Hannah's Assignment 5: Fabricating & CNC Milling a Box

Design

The goal of this assignment was to design a box with some type of joints and create it with CNC milling. I decided to design a box with fingerjoints. My expectation was for the male and female fingerjoints to match up and be able to pressfit together. I decided to make a square shaped box with the dimensions of 50mmx50mmx50mm. With Joshua's advice about how milling machines aren't as precise, I chose to add dogbones to the corners of the female slots. This will help the toolpath for the 1/32in flat end mill.

Process

Step 1: Designing the box on OnShape
First step was to create the sides of the box on OnShape. I knew I needed to design 3 sides - 1 for front and back, 1 for left and right, and 1 for top and bottom. Using the dimensions I chose, I created the 3 sides. My strategy was to evenly divide slots onto each side: 5mm male part, 5mm female slot, etc. I started with front side. Once that was complete, I started a new sketch for the left/right side/face.

OnShape sketch of front face of box

To be able to visualize the box I chose to use the left plane to make the slots match up at least on one side when compared to the front face of the box. I chose the same strategy. Once I got the side next to the front side to match up, I evenly divided the slots on the other sides of the left face. When that was done, I moved to the top plane to start the 3rd sketch for the 3rd face of the box. I chose to sketch on the top plane to visualize in 3D if the slots match to the front face and left face of the box. I did the same tactic for the slots for the top face as I did for the left face. After I finished creating each side of the box, I extruded each piece.

OnShape sketch of left and top face of box

Step 2: Assembling the box on OnShape
Now that I had the extrusions for each side, I assembled the pieces together on OnShape to see if my design worked. Sadly the slots didn't match up once I added the copies for each side that I made. This frustrated me as I had already spent a few hours creating the sides of the box. I chose to go back to the drawing board and start from scratch.

Failed assembly. Male fingerjoints sit on top of each other.

Step 3: Iterating on the slots and dimensions. AKA do math
I still wanted to use the same dimensions of 50mmx50mmx50mm, but I needed to redo the # of slots and actually do math. To do this, I used paper and pencil to sketch out an edge of the front face and what the slots would like. I then used the same measurements and slot numbers for the remaining edges of the front face. After I finished the front face, I moved on to the left face.

Front face sketch iteration

Since I had evenly divided the slots along the edges of the front face, I just had to do the opposite for the edges that would connect with the front and back. For the edges that would connect with the top and bottom pieces, I evenly spaced out the slots based on the calculations from the left and right edges. After finishing the left face, I moved to the top face.

Left face sketch iteration

For the top face, the edges needed to match the slots that are on the front/back face and left/right face. Using paper and pencil helped me keep the math straight.

Top face sketch iteration

Once each side was completed, I extruded them to the thickness of the FR-1 which was 1.55 mm.

Extruded sides of the box

I tested out my new box design on assembly, and it was a success!

Step 4: Adding dogbones
When I asked Joshua questions about the milling machine, he mentioned how the machines aren't as precise because of the shape of the tools. Unlike a laser cutter, milling has an inside corner problem where it has trouble getting all the way to an inside angle because of the tool shape. He suggested adding dogbones to the corner of the female slots and to make the diameter 1/32in in order to use the 1/32 flat end mill tool. I made new sketches using the extruded sides. On each of these new sketches I added dogbone circles with a diameter of 1/32 to the corner of female slots. Once all sides had dogbones, I extruded these new sketches. I then went into Assembly to see what the box would look like with dogbones.

Preview of box with dogbones

Step 5: Exporting to Kicad and BanTam
With the newly extruded sides that include the dogbones, I exported each side into a DXF file. I used the slide deck for reference on what to do on Kicad to get a Gerber file then BamTam to mill. On Kicad, I created a new PCB project for each file. On each corresponding PCB project, I imported the DXF file, set the origin, then exported into a Gerber file.

Kicad DXF import & Gerber export

I then opened BanTam and opened the front side file first. The tool was automatically set to 1/32in flat end mill. However, I saw red text saying that I would need a smaller tool to target the red areas. The red areas were where I had the 1/32in diameter dogbones. Not wanting to have to switch tools while milling since I had six sides to mill on separate FR-1s, I decided to fix the dogbone sizes.

