Tuesday, December 23, 2014

FreeCAD

I've been playing with FreeCAD for a few hours, since I need to make little Nylon brackets and things at Shapeways.

The program is absolutely amazing for its price - Zero Dollars - which is a nice way to say that there are a few hiccups, but once one knows how to avoid the hiccups, then it works perfectly... :)

Antique Radio Ham Speaker Mic, circa 2015

To aid with error checking of CAD files before uploading them to Shapeways, also install the free Meshlab.  Note that uploading a file to Shapeways takes a while, because they also do a conversion and error check, so be patient.  If you checked it with Meshlab, then it should work.


I made the above widget by drawing four cubes, resizing them and subtracting  three from the other, then subtracting four cylinders for the mounting holes and finally applying fillets to the edges.  That's all there is to it.  Made out of Nylon, it makes a strong base for a little step motor.

In general, I would say that FreeCAD is almost as good as Solidworks.   It is certainly good for hobby use and it may even be good enough in a small business for simple things.

The nice thing about FreeCAD, is that it is cross platform and it works exactly the same way on Windows, Linux and Mac - it even has the same bugs too...

Here are a few pointers on FreeCAD version 0.14:

The smallest features that Shapeways can produce is about 1 mm.  Widgets may also be somewhat uneven and may require sanding/polishing, but the resulting whatchammacallits are generally fine for prototype and hobby use as is.

If you keep the thickness at 1 mm, then the cost is about 50c per 10 square cm for the cheapest Nylon.  This schtuff is very strong and slightly flexible.  If you need it to be stiff - make curved surfaces or add some ribs.

The only CAD export format that I can get to work consistently with Shapeways, is STL, but FreeCAD cannot export to STL.  So, I Select All, then export to WRL, import that file into Meshlab and export it again to STL, then upload that to Shapeways.

To save a model in WRL, Click Part Workbench, Edit, Select All, Ctrl-E, then type filename.wrl, SaveThere is more information down below on how to make holy or unholy shapes...

I wanted to make an old fashioned Radio Ham handheld/standing Microphone/Speaker/PTT gadget, that looks like it was stamped out of brass, circa 1920 or so.  Here is the base:




It is also recommended to check the final file with Meshlab before sending it to Shapeways.  If the file is full of holes, or if the Shapeways uploader complains about the file being unusable, change the maximum mesh edge length to something smaller and try again as explained down below.

If you make a drawing using the 3D add/subtract method in the Part Workbench, then the whole hoosammawatzit seems to remain fully constrained all by itself.  This is a wonderful way to make a little wazzisname consisting of rectangular shapes and cylindrical holes very quickly and is the drafting method that I can recommend to occasional users.  Follow this tutorial: http://www.freecadweb.org/wiki/index.php?title=Basic_modeling_tutorial

If you make a watchammacallit using the 2D drawing and extrusion process in the Part Design WorkBench, then you have to keep an eye on the constraints box and ensure that every single line or shape is fully constrained before moving on to the next line or shape.  It is almost impossible to fix missing constraints later, but you may be able to find a loose end by clicking and dragging lines with the mouse - if it moves, then it isn't fully constrained.

The best way to draw a fully constrained line in the Part Design Workbench, is to draw the line close to where you want it to be, but not exactly.  Then click the end point and the point where you want it to be and apply a constraint (a dot 'coincident', or 'fix point to object' or some such), to make the line end point jump to the right spot.  You typically need to apply two to three constraints to get it exactly where it should be.  Watch the Constraints box on the left.

Here is the Speaker front and the tiny little hole at the top is for the Mic (The backside looks similar but without the rose of holes):



So now I can put my $20 of amplifier electronics inside a seemingly century old custom plastic enclosure costing $80 - ouch.  It is not a cheap hobby...

When making plastic/metal nurgles, it is important that you fillet/chamfer the edges.  Stress build up at edges can cause a plastic mcgafter to crack and metal thingammajigs may cut your fingers if you don't.  The fillet tool is a little unreliable, so save oftenDo this step last.

Sometimes (not very often!) FreeCAD exits, or the drawing disappears from view, leaving you rubbing your eyes and reaching for your mug of coffee in frustration.  The only solution is to restart it and get yet more coffee.  It is therefore important to save your work regularly and use an incrementing number for simple version control (filename-1, filename-2), so that you can go back to the last thingammabob that worked.

Manufacturing Issues At Shapeways

I ordered the above parts in black plastic and what I got, was white plastic, painted/dyed black.  That is not the same thing at all.

The problem being that if you have to rework a part then you need to repaint everything, so you can just as well order the parts in white plastic to begin with.

Some features that were supposed to be hollow, turned out solid.  All the carefully designed mountings for potentiometers and the speaker grill, came out solid.

So, be prepared to do some rework with a drill/dremel/rasp to get things into shape.  The white plastic of course melts if you use too much speed, but if you take it slowly, then you can drill holes and cut pieces to shape.

Even with rework, the final result should be much more satisfying than housing a project in a traditional tobacco tin or soap box!

Holy Shapes

I finally figured out how to make holy shapes.  The problem is that Shapeways uses mesh files, which are very simple things.  When you convert a CAD drawing to a mesh, much information is lost.  If the mesh is 'too coarse', then some legitimate holes in your object may get filled in by the automatic hole filling software that Shapeways uses and the object then becomes rather unholy...

The solution is to go to the Mesh Workbench in FreeCAD, select the whole object, then go to Mesh, Convert Object to Mesh and select a maximum mesh edge length of 1.5 to 2 mm, instead of the default 7.8 mm.  Then when you export the file to a mesh (Export as WRL file, then load that into Meshlab and export as STL file) the resulting object should be holy.  If FreeCAD quits unexpectedly, it is because it ran out of RAM while meshing - then you need to get a better computer.

Finishing

I finally got the Speaker Mic together and stuck a pic at the top of this page.  I burned out my Dremel in the process.  I have found that Dremels are really pathetic, having burned out two of them in 5 years working on plastic and balsa wood - need to find something better.

Shapeways offer many different materials apart from cheap 20c per cm3 plastic.  If you need something strong, then you could use steel at $5 per cm3, but reworking a steel part is difficult, while brass and bronze are expensive at $16 per cm3, but they now offer Aluminium at $7 per cm3, which is better.

Note that if your object quoted price turns out to be eye wateringly expensive, then the automatic hole filling software probably made some parts solid that should have been hollow.  In that case, go back and remesh with a smaller side length.

La voila!


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