| 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.
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.
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.
Now, we had a shell, a ghost of the original part. The really
amazing thing about solid modeling is that we can build a new solid
model with every parameter and critical feature defined and infinitely
adjustable. We built a huge model using most of the Solid Works
capabilities out into the ghost surfaces. This way we could immediately
see when we were above or below the original surface. This was
essentially digital sculpture with a digital pattern guiding us.
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.
We brought our patterns to Dan Holmes at Aluminum Precision Cast.
He poured right and left, front and back replica castings. After
heat treat and post machining processes, we had aluminum first
articles that we could fit up to the tube- frame chassis and check
our work.
Fortunately, the replicas fitted and worked perfectly as intended.
The patterns were then sent to Litemetals in Ohio to have real
magnesium alloy castings poured. The magnesium castings will match
the original weight and finish.
This replication process was very involved and would lend itself
to industrial or product development efforts. The Porsche 910 racecar
we were re-fabricating was so special to us that it justified these
lengths. Most castings are less complicated than these suspension
parts and most would be easier tests of the methods and tools we
used. With the MicroScribe, we were able to quickly come up with
a solution to what may have been an otherwise unsolvable dilemma.
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