Introduction: Boston Terrier Challenge Coin
My brother's birthday was last month, and he just adopted an adorable little Boston Terrier, so I decided to make a series of birthday gifts featuring the puppy.
I've been experimenting with copper and brass soldering recently and decided a coin shaped like his dog's face could be a fun way to use some of the CNC equipment and try out some metal inlay.
I used some 1/8 inch brass plate and .05 inch coper for this project as well as Stay-Bright solder and Stay-Clean flux. For fixturing of the brass on the CNC mill I used a block of scrap aluminum and some 3M double sided tape. More on that later.
As far as equipment, I used ProtoTrak CNC mill, an OMAX water jet, a butane torch and a series of files, sand paper and polishing wheels.
Modeling and CAM were both done in Fusion 360.
Step 1: Design
I designed this coin in Fusion 360, where I do almost all my CAD and CAM work. The design is based off of a logo I found on google that I imported as a canvas. Sketching on top of that canvas and using a mirror line in the center gave me the basic shape for the dog's face. Two extrude commands turned that into a 3D shape.
Since I knew I was going to be using a mill for the pockets and a water jet for the inlay I added a .05 radius on every corner. That way the water jet copper bits would fit without any hand work to adjust them. If I hadn't included that radius, then the copper would have had a sharp corner, but the milled brass part would not because of the round cutter used to mill it. The smaller tool, the closer to a sharp angle you can get, but then it's a trade off between geometry and time. 3/32 seemed small enough to me.
Step 2: Milling
The bulk of the coin is made from some brass 1/8 inch thick stock I had in the shop from an old project.
I've learned the hard way that fixturing thin material can be a pain in the butt. First off if you put it in a vice and clamp too hard it's likely to bend. Additionally, for this part, I wanted to cut a contour all the way around the piece. In the past I've either left tabs or left a thin layer of material all the way around and filed it off by hand. Both of those methods work, but for this project I wanted to try something new.
I'd heard of using super glue for fixturing, and figured that a good double stick tape could work just as well. I cleaned a scrap piece of 1/2 inch aluminum plate and the brass stock with acetone and then stuck the two together. The brass was slightly over sized, so I was able to use the offset edges to keep the work parallel in the vice.
Using a 3/32 end mill at 6 in/min I was able to cut out the pockets and the contour of the dog face. On the first attempt, I used mist coolant, but that piece failed when the coolant dissolved the adhesive on my double stick tape.
My advice for tape fixturing is go slow, get really strong tape, and don't use coolant.
Step 3: Water Jet
Name one thing cooler than a water jet cutter, I dare you. 60,000PSI of water cutting through metal! I use this machine almost every day for work, and it still blows my dang mind.
The copper bits for this piece were cut on the OMAX Micro Max water jet. Unlike most water jet machines, this one does not submerge the work in water before cutting, and uses plastic blocks instead of metal slats to rest parts on for cutting. The idea is that small parts don't get lost in the catchment tank. In reality, it works pretty well, but when the goal is a 3/16 inch dog snout, I lost a couple in the process. Aside from that, the only real issue I had was dialing in the offset to get a good fit with no gaps when I assembled the copper with the brass.
Step 4: Solder
Time for some fire. A couple months ago I was practicing copper and brass soldering for fun and watched a video by Adam Savage for inspiration. He recommended some specific solder and flux, so I tried it out. It's more expensive than the plumbing solder I was using before, but holly crap does it work better. Highly recomend Stay-Brite solder and Stay-Clean flux. I also recommend the video from Adam where he makes a brass watering can. I shamelessly copied this project and learned a ton in the process.
Anyway, back to the dog coin thing. Every good project has an ugly phase. This one is certainly no exception, and soldering is the ugly phase. I think I got the flux way too hot while trying to take photos and solder at the same time, but it still worked.
In order to get the best adhesion I could, I started by applying flux and a small amount of solder in each of the four pockets in the brass. I heated this and melted the solder to cover the whole bottom of the pockets first before placing the copper bits in at all. This was in hopes of getting full contact adhesion. After the pockets were fully covered, I heated the coin again and placed in the copper bits. Once in and hot, I used pliers to squeeze them into the bottom of the pockets. Some solder leaked out from the edges, which I think is actually ideal in terms of filling gaps.
Step 5: Filing and Polishing
As with many things, the final steps are the most tedious and time consuming. I think I ended up spending over an hour just filing and sanding this coin. Could have probably used a belt saner or something similar, but it's really hard to hold something this small. First started with a file, then ascending grits of sand paper, and finally used a leather knife polishing pad and fine grit polishing compound
Step 6: Ta Da!
Not sure what this thing is good for, but I like it a lot and learned a ton while making it.
Participated in the