Glossary
37 terms, defined the way they come up on an actual drawing or quote — not textbook definitions. Every entry links to the note or guide that goes deeper.
Materials and grades
- Ti-6Al-4V (Titanium Grade 5)
-
The titanium alloy most people mean by "titanium" — 6% aluminium, 4% vanadium, chosen for strength-to-weight and fatigue life. Its low thermal conductivity, not its hardness, is what makes it hard to cut.
Machining parameters → - Titanium Grade 2
-
Commercially pure titanium. Softer and easier to machine than Grade 5, with less strength — chosen for corrosion resistance where load is low.
Grade 2 vs Grade 5 → - Titanium Grade 23 (ELI)
-
Extra-Low Interstitial Ti-6Al-4V — tighter oxygen and iron limits than standard Grade 5, for better fracture toughness. Used where fatigue and crack resistance matter more than raw strength: implants, cryogenic structure.
Compared against Grade 5 → - Inconel 625
-
A nickel-chromium-molybdenum alloy chosen for corrosion resistance — seawater, acids, flue gas — rather than for strength at temperature. Machines slightly easier than 718, still work-hardens fast.
625 vs 718 → - Inconel 718
-
The default nickel superalloy for load-bearing structure to roughly 650°C. Behaves very differently solution-treated versus aged per AMS 5663 — the condition on the drawing changes the machining job, not just the hardness number.
Machining parameters → - Hastelloy C-276
-
A nickel-molybdenum-chromium alloy for the most aggressive reducing acids and chlorides. Work-hardens faster than 625 or 718; finishing passes need a minimum depth of cut to avoid riding the hardened skin.
Machining guide → - Monel 400 / K-500
-
Nickel-copper alloys chosen for seawater and hydrofluoric acid service. Long, stringy chips and a tendency to gall — chip control matters more than raw cutting speed.
Machining guide → - Duplex / super duplex stainless (2205, 2507)
-
Stainless with a mixed austenite-ferrite microstructure, giving roughly double the yield strength of 316 at similar or better corrosion resistance. Never stress-relieve super duplex in the 650–950°C range — it precipitates a brittle sigma phase.
Machining guide → - 17-4 PH stainless
-
A precipitation-hardening stainless that machines easily in Condition A and turns into a hard-machining job at H900. The condition code on the drawing is the single biggest cost lever on this material.
Heat treat conditions → - Invar 36 / Kovar
-
Low-expansion iron-nickel (Invar) and iron-nickel-cobalt (Kovar) alloys, chosen for dimensional stability or glass-to-metal sealing rather than strength. The stress-relief sequence matters more than the cutting parameters.
Machining guide → - Refractory metals (molybdenum, tungsten, tantalum)
-
High-melting-point metals for furnace and high-temperature structure. Failure mode is usually brittleness, not heat — recrystallised molybdenum in particular is close to unmachinable.
Machining guide → - Tool steel (H13, D2, A2, M2)
-
Steels formulated for wear resistance or hot-work service rather than corrosion resistance. Machined soft and heat-treated, or bought hard and machined with CBN — the process route decision that dominates cost.
Machining guide → - UNS number
-
The Unified Numbering System code that unambiguously identifies an alloy — for example N07718 for Inconel 718 — independent of trade name or which mill produced it. Worth putting on a drawing alongside the trade name.
Alloy comparison table →
Certification, documentation and quality
- EN 10204 (3.1, 3.2)
-
The European standard for material inspection documents. Type 3.1 is a mill certificate for the specific heat your material came from, checked by the mill’s own independent inspection department. Type 3.2 adds a witnessing third-party inspector — slower and more expensive, and only needed where the specification says so.
What each document proves → - Certificate of Conformance (CoC)
-
A supplier’s written statement that parts meet the drawing. Proves someone will say so in writing; proves nothing about the metal on its own.
What each document proves → - Heat lot / heat-lot traceability
-
A heat is one melt at the mill; every bar from it shares a heat number and a chemistry. Traceability means the finished part can be tied back to that number — which requires the shop to physically segregate and record material, and is worth asking for explicitly.
Heat-lot traceability explained → - Material Test Report (MTR)
-
The common name in North America for what EN 10204 calls a 3.1 certificate — a mill’s test report on the actual chemistry and mechanical properties of a specific heat.
Mill certs explained → - First Article Inspection (FAI) / AS9102
-
A dimensional report proving the first part off a new process meets every drawing characteristic, with each feature numbered and measured. A material document, this is not — it says the part is the right shape, not the right metal.
