Tuesday, September 23, 2014

Scaled 3D print of "ShopBot" project...

While learning how to use a ShopBot CNC router I was looking for a suitable starter project and came across the very popular Patio Chair project on the ShopBot site. It quickly occurred to me that I could assess quite a lot about the design simply by scaling and 3d printing the necessary parts. The conversion to STL file for slicing and printing was relatively straight forward, and is summarised below. I've broken down the steps, aiming the post at Sketchup beginners.

The downloadable drawing was in .DXF format and easy to view in a 2D drawing package such as DraftSight.

My starting point was to import the .DXF into Sketchup so I could use the Push/Pull tool to give each part some depth for printing. I first had to install a Plug-in into Sketchup to enable importing .DXF. There's a well known plug-in called "FreeDXF" which I found would only work with older versions of Sketchup than the free 2014 version I was using, but with some further digging I found I could manually install the plug-in by adding the files manually to the following location... (Windows7)
The plug-in still gave an error when starting Sketchup until I edited the "freedxf.rb" file, replacing ":" with ";" to clear up a minor issue.

When you import a DXF into Sketchup the shapes come in un-filled. This is easily fixed by drawing over any existing line. The part then turns dark grey, showing it is manifolded (no gaps), and ready to be extruded (push/pull tool). To remove internal shapes repeat the "draw over" process, then select the internal shape and 'delete'. There are probably plug-ins that can do this clean-up more quickly but, again, this walk-through is aimed at beginners. 
Selecting the Push/Pull tool, extrudes all parts to a depth of 3/4" to match the material specified in the original CNC cut design. If you have Sketchup set to millimetres you can still specify Imperial dimensions and it will conveniently do the unit conversion for you. Push/Pull to any distance, then simply type .75" and it will take the 3/4" distance and apply it.

To match the original wood design the joints need to be blind dog-bone pockets. Push/Pull the pockets to a depth of 1/2", matching the tenon depths on the chair sides. 
The final preparation step is to scale all parts down to a size that will fit on the 200x200mm print bed. This is best achieved by selecting all, then using the Scale Tool, drag one of the green corners of the yellow box inwards any distance, then type .15 and hit enter. This will scale all objects down to 15% of original size, to fit nicely on a typical desktop 3d printer.
Select and export the finished pieces from Sketchup in STL file format.
Because of the small pockets and little tabs on the objects I found it needs to be sliced at a reasonably high resolutions. I used .2mm layer height with a .3mm width in Slic3r, but you can choose your own slicing software and settings. The important thing is to preview the g-code to ensure you are getting good definition around the pockets and little tabs. Repetier Host is good for g-code preview, as is the on-line previewer gcode.ws. If you experience any difficulty slicing then I find the Cloud based Netfabb STL repair service very good (Netfabb.com, Service... Cloud Service). 

Here's a picture of the printed plate of parts. There was a little but of clean-up needed to be done around some of the tabs and pockets, but overall the parts printed well.

The chair was easily assembled with a dab of super-glue in the pockets and the parts pushed together, checking for square are you go. A trial dry fit is always a good idea too.
The finished miniature Patio Chair is proving to be a novel and popular item. Full credit TJ Christiansen who did the DXF drawings for ShopBot, and to ShopBot for sharing. The design was said to be found on Minwax.com and originally from American Woodworker Magazine. I'm sure it goes back a long way, and was made with traditional methods long before CNC or 3D printing!

Technical note: The parts were printed on a home made Mendel90, controlled by a Panucat Azteeg x3. It was printed with 3mm PLA using a .5mm J-head nozzle, active cooling on the J-head insulator and gentle cooling on the work. I use a glass print surface, cleaned with window-cleaner and a final wipe with vinegar. I have a Helios heated bed, which I set to 60Deg C for PLA printing.
The chair was sliced in Slic3r at .2mm height, and around 50mm/sec, but a much slower first layer.

I've shared my Sketchup and STL files on Youmagine.com should you wish to try print this chair yourself. 

Thanks for viewing!

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