When imported, message saying a smaller tool than 1/32in needed

Step 6: Fixing dogbone dimensions & re-exporting
I actually used tool sizes on BanTam to help me decide how I should make the dogbone circles bigger. I figured if a 1/32in flat end mill tool can't fully mill a 1/32in diameter dogbone, I should use the next size up as my diameter. The next sized up tool was a 1/16in flat end mill. I decided to try 1/16in diameter for the dogbones.

Adding dogbones

I went back to OnShape and changed the dogbone sizes on each of the sketch files that contained the dogbones. I re-extruded these shapes and re-exported to DXF files.

Extruded sides with dogbones

I followed the same steps to import each file to Kicad then export into Gerber. This time when I opened the Gerber file on BanTam, there were no red marks or a message that told me I would need a smaller tool. Pleased, I was ready to start milling!

Step 7: Finally milling time!
I followed the slide deck to set up the machine and BanTam settings and was able to set up easily with no problem. I first opened the Gerber file for the front and back sides of the box. I used my calipers to measure the width, height, & thickness of the FR-1 piece and entered the information in the appropriate area on BanTam. I had an extra FR-1 board that I chose to use as my sandwich piece for all the sides I needed to mill. I made my FR-1 sandwich, measured its thickness, then calculated for my Z offset. I entered that into the corresponding area on BanTam. I adjusted where I wanted the mill to start in terms of x & y on the board.

BanTam settings for front and back pieces

I applied double sided tape to the bottom of the sandwich and placed it on the platform, letting the edge hang over a little bit to be able to remove the board off the platform when finished. It was go time! I calibrated the tool and re-homed the machine and then clicked "Start milling".

Milling process for front and back pieces

About 10 minutes later, my first piece was done!

Successful first mill!

I have to say removing the sandwich off the platform then separating the sandwich was one of the more time-consuming parts. Once I freed the first piece, I started the whole process again for the next piece. Since my front and back piece are the same, I kept the same file on BanTam and just replaced the FR-1 sandwich. Once I had my front and back piece, I changed the BanTam file to the left and right face of the box. I checked the settings and started the milling process.

BanTam settings & mid-process for left and right pieces

Milling in progress

When the left and right pieces were done, I re-did the same process for the top and bottom pieces. Like before, I checked the settings first.

BanTam settings & mid-process for top and bottom pieces

Then I started milling!

Successful first mill!

Step 7b: One milling issue
Overall, milling went smoothly for me. I was able to use only one tool size, so I didn't have to switch tools during the process. The one slight problem I ran into was my sandwich not being taped strongly to the platform for one of the milling instances. After each piece finished milling, there was a lot of sawdust on the platform. When I was doing the set up process for milling the left/right face of my box, I didn't clear out some of the sawdust on the platform. I taped my sandwich onto the somewhat dusty platform, thinking "Oh it sticks, so it's okay". However, when the milling just started, a few seconds in my sandwich unstuck itself from the platform, and basically swung around in a 180 degree rotation. I hit the emergency power button, rehomed the machine, took the sandwich off the platform, wiped the platform with a paper towel, retaped the sandwich and stuck it to the platform as firmly as possible. Fortunately, this all happened at the very beginning, so I was able to reuse the same FR-1 sandwich. After this incident, I made sure to clear off all sawdust from the platform after each piece, and fortunately didn't run into this issue again.

Step 8: Putting the box together
I started assembling my box each time a piece finished milling. This was my way of making sure I did the right calculations and such. Each piece I created was a success! The front and back pieces connected with the left and right pieces. The top and bottom connected to these pieces easily as well. I was happy that my calculations were correct and that the pieces fit together nicely, yay!

Peer Teaching: Lukas S. for advice on CAD adjustments in order for the pieces to properly fit together since FR-1s aren't pliable like cardboard & Joshua for dogbone advice and slide deck guidelines!