Ferrari 360 Front Challenge Grill Frame

 

I decided to change out the front grills on my Ferrari 360 Spider from the stock to Challenge grills. The Challenge grills have a steel mesh to keep FOD out of the radiator cooling area, vs. the stock grills which just have some fins. 

I managed to find both left and right grills on eBay for about $150 each and thought that would be the end of it. After installing them, it seemed like there was a part missing. 

Checking the parts list, I found there's supposed to be a frame behind the grills (part #'s 66307300 and 66307400) to channel the airflow into the cooling duct for the radiators. Without the frame, air can spill around the duct and cause a loss of cooling flow. It's a large part with a maximum dimension about 15".

I had a choice. Spend over $1000 and purchase new frames, if I could even find them, or make my own.

I found a beat up frame on eBay for $80 and used that as a template. The eBay unit had most of the attachment tabs broken or missing but the frame itself was in good enough shape to use as a model.

The part has some interesting geometry that makes it a tricky part to reproduce. It's not flat and has an organic shape on the mounting side. Luckily, the other side is flat so only one side of the part has a difficult shape. The other side can just be traced onto graph paper.

To get the shape digitized, the part has to be "indexed" or registered properly so that the bottom and the top can be modeled and then built up into a solid part with the correct geometry. I did that by mounting a piece of cardboard with staples to my CNC machine table, then tracing the flat bottom onto that cardboard, then placing graph paper on the cardboard, making registration marks at the zero point and corners, tracing the inner shape again, then removing the graph paper and placing the part back exactly where it was. I secured the part to the cardboard with hot glue.

Here's the flat part traced onto fine graphing paper. I had to tape three sheets together to make a large enough tracing sheet. The cut outs allowed me to position the graph paper precisely on the cardboard to index the position of the base.

The tracing was corrected for spherical aberration in Photoshop, then imported into Fusion 360 as a canvas. Using the spline tool, I traced the outline to digitize the shape. 

Digitizing the part's mounting surface required using a probe placed precisely at the inner edge, around the circumference of the part and recording the 3D position in a spreadsheet. I'm sure there's a way to capture the points using Acorn, but I just typed them into a spreadsheet instead. Out of 200+ points, I made one single error, which is pretty remarkable. Humans usually have about a 10% error rate when manually recording data. The little box to the left of the computer controls the CNC machine and can be used to move the business end with extremely fine control. I measured to 0.01" accuracy. Difficult to do with a tape measure!

This is what I ended up with after importing the points into Fusion 360 as an inner and outer spline. I had to create the mounting tabs myself, which was fairly easy once I had the shape. The mounting surface is a complex organic curve in 3 dimensions. 

The mounting holes were recorded and imported into the model as well.

With the shape digitized, the splines were converted into a solid, meshed, and imported into Prusaslicer to prepare it for printing. Unfortunately, my 3D printer has a maximum dimension of about 9", so I had to scale down the print to fit the printer. I've got a new, much larger professional grade printer (Prusa XL) on order that will print this part full size, but for now, the smaller printer will do for testing the shape.

Note the green under the orange part. The green stuff are supports required because the mounting surface isn't flat. They rip off fairly easily and any rough spots can be sanded. My new printer will have multiple print heads so I can print the supports using a material like PVA which dissolves in water, making the cleanup job much easier.

Here's the finished part. It looks great and will be acceptable for use as a frame when printed at full scale. I should have my new printer in about a month, so at that time I'll take my front bumper off again and mount these frames in place.

Here's the old part with the new reproduction to show the scale. The part is about 15" at the wideset. Easily done with the Prusa XL which has a 14x14x14" print volume.

To make the other side, the part is simply mirrored in the slicing program. Only one model is required for both sides. The miracle of digital technology!