Holding Tight Tolerance on Titanium Spinal Implant: A Machining Case Study
How we held the required tolerance on a Ti-6Al-4V ELI implant with a roughened bone-contact surface: where the dimensional risk sat, the controls we put in place, how the result was verified and what the customer received at goods-in.
The tolerance requirement
The drawing called for a finished part described as controlled surface texture for osseointegration with full material traceability, which is tight enough that process drift alone can push parts out of tolerance.
The customer had experienced variation with a previous supplier and wanted evidence, not assurances, that the dimension would hold across the batch.
Where the risk sat
Risk came from three places: fixture repeatability, thermal growth during a long cut, and tool wear across the batch.
Each was measured before any attempt was made to compensate in the control.
The controls we used
We used a dedicated fixture with hardened locating pins, an on-machine probe to set the work offset and to check the critical feature, and a defined tool change interval based on part count.
Coolant temperature and concentration were controlled, because a few degrees of drift is enough to move a dimension of this size.
Verification
Every part in the sample plan was measured on a coordinate measuring machine, and the results were reported against the drawing with the measurement uncertainty stated.
The report showed capability on the critical characteristic rather than a simple pass or fail.
Outcome at goods-in
The batch passed incoming inspection with no non-conformances, and the customer removed its own incoming check for the following orders.
The agreed control plan is now the reference for repeat production of a Ti-6Al-4V ELI implant with a roughened bone-contact surface.
Summary
This case shows the pattern behind most successful CNC programmes: decide the datums early, prove the process once, then measure what you ship.
