Spherical Machining

When finishing spherical diameters to an extremely fine and accurate specification there are particular processes that need to be followed.

In the finishing of hip joints, for example, the steps required to get to the final sphericity and surface finish are very specific.

There are typically four to five steps that are required to finish the surface of the hip joint. The first step usually uses a CBN stone. The finish that is generated when using the Stone is a relatively rough surface finish. The second step uses a coarse aluminum oxide stone. This stone has a friable bond that breaks down during the cycle. The third and fourth is another step that reduces the finish even further and improves the sphericity even more. The final step in the process is generally a graphite-based tool the basically does not remove material but greatly improves the finish.

FINAL FINISHING BY PASTE POLISHING?

After this grinding processing is completed in the machine, it is taken out and thoroughly cleaned. The next process will utilize a felt pad and polishing paste.

This is a long and arduous process. The machine has up to 5 spindles which the newly finished hip joints are mounted to. The tooling for this operation is a soft felt sheet which is cut to be formed over a cup shaped tool holder. The sheet fingers are held in position with an elastic band around the outside diameter of the tool. The pad and tool is installed by hand and with the hip joints ball in place, the cycle of this machine starts. This cycle advances the tool to the hip joint and both spindles rotate counter to each other. The tool spindle also has an oscillating motion that will generate the final finishing. There is a paste added to perform the final finishing portion of the operation.

Shown below is a picture of the basic machine which completes this operation.

The spindles counter rotate and also oscillate. This motion generates a pattern that will remove all the microscopic scratches left from the previous machining operations.

This new process will eliminate this extreme manual operation. The use of precut finishing abrasive film is the key. The film pads are laser cut into a strip of finishing film which is then put into roll form. The strip has feed holes on the outer edges much like motion picture film uses. There is a unit that would swing into position and place the flower between the tool and the workpiece. The tool spindle would advance and capture the abrasive film. The process would maintain the same programming functions as the stone processes.

The holes are used to locate and accurately place the flower design over the workpiece and feed to the next flower pad after the cycle has completed. The spent flower would be flushed away with the coolant.

As previously mentioned, this flower is placed between a compressive tool and the workpiece and captures the flower in place. When the spindles start their counter rotation, the friable ends of the flower snaps free from the strip and the abrasive spins with the tool. This abrasive film will finish the hip joint while still mounted on the same spindle used for machining. The abrasive can be as fine as required and it acts like the process that uses a polishing paste and felt pads as in the current operation. This feature eliminates the need for a second machine and allows the workpiece to stay mounted on the same fixture.

What this new operation can do is eliminate the labor-intensive polishing operation. When the part comes out of the original machine, the hip joint is complete.

This operation can generate a more consistent and accurate finishing of the hip joints. The finishing film can be selected from any of the current developed films. This allows the finish to be varied for the application.

As with any spherical part, the finishing can either make or break the quality of the part. The manufacturing of spherical bearings can be improved by using this same flower abrasive film process. The friable flower film can be used to generate a more consistent quality. This will enable a higher level of roundness and surface finish. The former operation utilizes a grinding operation that does take a longer time in finishing.

The film used is a highly consistent Microfinishing film that can generate almost any surface finish with extreme consistency. Other machined components use this type of film for finishing.

Crankshafts and camshafts use this same Microfinishing film in the final finishing processes. The results in these workplaces are so fine and consistent that the longevity of the engines they are a part of has been improved. The wear rate of the engines are reduced to the point that they are not an issue in the life cycle anymore.

Manufacturing components to a higher level of quality with a reduction in cycle time is a huge process improvement.