Week 06 - Feb 22nd 2012 - Computer-Controlled Machining
Make Something Big!
This week's assignment was to design and fabricate something large using the Shopbot. The starting material was a sheet of 12mm plywood, dimensions 2440mm by 1220mm. I wanted to make some chairs so started by designing a chair in 3d using Sketchup.
Once I was happy with my design, I took each component and laid them flat on a 1220mm square within Sketchup (as I wanted to try and make two). Once I had a layout, I exported it to a .dxf file using this plugin. In DraftSight I edited the file and replaced Sketchup's faceted curves with real arcs.
This .dxf file was ready to load into the PartWorks software. This package generates the toolpaths used by the Shopbot. The workflow was as follows:
- Join vectors. This ensured all the paths were closed, meaning they could be treated as individual items in the software.
- Create fillets. This process ensures a curved remnant is not left inside any slots or holes cut into the material. All fillets were 3.2mm dogbones.
- Select all vectors and create profile. Set cut type pocket, and cut depth 13mm (to ensure the cut goes completely through the material). Select tool and specify a 0.25inch compression cut bit (part #60-100DE). Theoretical feed rate was caculated as 9 inches/sec, however the manufacturer recommended 6.2 inches/sec. In fact a classmate had just finished his Shopbot job and this feed rate had left a fairly rough finish, and the bit was not hot at all, so we reduced this to 5.5 inches/sec to improve the finish.
- Select all holes, select inside/left, climb cut and create path.
- Select all cutouts, select outside/right, climb cut and then add tabs - 3mm thick x 4mm deep. Add 3 at random to each piece then edit the spacing to ensure each piece will be securely held. Create path.
With the toolpaths exported, it was time to set up the Shopbot. The material was placed on the cutting bed, offset from the origin, and pilot holes were drilled for the fixing screws according to placement co-ordinates indicated by the software not to be in cutting areas. The material was then centred on the bed and fixed in position. The cutting bit was zeroed in the 3 axes then the job was run.
After removing the finished pieces from the bed and filing any rough corners and also removing the tabs, the chairs slotted together easily. All dimensions were nominal (ie. 12mm tabs and slots) and gave a strong interference fit that could be assembled by hand.
The chairs are comfortable to sit on thanks to the angled base panels, but a bit flexible. Obvious improvements would be use of a less flexible material, or inclusion of support struts in the design. However, as an excercise in the process of fabricating a design using the Shopbot I was pleased with how this week went. I will never have to buy furniture from Ikea again!