Case Studies
Arthroscopic Shavers
A tool used in orthopedic procedures to precisely cut and remove soft tissue, cartilage, and bone from joint spaces, an arthroscopic shaver features a “tube-in-a-tube” design with rotating blades. The inner tube rotates at high speeds against the outer tube while a vacuum sucks out the cut tissue and fluid as it cuts.
Blades and burrs vary depending on specific uses, such as full-radius blades for debridement of soft tissue or resector blades for precise cutting of cartilage. Those featuring serrated cutters with sharp teeth are effective at trimming tougher, fibrous tissues like meniscal cartilage.
Abrasive machining is a well documented tool to manufacture precision medical devices. Abrasive machining covers a wide variety of technologies, from centerless grinding to Abrasive/chemical grinding, to MicroBlasting. Each process plays a role in the precision required in today’s medical devices. Often, they are all used as complementary steps as seen with the machining of arthroscopic shavers. Let’s take a look at where each option fits.
Challenge: To grind two mating metal components to tight tolerances.
The building block of an arthroscopic shaver is two bull-nosed tubes, one nesting inside of the other. These tubes are ground to tight tolerances allowing the inner tube to move smoothly within the outer tube but preventing any foreign material to get between them. The standard tolerance is 0.0005″. The tips of the tubes typically have a rounded guiding surface, and the outer tube has a window cut into it to expose the sharp edges of the inner tube.
Recommended Approach
Equipment: GT-610 thru feed grinder
For the outer sleeve, a GT-610 thru feed grinder is used to grind the OD of the blank tubes, maintaining a surface finish between 3-6 RMS and maintaining a comfortable 2-3 Cpk on the outside diameter of the part. Material removal of 0.005″ with a tolerance requirement of +/-0.0005″ was easily met in a fully automated turnkey process.
Equipment: GT-610-CNC centerless grinder and P-4200 gauging system
To infeed the shape of the inner tube, using the Glebar GT-610-CNC, centerless grinding maintains a +/-0.0002″ tolerance on the tip dimensions. The GT-610-CNC is equipped with a patented movable work rest blade which ensures that the radius of the tube is positioned accurately to the wheel. Glebar also provides full automation using Fanuc Robot and step feeder which measures the part length before the grind and sends that dimension to the machine to adjust the movable work rest blade accordingly. This process drastically reduces scrap and is designed for high volume production of shaver.
Glebar developed the P-4200 gauging system specifically to measure the inner tube and more importantly the radius of the shaver. This data is fed back to the GT-610CNC to correct the wheel dress profile as well as provide diameter size compensation.
Challenge: To cut shaver teeth Efficiently
Solution: Electrochemical grinding provides a burr-free grind capable of meeting tight tolerances with heat sensitivity. This process combines a standard abrasive wheel with a DC voltage and a saltwater electrolyte. The material is anodically eroded away in addition to mechanical grinding so it can produce a fast burr free cut. This is unlike EDM or laser that vaporizes the metal surface. ECG causes no metallurgical issues.
Recommended Approach
Equipment: SG-1645 Electrochemical N/C Surface Grinder
For curved shaver teeth, parts need to be ground individually. For straight cut shaver teeth however, parts can be stacked up to 30 at a time with typical cycle times of 10 seconds per piece. Each machine features CNC controls and high precision components for high accuracy and repeatability. Optional pallet loader features a 180-degree index table allowing parts to be loaded during the grinding operation reducing loading time to almost zero, maximizing productivity.
ECG provides a burr-free cut in one pass with no heat affected zone, recast layer or work hardening of the material. The ECG technology can cut full depth in one pass requiring less wheel dressing with wheel life lasting up to 20x longer.
Alternate Solution: MicroBlasting as a secondary process for EDM or laser
Laser cutting and wire EDM enable the fabrication of highly complex tooth geometries with precise dimensional accuracy. These methods however, are slower than electrochemical grinding, and can create “Heat-Affected Zone” and leave behind slag or recast. Micro-abrasive blasting can be used as a secondary finishing process to remove the these imperfections.
Recommended Approach
- Equipment: AccuFlo AF10
- Nozzle: 0.030″
- Pressure: 80 psi
- Abrasive: 17.5µ Aluminum oxide
- Orifice: 0.025″
MicroBlasting the teeth removes the oxide layers and discoloration, smooths pulse marks and removes remelt restoring the device to uniformity. Aluminum oxide, a hard abrasive (registering at 9 on the Mohs scale of hardness), works well because it effectively transfers energy to a brittle oxide layer and shatters it away quickly.
Challenge: to texture the hub to improve adhesion to the overmold
If the shavers are installed in a polymer hub, MicroBlasting may also be used to engineer a surface finish to promote and optimal bond between the stainless and polymer. A textured surface finish increases the pull strength of the bond. MicroBlasting with a sharp cutting abrasive, like aluminum oxide efficiently textures the surface. Blasting properties can be controlled to match the specific surface requirements to best bond with a hub.
Recommended Approach
- Equipment: AccuFlo AF10 blaster equipped with a Ring Nozzle in the ProCenter Plus
- Nozzle: Ring Nozzle equipped with 5x 0.030″ nozzles
- Pressure: 60-80 psi
- Abrasive: 50µ or 100 µ Aluminum Oxide
- Orifice: 0.030″
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Questions about MicroBlasting or Grinding for Arthroscopic Shavers?
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