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On the ball with hip implants

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Orthopaedic hip surgery numbers are forecast to increase at a rate of 5% year on year to 2026. This is no surprise given a growing aged population around the world. With this comes increased demand for tools and components to support these procedures. ANCA’s process for femoral ball grinding, developed together with grinding wheel manufacturer Tyrolit, delivers quality and consistency in the finished femoral ball, using specialised machine control and application technology on the market leading MX7 Linear CNC grinding machine. 
Figure 1: orthopaedic hip implant that includes a femoral ball
The medical industry is one where customers and markets are acutely sensitive to product quality and consistency. This is even more the case for orthopaedic implants that are to be placed permanently inside the human body. As a result, customers buying and using orthopaedic implants need absolute confidence in the product, which in turn requires absolute confidence in the manufacturing process.
Performance of femoral balls used in hip replacement implants demands consistency of roundness, size and surface finish as key quality criteria. To achieve this, ANCA has developed a process on its MX7 Linear machine that applies a series of grinding, honing and final buffing operations. The result is surface finish on the ball of <0.01µm Ra and part roundness under 3µm.


Figure 2: surface finish measurement ​of finished ball is <0.01micron Ra 

The first step in the process is grinding the initial part accurately to size. Using plated CBN grinding wheels, excess stock material is removed from the rough turned part. This operation makes upstream processes easier by relaxing the tolerance demands for the blank part that the ANCA MX7 will grind.

Next are a series of fine honing processes that produce incremental improvement in the surface finish. Application Engineers from ANCA and grinding wheel manufacturer Tyrolit, teamed up to develop a process that delivered stable production results on the femoral ball. 

Tyrolit wheels have been specifically developed for the femoral ball applications but are known to naturally wear during the production process. ANCA was uniquely positioned to deal with this challenge, being the designer and manufacturer of its own LinX linear motors as well as the CNC and servo drive system that controls the machine and grinding wheel movement. ANCA’s System Engineers developed an entirely new control algorithm for this application. This allowed the operator to program the desired forces applied by the honing wheel on the femoral ball. With this programmed force, the wheel feeds into the ball at a constant rate as it wears away. The three different Tyrolit wheels used could be programmed with their own unique grinding force parameter to achieve a mirror surface finish and consistent part size tolerance. 

Figure 3: femoral ball honing operation

Additionally, automatic wheel measurement was developed that ensures each honing operation starts from the correct infeed position. This ensured consistency in the process regardless of the wheel wear that is experienced after each honing operation.

A final buffing operation is the last step, delivering mirror finish on the working surface of the ball. This is critical in ensuring mechanical friction and wear is minimised in the final hip implant.
Figure 4: Before and after results with the femoral ball production process on the MX7 Linear

ANCA’s MX7 Linear proved to be the ideal machine platform for this application. The six wheel pack changer ensures all the grinding, honing and buffing operations can be performed in one setup. In addition, the tooling used for femoral ball production can be easily switched over to cutting tool manufacturing applications. So rather than purchasing a dedicated machine for femoral ball production only, users can easily utilise their machine for other CNC grinding applications that complement their market and business model. Production of surgical rotary cutters, drills and reamers as well as femoral hip rasps are all ideal applications for production on the same MX7 Linear machine that are sure to grow your presence in the medical orthopaedics market.

7 December 2021