I have a process I use for every new gun drilling job. It starts with the bore print and works through diameter, shank, length, tip geometry, and coating in that order. Skip any of these steps and you risk a tool that wanders, breaks, or wears out before it reaches depth.
Here is my full sizing and selection process, along with the reference tables I keep pinned above my bench.
How Bore Diameter Determines Drill Diameter
The finished bore diameter on the print is never the drill diameter you order. A gun drill cuts slightly oversize by design — the outside diameter of the drill is smaller than the hole it produces. This oversize allowance is what gives the drill clearance in the bore and prevents the shank from rubbing.
The amount of oversize depends on the material, the drill geometry, and the tolerance of the finished hole. As a rule of thumb, I expect 0.001 to 0.003 inches of oversize per inch of drill diameter in steel. In softer materials like aluminum, the oversize can run higher — up to 0.005 inches per inch.
So if the print calls for a 0.500-inch bore in 4140 steel, I order a drill that is 0.498 or 0.499 inches in diameter. The drill cuts the remaining material and the hole finishes at nominal or slightly above.
The drill manufacturer’s catalog will list the expected oversize for each drill diameter and geometry combination. I always check this before placing an order. Guessing the oversize and hoping it lands on print is a fast way to scrap a part.
Shank Diameter Selection
The shank diameter controls how the drill is held and how much clearance it has inside the bore. Shank diameter must be smaller than the drill diameter to provide clearance for coolant return and chip evacuation.
The clearance ratio is the difference between the drill diameter and the shank diameter, divided by the drill diameter. I target a minimum of 0.010 inches of clearance on diameter for drills under 0.5 inches and 0.015 to 0.020 inches for larger drills. Too little clearance and chips pack up between the shank and the bore wall. Too much clearance weakens the shank and can cause the drill to whip.
Here is the reference table I use for nominal bore sizes, recommended drill diameters, and shank diameters:
| Nominal Bore (in) | Drill Diameter (in) | Shank Diameter (in) | Clearance (in) |
|---|---|---|---|
| 0.125 | 0.124 | 0.109 | 0.015 |
| 0.250 | 0.249 | 0.234 | 0.015 |
| 0.375 | 0.373 | 0.358 | 0.015 |
| 0.500 | 0.498 | 0.483 | 0.015 |
| 0.625 | 0.623 | 0.608 | 0.015 |
| 0.750 | 0.747 | 0.727 | 0.020 |
| 1.000 | 0.996 | 0.976 | 0.020 |
| 1.250 | 1.245 | 1.225 | 0.020 |
| 1.500 | 1.494 | 1.474 | 0.020 |
| 2.000 | 1.992 | 1.967 | 0.025 |
These numbers are starting points. I adjust them based on the material and the length-to-diameter ratio of the hole.
Length Selection
Gun drill length is specified from the tip to the back of the shank. The length you need depends on the depth of the hole plus the distance from the spindle to the workpiece.
The critical factor is the length-to-diameter ratio, or L/D. A gun drill with an L/D of 20:1 or less is straightforward — the drill is stiff enough to hold a straight line without much support. As L/D climbs past 30:1, straightness becomes a challenge. Past 50:1, you are in specialty territory and need a drill designed specifically for deep-hole work with reinforced shanks and optimized geometry.
Here is how I think about L/D:
- Under 20:1 — Standard gun drill, no special considerations.
- 20:1 to 40:1 — Use a drill with a reinforced shank or a larger shank diameter to improve stiffness. Support bushings become more important.
- 40:1 to 60:1 — Requires a drill with a welded-on reinforced shank section and a carefully matched guide bushing setup. Expect to regrind more frequently.
- Over 60:1 — Custom drill design. Contact the manufacturer with the exact bore requirements.
I never order a drill longer than necessary. An extra 2 inches of length reduces stiffness and increases the risk of whipping. Order the shortest drill that reaches the hole depth with the machine setup you have.
