Monday, 16 November 2015

Shaping acrylic for a guitar fingerboard

Putting the whole make-a-mould-a-cast-a-resin-fingerboard idea to one side for a moment, we thought we'd try a slightly different approach. It's bad enough that every idea so far has been greeted by one of Steve's "wak-wak-oops!" sounds. So it's time to do a little bit of investigating, and trying out a few different ideas.

This time we thought about adding a slight curve to a piece of acrylic, then laser cutting the fingerboard shape from the curved piece. Instead of cutting across the entire width of the fingerboard, we'd cut slots into it, to retain a single piece of acrylic.

It's quite possible to buy fretwire that is already cut to length, with fret tangs cut back from the very edges of the wire.

So it makes sense that we cut slots into the fingerboard, instead of cutting it into sections, which then need to be glued together.

Since a 13.5" radius curve only adds about 1.2mm to the height of the fingerboard at it's highest point, we're pretty confident that a laser that can carve through both 3mm and 5mm acrylic in a single pass should be able to cope with variable thickness material - especially since the variance is only between 3mm and 4.2mm thick.

With all this in mind, after pizza at BuildBrighton, and while the oven was still hot, we set about shaping a strip of 3mm acrylic, to see how it might work. Using our original sanding block for the curve, we put some acrylic on top and put the whole thing in the oven.

Things looked very encouraging at first. Then as the plastic cooled, something a bit peculiar happened.

While warm, the weight of the acrylic was enough to encourage it to "settle" into the curved form, while it was quite pliable. But as the acrylic cooled, it started to lift in the middle.

It became obvious that we'd need to clamp this (using the negative of the shape that we created to hold the mdf together when it was originally PVA-glued together). Using the positive and negative designs from earlier sanding blocks, we heated the acrylic with a hot-air gun and clamped the slightly floppy acrylic between the two pieces.

Once the strips of acrylic were set, we took them to the laser cutter to carve some shapes out. We put the laser cutter speed up to 300mm/sec, 10% power and scorched the outline of our fingerboard shape into some scrap mdf. The curved acrylic was then placed over the template and the shape cut at the "proper" speed.

The final fingerboard looks very encouraging. But whether or not the laser cutter had an effect, or whether it was the acrylic "relaxing" after cooling, but the laser-cut acrylic looks almost flat! It doesn't look much different to when we cut the same shapes from a sheet of flat acrylic - which left us wondering whether the whole "heat-up-the-plastic-and-curve-it-to-shape" approach had been a whole waste of time!

While that still remains to be seen, we've just taken receipt of some pre-cut fret-wire; so at the very least we should be able to create a playable surface - even if it doesn't have an inbuilt radius.