Creating implants for knees: from the foundry to finish: specific challenges are attendant with each step in processing medical prostheses, including the selection of the right abrasive at each stage--a critical element in achieving dimensional and final surface requirements.

Author:Reitz, Ed
Position:Medical: grinding

The goal of any manufacturing process is to maximize productivity and quality while minimizing total cost. Given that, there are specific challenges to meet during each stage of processing medical prostheses. Here, by way of example, we review the abrasive belt processing of femoral knee implants, but many of the considerations described here hold true for the production of other types of metallic orthopedic devices: tibial implants, hip implants, bone plates, tube plates, among others.

Various abrasive products are used throughout the process of refining raw castings into final prostheses, and selecting the right abrasive at each step is essential to reaching dimensional and final surface requirements.

Following are considerations to be addressed at each step:

Cropping castings--The trick to cropping investment castings from an investment tree is to be able to cut as close to the casting as possible without damaging it. Any leftover gate material must be ground away and generally becomes waste. Alloys used for orthopedic implants are generally quite expensive. Gate material left on the investment tree can usually be recycled. Fines are seldom recycled and may be categorized as special waste.

Following are some of the common errors committed during the cutoff process:

1) Failure to safely and securely clamp the work. Since significant force is generated by the downfeed of a cutoff machine, cropping parts from a "loose tree" can lead to a variety of problems.

  1. A part that rocks during the cut can "grab" and damage the cutoff wheel, sometimes to the point of causing complete wheel failure. While all cutoff machines should be properly guarded, flying wheel debris is a potential outcome and a potentially deadly problem.

  2. A part close to being re moved can be grabbed by the wheel and thrown.

  3. Even if the wheel does not fail, improperly secured parts will likely result in a crooked cut, which, at the least, will result in excessive gate material left on the part and, at the worst, may cut into the part, requiring that it be scrapped.

    2) Another common error is using a cutoff wheel that is too thin. Though reducing kerf loss by using a thinner wheel sounds logical, at some point the wheel will become too thin to remain stable in the cut. Once the wheel begins to cut off track, it will continue to veer even further in the direction it's cutting. Here, too, the result will be either excessive gate material or a ruined part.

    Here are a few other tips for the abrasive cutoff of parts from investment castings:

    1) Given that the part may require frequent repositioning for multiple cuts, manufacturers of cutoff machines have developed creative part-handling devices such as fixtures that can be "floated" into position on an air table, and then securely locked into position via a strong vacuum.

    2) ANSI standards call for wheel flanges to be at least 25 percent of a cutting wheel's diameter. Any less violates the standard and may result in a wheel that does not cut straight. Using flanges that are approximately 30 percent of wheel diameter will enhance the straightness of cuts and may actually reduce kerf loss by preventing the wheel from wobbling in the cut.

    3) Wheel flanges should be considered a maintenance item and be periodically checked to ensure that they are flat and parallel. Mismatched or uneven flanges may contribute to crooked cuts and wheel damage.

    4) While less powerful machines will get the job done, the most productive cutoff machines feature one-or-more horsepower per inch of wheel diameter. For example, a machine designed to use 25" cutoff wheels will be optimally productive when it has at least a 25hp motor. For highest productivity, use up to 2hp per inch of wheel. With this arrangement, not only are cuts completed in less time, the faster cut rate is usually straighter, produces less heat, and minimizes or eliminates burrs and metallurgical burns.

    5) Use a cutoff wheel that is properly sized for the machine's arbor shaft. This precaution will prevent the possibility of a loose bushing slipping out of place during the mounting of the flanges. A slipping, loose bushing can result in wheel damage at the arbor hole.

    6) If...

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