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3D modelling for multi-mode fabrication

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Combining additive and subtractive manufacturing with DesignSpark Mechanical

DesignSpark Mechanical is a powerful, free-to-use CAD package from RS that allows you to design in both 2D and 3D, as well as incorporating over 40,000 models of parts from the RS catalogue.

In this post I draw and fabricate a simple assembly consisting of laser-cut acrylic sheets and 3D printed joining parts, covering tools and techniques within DesignSpark Mechanical (DSM), designing and exporting files for manufacture.

It is assumed that you have DSM installed on your computer, if not, the installation files can be found here. It is recommended that you work through some of the tutorials to gain familiarity of the software and it’s capabilities.

Sketching out

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I decided that designing a cubic assembly with raised corners would be a great way to include both laser cutting and 3D printing in one project. With a new design open, I selected Sketch Mode and drew a square using the Rectangle tool. Starting by clicking and dragging, I could type in the dimensions to accurately size the square. I chose 50mm x 50mm as a nice size to work with.

With this sketch drawn, I could go back to 3D Mode, and use the Pull tool to give the sketched square a thickness. I want to use 3mm thick Perspex sheet for design. This 3mm sheet can have a 10% swing in thickness to still be within specification, meaning that a sheet can be anywhere from 2.7mm to 3.3mm thick and considered OK.

My aim is to model the flat sheets of acrylic and use them to carve, or cut slots in the parts that will make the corners of the cube. These corners will be 3D printed and hold the laser cut acrylic sheets in position with a push fit.

Bearing the variation of Perspex thickness in mind, a decision needed to be made regarding how thick to make my modelled sheets. I could opt to err on the side of caution and make them 3.4mm thick, that would make a universal design to allow any ‘3mm’ Perspex sheet to fit, or I could measure the sheet I intend to use, and work from there.

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I decided to make the design universal, and used the Pull tool to drag the rectangle and make the sheet 3.4mm thick. Once this operation had completed, I had a ‘Solid’ in the Structure panel in the top left of the display area, and a 3D model in the main drawing area. I renamed this to ‘Bottom’.

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To check that the shape was dimensioned correctly, I used the Measure tool, then the Dimension tool to add a visible dimension. These tools are handy to keep track of your design and make changes as you go along.

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Within the Structure panel, I selected the ‘Bottom’ solid and used ctrl-C, then ctrl-V to copy and paste a second version. This could then be moved up using the ‘Move’ tool, and renamed to ‘Top’. Next, again in the Structure panel, I selected both and moved them to a new component, which I named ‘Top and Bottom’.

I then copied this component of two parts, before using the Move tool to rotate them about 90 degrees in one plane, and renaming them as ‘Left’ and ‘Right’.

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I repeated this to give my final two sides, the front and back. Note that these names are of course dependant upon the viewing angle, and mostly useful for reference. Once I had done this, I realised that I had three components with the same name – Front and Back! It turns out that I was making clones of the first component – meaning that when I edited one, all three changed.

This is OK for my current design, but might be something I wanted to avoid in other circumstances. The way to do this is to right-click the component within the Structure panel and choose Source > Make Independent.

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I saved my progress before continuing. Next up was the task of creating the corner pieces. At first, I was tempted to use spheres for the corners, as they are interesting and something quite far removed from something that can be made in the laser cutter. However, I felt that they may not print very well, due to the large amount of overhang and lack of flat base upon which to print.

So I changed the design to using small cubes at each corner. These had to be big enough to accommodate slots for the sheet material in each side, so I used the transparency option to see through the sides and make sure that there was plenty of the corner piece on either side of the sheet sides.

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To centre the corner cubes, I used the ‘Measure’ tool to work out the gaps between each side, so that I knew where to locate the centre of the cube. I used the sketch rectangle tool again, this time selecting the Define rectangle from center option, to make positioning easier.

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With the sketched square centred in the correct place, I used the Pull tool to pull one face to half the cube’s size, then a second time on the reverse side, keeping the object centred as before. After double-checking the centre was correct in all three planes, I duplicated and moved the cube to each corner.

