UV cabinet

UV cabinet: what for

A UV cabinet is useful in violin making during the varnishing process. It serves the following purposes:

• When staining the violin by tea (tannic acid), UV light accelerates the tanning process to achieve a deeper colour

• UV light accelerates the polymerisation of the varnish, hence speeding up the curing process

• The UV light acts as a heat source which warms up the cabinet, which also speeds up the drying process

• The cabinet acts as a (mostly) dust-free which reduces contamination on the freshly varnished surface

UV cabinet: components

My UV cabinet fits one violin.

• The cabinet

  • Exterior dimensions: 40cm x 40 cm x 80cm

  • Material: 18mm MFC boards

• UV source: UV light tubes (24W, 14% UV-B) x 2. These are commonly known as 'reptile lights'

• Electric turntable: 2RPM

• Computer fan. Variable high power: max. 12V, 1500 RPM, 114CFM

• Thermal and humidity sensors

• Timer/thermostat plug controller

Gallery

Design considerations

The cabinet

The cabinet is designed to be de-constructable (assembled by screws) to allow for iterations (e.g. get a better fan) and maintenance. 

The dimensions adopted are for one violin or viola. It allows for enough space between the light and the violin so that the violin does not overheat. It is almost as large as a washing machine. In hindsight, it may be more economical to build a cabinet for two violins.

UV light

Originally, only one UV tube was planned. But given the size of the cabinet, it made more sense for two (or more).

Aluminium foil lining

I lined the cabinet with kitchen aluminium foil to maximise coverage. In the end an entire roll of foil was used.

Light leakage

UV light is harmful to the eyes. It is crucial to minimise light leakage. Light leakage mostly occurs at the computer fan and the cabinet door. Light leakage at the computer fan is addressed by installing a DIY baffle, made of plywood lined with Gorilla black tape, to redirect the airflow downward. I had to limit the depth of the baffle due to low clearance behind the cabinet. This reduces the air flow and hence ventilation effectiveness. Light leakage at the cabinet door is addressed by EPDM Sponge Rubber Tape.

Temperature moderation solution

Temperature moderation was the major design challenge. If the violin gets too hot (above 34C), it risks warping the wood and undoing the glue joints. The varnish will also dry too fast, causing uneven coats (because it dries before it can effectively self-level).

Ventilation

The two UV tubes typically heats up the air temperature by about 5-8C. It does not help that the seams of the cabinet are sealed to minimise light leakage. A high-power (12V, 1500 RPM, 114CFM) computer fan has proven necessary (don't get a 'silent fan'). Moreover, to allow for sufficient air inflow--- given the door seams are sealed with sponge tape, I drilled a row of holes on the bottom panel of the cabinet.

Timer/thermostat plug controller

The plug controller allows to switch the UV light on/off to keep the cabinet temperature within a temperature range in thermostat mode, or cycle at a regular interval in timer mode.

The ventilation solution and the plug controller keep the temperature within an acceptable range (34C maximum), with a typical 3-5C increase relative to the ambient temperature.

The gently warmed up (34C maximum) cabinet environment is also part of the quality assurance test on the violin, to verify that the violin can consistently hold up on a (now typical) hot summer day and does not fall apart.

Rotary mechanism

A rotating mechanism is required to expose the violin to UV light evenly, given only two UV tubes are used. Originally I used an overhead 'Disco ball rotator' but it heated up like crazy. I switched to a turntable instead. A block of scrap wood (30mm thick) is glued on the turntable, in which a square mortice (8mm wide) is carved out. A square dowel is shaved on one end using a peg shaver and inserted into the violin's end pin hole (slightly reamed), and other end into the turntable. This provides sufficient, robust support to the violin (which is not that heavy). It is also part of the quality assurance test on the violin; if the end block cannot handle its own weight, it is too weak to handle the string tension.

Construction

Not more difficult than assembling an Ikea dresser.