Saturday, 9 April 2016

Routing a guitar fingerboard with desktop CNC

It's been a long time coming, but we finally managed to get our desktop CNC actually routing again this weekend. With a super-whizzy spindle that practically purrs, it's so quiet, it was quite exciting to see our little 1.5mm cutting bit actually carving into the wood, following our Inkscrape designs.
Of which we'll address first.

Creating g-code for CNCs is a doddle with Inkscape. While the software has it's detractors (it's not exactly as nice to use as Adobe Illustrator, nor as fully featured as CorelDraw) for simple shaped-based CAD, it's really easy to use.

We drew our holes and cutting guide lines in Inkscape, then aligned them to make the whole image symmetrical. This way, we can flip the board over on the cutting bed and not worry about it needing to be re-centred, or to add (or subtract) and offset to compensate for it being flipped. Then - and this bit is important, or it might lock up the Python script - make sure all shapes are converted to paths.

With all the paths in the design selected, go to menu Extensions -> GCode tools -> Orientation Points

In the pop-up dialogue window, you can enter the total, final cutting depth of the tool, in the Z-axis.

After this has completed you should see an origin appear in the bottom left hand corner of the drawing. So now we need to add a tool/cutting bit. Menu Extensions -> GCodeTools again, and this time click the "tool library" and select "default".

Have a hunt around. Somewhere on your drawing should be a bright green panel, into which you can enter stuff about the cutting head.

The one we're interested in here is "depth step".
This tells Inkscape how many "passes" to create for the g-code. We're going to be plunging up to 2.5mm deep into our wood (that was the value set when we created the orientation points). But we might not want to do this all in one go. So we've set this panel up with a plunge depth of 1mm.

This means that our pattern will be routed to a depth of 1mm, then repeated, to a depth of 2mm, then finally a third time, to complete the cutting depth to make it 2.5mm in total.

Lastly, menu Extensions -> GCodeTools -> PathToGCode to actually generate the g-code file.

Here we've also selected the option to "sort paths to reduce rapid distance". This basically means that instead of routing the shapes in the order they were drawn, the cutting head will route one shape then move to the nearest - or whichever shape will reduce the overall travel of the cutting head over the entire job.

Just to be sure, we load our generated g-code into the software that used to be called OpenSCAM, and now goes by the name CAMotics.

After splitting our original drawing into two different files, here's our "placemarker" pattern - short cuts at the edges of each fret wire on the fingerboard to help up line up the piece for routing.
Then we drew our holes on a separate drawing, and set them to a much deeper cutting depth

The nice thing about OpenSCAM is that it includes a virtual cutting head. So you can play the cutting animation entirely from your generated g-code; the software not only shows you a 3d model of what your final piece will look like, but also animates the head and can also display the paths taken over the 3d image.

Convinced that we'd done a good job, we finally let our CNC run


And with the actual fingerboard in place, we drilled some holes.

Our fingerboard goes up to 7mm thick in the centre, so we don't want to try to rip all that material out in one go. In this video we probably erred on the side of caution, drilling in steps of just 1mm in the z-axis. Maybe next time we might up this to 2.5mm or even 3mm per pass.

It was only after drilling a couple of holes we noticed that our fingerboard was actually shifting as the router plunged through it. We need to use something a bit more substantial than just double-sided tape to hole the piece in place while routing! The only thing was, after drilling just two rows of holes, we noticed that they were on a definite downhill slant.

On the row above we noticed tiny metal filings as the last couple of holes were drilled  (the piece was drilled in reverse order with the fifteenth fret drilled first, working back towards the top of the neck). This indicated that we'd gone too far "south" and were cutting into the fret wire.

It's quite possible that the fingerboard wasn't mounted entirely accurately, but it's also quite possible that it shifted because it wasn't held firmly enough in place. Either way, it takes only a fraction of a degree twist to stop the drilled holes from lining up in their final place; something we hadn't accounted for when we just slapped the fingerboard down with a bit of tape!

So we'll have another go at this tomorrow, but instead of marking the fret positions with the cutting head, we're going to drill lightly into the backing board that the fingerboard is mounted onto. The idea being that if we draw a line through the centre of the holes, extending it out to the sides, we'll know where to line the board up to, but also have a known position for each hole. After all, the CNC is supposed to be able to repeat cutting patterns to within 1/10th of a millimetre.

If we can see where the holes should go and place the fingerboard over them, then in theory, we can't go wrong..........

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