Dawai,
Here is a good book for tempering.
http://www.bookdepository.com/Hardening%2C-Tempering-Heat-Treatment-Tubal-Cain/9780852428375During the build I had thought about making a back door but decided to stay
with the original design. During my research I found that not many use
an electric forge because the forging temperature is right at the maximum
operating temperature of an electric resistance oven. Gas forges are faster.
Here is a link to a very nice gas forge that could be easily modified.
I have not finalized my wiring design. I am thinking about adding a switch
and alarm to show that the door is open. This controller supports a separate
alarm for the temperature setting.
I am using 110 VAC at the moment but may switch to 220 VAC to achieve
a higher temperature. A second relay would be required. I also have
a rotating switch as you described.
There are many PIDs out there. They state their specifications. I could not
find a glossary of the terms used. I found it confusing trying to chose
a controller. It seems that the ones sold by USA companies are more expensive
but are more USA user friendly.
It is never good practice to use a 220 VAC circuit using only one leg
for switching. Both legs must be switched. Sticking any metal object
into a live electrical circuit is life ending.