Medical Manufacturing


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. Tere is a direct correlation between material hardness and tool life. Te 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 ex- cellent 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. Tis 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. Tis 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


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.


40 Medical Manufacturing 2014


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