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Hard Hips

By Patrick Loughney Product Specialist, Sandvik Coromant

The challenge of machining hip replacement implants out of cobalt chrome

Hip replacement surgery is currently the most common and most successful orthopedic operation in the United States. In 2013, more than 332,000 people traded in their painful hips for ceramic or metallic joints. And that number is rising steadily due to the needs of our aging population, innovative surgical techniques and recent material advancements. With more than 43 million people suffering from some form of osteoarthritis, the Academy of Orthopedic Surgeons predicts that the demand for total hip replacements will increase by 174% by the year 2030. This ever-growing need for prosthetic implants will require constant innovation and the highest machining efficiencies.

In the medical-manufacturing sector, meeting the stringent and specific requirements of the medical community and the FDA is a hefty responsibility. Components intended for the human body must be hypoallergenic, biocompatible and precisely machined. A top choice of surgeons, cobalt chrome is a desirable material for this extremely demanding application. Material strength is paramount due the amount of weight the joint must bear. Frequently subject to fatigue, medical prosthetic implant devices require cobalt chrome’s superior strength and hardness, corrosion resistance and excellent wear. Easily cast to complex shapes, its high stiffness can be polished to a mirror finish for friction-free joint movement.

However, the same properties that make cobalt chrome an excellent choice as a replacement hip joint, also make it problematic to machine. Although certain challenges exist for machining materials as hard and abrasive as cobalt chrome, it can be done precisely and effectively. As the demands for quality rise, stringent process control is essential in machining these types of components. An excellent surface finish must be achieved to avoid pitting, crevices and debris.

The cost of failure can be high when machining materials such as cobalt chrome. Between loss of output and wasted time, scrapping low-quality components due to inexperience and lack of training and knowledge is financially unacceptable for many shops. Using a secure and predictable process that ensures repeatability without compromising quality requires knowing the proper tool choice, cutting data, chip management and lowering the cost per part. Be sure to enlist the support and training from tooling company representatives for individual engineered solutions to help overcome any challenges. With high expectations of tolerances within microns at the lowest possible cost per component, the more knowledge and experience shops have will help them remain competitive in this industry.

Problems and Pitfalls

Depending on how the cobalt chrome starts out, whether the form is cast, forged or bar, difficulties machining it will vary. Cast blanks typically have less material to remove than other forms but tend to have a “tough skin” to break through. Bar stock normally needs a drilling operation to remove some of the excess material. But bar stock is easier to machine than forged or cast forms because the hardness levels throughout it are more consistent. According to the Rockwell C scale, the hardness of cobalt chrome registers between a 40–46 HRc but some particles in the structure can register up to 58 HRc. There is a direct correlation between material hardness and tool life. The higher the hardness of the alloy, the shorter the tool life and more rapid the wear will be to the cutting edge.

Proper Tool Selection

When using indexable tools for this type of material, now is not the time to be penny smart and dollar foolish. Skimping on quality tools will only lead to bigger problems down the road. From rough drilling to surface finishing, the insert can make and break a job. Swapping out prematurely worn inserts and constantly throwing away broken tools will not keep a shop operating in the green.

What types of inserts work best on this type of material? Look for inserts that impart a strong cutting edge and offer excellent resistance to avoid excessive notch wear. Round-shaped inserts with a positive rake angle offer multiple advantages. For internal turning of the spherical cut in a ball and socket hip joint, round inserts optimize the roughing process giving you a balance of security and productivity.

Notch wear is a common problem when machining these parts. Notching is mechanical wear which is concentrated at the depth of cut. It drastically reduces tool life and produces an unwanted burr on the component. This inferior quality leads to scrapped material and a reduction in productivity levels. When using round inserts, using an approach angle of less than Kr 45° will offer reliability and durability and require fewer tool changes. Round inserts also allow an increased feed rate and cutting speed to achieve maximum productivity. When using a round insert with the depth of cut well below the radius, the chip thickness hex is reduced relative to feed and the cutting edge length is increased. This results in lower temperatures being generated and the opportunity to increase both feed and speed for maximum output.

