gun drill be straightened?
Minor bends (up to 0.02 mm over 300 mm) can often be straightened using a precision press with V-block supports. The process requires skill — over-correction and spring-back make it iterative. Bends at the braze joint (where the carbide tip joins the steel shank) cannot be straightened — the brazed joint is brittle and will crack. Drills with bends exceeding 0.05 mm should be retired.
How does runout at the machine differ from drill straightness?
Runout at the machine is the combination of drill straightness, collet or holder concentricity, spindle runout, and mounting cleanliness. A perfectly straight drill can show high runout if the collet is worn or dirty. Always measure runout at the machine after mounting a drill — this is the actual runout that the hole will experience. Straightness is the drill's inherent geometry; runout is the total error in the mounting system.
Gun drill straightness is a fundamental quality attribute that directly determines hole straightness. Verify every drill before use — the 30-second roll test prevents hours of troubleshooting on bent-drill problems. This article reflects industry practice as of 2026.
