I select gun drill geometry based on the material I’m cutting. In my experience, the geometry is the single biggest factor determining whether a job runs smoothly or produces scrap. I have seen shops spend thousands on premium carbide drills only to get poor results because the geometry was wrong for the material.
The geometry of a gun drill’s cutting tip determines how the drill performs. Different geometries are suited to different materials and applications.
Here is how I approach gun drill geometry selection for every job.
Tip Angle
The tip angle is the angle of the cutting edge relative to the drill axis. The tip angle affects chip formation and cutting forces. I have found that getting the tip angle right is the first thing to check when a new job is not cutting cleanly.
| Tip Angle | Typical Use | My Recommendation |
|---|---|---|
| 30 degrees | Soft materials (aluminum, brass, bronze) | Best for chip flow in low-hardness materials |
| 20 degrees | Medium materials (carbon steel, alloy steel) | All-purpose for most steel work |
| 15 degrees | Hard materials (tool steel, stainless, 4140 > 30 HRC) | Better edge strength for abrasive materials |
| 12 degrees | Very hard materials (hardened steel > 45 HRC) | Minimizes edge chipping in hard cutting |
I use a 20-degree tip angle for most general-purpose work. It provides a good balance of cutting efficiency and edge strength. For aluminum, I switch to 30 degrees — the steeper angle produces wider, thinner chips that evacuate more easily through the flute. For hardened steel above 45 HRC, I go down to 12 degrees.
Clearance Angle
The clearance angle is the angle behind the cutting edge that prevents the tool from rubbing against the workpiece. Too little clearance causes rubbing and generates excessive heat. Too much clearance weakens the edge and leads to chipping.
The standard clearance angle on a gun drill is 8-12 degrees. I use 10 degrees for most work. For soft gummy materials like aluminum 6063, I bump the clearance to 12-15 degrees to prevent the material from welding to the relief face. For hard steel above 40 HRC, I reduce the clearance to 8 degrees to preserve edge strength.
I check clearance angles using an optical comparator. I set the drill in a V-block and project the tip profile onto the screen at 20x magnification. The clearance angle shows up clearly as the angle between the cutting edge and the relief surface. I verify the angle at three points along the cutting edge — the outer corner, the mid-point, and near the center. A variation of more than 1 degree across these points indicates the regrind needs correction.
Rake Angle and Inner-Edge Geometry
The rake angle on a standard gun drill is typically 0 degrees — the rake face is flat. This simplifies manufacturing and resharpening. However, I have found that a slightly positive rake (2-5 degrees) helps in soft materials by reducing cutting forces and improving chip flow.
The inner cutting edge has its own geometry considerations. The internal angle (phi angle) is usually 20-25 degrees, while the external angle (psi angle) is 30-40 degrees. The difference between these two angles creates the eccentric point that is characteristic of gun drills. The eccentricity is typically d/4, where d is the drill diameter.
I explain more about the cutting action in my article on how gun drills cut, which covers the mechanics behind the geometry.
Chip Former Geometry
The chip former — also called the chip breaker — is a groove or step behind the cutting edge that shapes the chip. The chip former geometry determines the chip size and shape. I have spent more time debugging chip former issues than any other geometry problem.
A well-designed chip former produces short, broken chips that evacuate easily through the flute. A poorly designed chip former produces long, stringy chips that pack up and cause problems. The chip former width and depth need to match the feed rate. For feeds of 0.02-0.05 mm/rev, a narrow shallow chip former works. For heavier feeds above 0.08 mm/rev, a wider deeper chip former is needed.
