Orthopaedic Implants — The Thread Form and the Surface Are the Product
Four defects that decide whether the part is scrap
- Torn thread flank. A thread cut with a dull or wrongly raked edge tears rather than shears. The pitch and the gauge pass, the flank angle does not, and the fastener will not hold torque as designed.
- Work-hardened surface layer. In 316L and cobalt-chrome a rubbing pass hardens the surface it just cut. The next pass cuts under a hard skin, and the damage accumulates along the feature rather than at one point.
- Embedded debris on a bone-contact surface. A chip pressed into a porous or textured surface cannot be cleaned out reliably. This is why the machining sequence and the coolant strategy are part of the acceptance discussion, not workshop preference.
- Heat damage in titanium. Titanium conducts heat poorly, so the edge and the surface see it first. Discolouration is the visible sign; a changed surface layer under a visually acceptable part is the expensive one.
The alloy cutting physics that produces these conditions is set out on Tough-Alloy Holemaking. This page addresses what an implant requirement adds on top.
What drives tooling on an implant
| Feature | What usually decides the tool | HENTRY entry |
|---|---|---|
| Bone and locking threads | Whether the form can be milled or must be cut, and the flank finish that follows | Thread Mills · Forming Taps |
| Plate contours and underside reliefs | Reach and clearance around the bone-facing geometry | Ball Nose End Mills · Corner Radius End Mills |
| Screw heads and drive recesses | The form geometry and whether the recess can be cut without a transition | Custom End Mills |
| Pilot and cannulation holes | Depth-to-diameter and whether the hole has to stay concentric through the length | Micro Carbide Drills · Deep Hole Carbide Drills |
| Bone-ingrowth and textured surfaces | Whether the texture is cut or added later, and what the cut surface must be before it | Reviewed per print — not a catalogue item |
Which alloy pushes you where
| Material | Route | Why |
|---|---|---|
| Ti-6Al-4V (and ELI grades) | Carbide, heat into the chip, no dwelling | Poor conductivity keeps heat at the edge; a pause welds material to the flank |
| Cobalt-chrome | Carbide with edge prep, light radial engagement | Work-hardens and resists clean shear, so thread flanks tend to tear |
| 316L stainless | Carbide, sharp and positive | Gummy and work-hardening; a dull edge smears the flank |
| Commercially pure titanium | Carbide, lower speed, higher feed | Softer and more adhesive than the alloy; built-up edge is the main failure |
| PEEK and PEEK-based composites | Sharp geometry with a defined exit | Melting and fuzzing at the edge rather than a conventional burr |
Boundaries, so you do not read the wrong page
- Jaws, serrations and instrument edges — mated condition and edge geometry → Surgical Instrument Machining.
- Cannula, hypotube and catheter shafts — lumen burr and thin-wall deflection → Cannula & Lumen Machining.
- Threads on general industrial parts → Taps and the General Mechanical hub.
- Titanium and cobalt-chrome physics without a device requirement → Tough-Alloy Holemaking.
What makes an implant quote reviewable
- Drawing and revision, with the thread specification called out in full
- Material and condition, including ELI or medical-grade designation where it applies
- Which surfaces are bone-contact and which are not
- Surface requirement after machining, and whether the texture is cut or added
- Machine, holder, stickout and coolant — titanium heat control starts here
- The cleaning and finishing step that follows, and what it must not find
Quick answers before you RFQ
Thread milling or thread cutting for a bone screw?
Thread milling gives a cleaner flank on difficult alloys and handles a thread that runs out into a shoulder, which is common on screws. Cutting is faster on long runs where the form is simple and the material behaves. On cobalt-chrome the flank finish usually decides it, and that is checked on the first part rather than assumed.
How is discolouration on titanium treated?
As a warning rather than a defect in itself. Colour indicates a temperature excursion, and the question is whether the surface layer underneath changed. Where the print controls it, the process is set so the excursion does not happen — coolant delivery, edge sharpness and the absence of dwelling do more than any downstream correction.
Can you produce a textured bone-contact surface?
It depends on how the texture is specified. A cut texture — grooves, pockets, a defined roughness — is a tooling question and can be quoted. A porous or additive coating is a different process and is not something this page covers. Send the print and it is reviewed as its own case.
Where do the instrument parts belong?
On Surgical Instrument Machining. An implant thread carries load into bone; an instrument serration grips tissue. The tooling, the inspection and the failure modes are different enough that they are kept apart.
Send the thread specification and the bone-contact surfaces
Primary tooling entries for implants: thread mills, carbide end mills and corner-radius geometry, ball-nose cutters for plate reliefs, micro and deep-hole drills for cannulation. Surface requirements are confirmed before quoting.