Again, I saved the model, this time using the Save As > Save As New Version menu. I next tried to use the sheet sides to cut out slots in the cube corners. First, clicking the Combine tool, then selecting one of the corners, each one was highlighted, as they are all linked objects. When selecting the sides as the cutting objects, this caused problems as shown in the screen shot below.

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To solve this, I made each corner independent, then tried again. Each cube was selected, then the sides used as cutters, with the remainder pieces chosen to be removed. After cutting each corner, I moved them a known distance (I picked 30mm) away from the main assembly, to keep track of which cubes had been cut.

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With each corner piece now modified with slots for the side panels, I saved a new version of the file. Happy with the design, we can now look to exporting files for laser cutting and 3D printing.

Firstly, for 3D printing, since each corner is identical, it makes sense to export just one and print it multiple times.

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Using the Structure panel again, I made all parts except one corner invisible by clicking the check boxes to the left hand side. With only one part visible, this will be the only part exported when using the Export Options’ > ‘3D Print (*.STL) menu.

Exporting for laser cutting requires more attention, as the output depends on the viewpoint in the drawing area. To simplify this, I made sure that only one side piece was visible, again using the check boxes in the Structure panel, and used the ‘Sketch Mode’ and ‘Plan View’ to ensure the viewpoint was perpendicular to the design.

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I then exported the file using Export Options’ > ‘2D AutoCAD (*.DXF) menu. Now the exported files can be opened with different software packages before fabrication.

3D printing - Cura

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Cura is a free software package from Ultimaker, a company that sells a range of easy-to-use, well developed 3D printers. It allows you to position your 3D model on a virtual bed, re-size it if necessary and configure your print settings, before exporting a file suitable for the 3D printer. More information on this process can be found in a recent post, as well as on the Ultimaker website.

The 3D model loaded into Cura without trouble, and after scaling the model up to 105%, and duplicating it so that I could print two models at once, I used a low quality print setting for a quick result and started the print. Note that more detailed instruction and information on 3D printing is beyond the scope of this post, and I would recommend the Ultimaker forums as the best place to start when looking for advice and tips on improving your 3D prints.

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Despite some printing anomalies, I was generally happy with the quality and form of the printed parts, and so continued to print the remaining six parts using the same file as the first two.

Laser cutting - Inkscape and Laser Cut 5.3

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I often use Inkscape for drawing 2D designs for laser cutting, and since it is a known process to me, I opened the file exported from DSM to check the dimensions. This showed the square sides to be 210mm – not 50mm as expected! However, dimensions can sometimes be lost when importing files into Inkscape, so all is not lost. Importantly, there is a single square in the design file, as expected.

I next imported the file into the software used to control our laser cutter, Laser Cut 5.3, to see if the dimensions carried through to there, which they had, meaning I could cut the parts without needing to make any changes.

If I had needed to make any changes within Inkscape, I could have simply scaled the entire design file from 210mm to 50mm without too much trouble.

Assembling the parts

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With all the parts fabricated, the protective film peeled from the laser cut acrylic and the excess filament tidied up from the 3D printed pieces, I could push the pieces together. Having some understanding of 3D printing and laser cutting, how each process works and the results you can achieve allows you to design with some foresight.

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For example, expecting some droop on overhanging parts, and some slight shrinkage with 3D printing, I had made adjustments to my design before printing, most specifically with making the slots wider than they needed to be. Though printing is still reasonably quick, roughly fifteen minutes per part, it is helpful to keep rapid prototyping as rapid as possible, avoiding re-designing and re-printing if possible!

Conclusion

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DesignSpark Mechanical is a free and easy to use CAD package that makes drawing parts and exporting files for both laser cutting and 3D printing quick and simple to do. This example project should give you some helpful tips and hints towards getting your own projects made, for further information, head over to the DSM reference centre, follow some of the tutorials and have fun experimenting!

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maker, hacker, doer

19 Sep 2016, 17:39