Orthopedic component machining covers a broad set of parts: bone screws, plate trial components, fixation hardware such as washers, nuts and pins, and the instrument cases that hold them. The materials are narrow by comparison. Most drawings call for Ti-6Al-4V ELI, commercially pure titanium, 316L stainless, or PEEK for trials and prototypes.
This article covers how those materials machine, how bone screw threads are produced, what titanium anodizing does and does not do, how burrs and particulate are controlled, and which tolerances are realistic. It is written for engineers and sourcing people at orthopedic device companies who send drawings to contract machine shops.
The materials
Ti-6Al-4V ELI (ASTM F136)
Ti-6Al-4V is an alpha-beta titanium alloy and the most common choice for load-bearing orthopedic hardware. The ELI (extra low interstitial) version, specified for surgical implants in ASTM F136, limits oxygen to 0.13% maximum and tightens other interstitials, which improves ductility and fracture toughness compared with standard Grade 5.
Annealed F136 bar has a minimum tensile strength around 860 MPa and an elastic modulus around 110 GPa, about half that of stainless steel. That lower modulus is one reason titanium is preferred for bone contact, and it is also why titanium parts spring away from the tool during machining.
CP titanium Grade 2 (ASTM F67)
Commercially pure titanium, covered for implants by ASTM F67, is weaker (Grade 2 minimum tensile strength around 345 MPa) but more ductile and easier to form. It is used for thin plates that are contoured in surgery, some washers, and parts where the alloying elements are not wanted. It machines more like a gummy stainless: long chips, built-up edge, and burrs that fold over rather than break off.
316L stainless
316L (ASTM F138 when specified for implants) still appears in fixation hardware, trial components and instrument cases. It is cheaper to buy and machine than titanium, but its higher modulus and nickel content make it less common for new permanent implant designs.
PEEK
PEEK is used for trial components, spinal cage prototypes, radiolucent instrument parts and some cases. Medical grade PEEK is bought with certificates; for implant use the buyer typically specifies a particular implant grade and supplier documentation (ASTM F2026 covers PEEK polymers for surgical implants). PEEK machines cleanly but is sensitive to heat and internal stress: thin walls can move after machining unless the stock is annealed and the process leaves material for a finish pass.
Material comparison
| Property | Ti Gr5 ELI (F136) | Ti Gr2 (F67) | 316L (F138) | PEEK (unfilled) |
|---|---|---|---|---|
| Min tensile strength, annealed | About 860 MPa | About 345 MPa | About 490 MPa | About 90 to 100 MPa (typical) |
| Elastic modulus | About 110 GPa | About 105 GPa | About 193 GPa | About 3.6 to 4 GPa |
| Density | 4.43 g/cm³ | 4.51 g/cm³ | 8.0 g/cm³ | 1.3 g/cm³ |
| Machinability | Difficult: heat, tool wear | Moderate: gummy, burrs | Moderate: work hardens | Good: watch heat and stress |
| Color anodize | Yes | Yes | No | No |
| Radiolucent | No | No | No | Yes |
| Typical orthopedic use | Screws, fixation, trials | Thin plates, washers | Trials, cases, fixation | Trials, cage prototypes |
Values are indicative. The standard and condition on your drawing govern.
Why titanium is hard on tools
Titanium’s thermal conductivity is roughly 7 W/m·K for Ti-6Al-4V, compared with about 16 for 316L and over 150 for aluminum alloys. Heat generated at the cut does not leave through the chip or the part; it stays at the tool edge. Titanium is also chemically reactive at high temperature and tends to weld to the tool.
The practical consequences for orthopedic component machining:
- Lower cutting speeds than stainless, with a higher share of machine time in the quote.
- Tool wear is the main cost driver. Worn tools raise temperature further, which can discolor the surface and change dimensions mid-batch. Shops set tool life limits by part count rather than waiting for visible wear.
- Coolant matters. High-pressure, through-tool coolant helps break chips and move heat, especially in deep holes and cannulations.
- Springback. The low modulus means thin plates and long slender screws deflect under cutting force. Fixturing and finish passes are planned around it.
- Surface integrity. Excessive heat can leave an altered surface layer. If your specification restricts this, state the requirement and the inspection method.
Bone screw threads: whirling vs single point
Bone screws combine a long thread, often with a large pitch and a thin root, a drive recess (hex or hexalobular), a head with a spherical or conical underside, and sometimes a cannulation along the axis. Thread forms are often based on ISO 5835 or ASTM F543 profiles, or on the maker’s own design.
Thread whirling is done on Swiss-type lathes. A ring carrying several inserts rotates at high speed around the slowly turning bar, offset from its axis, and cuts the full thread form in a single pass. The guide bushing supports the bar right at the cut. Whirling suits long cancellous screws because deflection is low, cycle time is short, and the thread surface is usually consistent.
Single-point threading cuts the thread in several passes with one insert. It is slower on long threads and more sensitive to deflection, but needs no dedicated whirling tooling. It is a reasonable route for short cortical screws, prototypes and low quantities.
