I get more questions about drilling small holes in Inconel 718 than almost any other topic. A recent Practical Machinist thread about drilling 3mm holes 160mm deep in Inconel 718 at HRC 40-45 got over 3,500 views. The user was struggling with drill breakage, centerline deviation, and surface finish.
The root cause in that thread was the same one I see most often: coolant pressure at 8 bar (116 psi) when the job needs 100 bar or more. Here is what I have learned about getting this application right.
Coolant Pressure Is Not Optional
Small diameter gun drilling in Inconel 718 requires coolant pressure that most shops do not have. The chip evacuation challenge at 3mm diameter is fundamentally different from 10mm or larger.
| Drill Diameter | Minimum Coolant Pressure | Recommended Pressure |
|---|---|---|
| 3mm | 100 bar (1500 psi) | 150-170 bar (2200-2500 psi) |
| 4mm | 80 bar (1200 psi) | 120-150 bar (1750-2200 psi) |
| 5mm | 70 bar (1000 psi) | 100-130 bar (1500-1900 psi) |
Below these pressures, chips do not evacuate reliably. The chip flute on a 3mm gun drill has a cross-sectional area of roughly 2-3mm². At inadequate pressure, chips pack in that tiny flute, torque spikes, and the drill breaks.
The user in the Practical Machinist thread was running 8 bar. I have seen this pattern before. A shop buys a small gun drill, runs it on a machine with standard coolant, and wonders why it snaps. The coolant system is the first thing to check.
I cover coolant system requirements in Minimum Coolant Requirements by Diameter.
Cutting Parameters for Small Diameters
Once coolant pressure is adequate, the next step is parameters. Small diameter gun drilling in Inconel 718 needs conservative settings:
| Parameter | 3mm | 4mm | 5mm |
|---|---|---|---|
| Cutting speed | 20-30 m/min | 20-30 m/min | 20-30 m/min |
| Spindle speed | 2100-3200 RPM | 1600-2400 RPM | 1300-1900 RPM |
| Feed rate | 0.003-0.008 mm/rev | 0.005-0.012 mm/rev | 0.008-0.015 mm/rev |
| Coolant pressure | 1500+ psi | 1200+ psi | 1000+ psi |
Research by Biermann and Kirschner (2015) on single-lip gun drilling of Inconel 718 at 1.3-3mm diameter found that feed rates of 0.003-0.005 mm/rev produced the best chip formation. At 0.007 mm/rev, tool life dropped sharply due to unfavorable chip formation and abrasive wear.
I start at the low end of the feed range and increase until I see consistent chip breaking. If chips are longer than 5-6mm, I increase feed slightly. If the drill squeals or the pressure fluctuates, I reduce feed.
The Feed Rate Trap
There is a trap at very low feed rates. Below 0.003 mm/rev, the chip thickness approaches the cutting edge radius. The tool stops cutting and starts rubbing. This generates heat, work-hardens the material, and accelerates edge wear.
I have seen operators reduce feed to avoid tool failure, only to make the problem worse. The minimum feed for Inconel 718 with a sharp gun drill is about 0.003 mm/rev. Below that, the edge rubs and fails faster.
If the machine cannot maintain a steady feed below 0.005 mm/rev, use a feed reducer or a different machine. An inconsistent feed in this range is worse than a slightly higher steady feed.
Straightness Expectations
At 50:1 L/D ratio (3mm × 160mm), straightness of 0.2mm over the full length is achievable with proper setup. Exceeding 0.5mm deviation indicates a problem.
The factors that affect straightness at this scale are the same as for larger diameters, but the tolerances are tighter:
| Factor | Effect at 3mm Diameter |
|---|---|
| Guide bushing clearance | 0.003-0.005mm max |
| Bushing concentricity to spindle | Under 0.010mm TIR |
| Feed rate | Lower feed improves straightness |
| Coolant pressure | Inadequate pressure causes chip packing which pushes the drill off-center |
I cover straightness in more detail in Deep Hole Straightness Mechanics.
Surface Finish Expectations
The user in the forum thread was targeting Ra ≤ 0.3μm. This is achievable with gun drilling in Inconel 718, but it requires everything to be right:
- Sharp tool with appropriate geometry
- Adequate coolant pressure and flow
- Steady feed rate
- Good chip evacuation
In practice, I expect Ra 0.4-0.8μm as-drilled in Inconel 718 at 3mm diameter. Achieving Ra 0.3μm or better typically requires a secondary operation like reaming or honing.
I cover surface finish expectations in How to Improve Gun Drilling Surface Finish.
Tool Coating Selection
Inconel 718 is abrasive and work-hardening. The coating on the gun drill makes a significant difference.
| Coating | Performance in Inconel 718 | Recommended |
|---|---|---|
| TiAlN | Good | Standard choice |
| AlCrN | Better at high temperatures | Preferred for production |
| TiN | Poor | Not recommended |
| DLC | Not suitable | High temperature degrades DLC |
I specify AlCrN coating for production runs of small diameter Inconel 718 drilling. The higher oxidation resistance at the cutting edge translates to 20-40% more holes per regrind compared to TiAlN.
I cover coating selection in Gun Drill Coatings.
What to Check First When Things Go Wrong
If you are struggling with small diameter Inconel 718 gun drilling, check in this order:
- Coolant pressure at the tool tip. Not at the pump. If it is under 100 bar for 3mm, nothing else matters until this is fixed.
- Guide bushing condition. Measure the ID clearance. At 3mm, even 0.01mm wear causes problems.
- Feed rate. Too low causes rubbing. Too high breaks the drill. Start at 0.005 mm/rev and adjust.
- Tool condition. A reground drill with edge damage will not cut Inconel 718 reliably.
- Spindle runout. At 3mm diameter, 0.005mm TIR at the spindle nose is the maximum acceptable.
Steps 1 and 2 catch 80% of problems. Step 3 catches most of the rest.
Key Takeaways
- Coolant pressure is the #1 requirement for small diameter Inconel 718 gun drilling. 3mm holes need 1500 psi minimum. Below that, expect chip packing and drill breakage.
- Feed rate below 0.003 mm/rev causes rubbing rather than cutting. Do not reduce feed to solve a problem — it will make the problem worse.
- Straightness of 0.2mm over 160mm at 3mm diameter is achievable with proper bushing and alignment.
- As-drilled surface finish of Ra 0.4-0.8μm is realistic. Ra 0.3μm or better typically needs a secondary operation.
- AlCrN coating outperforms TiAlN for production runs in Inconel 718.
- Check coolant pressure first, bushing condition second. These two factors cause the majority of failures.