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Reverse Engineering:

Reproduction of Rare Porsche 907 Castings
Erik Shahoian

Restoring vintage racecars can be very difficult when parts like castings are no longer available as spares from the manufacturer. In some cases, the castings are made in only the number required to build 25 to 50 cars plus a handful of replacements for crash repair.
 
 
Figure 1
The original Porsche part

This was the case with our Porsche 910 restoration. Our original 910 castings were damaged and repairs would be a marginal solution at best. No driver wants to race along at 180 mph with a doubt as to the integrity of his suspension parts.

A brand new spare upright casting was loaned to us for a few weeks. This casting was from a 907, which was an improved part with improved geometry. We knew that we would have to go to great lengths to reproduce the rear uprights, so we chose to put the time and money into the revised 907 part and go for the improved suspension geometry that would result. The 907 was better than the 910 not a difficult decision.

We only had a left side part and we needed the other side. We also wanted to make absolutely sure the shape was copied with fidelity and all of the critical geometry was preserved. There are no easy technical references available for working on such rarified limited production machines of automotive warfare. The part we had in hand was over 33 years old and Porsche has moved on to building SUV's.

 
Figure 2
The cast made from the digitized data.

The original part was beautiful and exquisitely complicated. It also had no flat surfaces or easy features to make a geometrical reference from. We visited several foundries and pattern makers and they would scratch their heads. No one agreed on exactly how the part was cast. All agreed that the skill level of the original craftsman was extremely high and that it would not be easy for a modern pattern maker to reproduce such a part. This was getting complicated fast.

We purchased a MicroScribe 3D with the idea that we would digitize the original part. We would turn this four pound hollow magnesium piece of sculpture worth the price of a new street car by itself, into a database that would allow us to make more parts using more accessible modern machining technology After some careful consideration and strategizing, we embarked on a precise digitizing effort using the Microscribe. We used Rhino to generate all of the outsidee surfaces of the casting.

It was not practical to knit all of the surfaces into one surface that could be imported into Solid Works and manipulated this part was far too complicated for such an approach. Instead, we knitted as many of the surfaces into each other as we could and imported the IGES surfaces into Solid Works as references


Figure 3
The part in SolidWorks.

SolidWorks also allowed us to hollow out the part in such a way as to maintain a very thin 5mm casting thickness everywhere inside. With a few keystrokes we were able to make the mold parts as derivatives of the hollow spaces inside the part. Later we would make the mirror images of the rear upright part and the molds and send the final IGES file to Dave Aspenwall of Marty Manufacturing.

Using his Mazak CNC machine, Dave took huge blocks of pattern plastic the size of Texas, and machined them down to perfect polished pattern parts and core box molds. Being a pattern maker by training, he was able to help us make some small changes to the patterns and cores to ensure success at the foundry.

 

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