A Single System to Meet High Quality Challenges

Many medical components contain free form surfaces that need to be machined. Today this is done by using manual processes, robots or CNC machines in different constellations.

These operations contain different steps in many stations and various kinds of programming, like teaching a robot or programming a CNC machine. The requirements in modern implant production are for the highest quality with the utmost possible efficiency. There is great potential for a closed manufacturing chain on a single production cell. All operations in this chain can be programmed by a single CAD-CAM system. Under these conditions, turning a blank into a finished work piece is simple and efficient. NC technology in multiple axes is the basis of this procedure. It provides a superior level of accuracy of the component and is the standard of the future.

The following shows the principle of a complete CAD-CAM chain starting from a 3D-Model and going up to a finished part with a femoral knee as an example (figure 1).

Figure 1

The steps of this process (figure 2) are:

• creating a 3D CAD Model;
• defining tools and working strategy in a CAM system;
• generating the tool path;
• post processing of the tool path with feed commands for a certain machine as a result;
• inclusion of these feed commands in an NC-Program;
• manufacturing the part on the machine.

Figure 2

Figure 3: A single electroplated CBN grinding wheel is used for all sizes of a part family.

The first step when manufacturing a femoral casting is removing most of the stock by grinding. With different sizes left and right there is great advantage in using a single grinding wheel to work on the complete part family. The solution is an electro plated CBN grinding wheel with a full radius shape and grinding in lines.

Today there are no CAM systems that support multi axes grinding as a special operation. So, using a full radius grinding wheel as a tool in a CAM system requires the ability of the software to provide a unique tool shape and the special capabilities of the programmer, because grinding differs totally from milling. As can be seen in figure 4, the relative positions of tool and the work piece will influence the contact.

Keeping this in mind, the programmer has to decide how many axes should be used for the machining of the part. Two possible strategies are shown in the figure 5 and figure 6 examples. Manufacturing in three axes, for example, leads to another point that the programmer can vary: The method of post-processing. First, the post-processor must be able to translate the tool paths generated by the CAM system into feed commands fitting the kinematics of the production machine. The post-processor determines how many axes are needed for the process. In some cases there is more than one possibility to combine axes movements and an intelligent post-processor can be switched to give the desired output (figure 7). A polar or linear post-processor method will influence the contact situation and the dynamics of the moving axes, as well. An experienced developer can best decide which method fits the task.

Figure 4: Differences between a ball mill and a grinding wheel.

Figure 5: Manufacturing in three axes.

Figure 6: Manufacturing in five axes.

Grinding is the first and most important step in a row of downstream operations after casting. The surface after grinding provides the desired shape to the work piece and has to fit the tolerances. The following operations mainly improve the surface quality. Therefore, the grinding surface quality determines the efforts for belt grinding and polishing.

Figure 7: Postprocessor methods "polar" and "linear".

Roughness after grinding is influenced by grit size of the grinding wheel, cutting speed and feed rate. But even the parameters of a CAM system, like Unigraphics, affect roughness by varying the calculating tolerances, scallop height or tool shape (figure 8, figure 9). These parameters are set at an early stage of the developing process, so the programmer not only decides the rough structure of manufacturing, but influences the machining parameters, as well.

Figure 8: Roughness in circular direction.

The operating procedures for machining a femoral knee could be grinding, disc cutting, belt grinding, milling and buffing. Every type of tool has its own requirements within the working strategy. This can be the relative position of tool and work piece, overlap, cutting direction etc. Belt grinding and buffing are operating procedures that have a tool shape similar to a grinding wheel. But the contact wheel that drives the grinding belt must touch the surface at its normal vector, and the buffing wheel has to dig into the work piece to get a shining finish. Because of these requirements the operation is different than grinding.

Figure 9: Roughness in crosswise direction.

Milling might be necessary for surfaces that cannot be reached completely by a grinding wheel, like the box section or the outer diameters (figure 11). Generating milling tool paths is no problem for the CAM system, which is in its element here. But the machine has to be able to realize the degree of freedom necessary to mill a free-form shape.

Figure 10: Belting Unit, specially designed by Schütte and treated buffing wheel.

Using a five axes grinding machine for this kind of application provides the ability to follow almost any shape, but at the same time dynamic movement of all axes is requested. Schütte is a manufacturer of tool and universal grinding machines (figure 12).

Figure 11: Milling the box section of a femoral implant.

Figure 12: Schütte grinding machine WU305 linear.

For producing and regrinding complex tools, precise CNC grinding with simultaneous interpolation of up to five axes is needed (figure 13). These are requirements that also fit for machining free-form shapes.

Figure 13: Kinematics of a Schütte five axes grinding machine.

A compact and rigid machine design is completed by five direct driven axes for most possible dynamics, which is the key to free-form shape manufacturing. Two rotary and three linear axes are driven by synchronized motors, which are backlash-free and do not have to bear lateral forces. To be flexible in different operating procedures the machine also features automatic tool change. A magazine for four or five different tools is provided, so that grinding, belt grinding, polishing and milling tools can be used in one setup (figure 14). The supply of coolant and polishing compound is controlled by the NC program and exactly fits precisely to each tool, which is an important aspect for grinding of difficult machineable materials, like titanium. Depending on lot sizes the automation of handling the work piece is needed as well. Therefore, a pick up loader can be provided in this machine (figure 15).

Figure 14: Magazine for automatic tool change with grinding wheel, belting unit, buffing wheel and end mill.

Figure 15: Pick up loader for handling of work pieces.

Containing all these features, the Schütte WU305 linear is a complete manufacturing cell and highly suitable for implant manufacturing. The missing link from a tool-grinding machine to a CAD-CAM system is a full five axes post-processor that was specially designed for this machine and can be used with Unigraphics as CAD-CAM software. The advantages of this CAD-CAM system are:

• the programmer uses only one programming platform (CAM System) and overviews all process steps;
• the operation structure of the CAM file can be used as a template for all other part sizes;
• the programmer defines input and output of each machining step;
• the process chain only contains a single manufacturing cell and the programmer needs to know the kinematics of only one machine;
• costs are reduced because one tool setup for a complete part family is stored in one machine;
• one chucking for all process steps is required and automation of handling is possible, which reduces costs;
• a higher repeatability of process output in dimension and surface roughness.

The Schütte grinding machine can be used for universal applications. The complex kinematics and various features predestinate it for the production of medical components.