FAI vs mill cert → - AS9100
-
The aerospace quality management system standard, built on ISO 9001 with configuration control, key characteristic management and traceability added. It certifies a shop’s system, not any one process or operator — and it is easy to overstate.
What it does and doesn’t prove → - OASIS registry
-
The IAQG’s public database of AS9100 registrations. The only way to confirm a claimed certificate is genuine, current and correctly scoped, rather than trusting a PDF.
How to check a claim → - Key characteristic / CTQ
-
A dimension or feature flagged as critical to fit, function or safety, controlled and measured more tightly than the rest of the part. Naming these explicitly lets a shop sample the remaining features instead of inspecting everything.
Choosing what to inspect →
Machining and process
- Work hardening
-
A material getting harder at the surface as it is cut, from the mechanical deformation of the cutting action itself — the defining problem of nickel superalloys. Light finishing passes make it worse, not better.
Why it happens, how to avoid it → - Distortion / stress relief
-
Parts moving out of tolerance as internal stress — from rolling, forging or heavy material removal — releases during machining. Managed with a stress-relief heat treatment step placed between roughing and finishing.
Managing distortion → - Hard turning
-
Turning a part after it has been hardened, typically above 45 HRC, using CBN tooling instead of removing material soft and hardening afterward. An alternative process route to grinding, with different cost and tolerance trade-offs.
Hard turning vs grinding → - CBN (cubic boron nitride)
-
A tooling material harder than carbide, used for hard turning and finishing hardened steel. One of three tooling choices, alongside carbide and ceramic, whose right answer depends on the alloy and the operation.
Ceramic vs carbide vs CBN → - High-pressure coolant
-
Coolant delivered through the tool at high pressure to break chips and pull heat away from the cutting edge — often the difference between a viable cycle time and tool failure on titanium and Inconel.
Why it matters on superalloys → - 5-axis machining
-
Machining with two additional rotational axes beyond standard X/Y/Z, letting a tool reach complex geometry in fewer setups. Pays for itself on hard materials mainly by cutting the number of times a part is re-fixtured, not by cutting faster.
When it actually pays → - Thin-wall machining
-
Machining features thin enough that cutting forces and released internal stress deflect or distort them — a particular problem in titanium, where low stiffness and springback compound the issue.
Thin-wall titanium → - Surface finish (Ra)
-
A roughness average measured in micrometres or microinches. A tighter Ra callout than a part functionally needs is one of the most common ways a drawing quietly doubles the finishing time — and the price.
Specifying Ra correctly → - Springback
-
Elastic deformation that reverses itself after the cutting force is removed, common in titanium because of its low stiffness — a thin wall can measure in tolerance under load and out of tolerance once released.
Distortion in thin titanium walls →
Sourcing and commercial
- RFQ (Request for Quote)
-
The package a buyer sends to get a price: drawing, material, quantity, documentation requirements and a real date. What is missing from most RFQs, not what is in them, is usually why they sit unquoted.
RFQ checklist → - Incoterms (EXW, FOB, DDP)
-
Standard trade terms defining where cost and risk transfer from seller to buyer. EXW hands the goods over at the shop door; FOB adds export loading; DDP delivers cleared through customs. Comparing quotes without checking the Incoterm compares different products.
Get a quote → - Lead time (material vs machining)
-
On exotic alloys, the material lead time — how long it takes to get the bar or plate — is almost always longer than the machining lead time. A quote that states only one combined number is hiding which one is actually driving the date.
Where the lead time actually goes → - Approved source list
-
A specification-mandated list of mills or suppliers a material must come from — common in aerospace and energy. Narrows available stock regardless of certificate quality, and is worth checking before, not after, quoting.
How this affects lead time → - Landed cost
-
The total cost of a part delivered to the buyer’s door — price plus freight, duty and customs clearance — as opposed to the quoted EXW or FOB price. The only number that makes two quotes with different Incoterms comparable.
Comparing quotes properly → - Machinability index
-
A relative ranking of how easily a material cuts, indexed against free-machining steel (B1112) at 100. Approximate by nature — useful for ranking alloys against each other, not for calculating a cycle time.
Twenty alloys compared → - Trading company (machining)
-
What this site is: a company that buys the machining from shops in a network it manages and sells the finished part on its own quote, order acknowledgement and invoice, carrying quality and schedule. Not a factory, not a broker: the difference is the production management between the PO and the delivery.
How we work →
Where this comes from
This glossary is a lookup for the guides and notes, not a substitute for them — start at the guides for the full argument on any one material, or browse all notes. Corrections to a definition are welcome; see the editorial standards.
Replies within 24 business hours.