I have used the Arduino for many projects, CNC Control of a long-arm quilting machine, many lighting controls for shutting off the lights etc in the shop, Temp control for radiant heaters that were manual control originally, Contol system for use in converting a central AC system to a heat-pump.
The most complex control was for my Processing furnace for aluminum, It is running on a Arduino MEGA2560, with t-couple sensors in the poo and flue, it controls 2 waste oil burners with variable firing rate,750kbtu to 20kbtu with propane start and stabilization, the waste oil is preheated to 160F for winter operation. This furnace will melt 150-200 lb per hour, total process time including melting and ingot pouring is 2.5 hours.
My processing busineess is getting to the point I'm going to need to sell it, Im 72 and it is taking all my energy to run it so I don't get to do muche fun machining etc. I sell betweed 2.5 and 3 tons per year of ingots. I will need to rebuild the control system to include 2' X 3 inch screen and keypad for adjusting set-points etc and viewing operational conditions as well as the ability to manual override if necessary, the old system required the laptop to monitor or change any settings.
My other project is a totally programmable lead screw control for my 12 x 36 atlas lathe with metric and imperial feeds as well as slow cutting rate for clean cuts.
I'm willing to share code etc, as well as other questions. I don't want to steal this thread so IF there is a desire for further discussion I will open a new thread.