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How to CNC machine a scale model of a car

Scale 1:4 car model for a NASCAR team presentation

The N in NASCAR (the National Association for Stock Car Auto Racing) suggests that this is a National sport in the USA. While this is of course true (NASCAR is the second-most popular professional sport in terms of television ratings inside the USA), there is much more NASCAR racing worldwide.

In Argentina NASCAR is a well-known event, and the major racing teams carefully plan the races to get a maximum publicity. The racing team of Canapino Sports presented the car to be used in 2008 at the Auto Performance 2007 exhibition in Buenos Aires, end of 2007.

For this presentation a scale 1:4 scale model was needed, and that model has been created by PR agency A4 Color in Buenos Aires, using their DeskProto software.

The frame inside the car
Side view of the car in Rhino
Drawing of the frame, and the CAD design in Rhino.

Very important for the actual car is the internal frame structure, to make the car more rigid and to protect the driver as crashing during a race is not uncommon. The car model as shown here just features the outside geometry: the frame is not present.

Animation: walk around the car
The car model split up in Rhino
A simple animation, and next the parts to be machined.

The geometry for the racing car has been modelled in the Rhino CAD software, which is very suited for the free-formed car body design. This geometry later has been split into a number of separate parts, making sure that no undercuts were present in any of these parts.

Toolpaths in DeskProto for the left side panel
Toolpaths in DeskProto for one wheel
Toolpaths in DeskProto.

The geometry of each part has been saved as STL file, and next imported into DeskProto in order to calculate the toolpaths. As can be seen in the DeskProto screenshots shown above, a small distance between the toolpaths was used for a smooth resulting surface. The toolpath is drawn as a red line, however as so many toolpaths are present the separate red lines can no longer be distinguished.

Milling the PolyStyrene model base
Milling the epoxy top layer
The top of the car being machined: first the foam core, next the epoxy shell.

A4 Color use a large CNC Router called Excitech, made by SH-CNC in China. Working area is 246 x 126 x 20 cm, and special attention was needed to keep the height of all car parts small enough for this machine.

It was possible to create a high quality model still having a low weight as A4 Color applied a special technology, the sandwich construction: the car model was machined in light PolyStyrene foam using a negative skin value (so a bit too small). Next the foam model was covered with a 4 mm thick layer of epoxy resin (bondo, the same is used by car painters). After that the model was machined again, at a much higher accuracy for a smooth surface. The result is a model that combines a low weight with a durable and high quality outside.

The two side parts, just milled
Manual finishing a side panel
Finishing the two side panels.

Of course after the machining was completed manual finishing was needed too: sanding and polishing to get the required high gloss car finish.

A partial assembly, unfinished
The resulting model, before painting
Building the model by combining all parts.

Further work the model included assembly of the complete car model.

The resulting model, after painting
The resulting model, after painting
Now in glossy black.

Finally the model was sprayed in black, the wheels were mounted and product graphics (striping) were added.

Striping design: Adobe Illustrator screenshot
The striping being printed on a Roland large format printer
The striping: in Adobe Illustrator and next on the Roland color printer.

The graphic design has been created by Dario Canovas and German Katz, using Adobe Illustrator. Printing on vinyl sheets (3M) has been done using a Roland VP 540 large format printer.

The model at the exhibition
The resulting model on display.

The resulting model has been on display at the Auto Performance 2007 exhibition