Machining Inconel 718

Why Inconel 718 is difficult

Inconel 718 is a nickel-chromium superalloy (52.5% Ni, 19% Cr, 18.5% Fe) with a yield strength of 1,035 MPa in the aged condition. It retains mechanical properties at temperatures up to 1,300°F — which is exactly why aerospace uses it for turbine disks, combustion chambers, and exhaust components, and exactly why it destroys tooling.

Three properties make it difficult. First, thermal conductivity is 11.4 W/m·K — roughly 1/4 of carbon steel. Heat doesn't leave through the chip. It stays in the cutting zone and cooks the insert. Second, work hardening. The surface ahead of the tool hardens as it's cut, which means any rubbing or dwelling creates a hardened layer that the next pass has to fight through. Third, abrasive carbide particles (NbC, TiC) in the matrix act like microscopic grinding wheels on your cutting edge.

Turning parameters

Carbide inserts (PVD-coated, positive rake): 110–150 SFM, 0.006–0.012" IPR, 0.040–0.100" DOC. This is the production baseline for solution-treated 718 (28–34 HRC). Use CNMG or DNMG with aggressive chipbreaker geometry — you need a sharp edge and positive rake angle. Negative rake inserts work but require more horsepower and generate more heat.

Age-hardened 718 (36–44 HRC): drop to 80–120 SFM with carbide. This material is harder but actually produces better chips — less gummy, more predictable. Sandvik GC1105 and GC1115 in CNMX geometry with the SM chipbreaker work well here. Kennametal KC5010 is another proven grade.

Ceramic inserts (SiAlON, whisker-reinforced Al₂O₃): 600–1,000 SFM, 0.004–0.008" IPR, 0.020–0.060" DOC. Greenleaf WG-300, NTK WA1, and Kennametal KY1540 are production-proven grades. Ceramics require rigid setups, high spindle horsepower (15+ HP at the tool), and absolutely no coolant — thermal shock destroys ceramics. Run dry with chip evacuation by air blast. Ceramics make economic sense on long production runs where the 3–5x speed increase offsets the higher insert cost ($15–30 per edge vs $5–10 for carbide).

CBN inserts: 500–800 SFM for finishing. Achieves Ra 16–32 µin on hardened 718. Cost is 2–3x carbide, but the speed increase and surface finish often eliminate secondary grinding. Production shops use CBN for finish passes and carbide or ceramic for roughing.

Milling parameters

Indexable milling (carbide): 80–120 SFM, 0.003–0.006" IPT, 0.040–0.080" axial DOC, 30–50% radial engagement. PVD-coated grades with sharp positive geometry. AlTiN coating outperforms TiAlN on nickel alloys because it forms a stable aluminum oxide layer at cutting temperatures.

Solid carbide end mills: 80–100 SFM, 0.001–0.003" IPT, variable helix (35/38°) to reduce chatter. 4-flute for slotting, 5-flute for profiling. Through-tool coolant is mandatory. Chip thinning at radial engagements below 25% allows higher feed rates — calculate effective chip thickness and adjust IPT accordingly.

High-feed milling: 200–350 SFM with round inserts (RCMT/RPMW) at shallow DOC (0.010–0.030") and high feed per tooth (0.020–0.040" IPT). This strategy works because the shallow DOC shifts cutting forces axially into the spindle rather than radially. It's faster for roughing open pockets and faces than conventional approaches.

Achievable tolerances

Standard production tolerances on Inconel 718:

Turning: ±0.001" on diameters is routine with carbide tooling and stable setups. ±0.0005" requires in-process gauging and slower finish passes. ±0.0002" is achievable but demands CBN or ceramic finishing, temperature-controlled environments, and post-machining stress relief.

Milling: ±0.002" on profile dimensions with carbide tooling. ±0.001" with rigid workholding and finishing passes at reduced engagement. Thin-wall features (wall thickness < 0.060") are the challenge — thermal expansion and cutting forces cause deflection. Leave 0.010–0.015" for a finish pass with reduced radial engagement.

Surface finish: Ra 32–63 µin is standard with carbide. Ra 16–32 µin with CBN or ceramic finishing. Ra 8–16 µin requires grinding or polishing as a secondary operation.

Flatness/straightness: 0.002" over 6" is production-realistic. Tighter than 0.001" over 6" requires stress-relief heat treatment between roughing and finishing.

Coolant strategy

High-pressure coolant (1,000+ PSI) is not optional — it's the single biggest factor in tool life after insert grade selection. At 1,000 PSI, coolant penetrates the tool-chip interface and hydraulically lifts the chip off the rake face. This reduces crater wear by 30–50% compared to flood coolant at standard pressure (50–150 PSI).

Systems like ChipBLASTER and MP Systems deliver 1,000–2,000 PSI through-tool coolant. Retrofit cost is $15–30K per machine, but the tool life improvement pays for itself in 3–6 months on steady 718 work. If high-pressure isn't available, use flood coolant with maximum volume directed precisely at the cutting zone. Emulsion at 8–10% concentration (higher than the typical 5–7% for steel).

Exception: ceramic inserts. Never use coolant with ceramics. Thermal shock from intermittent coolant contact causes immediate edge failure. Run dry with an air blast for chip evacuation.

Work hardening — how to avoid it

Every time the tool rubs instead of cuts, the surface work-hardens. The hardened layer can reach 50+ HRC — harder than the tool's comfort zone. Three rules prevent this:

Never dwell in the cut. Constant feed rate at all times. If the tool stops moving while in contact with the workpiece, the surface hardens. Program smooth toolpaths with no hesitation at direction changes.

Climb mill, never conventional mill. In climb milling, the chip starts thick and thins out. In conventional milling, the chip starts at zero thickness — the tool rubs before it cuts. That rubbing hardens the surface. Always climb mill Inconel 718.

Never re-cut a chip. Chips that stay in the cutting zone get re-cut and work-harden the surface. High-pressure coolant and proper chip evacuation prevent this. On turning operations, use chipbreaker inserts that curl and break the chip reliably.

When to use EDM instead

Wire EDM cuts Inconel 718 at any hardness with no tool wear concerns, no work hardening, and no cutting forces. For thin-wall features, small slots, and complex profiles where mechanical cutting would cause deflection or require multiple light passes, EDM is often faster and cheaper than milling.

Sinker EDM produces cavities, ribs, and complex 3D forms in hardened 718 that would require hours of milling. The electrode (graphite or copper) wears, but electrode cost is a fraction of the carbide insert cost for equivalent material removal in tight geometries.

The decision: if the feature is accessible to a cutter and the volume justifies the setup, machine it. If the feature is deep, thin-walled, or geometrically complex — EDM it.

Cost impact

Machining Inconel 718 costs 3–5x more than machining 4140 steel for equivalent geometry. The drivers: 5–10x slower cutting speeds, 3–5x higher tooling consumption, longer cycle times, and the need for high-pressure coolant systems and rigid machines. A part that costs $50 in 4140 will cost $150–250 in Inconel 718.

Material cost compounds this. Inconel 718 bar stock runs $15–30/lb depending on form and quantity. Plate is $20–40/lb. Compare to $0.50–1.00/lb for 4140. A part that uses $5 of steel uses $50–100 of Inconel.

The shops that machine 718 efficiently invest in the right equipment: high-pressure coolant, rigid machines (40+ taper minimum, often 50-taper or HSK-A63), and operators who understand the material. These aren't commodity CNC shops — they're aerospace-grade operations with the tooling budgets and process discipline to match.