Medical Grinding Takes More Than a Machine

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Columns From: 12/28/2009 Production Machining,

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Russell Riddiford

Russell Riddiford, President ANCA Inc.

Medical grinding is attracting new players, partly because of opportunity, but also because it is possible to use the same equipment used to produce metalworking and woodworking tools. It is viewed as a diversification play for tool grinding companies.

For grinding houses and medical OEM, medical grinding is two segments: medical cutting tools, such as reamers and drills; and replacement parts, such as knee or hip parts. The latter are the type of part that is commonly designed using a solid modeling package (typically NX).

To be successful, shops must invest in a machine tool that is efficient and also flexible, ensuring productivity even when the market changes. Accessories such as automation, in-process dressing, full grinding machine simulation and integrated CAD/CAM software help to achieve these goals.

For grinding shops and OEMs, a lot of the work is short run; it’s not one size fits all for bone or joint replacement parts. This is why machines must be flexible and easy to set up using powerful software. From drills and reamers to hip rasps, knee implants or femoral balls, one machine and software package with only one operator must be capable of producing all these different parts accurately and efficiently.

Often operating more than one grinding machine due to overall part volumes, the medical industry may use a cell of three to four machines to complete parts without human intervention. Workholding fixtures are key, as are special accessories: full grinding machine simulation for path verification, NC steady rest for support, in-process dressing capabilities and robotic automation (typically a Fanuc robot) to change workpieces and perform secondary operations such as polishing or deburring.

In-process wheel dressing is essential for trouble-free medical part grinding because the material, normally stainless steel, needs to be cleaned from the wheels frequently because of a high wheel loading coefficient. This keeps the grinding wheel open and cutting freely, thus permitting the uninterrupted production of a batch of tools, to within specification.

Because it is not one size fits all, the current approach is to generate the grinding paths from a designer-supplied solid model using a third party CAD/CAM package such as NX. The major disadvantage is that any small change in process parameters or grinding wheel diameter requires that the grinding path be recalculated, ultimately decreasing productivity while the grinding machine waits for the new path to be generated.

A new approach is to split the CAD/CAM responsibilities between the solid modeling package and the grinding machine, taking full advantage of the strengths of both. The solid modeler’s capability to generate free form paths is coupled with the grinding machine’s capability to set process parameters such as machine feed rates, spindle RPM, in-process dressing frequency, tool offsets and compensating for changes in grinding wheel diameter. This redistribution of responsibilities gives the machine operator the usual level of control he expects without needing to return to the solid modeling package to regenerate the grinding path every time he makes a change in a process parameter.

Grinding machine simulation and integration between the grinding machine software and solid modeling package gives the designer the tools required to perform the machining process on a desktop PC. The ultimate outcome of this coupling is the direct comparison of the original solid model with the solid that the grinding machine would produce. This offline process not only means that the designer can be confident that an accurate part will be produced, but that the grinding machine can be in production while the designer generates the tool path for the next job.

For medical implant and tool grinding, machine flexibility, simulation capability and programming flexibility are essential to being competitive. Opportunities exist for shops, captive as well as independent, to participate successfully in this field, but only if the right level of integration between design and manufacturing components is achieved. 



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