Swiss Turning Medical-Grade Stainless

The two medical stainless steels

316L (UNS S31603): The implant-grade stainless. "L" means low carbon (0.03% max) — this prevents carbide precipitation during welding and heat exposure, maintaining corrosion resistance. Used for bone screws, bone plates, spinal rods, surgical staples, and any implant that stays in the body. ASTM F138/F139 is the medical specification. Machines readily on Swiss — similar to 303 but slightly gummier with longer chips.

17-4PH (UNS S17400): Precipitation-hardened martensitic stainless. 17% chromium, 4% nickel, with copper and niobium for age hardening. Achieves 28–44 HRC depending on heat treatment condition. Used for surgical instruments, endoscopic components, orthopedic tools, and structural medical device components where higher strength is needed than 316L provides. ASTM A564 Type 630. Machines differently depending on condition — solution-annealed (Condition A, 28–38 HRC) is gummy; H900 (44 HRC) is hard but produces better chips.

316L on Swiss — parameters

Turning: 200–350 SFM, 0.003–0.008" IPR, 0.010–0.040" DOC. 316L is austenitic and work-hardens under light cuts — maintain minimum chip thickness by keeping IPR above 0.003". Use sharp positive-rake inserts with polished chipbreakers. PVD TiAlN coating. Kennametal KC5010, Sandvik GC1115, Iscar IC806.

Thread whirling (bone screws): 300–500 SFM peripheral speed on the whirling ring. 316L thread whirling produces excellent thread form with minimal burr. Cycle times for a standard 3.5mm cortical bone screw: 25–40 seconds including cutoff. For cannulated screws (hollow center), add 10–15 seconds for gun drilling the center hole.

Cross-drilling: 316L galls badly in deep holes. Use through-coolant carbide drills with TiAlN coating. Maximum hole depth without pecking: 3D. Beyond 3D, use peck cycles with full retract to clear chips. For holes under 0.040" diameter (common on medical devices), use micro-drills at 8,000–12,000 RPM with 0.0005–0.001" IPR.

Cutoff: 316L tends to burr on cutoff. Use sharp cutoff inserts with positive rake and high-pressure coolant directed at the cutoff zone. Programmable bar feeder timing matters — if the bar pushes during cutoff, the burr is worse. Some shops add a facing operation in the sub-spindle to clean up the cutoff face.

17-4PH on Swiss — parameters

Condition A (solution annealed, 28–38 HRC): 150–250 SFM, 0.003–0.006" IPR, 0.010–0.030" DOC. This condition is gummy — long continuous chips and BUE tendency. Many medical shops prefer to machine 17-4PH in the H1025 or H1075 condition (34–38 HRC) where it produces better chips with less BUE.

H900 condition (44 HRC): 80–150 SFM, 0.002–0.005" IPR, 0.010–0.025" DOC. The hardest standard condition for 17-4PH. Produces short, predictable chips but eats tooling faster. Insert life is 40–60% of Condition A. Use harder substrate inserts — Sandvik GC1125, Kennametal KC5025.

H1025/H1050 condition (34–38 HRC): The sweet spot for Swiss machinability. 120–200 SFM, 0.003–0.006" IPR. Hard enough for good chip breaking, soft enough for reasonable tool life. Most medical device companies spec H1025 or H1050 for surgical instruments precisely because it balances machinability with the mechanical properties needed for the application.

Surface finish requirements

Medical devices have specific surface finish requirements driven by function and regulatory expectations:

Implant surfaces (316L): Ra 8–16 µin is typical for bone screw shanks and plate surfaces. Achieved by Swiss turning with sharp carbide tooling at finishing speeds (300+ SFM) followed by electropolishing. The electropolish removes 0.0002–0.0005" of material and produces a passive, biocompatible surface. Some implants require Ra 4 µin or better — this needs mechanical polishing before electropolish.

Instrument surfaces (17-4PH): Ra 16–32 µin is standard for surgical instrument shafts and bodies. Passivation (citric acid or nitric acid per ASTM A967) is the standard post-machining treatment — it removes free iron from the surface and promotes the chromium oxide passive layer. No electropolish required for most instruments.

Thread surfaces (bone screws): Thread form quality matters more than Ra number. Thread whirling produces Ra 16–32 µin on thread flanks. The thread root must be free of burrs and tool marks — these act as stress concentrators in fatigue-loaded implants. Optical inspection at 20–40x magnification is standard QA for bone screw threads.

Tolerances for medical

Bone screw major diameter: typically ±0.001" (±0.025mm). Achieved in-process on Swiss with standard tooling.

Bone screw thread pitch: ±0.0005" (±0.013mm). Thread whirling achieves this routinely.

Instrument shaft OD: ±0.0005" (±0.013mm) is common on endoscopic instrument shafts where the OD mates with a tube or housing. Swiss turning with in-process gauging holds this across production runs.

Concentricity: 0.001" TIR between main spindle and sub-spindle features. Critical for cannulated screws where the center hole must be concentric with the thread OD.

Length: ±0.005" (±0.13mm) on overall length from cutoff. ±0.002" with sub-spindle facing. Bone screw lengths are typically specified in 2mm increments (surgeons select length intraoperatively), so the length tolerance applies within each size increment.

Passivation and cleaning

Every medical stainless part gets passivated. This is non-optional — FDA and ISO 13485 require it for any device contacting the patient.

Passivation process: ASTM A967 or ASTM A380. Citric acid passivation (20–50% citric acid, 120–150°F, 20–30 minutes) is replacing nitric acid in many shops due to environmental and safety advantages. The passivation removes free iron, machining contaminants, and embedded particles, leaving a clean chromium oxide surface.

Cleaning validation: Parts must be cleaned to remove all machining oil, coolant residue, and particulate before passivation. Ultrasonic cleaning in an enzymatic detergent followed by DI water rinse is standard. For implants, cleaning validation per ISO 19227 is increasingly expected.

Material traceability: Medical device regulations require material traceability from bar stock mill cert through finished part. The Swiss shop needs to maintain lot control — every bar of 316L or 17-4PH traces back to a specific heat number. This means no mixing bar stock between lots and maintaining records that link each production run to a specific heat cert.

Cost comparison

316L bar stock: $5–12/lb in Swiss diameters (3–20mm). Premium over standard 316: 30–50% due to ASTM F138 certification, tighter chemistry, and mill cert requirements.

17-4PH bar stock: $4–10/lb in Swiss diameters. Condition A (annealed) bar is standard — most shops machine in Condition A and heat treat after machining if a specific aged condition is required.

Production cost for a typical bone screw (316L): $1.50–4.00 per piece at volumes of 5,000–50,000. This includes material ($0.10–0.30), machining ($0.80–2.50), passivation ($0.15–0.30), and inspection ($0.20–0.50). Price decreases significantly at higher volumes — 100,000+ screws drop to $1.00–2.00 per piece.

Production cost for a surgical instrument shaft (17-4PH): $3.00–15.00 per piece depending on complexity. Instruments are typically lower volume (500–5,000 per run) and require more operations than screws.