7. Computer-controlled cutting¶
Design¶
For this week, I decided that a shelf would be helpful furniture, so I designed a shelf with circular tables in Fusion 360:
To design this shelf, I first made a sketch for the “leg” part which the individual shelves would rest on, and then extruded that, using a circular pattern to duplicate the same piece four times.
I also used the parameter menu in Fusion to define the parameters such that they could be easily changed later if I needed to. This included the dimensions for the circles and the dimensions that defined the shape of each wood leg.
I then created circles on offset planes to extrude them up, projecting the legs’ faces into the circle sketch so that the circle would properly be able to fit in here:
After repeating that three times, I moved to the Manufacture workspace in Fusion in order to access the Arrange tool, which would automatically lay out each body on a sheet to be cut. In this workspace I created a Manufacturing Model for the arrangement, the purpose of which was to allow arranging each body and then later making changes to them. This effectively acts as a separate copy of the model.
Then, I used the Arrange tool, converting each individual body into its own component, selecting the components, and setting the relevant settings for the size of the wood sheet (48”x96”) and spacing between components:
Finally, to convert the arrangement into a 2D vector, I created a sketch at the bottom of the model and projected each part into the sketch. I then selected the sketch in the fusion browser (at the left) and exported it as DXF. I made sure not to export the entire design as DXF, but just the individual sketch, to prevent the vectors being duplicated.
CAM¶
To actually be able to mill the design, I had to use CAM software to convert the vector file from Fusion into a toolpath for the ShopBot machine. Our lab uses Vectric Aspire for this purpose.
To start, I opened Aspire and set up settings for the job. The job size was set as 96” x 48” from the wood size. The thickness was 0.48 inches based on what I had measured initially, but I had to revise this to 0.45 inches later after getting more accurate measurements. The other settings (Z Zero Position, XY Datum Position, etc.) were either all based on the specifics/conventions of the lab CNC machine or the Aspire defaults.
After that, I imported the file:
Obviously, this was not oriented correctly. Most likely this was caused by a mismatch between X/Y/length/width between the Arrange setup in Fusion and Aspire. I decided to fix this by just using the Rotate tool in Aspire (at the Drawing tab at the left):
Moving it so that the cut path was actually in the wood:
I had to manipulate the drawing a bit more in Aspire by adding dog-bone and T-bone fillets. Basically, the CNC machine cannot mill internally 90º corners (or 270º from the perspective of the wood, I suppose) because the bit removes a circular area from the material. Instead, it produces some curving around the corner. This meant that if I had proceeded without modifying the holes and corners, I would essentially be trying to fit square pegs into round holes, and things wouldn’t fit. So to make the holes capable of being able to have the wood legs fit into them, I used the Fillet tool in Aspire, set it to T-Bone, set the bit radius (3/8” diameter bit, so 1/2 of that is the radius, which is 0.1875 inches; this is necessary because otherwise Aspire would not know how big to make it), and then used it on all the holes. If I had used the Dog-Bone fillet on the holes rather than T-Bone, then the two dog-bone fillets on the near pairs of corners would intersect, and it would actually increase the length needed for the piece to fit in, resulting in a less smug fit. I used the Dog-Bone fillet on the necessary corners for the wood leg parts, though.
Now that the drawing was done and accurate, to actually get Aspire to generate the path for the CNC to follow, I used the 2D Profile Toolpath. The CNC bit I was using for this was the 3/8” compression, which I made sure was selected (The 2D Profile Toolpath was found in the Toolpaths tab on the right; also, note on the following image: the cut depth had to be changed to 0.45” later):
I actually had to create two separate toolpaths due to how the design was set up. Basically, the inner holes on the circles needed to be milled with the Machine Vectors setting set to “Inside”; this would ensure that the inside distance between the edges inside the holes would measure to be the same after it was cut, compared to the design. For a similar reason, everything else needed to be cut as “Outside” so that the CNC would remove the right amount of material.
For everything but the inner holes, I needed to add tabs so that the individual pieces of my design did not separate from the rest of the wood while cutting (this would have been bad). The inner holes did not actually leave any wood behind in them, so they did not need tabs (the tabs would not connect to anything). In the 2D Profile Toolpath settings, for the toolpath containing the outside cuts, I clicked on the tab settings, and set the tabs to “Constant Number”: 7, then used “Add Tabs”. In the following image the tabs are visible. I determined that 7 provided a sufficient amount of tabs.
