Design Parts
After rough sketches on index cards, it is time to make a more exact plan. We do this in Aspire. One approach we’ve been experimenting with is laying out the project in a block diagram where the positions and dimensions are are mapped into a master template for each type of piece. By removing and adding pieces of the template, the final part can be created with the correct tabs, slots, and other details.
In this project, we have a number of actual parts to layout. The process of figuring out the parts we need and creating the templates took about 16 hours of design time. The templates below are for Ext X, Int X, X Ribs, X/2, Ext Y, Y Ribs, Top Face, and Caster Legs. (Hopefully with practice we can explain this process better.)
Plywood Forklift v1 0 Layout
By adding and subtracting the rectangles in the topmost template in the diagram above, the final Ext X part can be created. Two of this part form the exterior sides and legs of the project.
Plywood Forklift v1 0 Ext X
Layout Sheets
In Aspire we put each part in it’s own layer. Once all the parts are created, we make copies of the those layers to build up the sheets for all parts in the project. Since sheets represent actual pieces of material, we spend time nesting parts to use as much of the material as possible. Below is Sheet 1 of the project that includes 2x Ext X, an Int Y, and some Caster Leg components.
Plywood Forklift v1 0 Sheet 1
Laying out sheets added another 6 hours to design time.
Test Design
At this point we used to cut the entire project and see if it works. In carpentry there is a saying “Measure Twice, Cut Once.” As amateur wood people, we are usually unable to repair the mistakes we make. So, we treat it like software and fix the code and run it again. This means so far we have cut every project at least twice.
Recently we have been able to insert a laser cutter into our process. Our friends at Venture Lab loaned us their 50Watt Epilog. Instead of cutting full sheets of OSB, we cut prototypes in 3mm fiberboard. We bought the fiberboard from Roddis in full sheets for $7 and cut them into 12x24in pieces to fit on the laser.
Laser Cutting
To cut with the laser, we just exported our designs as SVGs, brought them into Corel Draw and “printed” them. With an Epilog a special print driver cues up jobs to be sent to the laser. The first difference about using the laser is that the “kerf” of the laser is practically zero so it can cut sharp internal angles. With a router, the “kerf” is a quarter inch so parts have to be filleted so parts can fit correctly.
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This is why we test
The parts were cut to spec. Everything fits. There is only the small problem that what works on the screen cannot be assembled as drawn. The internal ribs that form the torsion box are blocked by the other parts. FAIL.
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Staring at the problem for a while, we grabbed a pair of flush cutters and were able to whittle away the issue. Only one part needs re-designed.
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X Rib Re-design
To fix the X Ribs, we need to reduce their length to avoid the mid section of the frame and round the leading corners of the fins on the ribs. The original X Rib is on the left, re-design on right.
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After cutting the new design we were able to test again. The X Rib slips under the Int Y frame to lock and lays down over the Y Ribs.
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Scale Model Complete
Cutting the rest of the pieces, our scale model is assembled and working. The design of the Top Face needs to be done, but the hard part is finished.
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Full Scale Test
As we learned on another project, just because the scale model works doesn’t guarantee the full-size project will go perfectly. Again from software, we want to fail fast so we test the most complicated part first. To test at scale we slice a 24inch corner section of the project. This shows us if the new X Rib design and the supporting parts will connect correctly.
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Since OSB can swell with humidity, the fit is snug but it works!
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Next
With what we’ve learned so far, we’ve updated the designs. Next we will define the toolpaths and start cutting the project on the ShopBot. It will take a few hours to machine the 7 sheets of OSB.
