Difficult materials need different parameters than standard steel. I have drilled all three of these materials extensively. Each one presents unique problems that require specific solutions.
The common thread across all difficult materials is heat management and work-hardening. Here is my detailed approach for stainless steel, titanium, and Inconel.
Material Difficulty Rating
Before I get into the specifics, I want to lay out a difficulty framework. Not all difficult materials are equally hard to drill, and the ranking matters for quoting and tooling decisions. I rate materials on a 1-10 scale based on tool wear rate, achievable penetration rate, and coolant pressure requirements.
| Material | Difficulty (1-10) | Primary Challenge | Secondary Challenge | Relative Penetration Rate (%) |
|---|---|---|---|---|
| 1018 mild steel | 2 | None (baseline) | — | 100% (baseline) |
| 4140 alloy steel | 4 | Heat at deeper depths | Tool wear at high speeds | 75% |
| 304/316 stainless | 6 | Work-hardening | Chip evacuation | 40% |
| Ti-6Al-4V titanium | 8 | Heat retention at cutting edge | Chip welding, stringy chips | 25% |
| Inconel 718 | 9 | Combined work-hardening + heat | Extremely short tool life | 10% |
| Waspaloy / Rene 41 | 10 | Severe work-hardening | Rapid edge degradation | 8% |
This table tells me what I am up against before I touch the machine. Stainless is about twice as hard as 4140. Inconel is about ten times harder than stainless in terms of tool life. These ratios drive my quoting and tooling budgets.
Parameter Comparison
Here are the parameters I use for each material. These are starting points — I adjust based on chip formation and tool wear.
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (psi) | Tool Coating | Expected Tool Life (m) |
|---|---|---|---|---|---|
| 304/316 stainless | 50-75 | 0.02-0.06 | 1200-1800 | TiAlN | 8-15 |
| Ti-6Al-4V titanium | 25-45 | 0.01-0.04 | 1500-2500 | Uncoated carbide, sharp edge | 3-8 |
| Inconel 718 | 15-30 | 0.01-0.03 | 1800-3000 | AlTiN | 1-2 |
The speeds vary by about 5:1 from stainless to Inconel. The coolant pressure varies by about 2:1. The tool life varies by about 10:1. These ratios tell you which material is hardest on tools.
When depth exceeds 2 times the diameter, I reduce cutting speed by another 20 to 30 percent. The deeper the hole, the harder it is to get coolant to the cutting edge and chips out of the bore. This derating applies to all three materials.
Stainless Steel (304/316)
Stainless steel work-hardens faster than any other common material. The key is keeping the tool advancing steadily with no dwell time. I program a continuous feed from start to finish.
The cutting speed range of 50-75 m/min works well for 304 stainless. For 316, I use the lower end of the range because it work-hardens more aggressively. I have broken tools at 80 m/min in 316 when the material had a higher carbon content.
The feed rate needs to be high enough to cut below the work-hardened layer. I use a minimum of 0.025 mm/rev. Below this, the tool rubs and creates a hard surface that dulls the edge quickly. At larger diameters above 16 mm, I increase the feed to 0.04-0.06 mm/rev depending on chip formation.
I use TiAlN-coated carbide tools for stainless. The coating reduces friction and heat buildup. The chip formation should be short and slightly colored — blue chips indicate the speed is too high, silver chips indicate good cutting. Advanced coatings like AlTiN or AlCrN can extend tool life by 15-22 percent in stainless by providing better thermal barrier properties.
Tool life expectation: 8-15 meters of drilling per edge in 304/316 stainless. The shorter end applies to small diameters under 6 mm and deeper holes over 100x diameter. I change tools at the first sign of increased load or chip color change.
For more on work-hardening management, see my guide on heat management in high-temp alloys.
Titanium (Ti-6Al-4V)
Titanium has low thermal conductivity — about 7 W/mK compared to 50 W/mK for steel. The heat stays at the cutting edge instead of dissipating into the chip. This is the main challenge.