For passes, the ‘rule of thumb’ is each depth increase per pass ≤ the tool diameter. Since the bit diameter was 3/8” = 0.375 in, I decided to have two 0.25 in passes.
At this point, I had realized that my previous measurement of 0.48 in was inaccurate since the wood was 0.45 in thick, so the toolpaths and job setup had to be modified. After modifying, each pass was 0.225 inches down.
I also previewed the toolpaths to make sure nothing was wrong with them:
To actually get the file to use with the shopbot, I selected both toolpaths (on the bottom right) and then hit the save icon, using the shopbot postprocessor to save the sbp file for the toolpath.
Milling and assembling the shelf¶
To start on using our lab’s ShopBot Alpha ATC machine, I got the piece of wood I was using and placed it on the ShopBot machine. Initially I made a mistake with the way the wood was placed (I placed it upside down relative to the way it bent, so it wasn’t actually flat on the table) so I had to take it out and turn it around to fix that. Since the wood was so large (48” x 96”) I had a friend help me transport it from the shed to the shopbot machine (thanks Collin!) while also wearing gloves to make sure we didn’t injure our hands.
To make sure that the wood wouldn’t fly off in random directions once the CNC started to cut it, I used the pneumatic brad nailer in order to fix the wood. (Here’s an image of it:)
Also, here’s a close-up of one of the nails that held the wood in place. I distributed enough nails in places on the edge so as to not conflict with the cut and also hold the wood.
To start the cut, I followed the workflow/routines documentation that our lab has: https://docs.google.com/document/d/13KMj7j8VIOVJ1x5KJyU1tCYY6a3ldDfN6Uc6bT5ok20/edit I also got help from Mr. Nelson and Mr. Dubick throughout the process to make sure I was doing everything correctly.
I started off by jogging the CNC spindle home, then using the JZ1.5 command to set the Z zero one inch above the top of the wood. This was so that the job would run an “air cut” to make sure that the CNC machine would follow the correct path without actually touching the wood yet. Then, I continued by loading the file from the Part File Load command in the ShopBot Command Console:
After going through the Part File Load menu and selecting my file (the sbp file which I had previously exported from Aspire), and running the air cut, I stopped it midway after verifying that the path in the air above the wood seemed similar to the intended cut path. Then, I used the C3 command to rehome the axes. Despite the command name in the ShopBot Command Center only mentioning the X and Y axes, it also reverted the earlier change of the Z-axis zero, so that the next cut would run and cut through the wood.
Running the job went well. Here is a photo of the wood while the CNC machine was cutting it:
And here is a photo of the wood after the CNC machine finished the path:
Assembly¶
When I first tried to assemble the shelf, the pieces of wood were not fitting correctly in the slots, since I had designed the slot and the legs to fit exactly together in the design without leaving any space. This was a mistake and I should have left some tolerance in the design; Mr. Nelson advised that I use a belt sander to fix it, so I used that and got the pieces to fit in the slots. Still, assembling the shelf was somewhat annoying as I had to move it around a lot and often hammer the pieces in. I also rotated the shelf around a lot to prevent it from falling apart.
Here is how the shelf looked halfway through assembly. At this point I had gotten the bottom platform put in. Putting the middle piece in was the hardest, since the legs were prone to unaligning themselves and all four had to be aligned correctly.
At this point I had to start hammering the circle down, as it would not fall down by itself due to friction:
Getting the top piece on was a bit easier. This was the final, assembled, shelf:
Also: Fusion 360 has an option to export a file as a USDz file. (link to Autodesk blog post about this!) Opening this file on an iPhone will bring up the iOS AR viewer app which lets you see the object in a real scale. I decided to export this file from Fusion and AirDrop it to my iPhone so I could see the AR version of the shelf (right) in comparison to the final assembled shelf (left), which was cool to see.
Files¶
Here is a link to my relevant files from this week. This includes my Fusion 360 design file and my Aspire file.
Group documentation link¶
Reflection¶
This week, I learned a lot about how to use the ShopBot Alpha ATC machine in our lab, Vectric Aspire, and the ShopBot Command Console. I also discovered some nice Fusion 360 features (Arrange, USDz export, Manufacturing Models) and learned how to use a belt sander. I did run into problems during this week, but the most significant and annoying one was the lack of tolerance in my slots for the shelf design, which resulted in lots of extra time and pain during the assembly stage, so in the future I will have to remember to consider assembly in the design process. Overall, I feel a lot more confident in my ability to correctly use and run the CNC machine and build large-scale structures using wood.