Materials Machining Guide
Inconel 718 Machining Guide — Feeds, Speeds, Tools, and Common Mistakes
Inconel 718 is a discipline problem, not a horsepower problem. The alloys that give shops trouble in aged (H900) condition machine cleanly with the right setup — sharp coated carbide, aggressive feed, high-pressure coolant, and no dwelling. This guide covers the specifics: SFM ranges, chip load, tooling grades, and the failure modes that show up when discipline slips.
Condition matters — aged vs solution-annealed
Inconel 718 in solution-annealed condition (as-received from the mill, ~30 HRC) machines almost like a well-behaved stainless steel: moderate SFM, standard positive-rake carbide, flood coolant. Roughing takes real chip load; finishing takes sharp inserts.
In aged condition (H900, ~40–45 HRC after precipitation-hardening heat treatment) it becomes a different alloy: harder, tougher, work-hardening under any tool-material contact that doesn't remove chip. Wrong feeds turn a good tool into a scrap tool in three passes.
Recommended tooling
Coated carbide is the workhorse. Grade selection matters — AlTiN or TiAlN coatings on a tough carbide substrate are the reliable default. PCD isn't cost-justified for most oilfield-scale work. HSS is not viable for aged 718.
| Operation | Insert geometry | Coating | Notes |
|---|---|---|---|
| Roughing (turning) | Positive rake, chip-breaking | AlTiN | Aggressive feed to stay below work-hardened layer |
| Finishing (turning) | Sharp positive rake, small nose radius | Fine-grain AlTiN | Fresh insert per feature; don't push tool life |
| Milling (roughing) | High-feed insert, positive geometry | AlTiN | Trochoidal path preferred for HSM strategy |
| Milling (finishing) | Solid carbide end mill, 4-flute | AlTiN | Climb-mill; light DOC, high feed |
| Drilling | Solid carbide, coolant-through | AlTiN | Peck sparingly; through-tool coolant essential |
Feeds and speeds (typical starting points)
These are starting-point ranges from B&R shop practice. Adjust up or down based on rigidity, tool life, and finish requirements. Always start conservative on new work and step up when the process is stable.
| Condition | Turning SFM | Turning IPR | Milling SFM | Chip load per tooth (in) |
|---|---|---|---|---|
| Solution-annealed (~30 HRC) | 100–150 | 0.008–0.015 | 150–200 | 0.003–0.006 |
| Aged H900 (~40–45 HRC) | 60–100 | 0.006–0.012 | 80–150 | 0.002–0.005 |
| Aged H1150 (~35 HRC) | 80–130 | 0.007–0.014 | 120–180 | 0.003–0.005 |
Coolant strategy
Flood coolant is minimum. High-pressure through-tool coolant (>1000 psi) is what enables consistent finishes and long tool life on aged 718. Water-soluble oil emulsion is the standard choice — synthetic coolants are marginal on nickel-based alloys.
Coolant direction matters: for turning, aim behind the cutting edge to break chips and reduce heat. For milling in pockets, through-tool + copious flood keeps chip evacuation and prevents welding.
Work hardening — the failure mode to avoid
Aged Inconel 718 work-hardens under sliding contact from a dull tool. Once the surface layer is work-hardened, subsequent passes cut in an even harder material — accelerating tool wear until the tool fails and the part is scrapped.
Prevention: sharp inserts, aggressive feed to keep the cutting edge below the surface, and no dwelling (rapid retract; never let a rotating tool sit at a stopped feed). If you see chatter, don't slow the spindle — that makes work hardening worse. Increase feed or increase depth of cut instead.
Common mistakes to avoid
The mistakes that cost shops on Inconel 718 are almost always process discipline, not equipment. Here's what to avoid.
- Running dull inserts to save cost — One scrapped Inconel part costs more than a case of inserts. Replace inserts per feature on finish work; per predefined tool life on roughing.
- Reducing speed when chatter starts — This work-hardens the surface. Increase feed or DOC instead.
- Skipping through-tool coolant on deep bores — Chip evacuation problems compound. Buy the through-tool coolant if the operation requires it.
- Machining aged 718 with solution-annealed feeds — Different alloy behavior. Always ask the customer which condition.
- Dwelling at feed hold — Any pause on rotating stock with tool in contact work-hardens the surface. Complete the feature or retract fully.
Frequently asked questions
Where can I get Inconel 718 CNC machined in Texas?
B&R Productions in New Waverly, TX runs Inconel 718 weekly. Direct number for RFQs and rig-down work: (936) 291-7827. Serving Houston, Conroe, Huntsville, The Woodlands, Dallas, San Antonio, Austin, and the Gulf Coast.
What tolerance can be held on Inconel 718?
±0.0005" routine on critical features with sharp coated carbide, controlled feeds and speeds, and CMM verification. Tighter possible with the right fixture and setup discipline.
Can Inconel 718 be machined in the aged (H900) condition?
Yes — routine for finishing operations where the customer wants final dimensions after aging. Requires sharp coated carbide (AlTiN), aggressive feed to stay below the work-hardened layer, and high-pressure through-tool coolant. Feeds and speeds are more conservative than annealed condition.
What's the best insert for machining aged Inconel 718?
Fine-grain AlTiN or TiAlN coated carbide with a sharp positive-rake edge and small nose radius for finishing. Kennametal KCU25, Sandvik GC1005 or GC1105, or equivalent grades are common shop choices. Replace inserts before they dull — dull inserts work-harden the material.
Why does Inconel 718 work-harden so aggressively?
The nickel-chromium matrix hardens under any plastic deformation without cutting. A dull tool or a tool at low feed slides across the surface, deforms without removing metal, and leaves a work-hardened layer harder than the base material. Subsequent passes then cut in even harder material.
Can I use HSS on Inconel 718?
No — HSS lacks the hot-hardness to survive cutting temperatures on nickel-based superalloys. Coated carbide (AlTiN, TiAlN) is the minimum. Solid carbide end mills for finishing; brazed carbide or indexable insert tooling for turning.
What tolerance can I hold on aged Inconel 718?
±0.0005" routine on critical features with proper tool selection, controlled feeds, and CMM verification. Tighter (±0.0002") possible on the right feature with the right fixture, but requires more setup discipline and process time.
How much does Inconel 718 machining cost vs stainless steel?
Rule of thumb: 3–5x the cutting cost of 304/316 stainless on similar features, plus material cost (Inconel is 5–10x the raw material). Comes from lower feeds, more insert wear, more setup time, and required CMM verification. See our cost-drivers guide for details.