Tip Geometry Selection by Material
Tip geometry is where experience separates a working drill from a problematic one. The point angle, primary clearance, and secondary clearance all need to match the material. I covered the full breakdown of angles in my earlier article on gun drill geometry, but here is the cheat sheet I actually use when selecting a drill for a new job:
| Material | Point Angle | Primary Clearance | Secondary Clearance | Recommended Coating |
|---|---|---|---|---|
| Low-carbon steel (1018, 1020) | 20-22 deg | 8-10 deg | 12-15 deg | TiN |
| Medium-carbon steel (1045, 4140) | 18-20 deg | 8-10 deg | 12-15 deg | TiAlN |
| Tool steel / Die steel | 15-18 deg | 6-8 deg | 10-12 deg | TiAlN or AlTiN |
| Stainless steel (303, 304, 316) | 15-17 deg | 6-8 deg | 10-12 deg | TiAlN |
| Inconel / Superalloys | 12-15 deg | 5-7 deg | 8-10 deg | AlTiN |
| Aluminum (6061, 7075) | 25-30 deg | 10-12 deg | 15-18 deg | DLC or uncoated |
| Brass / Bronze | 25-30 deg | 10-12 deg | 15-18 deg | Uncoated |
| Cast iron | 18-20 deg | 6-8 deg | 10-12 deg | TiN |
| Titanium alloys | 12-15 deg | 5-7 deg | 8-10 deg | AlTiN |
A steeper point angle (higher degree) reduces cutting forces and works well in soft materials. A shallower point angle strengthens the cutting edge and is necessary for hard or abrasive materials. The clearance angles follow the same pattern — more clearance in soft materials, less in hard materials to preserve edge strength.
Coating Selection by Material
The coating is the second most important decision after geometry, and I have a dedicated article on gun drill coatings and materials that goes deeper into each option. Here is the short version of what I use and why:
- TiN (titanium nitride). My go-to for general-purpose carbon steel. Good lubricity, moderate heat resistance, and affordable. It is a yellow-gold coating and works well below 1000 degrees Fahrenheit at the cutting edge.
- TiCN (titanium carbonitride). Harder than TiN and lower friction. I use this on drills that will see abrasive conditions like cast iron or high-silicon aluminum. The blue-gray color makes it easy to identify on the shelf.
- TiAlN (titanium aluminum nitride). The workhorse for tough jobs. It handles heat up to 1400 degrees Fahrenheit by forming an aluminum oxide layer at the cutting edge. I run this on almost everything in the medium-to-hard steel range. It is violet in color.
- AlTiN (aluminum titanium nitride). Higher aluminum content than TiAlN, which means even better oxidation resistance at extreme temperatures. I reserve this for superalloys and hardened steels above 40 HRC. Dark gray coating.
- DLC (diamond-like carbon). The best coating for aluminum and non-ferrous materials. Extremely low friction coefficient — chips practically slide off. No chemical affinity with aluminum, so built-up edge is rare. Black coating.
I do not coat drills for brass, bronze, or most cast irons. These materials are naturally lubricious and the coating adds cost without meaningful benefit. Uncoated carbide is the right call there.
What to Do When You Need Non-Standard Sizes
Standard gun drill diameters come in 1/64-inch and 0.5 mm increments depending on the manufacturer. But real-world jobs do not always line up with standard sizes. When I need a non-standard drill, here is my process:
First, check whether the finished bore tolerance is wide enough to accept a standard drill running slightly oversized. If the print calls for 0.4375 inches and the tolerance is +/- 0.002, a 0.4370-inch standard drill that cuts 0.001 oversize lands at 0.4380 — right inside the band. I do this first because it saves a lead time of two to four weeks.
If the tolerance is too tight for a standard drill, I order a custom grind. Most gun drill manufacturers will grind a custom diameter on a standard blank for a moderate upcharge. Lead time is typically 10 to 15 business days. I have done this for diameters like 0.378 inches and 0.562 inches when the job demanded it.
For shank diameters, custom is rarely necessary. A standard shank with the correct clearance ratio and a custom bushing is almost always the faster and cheaper solution. I keep a set of bushings in 1/64-inch increments for exactly this purpose.
The one case where I always go custom is L/D above 60:1. Standard off-the-shelf drills at extreme L/D ratios are a gamble. A custom drill with the exact reinforced shank geometry and tip design for the application pays for itself in reduced scrap and longer tool life.
Key Takeaways
- Order the drill diameter below nominal bore size by the expected oversize for the material. Do not match the print dimension.
- Shank diameter needs a minimum of 0.010 to 0.020 inches of clearance depending on drill size. Too little clearance causes chip packing.
- Keep the drill as short as the machine setup allows. Every extra inch reduces stiffness at high L/D ratios.
- Match tip geometry to the material using the reference table above. Point angle and clearance angles must change with material hardness.
- Use TiAlN for most steel work and DLC for aluminum. Skip coatings for brass and bronze.
- Check if a standard drill fits the bore tolerance before ordering a custom grind — it saves time and money.
- For L/D ratios above 60:1, always go with a custom drill designed for the specific application rather than a standard catalog tool.