Monday, 1 September 2014

FreeCAD for assembling 3d laser cut shapes designed in 2d in Inkscape

During the construction of our cnc frame (for a pick-n-place machine) we had a number of revelations. The first being that we're actually not that great at realising 2D drawn designs in 3D!
All too often, we'd line parts up and realise they were out by about 3mm - usually the thickness of a piece of mdf, where we'd forgotten to allow for abutting edges or to compensate where one piece meets another.

Rather than go to the trouble of cutting and assembling (and later, trying to fudge, fix, and eventually throw away) pieces, only for them to end up in the bin, it'd be far easier - in the long run - to assemble everything virtually, on the screen.

Steve does this a lot, using 3DS Max. We looked at that a few years ago, but it has far too many buttons for us! What we needed was something simple to use. Google Sketchup is great for layout out shapes into a 3d design - we'd already done this quite a bit with our spaceship terrain a few months back - but it doesn't have the ability to import and extrude your own CAD drawings; everything has to be created from scratch in Ketchup, which is a bit of a chore.

You can import your own drawings in Sketchup, but it requires a Pro licence - costing about £500. So we need something simple, and cheap, to create our 3D master pieces.

OpenSCAD is a popular 3D package used by a lot of nerds and geeks and is great for creating shapes using a programmatic approach, for boolean operations, additions and subtractions and the like. It can import flat svg files and extrude them, to create 3D models from 2D cad files. The only issue with this is programmatically placing each shape gets complicated, very quickly!

So we need something easy, cheap, able to import svg (or other vector formats) and with a simple interface for moving the pieces around.

Luckily FreeCAD ticks all the boxes! Of course, like starting out with all new applications, the interface takes a bit of getting used to, but it's actually really easy to turn a 2D CAD drawing into a 3D representation of the project. To start off, simply import the SVG of the CAD drawing

Now, make sure the drop down menu is set to "Parts" (for parts editing). Without doing this, half of the toolbars either disappear, or are greyed out. With all of the CAD drawing selected, hit the "extrude" button (a box with an up-arrow against it) and enter the height 3mm. The CAD drawing will magically become a 3D image!

Even if the CAD drawing has "removed parts" from planes (e.g. a drill hole cut into a rectangle) these don't come through in FreeCAD. The shapes exist, but they need to be removed, using a boolean operator. Luckily this is easy enough. Select the part you want to keep, then - holding down the control key (not shift, as is common in most applications for selecting two or more parts) - select the part to be removed

Hit the boolean subtract button, and the selected piece is cut out of the main shape. It's a simple enough job, just a little time-consuming if you've got loads of tabs and slots to cut out.

With all the slots and tabs cut out, we're ready to start assembling. Having only Sketchup as a comparison, in which we had to install an edge-align tool as it doesn't have one as standard, we've no idea if how FreeCAD works is typical, but it's certainly intuitive enough to get going really quickly.

Start off by selecting a piece - the first piece selected will remain in place. Then, with the control key held, select a second piece. The second piece will be moved to join the first, stationary piece. With both pieces selected, use the Edit -> Alignment menu option

There is no toolbar for this, nor is it available in a pop-up menu, so you must use the Edit menu. This opens the alignment screen(s).

Now it's a case of stretching your head around which parts will be touching, once the second piece has been moving into its final position (strangely, the second/moveable piece appears on the left of split screen).
In this case, we want our end piece to stand up at 90 degrees to the base. Rather than mess about rotating pieces, you simply select enough points to allow the application to work out the required rotation.

In the example above, we've selected two points, which will line up the tabs on the vertical piece, with the slots in the base. But we need to provide at least one more point, to align the bottom edge of the vertical piece with the bottom edge of the base.

You can align faces, edges or vertices (corners). By selecting a vertex, you're often actually aligning two edges at once, so we tend to stick with vertices where possible - though sometimes it's handy to be able to say "this edge should run along this edge" or "this face should be touching this face" to help get the rotation correct.

After successfully aligning a few pieces, our 3D design is starting to take shape

And finally, we get to the whole point of learning how to use a completely new CAD package, and the reason behind spending hours longer in the design process - it's cock-ups like the one below we're looking to eliminate by assembling everything on screen before sending our designs to the laser cutter

We've done the classic trick of offsetting our tabs by 3mm (the thickness of the mdf) and forgetting to adjust it in other parts of the drawing. This actual mistake cost us two sheets of mdf. Not much in terms of materials costs (it would have been a lot more if we were using acrylic!) but a lot in terms of time and effort (especially since we wasted about half an hour trying to bodge/fix it, instead of just cutting a fresh piece!).

3D CAD is a great way not only to visualise the final project, but also a really useful way of ensuring everything lines up properly before even going anywhere near a laser cutter (or cnc router, if that's what you're using). This is especially useful if you're going to be using slightly more expensive construction materials.

But one of the great things that 3D CAD allows us to produce is a set of assembly instructions. So in the true spirit of sharing designs and ideas, we can now easily share instructions on how to assemble the pieces, with anyone daft enough to download our plans and use them in the first place!

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