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Manufactured for precision fit and quality of life

Artificial hip joints must be manufactured with high precision, especially in the area where the hip stem and the ball joint connect. To achieve this CERATIZIT has developed a machining solution based on a u-axis system and a stepped reamer. When combined with other precision tools from CERATIZIT the required tolerances are met, while increasing productivity compared to conventional production methods.

If a hip joint is affecting quality of life by restricting movement and causing chronic pain, and if conservative treatment methods are no longer helping, the only option is to have an artificial replacement joint implanted – over 200,000 such operations are performed in Germany-alone each year. Those who take this route are hoping for long-lasting improvements to their quality of life. To make this hope a reality, as well as a good surgeon and first-rate care, the highest quality ‘replacement parts’ are needed.

Prosthetics usually consist of a hip stem with ball joint, a hip socket and, an intermediate piece to ensure movement is as smooth as possible. During manufacture, particular attention must be paid to the connection between the hip stem and the ball joint; to ensure the conical surfaces fit together perfectly they need to be produced with the highest precision and surface quality.

Therefore, the tools used play a crucial role when manufacturing these components. Dirk Martin, Application Manager Medical at CERATIZIT and member of Team Cutting Tools, explains: “An artificial hip joint is manufactured from difficult-to-machine materials, which not only need to be machined within the narrowest tolerances, but also economically. Ultimately, an artificial hip replacement should be affordable for as many people as possible. Ceratizit’s role is to develop suitable tool solutions for these tasks.”

CERATIZIT experience as in the machining sector brings in-depth knowledge of a wide range of standard and specially-made tools as well as associated machining expertise. “With our huge product range and the experience of our application specialists, we are extremely well placed to provide support and to develop machining strategies for applications such as where the hip stem and joint ball connect,” says Dirk Martin. “With our range of tools, we can test all manner of approaches to ultimately find the optimal solution.”

In the case of the artificial hip joint, the components have particularly demanding and varied requirements. For the hip stem, made from high-strength titanium alloy Ti6Al4V, an angle tolerance of just +/-5‘ must be achieved in the conical connection area. Other tolerances are 3 µm for straightness, 8 µm for roundness and 60 µm for the diameter. It is also important that the specified contact ratio for the cone is achieved and a precisely defined groove profile produced.

The ball joint is made from a cobalt-based alloy (Co-Cr-Mo). Its conical hole must have the same shape, angle and dimension tolerances, as well as the specified contact ratio. However, there must be no marks, ridges or grooves made during machining. Dirk Martin points out another crucial factor: “We need a machining solution that is suitable for mass production. This means the machining must be process-secure and require as little monitoring as possible.”

Flexible u-axis and special conical reamer ensure success

To produce the conical outside profile, CERATIZIT’s application specialists opted for pre-machining with a solid carbide conical milling cutter. The subsequent roughing and finishing are then completed using a CERATIZIT u-axis system. “This is an interchangeable, freely programmable NC axis for machining centres, which can be used to machine contours or for turning.” explains Dirk Martin. “Attachment tools and indexable inserts can be used to create contours in holes and external machining work. This usually means that production times can be reduced considerably, while providing optimal surface quality and higher shape accuracy than usual.” This means the desired groove structure can be produced on the stem cone even on a machining centre. The benefit of this is that all machining processes can be done on a single machine. Using the conventional process, a lathe and a milling machine would be required, which means additional clamping, aligning, time and money.

To make the conical hole in the ball joint, CERATIZIT’s solution involves the following steps being carried out on a lathe: First, the part is faced to provide a flat surface for the subsequent special solid carbide 180° drill with four cutting edges. This is then used to make a hole with a flat bottom. After this an EcoCut Classic drill and turning tool is used to produce the cone with close-contour boring, while a special solid carbide conical reamer ensures the ideal contact pattern as well as perfect surface quality and tolerance are achieved. The regrinding capability also saves the user further production costs. Dirk Martin comments: “We were also able to produce the internal contour to the required precision with a u-axis. However, using the reamer is the better method for the ultra-smooth surface quality required.”

All in all, CERATIZIT’s approach enables the critical contours of the artificial hip joint to be produced process-secure and extremely economically. The contact pattern achieved (contact ratio and contact surface) and the tolerances for both machining processes are well below the specified limits. Dirk Martin is proud of this team achievement: “With this approach, we were able to position ourselves as a full-service provider for our customer, a renowned medical technology company, with the prospect of further orders.” CERATIZIT’s expertise in the medical sector also extends to other similar joint replacements, such as artificial shoulder joints, as well as for attachments for artificial knee joints.

The CERATIZIT solution at a glance and specific values

Conical outside profile of the hip stem (produced on a machining centre)

  1. Pre-machining by special solid carbide milling cutter with incorporated angle (Ø 12.0 mm/z4) Vc 90 m/min, Fz 0.12 mm, Ap max 16 mm
  1. Further machining with u-axis with special indexable insert optimised with R0.8 Rough cut with Ap(1): 0.20 mm and Fz(1): 0.10 mm Finishing cut with Ap(2): 0.03 mm and Fz(2): 0.24 mm

 

Conical hole in the hip joint (produced on a lathe with a reamer):

  1. Pre-machining using special 180° drill (special solid carbide D12 180° drill with four cutting edges) For creating a hole with a flat hole bottom
  1. EcoCut used for close-contour boring (EcoCut Classic 3xD [#70804608] with XCNT 040102ER CTPP430 [#70386922])
  1. Precision machining special solid carbide conical reamer for hole with 5°42‘30“ +/-5‘