5Axis CNC Machining 17-4 PH Stainless Steel Surgical Instrument:

At Flow Wing Metal, we specialize in CNC machining for high-precision medical devices, aerospace components, and industrial parts. Our expertise in 5-axis CNC machining allows us to produce complex, monolithic parts with tight tolerances, minimizing assembly and improving reliability.

CNC Equipment at Flow Wing Metal:

Our machine shop is equipped with a range of CNC machining to address diverse part geometries, volumes, and tolerances.

Client: OS Medical (ISO 13485 certified, Germany)

Industry: Medical Device (Spinal Surgery)

Challenge: OS Medical required a spinal pedicle screwrod reduction instrument used in minimally invasive spine surgery. The part (150mm total length, 12mm maximum diameter)

featured:

A complex, ergonomic handle with undercut finger grooves.

A distal “reduction tab” with a precise 2.5mm × 3mm rectangular slot (tolerance ±0.01mm).

A cannulated (hollow) shaft with a 2.0mm internal throughhole.

A ratcheting engagement interface requiring a 0.15mm radius root (sharp internal corner impossible with standard ball end mills).

Original challenge:

The instrument was previously manufactured as a 3component assembly (handle machined separately, shaft with broached slot, ratchet insert welded in place).

This resulted in:

High assembly labor cost.

Weldment weak points (potential for crevice corrosion after sterilization).

Inconsistent slot positioning (±0.08mm across batches) leading to poor instrument fit intraoperatively.

8 weeks lead time.

OS’s request:

A single piece, weld-free, monolithic instrument machined from 174 PH stainless steel (precipitation hardened, high strength, corrosion resistant), with:

No assembly required.

Slot tolerance ±0.01mm (half of previous tolerance).

Internal ratchet feature with 0.15mm root radii (EDM or 5axis machining).

Lead time ≤ 4 weeks for 200 units.

The Flow Wing Metal Execution:

Step 1: Material Selection & Preparation of CNC Machining

Material: 174 PH stainless steel bar, Condition A (solution annealed – softer for machining), then agehardened to H900 condition postmachining (yield strength ≥ 1,170 MPa / 170 ksi).

Bar size: 16mm diameter × 250mm length blanks.

Certification: Material traceable per ASTM A564 with mill test reports (MTRs) and biocompatibility verification (ISO 109935 noncytotoxicity).

Step 2: CNC Programming & Simulation

CAM Software: Mastercam 2024 with 5axis simultaneous module.

Machine: DMG MORI DMU 65 (5axis machining center with 20,000 RPM spindle, HSK63 tool holder, throughspindle coolant at 70 bar/1,000 PSI).

Workholding: Custom soft jaws with a dovetail fixture providing 5sided access.

Toolpaths:

Roughing: High efficiency dynamic milling (trochoidal toolpaths) with a 10 mm 5 flute carbide end mill.

Semifinishing: 6mm ball end mill (scallop height 0.02mm).

Finishing (grooves/slot): 1.5mm miniature carbide end mill (7flute, necked reach).

Ratchet feature: 0.5mm tapered ball end mill (0.15mm corner radius programmed).

Internal throughhole: Gun drilling (2.0mm diameter, 120mm depth – L/D ratio 60:1) followed by carbide reamer.

Step 3: Machining Process (5Axis Simultaneous of CNC Machining)

Operation  Tool  Parameters  Result:

1. Face & turn OD  CNMG 432 insert (turning)  200 SFM, 0.1mm/rev  15.8mm diameter
2. Drill center hole  1.8mm carbide drill  4,500 RPM, 0.025mm/rev  Pilot hole
3. Gun drill throughhole  2.0mm gun drill (coolantfed)  8,000 RPM, 0.02mm/rev  2.0mm +0.00/0.02mm
4. Dynamic roughing  10mm end mill  8,000 RPM, 2,000mm/min  80% material removed
5. 5axis handle finishing  6mm ball end mill  12,000 RPM, 1,500mm/min  Handle surface RA 0.8µm
6. Rectangular slot (reduction tab)  1.5mm end mill (7flute)  18,000 RPM, 400mm/min  2.5mm × 3.0mm ±0.008mm
7. Ratchet feature  0.5mm tapered ball (0.15mm rad)  22,000 RPM, 200mm/min  0.15mm root radius retained
8. Ream final throughhole  2.0mm carbide reamer  2,000 RPM, 0.1mm/rev  2.0mm +0.005/0.000mm

Step 4: Post Machining Heat Treatment & Finishing

Heat treat: Age hardening to H900 (480°C / 900°F for 1 hour, air cool). Achieved hardness: 44 HRC (target 43–46 HRC).

Surface finishing:

Electropolishing: 30 minutes in an electrolytic bath to remove a 0.01mm layer, resulting in a bright, smooth (RA ≤ 0.2µm), and passivated surface. This eliminated any burrs from the 0.5mm ratchet tool.

Passivation: Subsequent citric acid bath to ensure chromium oxide restoration (per ASTM A967).

Inspection:

CMM (Coordinate Measuring Machine) for full 5axis dimensional inspection.

Vision system for 0.15mm ratchet root radius verification.

Optical comparator for slot tolerance.

Surface roughness profilometer (RA 0.18µm achieved).

CNC Machining the Result:

Cost savings: 38% lower per unit cost compared to the previous 3 component assembly ($48 → $30 per instrument).

Lead time reduction: From 8 weeks to 3.5 weeks for 200 units (factory to sterile ready).

Quality improvement: Slot positioning tolerance ±0.006mm vs. previous ±0.08mm (13× more precise). No rework or scrap in production run.

Reliability: Zero field failures after 24 months (3,500+ surgical uses). No crevice corrosion, no weld fractures (there are no welds), and the electropolished surface cleans faster in sterile processing.

Clinical feedback: “The single piece instrument feels more solid, engages the pedicle screw with zero play, and the ergonomic handle reduced surgeon hand fatigue by approximately 40%.” – Lead Spine Surgeon, University Hospital Munich

Client testimonial:

“Flow Wing Metal transformed a three part welded assembly into a monolithic 5 axis CNC machining masterpiece. The 0.15mm internal corner on the ratchet feature – achieved with a 0.5mm end mill – is the reason we chose them over five other CNC machining shops. They understand medical quality.”