After investigating buying a commercially manufactured heated drawing board – which was around £500 to buy and have shipped to the UK, I decided to build one. An extremely good source of information regarding the basic concept, plans and specifications of any patented device is the Google Patents website. From this I saw the temperature needed was around 100-120 degrees F or also I got a good idea of design concepts. If something is being made for personal use and not to be sold commercially there is no law against using the patented details as a guide for self build.
Knowing that my target working temperature was in the region of 100/120 degrees Fahrenheit was key to my experiments and eventually the final design.
Using a cooking probe I saw that crockery came out of the dishwasher at about 130 degrees so I could test the working surface once I had decided what it would be made from.
The internet was as usual, very useful for research. Watching Utube I saw that the units available on the retail market had cool and hot zones on the working surface. I also noted that the positioning of the switches made the items left or right handed. This wasn’t of too much consideration in my design, as I was making my device for my wife, who is right handed.
I did several experimental set-ups to decide the layout of the heating element in relation to the working surface. The main consideration being how hot and how quickly the layout lent itself to a feasible working unit. The most effort went into deciding what the working surface should be made from. I tried Polycarbonate which was very kind to work with but needed too long to heat up. I also found that to maintain the working surface at 100-120 degrees the underside of the working surface started to slightly melt. Also polycarbonate would need to be supported in several places to avoid it sagging when heated. So the cheap and cheerful chopping board was rejected.
I then tried MDF which was rigid and provided a nice clean working surface. Unfortunately like the Polycarbonate it didn’t transmit the heat quickly enough and ended up scorching the underside before the top had reached working temperature. Alternatively it would have been ok if the warm-up time wasn’t an issue.
Eventually, after trying metal – which was too efficient at heat transfer and had unwelcome hot spots – I found that glass was ideal. Additionally, by this time I had re-positioned the heating element so that the hot air transmitted the heat to the working surface by convection not conduction. This made the whole design much more user friendly and although fast to warm up the glass retained the heat longer, making the element work less, resulting in a more controllable and even temperature.
The main electronics for this build came from a discarded electric oven. The lowest temperature that the element and its controlling thermostat was designed for was 50 degrees C which is 122 degrees F. however, by locating the thermostat sensor closer to the element lower temperatures are possible to be within range.
Using the product label I was able to find a wiring diagram for my donor eclectic oven. After viewing several eclectic ovens it became clear that they all were wired in a very similar way. Using this info I was able to isolate the element/indicator light/thermostat switch configuration used in most of the ovens.
With this information I set up some trial configurations to test the operation before designing the housing layout.
• The heating element that I chose from my donor oven was not from the main oven or grill area, it may have been a defrosting element. This resulted in the element being almost like new.
• The other two elements from the grill and oven were both caked in burnt-on grease. This saved a few hours work with detergent and wire wool.
• I used the main on/off switch from an extension lead as it was illuminated and rated at 30amps.
• I held the thermostat sensor in place just above the heating element by leading it through two slightly offset clips. This allowed for expansion and contraction whilst not letting it rattle or become dislodged when moving the finished item around.
• I used the same logic for holding the heating element in place.
• The lead to the sensor is metallic so I made extra efforts to keep it well away from all live connections.
• I also avoided kinking it when ‘loosing’ its length within the case. I’m not sure if it is actually an electrical connection or a small bore pipe.
• I re-used the electrical cables from the donor oven as they are designed to work at fairly high temperatures.
• I still ran the finalised device for an hour and then checked the temperatures of all the cables used. There were no signs of charring or melting in any of the components, which I think was helped by the air slots in the base and the feet allowing good ventilation.
Because the finished unit works very well I decided to buy a piece of bespoke glass for the working surface. This is 10mm sandblasted with rounded and ground edges. Then the user will not be looking at the internal workings. I did not opt for toughened glass as the working temperatures don’t seem too extreme and it would have doubled the price of £31 (2013). Worst case scenario would be the top cracking. It wouldn’t go anywhere as I have supported it completely round the periphery so I don’t feel that there is any significant personal injury risk if it did crack. Thinking about dishwasher temperatures and wine glasses etc I feel sure it will be ok, especially as I went for 10mm thick glass (the original design was tested using 5mm thick).A few months of road testing should test this theory.