Week 10: Make something BIG

Assignment

Make (design+mill+assemble) something big (~metre-scale)

This week I learned how to use the ShopBot CNC machine to try and make some furniture for the first time. My initial idea was to make something that could be a fun home for my plant-satellite dancing project (code-named Ground Control).

I share an office with Jack Forman, so I had a great example of an output from this very week (the bunk bed he built). I looked at the joints and the way that it was fitted together which came from an open-source bunk bed building website.

So with this as inspiration, I made a test joint file during the group excercise. We were told we would be using 0.5 inch plywood stock, so i modelled a system with slots that would accomodate this thickness. During the training we tested some dogbone style joints that seemed to work well with slots of 0.5 inches and 0.49 inches. So I stuck to my initial design.

I cut the parts by operating the ShopBot following the instructions provided by Alfonso here

Going over the main points:

• We were given 4x8 feet plywood stock (0.5 inches thick).
• The tool we were milling with had a diameter of 0.25 inches.
• We would run two processes; a downcut and an upcut.
• We used a Fusion Tool Library: CBA Shopbot.

Design

In Fusion, using the design from Better Block as a reference, I created a new version to fit onto 2 sheets of plywood and to work with the thickness of the wood we were provided (0.5 inches).

For the design of the slots, I revisited earlier weeks’ work using offsets to ensure that slots could be adjusted parametrically. Once done, I outputted the gcode for the machine within the manufacture section of Fusion. Here’s a step by step instruction of what I did to create the g-code:

Steps to Create the G-code:

1. Switch from design to manufacture.
2. Create a new setup.
3. Select bodies to cut (hide the stock during selection)
4. Set up coordinates: the CNC we used models x (long dimension) and y (shorter dimension) which also needed to be the setup in Fusion.
5. Setting the z-axis to the correct orientation
6. Within ‘stock’ tab, added the stock via ‘stock from a body’ or equivalent literally selecting the stock. This should automatically populate the dimensions of the stock (verify)
7. Set the origin coordinates to the top right corner of the stock (imagining the 0,0 of where you want to start your cut)

Next we run a 2D Contour Operation for our Downcut

• First tab:
  • Select the tool imported from the library.
  • We set the cutting feedrate to 150 in/min.
• Geometry tab:
  • Create a ‘closed chain’ selecting the top contours only (e.g., the top edges of the bodies).
  • Select each path one by one. Zoom in to see what it’s cutting and make sure it is inside or outside of your shape as needed. For the down-cut it is not necessary to be precise on the order of cuts, but it’s good practice to always do inside cuts first.
• Heights tab:
  • Set Bottom Height value as -0.2 in.
  • Check values are as below:
• Simulate:
  • Once done, we simulate to check that we’re not cutting the whole way through.
  

Next we create the operation for a 2D Contour for our Upcut

• First tab:
  • Select the tool imported from the library (should already be done).
  • We set the cutting feedrate to 150 in/min.
• Geometry tab:
  • Again we select closed chain but this time we select the bottom edges of our bodies.
  • This time be sure that inside cuts are chosen first.
  • We add tabs (manually) to stabilize the cut, especially on smaller pieces.
  • The default tabs are probably not wide enough, so make them wider!
• Height tab:
  • We also modify our Height tab for this operation to ensure path planning does NOT occur in the area where material has already been removed:
  1. Top Height: -0.2 in
  2. Bottom Height: -0.025 inches
• Passes tab:
  • Add multiple depths.
  • Click on preserve order (maintains the order of the chain).

We then export each process individually as its own pathplanning.

• To do this simply right click on the operation, select post-process.
• Be sure to also select the post-processor as the ShopBot Open SBP.
• Do this for both files.

Next we head over to the CNC. The software for the ShopBot is a bit fussy. Run operations slowly. I found that talking through each step out-loud helps to avoid moving too quickly and overloading the software. Open the yellow dialogue to move things around.

1. Put on safety equipment.
2. Add the stock to the bed and hold down with screws.
3. Insert a downcut endmill.
4. Open the yellow dialogue and zero the X and Y axes using the software (run z2 in the grey part of the software).
5. To zero the Z axis:
   • Take out the metal probe.
   • Connect the metal contact to the endmill and run C2 on the ShopBot software.
   • Once done, put the probe and contact back.
6. Put the shoe on the mill to clear up as you cut.
7. Load the g-code for the down-cut and hit cut.
8. Turn on the extractor, put on ear protection, and start the spindle.

Once the job is done, change the end-mill. The end-mill should be correctly inserted to avoid breakage. Insert enough of the endmill to make sure that it doesn’t snap off, but not too much so that it is above the ends of the shoe.

Assembly and Finishing

I measured my wood before cutting to be 0.45 inches thick on average. So in my Fusion file, I set the joints to 0.46 inches (based on the testing this seemed appropriate). After cutting, I found however that my joints were extremely tight. Of course, I was only allocated 1 piece of plywood, so I could not make a recut which meant a lot of sanding!

I sanded the surface of the wood to give it a good finish and also reduce some of its thickness. I also sanded joints by hand to allow pieces to fit together better. This process allowed me to create the base for my design which was very stable as the slots were very tight.

To complete my design (as it required 2 sheets), I also managed to get some additional pieces of wood by using parts that other people were not using. The main pain point was trying to fit everything into weird shapes in Fusion! For the remaining slots, I made everything 0.49 inches, which worked well and allowed things to slide in and out easily.

I was then able to get some plants from around Media Lab to fill my planter for the demo. It now lives by the TMG window where hopefully it will cultivate some plants.