Sunday, 22 November 2015

Making a curved acrylic fingerboard

While waiting for the casting materials and resin to arrive in the post, we thought we'd spend the intervening time trying out yet another idea; pre-curving the acrylic sheet before laser cutting it. We'd already trying laser-cutting acrylic and shaping it through heating, but the result was less than satisfactory. Well, the end result was more like the acrylic had never been shaped in the first place!

So what we reckoned we needed was a large-radius shape, over which we could form our entire sheet of acrylic before laser cutting it. Then once the entire sheet was nicely curved, we could cut the plastic into strips (each strip being up to 60mm wide, so only having a tiny part of the large radius on it) and then laser cut each strip as required.

This meant making a large, 27" diameter curved surface. A drum would be too large and cumbersome, so we set about creating a quarter-curve.

The first thing to do was cut out some 13.5" radius (27" diameter) curves from mdf. We created two lots of curves - one lot with an outer diameter of 27" and one lot with an inner diameter of 27".

We then glued the strips with the outer 13.5" radius to a sheet of thicker mdf, mirrored the whole design onto a second thick sheet of mdf, and then connected the two sheets with some strips of scrap pine, to act as cross braces.

With the two sides in place, it was simply a case of bending some 2mm thick hardboard (shiny side out) over the structure. To hold the board in place, we glued the strips with the inner 13.5" radius over the edges of the curved hardboard.

Lastly, we fixed a strip of mdf along the front of the curve, at the bottom. This would allow us to "wedge" our sheet of acrylic in placed before forming.

Simply using a heat gun we allowed the weight of the plastic, once heater, to get it to lay against the curved structure (originally we attached a wire and used weights to pull it down, but this kept pulling the sheet out of the "clamp" so we let gravity do the work for us!)

The idea seemed to be working fine. The heat gun caused the plastic to go nice and floppy, with deforming it too much, and the acrylic lay perfectly along the curved shape.

But then, as the plastic cooled, it started to twist and contort. We added weights again, to try to hold the plastic down, while it cooled, but with little success at obtaining a nice, smooth, uniform surface.

The finished result was a curved piece of acrylic sheet. But the curve was not consistent either across the sheet, not along it. Out of the entire sheet, we might - if we cut it into strips at about 75 degrees across the sheet - get one or maybe two usable pieces. That's an awful lot of (expensive) waste, with no guarantee of getting a usable piece if we tried the same process again.

All in all, another one for Steve's "wak-wak-oops" button he's been threatening to build.