Either way, the drawing should define the thread profile, major and minor diameters, pitch, lead, and how the thread is inspected (optical comparator, thread profile projection, or gauges). The shop chooses the process; the drawing controls the result.
Drive recesses are usually produced by rotary broaching or milling. Cannulations in long screws need deep-hole drilling and careful burr removal at both ends and at any cross holes.
Anodizing titanium: Type II and Type III
Anodizing on titanium is often referenced to AMS 2488, which uses types that differ from aluminum anodizing:
- Type II (titanium): a gray, thicker conversion layer used mainly to reduce galling and improve fatigue behavior on threaded and sliding parts. Appearance is uniform gray.
- Type III (titanium): a thin oxide grown at a controlled voltage. Its thickness produces interference colors such as bronze, blue, gold, purple and green. Used to color-code screw diameters or lengths, or to distinguish instrument sets.
Color anodizing does not add meaningful thickness for most tolerances, but color is sensitive to surface finish and cleanliness. Two parts from different batches can differ in shade if the incoming finish differs. If color matching matters, provide a reference sample and an acceptance method. Anodizing Type II and III is available through the partner shops’ secondary process vendors where applicable, and is confirmed in the quote.
Burrs and particulate
Orthopedic hardware has many burr-prone features: thread start and runout, drive recess edges, cannulation entry and exit, cross holes, plate countersinks. Burrs matter for fit and function, and loose material can become particulate that the device maker’s cleaning process must then remove.
Good drawing practice:
- Define edge break ranges, for example 0.05 to 0.2 mm, and mark any edges that must remain sharp.
- State the inspection method and magnification, for example “no burrs or loose material visible at 10x in zones A and B.”
- For cannulated screws, state how the bore is inspected (pin gauge pass-through, borescope, or light check).
- Specify the pre-shipment cleaning and packaging requirement, such as individual bagging, so parts are not contaminated in transit.
The shop removes burrs and machining residue to the drawing. The final cleaning process that brings the part to its implant or instrument cleanliness level, and the validation of that process, belong to the device maker.
Features, tolerances and the capability window
The table compares typical drawing callouts on orthopedic parts (illustrative) with the partner shops’ capability window. The window is confirmed per drawing, never assumed.
| Feature | Typical drawing callout (illustrative) | Capability window |
|---|---|---|
| Bone screw thread major diameter | ±0.02 to ±0.05 mm | ±0.01 mm typical |
| Screw head sphere / underside profile | Profile 0.05 mm | Profile verified on CMM or comparator |
| Drive recess (hex / hexalobular) | Gauge fit, depth ±0.1 mm | Gauge checked, depth ±0.01 mm possible |
| Plate trial hole position | Position 0.05 to 0.1 mm | ±0.01 mm typical, ±0.005 mm on 5-axis |
| Plate trial thickness and flatness | ±0.05 mm, flatness 0.05 mm | ±0.01 mm, flatness on CMM |
| Bearing surfaces | Ra 0.8 µm | Ra 0.8 µm best, Ra 1.6 µm typical as machined |
| Instrument case features | ±0.1 to ±0.2 mm | Well within window |
Holding a tighter tolerance than the function needs raises cost, mainly through inspection time and scrap. DFM feedback returned with the quote will point out features where a looser callout would not affect function. The full list of materials, processes and inspection equipment is on the capabilities page.
What stays with the device maker
Implant classification, the regulatory pathway, biocompatibility evaluation per the ISO 10993 series, cleaning validation, sterilization validation and packaging validation are the device maker’s responsibility. The same goes for design controls and material selection rationale. The machine shop supplies parts to the buyer’s drawing and quality requirements, with the records requested: first article inspection, material certificates traceable to heat number, finish certificates. We review drawings and match them to shops; we do not hold or share in the manufacturer’s regulatory obligations.
If you need a specific supplier certification, verify the shop’s certificates yourself as part of supplier qualification. Put quality agreement terms, inspection sampling plans and documentation requests into the RFQ so they are quoted.
Lead times
Prototype lots of 1 to 10 pieces typically take 2 to 3 weeks including the FAI report. Small batches of 10 to 500 pieces typically take 3 to 5 weeks. Titanium bar in F136 condition and medical grade PEEK with certificates can add time if the diameter is not stocked locally, so state the bar size or let the shop propose one.
What to include in your RFQ
For an orthopedic component machining quote, send:
- 2D drawing (PDF) with tolerances, GD&T and inspection notes, plus a STEP file
- Material and standard (for example Ti-6Al-4V ELI per ASTM F136), bar condition, and certificate requirements
- Thread definition and inspection method for screws
- Anodize type and color per part number, with reference samples if color matching matters
- Edge break, burr and cleanliness criteria, and packaging per part
- Quantities for prototype and batch, and documentation needed (FAI, certs, PPAP-style package on request, quoted separately)
Upload it through the RFQ form. A mutual NDA can be signed before review, and you will get a first response within 24 business hours with DFM feedback at no charge.