I match the chip former to the feed rate using this guideline:
| Feed Rate Range | Chip Former Width | Chip Former Depth | Expected Chip Form |
|---|---|---|---|
| 0.01 - 0.03 mm/rev | 0.3 - 0.5 mm | 0.15 - 0.25 mm | Dust to fine powder |
| 0.03 - 0.06 mm/rev | 0.5 - 0.8 mm | 0.25 - 0.40 mm | Short comma chips |
| 0.06 - 0.10 mm/rev | 0.8 - 1.2 mm | 0.40 - 0.60 mm | Half-moon chips |
| 0.10 - 0.15 mm/rev | 1.2 - 1.6 mm | 0.60 - 0.80 mm | Spiral segments |
Guide Pad Geometry
The guide pads are the two bearing surfaces on the drill head that stabilize the drill in the bore. Their geometry is just as important as the cutting edge geometry. The pads have a back taper — the diameter decreases from the cutting tip toward the shank — typically 0.02 x d per 100 mm of length.
The pad lag — the distance the pads trail behind the cutting edge — is critical for stability. I set the lag at 0.5-1.2 mm for most applications. Too little lag and the drill chatters. Too much lag and the pads rub excessively, creating heat and surface finish problems.
I discuss the role of guide pads in depth in my article on gun drill stability and vibration control. The pad geometry is often overlooked but causes more quality problems than the cutting edge does.
Tip Types
Gun drill tips come in different configurations. Here is how I choose between them:
| Tip Type | Cutting Edges | Coolant Holes | Best Application | My Experience |
|---|---|---|---|---|
| Standard tip | 1 | 1 | General purpose, most materials | Good for 80% of jobs |
| Double-margin tip | 2 | 2 | Unstable materials, deep holes > 100:1 | Better hole straightness but higher torque |
| Step tip | 2 | 2 | Large diameters > 25 mm | Reduces cutting forces per edge |
| Special profile tip | 1-2 | 1-2 | Specific materials (cast iron, Inconel) | Custom geometry for difficult materials |
Coatings
Coatings improve tool life by reducing wear and friction. I have tested every coating type listed below across different materials. Here is my real-world experience:
| Coating | Color | Best For | Typical Tool Life Improvement |
|---|---|---|---|
| TiN (titanium nitride) | Gold | General purpose steel | 50-80% over uncoated |
| TiCN (titanium carbonitride) | Blue-gray | Hard materials, abrasive steels | 80-120% over uncoated |
| TiAlN (titanium aluminum nitride) | Violet | High-temperature materials | 100-150% over uncoated |
| AlTiN (aluminum titanium nitride) | Dark gray | Very hard materials > 45 HRC | 120-180% over uncoated |
| DLC (diamond-like carbon) | Black | Non-ferrous materials, aluminum | 200-300% over uncoated |
I use TiAlN for most steel work. It provides good wear resistance at moderate cost. For aluminum, I use DLC-coated drills exclusively — the tool life improvement is dramatic and the reduced friction prevents built-up edge.
Geometry Troubleshooting Reference
When a gun drill is not performing, I check these geometry issues in order:
| Problem | Likely Geometry Cause | What I Check First |
|---|---|---|
| Poor surface finish | Incorrect clearance angle | Measure outer clearance with comparator |
| Short tool life | Tip angle too acute for material | Verify tip angle vs material hardness |
| Chip packing in flute | Chip former too narrow for feed rate | Check chip former width against feed rate |
| Drill wanders off-center | Guide pad geometry or back taper | Measure pad clearance and back taper |
| Chattering noise | Pad lag too short | Increase pad lag by 0.2-0.3 mm |
| Built-up edge on tip | Rake angle too low for material | Increase rake to 2-5 degrees positive |
Key Takeaways
The tip geometry is the most important factor in gun drill performance. A drill with the right geometry for the material cuts well and produces good surface finish. A drill with the wrong geometry causes problems regardless of the machine or parameters.
I select the tip geometry based on the material and the hole requirements. For general-purpose work, a standard 20-degree tip with TiAlN coating works well. For specialized work, I match the clearance angle, chip former geometry, and coating to the specific material.
The geometry troubleshooting table I shared above has saved me hundreds of hours of trial and error. I printed it out and taped it to my toolbox — I recommend every shop do the same. When a drill is not cutting right, check the geometry first, before changing speeds or feeds.