Components machined for hip joints are the internal and external spheres of the acetabular cup, the femoral head and the stem. Operations for these types of parts are typically rough drilling, rough turning, semifinishing, finish turning and parting off. Choose inserts for these operations that offer process security over long runs. Important factors include insert nose radius, coating, geometry and grade.

Selecting a proper nose radius is a key factor in turning operations. A larger nose radius is advantageous for heavier feed rates, larger depths of cut, stronger edges and increased radial forces. In contrast, the smaller nose radius is better for small cutting depths, less insert strength and also reduces vibrations. Using an insert that is wear resistant, heat resistant and increases security will machine more components with fewer inserts. Coatings improve time in the cut by creating a heat barrier towards the cutting zone and chip. From groove milling to surface finishing, select geometries and grades that are designed to work with cobalt chrome. Razor sharp, high edge-line toughness and versatility will provide peak performance when machining these components.

Cutting Tool Data Knowledge

Precise cutting tool data plays a key role in controlling and lengthening tool life when handling metal alloys this hard and abrasive. Knowing the proper cutting tool data and choosing the correct tools are important for process security and productivity and can extend insert and tool life. Productivity is increased when inserts can run a full batch without needing to be replaced. Knowing the correct feed rate and cutting speed are important as well. Using a feed rate that is too high causes tool vibrations and pressure that can cause the insert to break and chip. If your cutting speed is too high it creates heat and friction that quickly leads to work hardening. Once work hardening takes place, a domino effect can happen and tool life takes a nosedive as it causes premature wear, inadequate surface finish and insufficient tolerance requirements.

Hips and Chips

Chip management is another obstacle when machining cobalt chrome. Often producing long chips that don’t break properly and tend to wind and tangle, cobalt chrome chips can cause unnecessary downtime. Constantly stopping the operation to untangle chips is counterproductive and leads lower output levels and ultimately less profitability. Using high-pressure coolant can help evacuate the chips and at the same time help avoid work hardening by preventing the transfer of heat from the insert back to the material. Using a high-precision coolant will channel coolant directly to the cutting edge. Such accurate coolant delivery aids in the breaking and evacuation of the chips increasing process stability while helping to lower the temperature in the cutting zone and increase tool life.

Radial chip thinning techniques can also help and is another reason for choosing a round insert. Using a round insert for the internal and external acetabular cup will help create thinner chips so evacuating them is no longer a problem. The radius of the round insert allows the cutting edge to approach the material more slowly increasing engagement. And with longer engagement, chip thickness is reduced.

Importance of Surface Finish

Surface integrity is an important aspect as to why cobalt chrome is an appropriate material for hip replacement material. It’s medically necessary to use a material that can hold up to the wear and tear of the human body for a lifetime. The surface finish must be excellent not only for a patient’s comfort levels and mobility because it creates less friction, but it must also be primed for any secondary processes that the components must undergo before they arrive in the operating room. Without a proper surface finish, codes and numbers that are required to be etched into each component for identification and tracking will not be visible.

This year, medical devices manufactured in the United States, including hip replacement components, will include a unique device identifier (UDI). This unique code will allow patients, health care professionals, manufacturers and federal regulators to track each device. By 2018, every component manufactured in the US must carry this UDI.

Shops need to turn out the highest quality components at the lowest possible cost per part to remain competitive. With an increasing need for prosthetics due to our aging population, conforming to the uncompromising guidelines and rules of the medical community presents some challenges. While cobalt chrome is one of the hardest, most abrasive materials to machine, those demanding qualities are exactly why it is a desirable material for hip replacement parts. Absorbing weight and allowing mobility, resisting corrosion and providing excellent wear, this material is hypoallergenic, biocompatible and able to be precisely machined for friction-free joint movement. By understanding key factors like tool selection, cutting tool data and chip management, machining a high-quality component may be difficult to achieve, but through proper knowledge, education and technical support shops can overcome these challenges and be successful in this industry.

This article was first published in the 2014 edition of the Medical Manufacturing Yearbook.

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