High coolant pressure is essential for titanium. I run at 2000-2500 psi and check the pressure at the tool tip. The coolant removes heat from the cutting edge and flushes the chips. Without adequate pressure, the chips weld to the edge within seconds. Some high-performance carbide drills with specialized coatings like ALtima Plus have been shown to achieve 46 m/min at 0.36 mm/rev in titanium alloys, which is substantially faster than standard uncoated tools.
I use uncoated carbide tools with a sharp cutting edge for titanium. Titanium reacts with many coating materials, causing chemical wear. A sharp edge cuts with less force and generates less heat. I inspect the edge after every 10 holes and replace it at the first sign of wear. For production runs over 500 holes, I evaluate AlTiN-coated tools because recent coating technology has improved chemical stability in titanium.
The cutting speed of 25-45 m/min seems slow compared to steel, but titanium requires it. At 50 m/min, the edge temperature rises above 600 degrees C and the tool fails rapidly. I stay at the lower end for small diameters under 8 mm and push toward 45 m/min for diameters over 12 mm.
Titanium chips are long and stringy. I use peck cycles to break the chips and prevent them from wrapping around the drill. For a 10mm hole at 200mm depth, I use 5mm pecks. The peck depth should not exceed half the drill diameter in titanium to prevent chip packing.
Tool life expectation: 3-8 meters of drilling per edge. Titanium is gentler on tools than Inconel but harder than stainless. I replace tools proactively at 3 meters for diameters under 8 mm and at 6 meters for larger diameters.
Inconel 718
Inconel 718 combines the worst properties of stainless and titanium. It work-hardens like stainless steel and retains heat like titanium. The combination makes it the most difficult material I drill regularly.
The cutting speed of 15-30 m/min seems impossibly slow when you are used to drilling steel at 100 m/min. But at these low speeds, the tool survives and the hole quality is good. Above 30 m/min, the edge degrades in seconds. Recent research with PCBN-tipped gun drills has shown that cutting speeds above 50 m/min are possible, but the tooling cost is significantly higher.
I use AlTiN-coated micrograin carbide tools. The coating provides the thermal barrier needed for Inconel’s high cutting temperatures. The micrograin substrate provides edge toughness. Advanced coating systems that combine AlTiN with a top layer of AlCrN can increase maximum operating temperature by 22 percent compared to standard AlTiN alone.
The feed rate must balance two competing requirements: high enough to cut below the work-hardened layer, but low enough to avoid edge chipping. I have found that 0.015 mm/rev is a good starting point for most diameters. For diameters under 6 mm, I reduce the feed to 0.008-0.010 mm/rev.
Coolant pressure at 2500-3000 psi is non-negotiable for Inconel. I use straight oil coolant with a chiller to keep the temperature below 45 degrees C. Inconel generates so much heat that the coolant temperature can rise 15 degrees C during a 30-minute run.
Tool life expectation: 1-2 meters of drilling per edge in Inconel 718. This is the shortest tool life of any material I regularly drill. At these rates, tool cost is a significant factor in part pricing. I track tool usage per hole to maximize edge life before resharpening.
Key Takeaways
- Each difficult material needs a tailored approach — one size does not fit all.
- Difficulty ranking: Stainless (6/10) < Titanium (8/10) < Inconel (9/10) < Waspaloy (10/10).
- Reduce cutting speed by 20-30% when depth exceeds 2x diameter for all materials.
- Stainless steel: continuous feed, minimum 0.025 mm/rev, TiAlN coating, 8-15m tool life.
- Titanium: 2000+ psi coolant, uncoated carbide or AlTiN-coated, 25-45 m/min, 3-8m tool life.
- Inconel: 2500+ psi coolant, AlTiN or advanced coating, 15-30 m/min, 1-2m tool life.
- Tool life varies enormously: 8-15m in stainless, 3-8m in titanium, 1-2m in Inconel.
- Advanced coatings (AlCrN, ALtima Plus) can extend tool life 15-200% depending on material and application.
- PCBN-tipped gun drills offer higher speeds for Inconel but at significantly higher tooling cost.
For detailed parameters on specific materials, see my guides on Inconel 718 and drilling Waspaloy and Rene 41. For more on coolant system requirements, see my guide on coolant filtration system design.