How To; 3D printing & bronze casting
For the Velocette Rally, I volunteered to make the trophies for the various classes of bikes at the Concours d’ Oilegance, our not-at-all-formal gathering on the Saturday afternoon where we push all the bikes into a pile and gaze at the oil leaks, broken mufflers and occasional shiny bits.
Step one was to get a good digital file of the logo that the Veloce factory put on the headstock of the motorbikes. It is often referred to as The Naked Lady, although she isn’t showing all that she could.
I scanned that image into Solidworks, then built a CAD part with the various lines lifted up into a relief. This was all very simple, but it took about 5-6 hours to get all the little lines traced and sketched, then extruded to a depth and 3 degrees of draft added for the sand casting process.
The CAD file was exported to a 3D printing machine that a friend has. This machine was an “affordable” type that costs around $2,000. Others are available for $500.
3D printing is a fairly new technology that has a lot of press lately. It can be used to make patterns like this, or to make up finished plastic or even metal parts. Some printers can even use materials like rubber or similar to ceramics.
It may be difficult to imagine how the machine works, so I’ll explain it a bit. Imagine your typical office printer on your desk. It feeds the paper in one direction, and the printer head moves back and forth across the paper, dropping little drops of ink in specific places. The paper feeds in a little more, and the printer head moves across again, dropping drops. Eventually the head has passed over every square inch or square millimeter of the paper, and it either drops ink or leaves that spot empty. The drops add up to form words or images. A 3D printer does the same method. But the paper is replaced by a flat plate of aluminum or hard plastic. And the printer head moves front to back as well as side to side. The printer head drops a little bit of melted plastic instead of ink. Once the head has travelled over the full area of the base plate, a motor lowers the plate a small amount, say .007”. The printer head then travels back and forth all over again. Then the plate lowers .007” again, and it repeats. Eventually the little dots of melted plastic form layers of a solid part, with each layer being that .007” dimension.
Check on the internet, there are a number of these 3D printers now available. But the quality of the small features will vary for different machines. (mostly due to the size of the dot of melted plastic, and how small was each step movement of the head). For medium sized parts or medium features, the only concern will be the waviness of the surfaces due to the layer height of .003” or .007” or .020”, etc, and these surfaces can often be sanded smooth as needed.
Or don’t buy a machine, just pay somebody to make your parts. At my day job I’ve used these guys before, and there are many more out there.
That green part didn’t have enough resolution on the small ribs of the relief, so I had another friend make up two on his 3D printer machine. That machine uses a better process with more accuracy, but has a $100,000 price.
The parts were glued onto an old board, and gates and runners were installed. You might recall this stuff from the How To: Spark Plug Holder article.
Here is the backside of the plate. It was then placed into a box of sand to produce a negative gap with the specified shape. Another box of sand was used to form the backside surfaces. Then the board was removed and the two boxes were placed together. The empty hollow spaces in-between the two sand boxes were filled with molten red brass to make two trophies. Here is a link showing more info on sandcasting.
The kids and I spray painted the castings, then wiped off the high edges with a rag soaked in paint thinner. A quick drill and tap operation made threads, and another volunteer made up oak bases and brought some screws and spacers to the rally. K ordered some lettered plaques from a local trophy shop, and we were ready for some cold beverages.
I’d like to give special thanks to Terminal Brass and Foundry for their work on the wood plates and the castings. And to Derek at Moss Machine for the super fine 3D prints.