Achieving the right finish

The orthopedic device market continued to grow at a rate of 7.6% during the past decade. It is expected to reach a value of $61.3 billion by 2017 from $36.7 billion in 2010. Market competitiveness to increase as companies want a piece of the growing industry. The need for increased quality standards and improved manufacturing practices has been amplified. These improvements will help to ensure the prevention of recalls and litigation as well as to decrease manufacturing costs.

Current methods of manufacturing medical implant devices require exceptional finishes from materials such as cobalt chrome, titanium, zirconium, and ceramic. Using a knee as an example, the process is typically: 

• Forging or investment casting

• Gate removal by machining with carbide tools or rough grinding with plated superabrasives

• Finish grinding using conventional or vitrified cBN wheels

• Polishing utilizing coated abrasive belts and/or nonwoven wheels

• Final finish process involving buffing or drag/chemical vibratory finishing. 

The goal, the process

The ultimate goal is a mirror finish. In a market with increased focus on lean manufacturing, multiple steps create an opportunity for manufacturers to look at their processes and understand what is required to reach their manufacturing goals.

The finish requirements create reliability and quality by removing surface imperfections. The process to accomplish this should be as efficient as possible while meeting geometrical and quality specifications.

The process of rough machining or grinding generates lines in the knee that need to be removed in subsequent steps. Walter, a division of United Grinding North America, and Norton | Saint-Gobain Abrasives have partnered to develop a process to eliminate manufacturing steps while producing an orthopedic knee implant that is ready for a final finishing process of buffing, chemical vibratory finishing, or drag finishing.

With Walter’s NXis Ortho software and a Norton plated, vitrified cBN superabrasive grinding wheel alongside a Norton Bear-Tex non-woven wheel, operators can reduce overall cycle times by more than 30%. On a typical orthopedic knee, limitations of three to four components per tool are obtained before the tool needs to be replaced for the machining process. Using Norton vitrified cBN wheels allows Walter’s integrated wheel dressing and compensation to automate production, allowing flexibility and control of surface finish through parameter adjustments – an important benefit of grinding versus machining.  

One step further

The above process readies the component for a couple of different finishing options:

Option 1: A robotic cell with coated abrasive belts for a multi-step polish- ing sequence

Option 2: Clean and finishing room, various manual off-hand processes

These subsequent steps, within the process, increase overall cycle times and costs. Walter and Norton developed a grinding path approach using Norton vitrified cBN to minimize the rough step-over lines, eliminating the need for belt grinding that is conventionally used. The knee now has a surface condition that allows for the use of a Norton Bear-Tex non-woven wheel trued to the knee geometry using MSL superabrasive block truers.

This approach enhances the surface finish from an incoming 30µ" to 35µ" Ra to 4µ" to 8µ" Ra.

The ability to process the knee in this manner reduces process wait time (queue time), reduces high labor off-hand polishing, and streamlines the process. The resulting part finish provides a surface condition ideal for post-process buffing, and chemical vibratory or drag finishing. Figures 3 and 4 are additional examples of orthopedic devices that were first ground utilizing vitrified cBN wheels. The vitrified cBN process produced a finish of 6µ" to 9µ" Ra. They were then polished with a non-woven wheel to obtain a finish of 1.5µ" to 2.0µ" Ra. The final buffing process obtained a finish of less than 1.0µ" Ra.  

Product advancements

Improvements in processing can come in several forms from new product advancements, added machine capability, and in this case, a combination of both product and machine. The data on page 26 shows a clear picture of the potential cost savings when a process solution is created.