CNC Machinable Materials — the Factory's Grade-by-Grade Table

Three Real Alloy-Swap Stories

6061 → 5052 for a marine bracket: a customer spec'd 6061-T6 for a boat-accessory bracket; the loads read 40% of capacity with margin to spare, and the real enemy was chloride pitting at the weld. 5052-H32 — the marine sheet alloy — bent instead of cracking during their install abuse, took the same black anodize, and quoted 15% under because stock plate is cheaper here. Nobody paid for strength the boat never asked for.

304 → 17-4PH, then back: a medical shaft was upgraded to 17-4PH H900 by a cautious engineer — triple the machinability penalty. A load-cell test showed 304 cold-worked journal zones at 3× actual duty; they went back to 304 with a nitride treatment at the bearing band, saving the program its annual margin.

POM → PEEK at 121 °C: a packaging wear-plate lived 200 hours on a steam-adjacent line at 110–125 °C; POM creeps when hot. A 40% cost jump to PEEK bought 6,000-hour life — the right kind of expensive. Material selection is a duty-cycle equation, and the duty cycle, not the datasheet, should sit in the driver's seat.

Stock Reality: How Availability Moves Lead Time More Than Machining

Ask any machinist what actually blows schedules: not spindle speed, purchasing. In-stock grades (6061, 304, brass 360, 12L14, POM, most of the table's core band) start cutting same-week. On-order mill purchases add 3–7 working days in the Shenzhen supply belt — times two if your grade is special thickness or a certified heat with no local stock. Titanium and Inconel sit beyond that on mill lead. Practical countermeasures that cost nothing to ask about at RFQ: (1) Can we build from an adjacent plate/bar size with a mill pass you'd otherwise pay for anyway? (2) Does an equivalent national standard (e.g. GB 6061 vs ASTM 6061 composition windows) satisfy your documentation, for a bigger stock pool? (3) For certified programs, quote the cert route now — the 5–10 day heat-sourcing cycle belongs in your launch plan, not in an apology email later.

One more honesty item on the table's Machinability column: it is a machine-rate multiplier, not a trivia score. “Excellent” brass and steel can run 400+ sfm class cuts; “difficult” titanium runs a tenth of the metal-removal rate with three times the tool consumption, and the quote should show it. If two identical parts in different alloys come back at only 20% apart, someone is hiding risk in the price; when we machine Ti we show the multiplier and explain the toolpath discipline that contains it.

Why machinability moves price more than material cost. On a typical mid-size bracket, stock is 10-25% of the quoted price; the rest is the hours the part spends in the machine and the tool wear it consumes. That ratio is why the table's Machinability column is the price column in disguise: brass C360 runs at the top of the speed range and its chips clear themselves; 304 stainless work-hardens under a timid feed and punishes it with work-hardened ridges and blown inserts the next pass; Ti-6Al-4V holds heat in the tool and demands slow, constant engagement with high-pressure coolant; PEEK machines almost like aluminum but its stock costs ten times a bar of POM. The 25% of your quote that is metal is largely fixed by the alloy's commodity price; the 75% that is machine time is a negotiation between the machinist's parameters and the alloy's temperament — and that negotiation is the quote. Two identical drawings in two alloys do not price proportionally to density or datasheet cost; they price proportionally to metal-removal rates, tool-life multipliers and the number of setups the material's behavior forces. This table gives the multipliers; the quote multiplies them against your geometry, and if a material swap is on the table, both lines arrive printed side by side.

Machinability ratings here reflect shop-proven parameters on our machines in our grades, with our tooling; national-standard equivalents (GB/AISI/DIN) of the same composition window perform the same at cut, and the stock pool is larger than the brand names in the table suggest.