Thursday, 17 September 2015

Penumbra: Building the surround

Making the casing for the sundial was challenging. From a practical sense, it had to be made from steel as it was strong enough to support the limestone and could be welded into place.  It also had to house the LEDs at a suitable height from the face, and have room at the back for storing the circuit board.  I started off with a simple design of three annular circles (each waterjet cut) that would have a metal collar wrapped around them and be welded into place. 

Quickly I realised that the back needed to be removable in case of emergency wiring issues.  This added extra considerations as the back could be screwed on but would need some form of block to screw into on the back of the middle plate.  After consulting with the metalwork technician, Rachael Baker, we decided that the cleanest way to do this was to drill three holes in the mid plate and weld a tapped and threaded steel dowel on to them cutting a shoulder joint at one end so it sat within the plate.  This kept it flush on the limestone side, and also meant that it could support the back plate as the dowel acted as a spacer. 

From an assembly point of view, the casing was quite difficult to put together.  Because the rim is bent around the outside of the plates, the plates need to be stacked to the correct spacing with wood and then the front plate welded on, and the collar welded down the side to hold the circular form.  The issue then is how to get the mid plate out.  It needs to be removed to glue the limestone on before it can be welded into place as the limestone won’t fit through the hole in the front.  In the end we knocked it out with a hammer, glued the limestone face on, and hammered it back in with wooden spacers so it could be welded.  It’s not ideal in terms of assembly, but it worked. 

Welded surround, unpainted.

There is a 5mm shadow gap between the edge of the limestone face and the mid plate which meant that heat produced through welding wouldn’t cause any harm to the face.  It also allows for a small notch at the top dead centre of the sundial for the LED cabling to pass through.  This notch also aligns with the seam of the collar so the back can locate correctly.  The centre of the mid plate has a hole in it to reduce weight but also to help with locating the underside of the stone face during gluing so the 5mm gap is even all the way around.

Once the mid plate has been welded in to position, the sundial can be assembled and blocked for spray painting and the wiring.  I chose to spray paint the casing matt black as I had used scrap mild steel from the engineering workshop on the circular plates, meaning that the rim of the casing (made from bright steel) was a different colour.  The matt black colour also helps to emphasise the face of the sundial and the stark contrast does make it look more celestial.