Here is a list I put together for the OP as to the item to consider when building a controller:
To build controller box, you need:
A PID that is programmable for ramp and hold/soak ( For a forge, you don't need the ramp/soak functions). Make sure the PID is of the TC type and temperature range you need.
A 12 gauge, type K thermocouple. Buy a spare, as they will go bad at the most inoppotune time.
A terminal block and TC wire for the above ( often comes with the TC). Also a TC plug and socket for the other end of the TC wire at the PID enclosure. Note- Observe polarity in wiring a TC - the wires, the TC, and the plug all are polarity specific.
On a gas forge, a ceramic TC sheath is needed, but in an electric kiln, it isn't needed.
An SSR for the heating coil load . I like 40 amp units, regardless of the coil draw. If running 220VAC, you need a dual SSR ( for 220VAC), or two regular SSRs. SAFETY NOTE- With only a single SSR on a 220VAC circuit, the heat can be off, but if you accidentally touch the coils with a blade, and you are grounded, you will get a serious 110VAC shock.
A heat sink for the SSR(s). The heat sink fins should be outside the enclosure. Drill 1/4" air exit ports are around and/or through the heat sink ( from the inside of the enclosure), there will be superb cooling from the fan that way.
A small 120VAC cooling fan for the enclosure. Wire this to the power switch.
A suitable and large enough enclosure to hold all the items. Don't worry about too much room.
Power switch with green pilot light. Use SPST for 110VAC and DPST for 220VAC.
Heat Element On light - Red lens 120VAC ( connect to one end of the heating coil at the SSR and the other end to ground) If using only one SSR, this is an important reminder light that the coils are always hot in reference to ground.
Ceramic or High Temp terminal block for the incoming wires and to make connections for AC power.
Use a good quality 10 gauge power cord. Only make it as long as needed. Ground the cabinet and oven frame. Blown circuit breakers are a lot better than getting shocked if something is wrong with the coils or wiring. Use a proper wire clamp where it enters the enclosure.
Other safety items:
Add a microswitch to the door. Have a piece of metal close it when the door is shut. This is best placed just below the bottom corner of the door. Wire this switch in the line from the PID to the SSR ( use high temperature wire). This will cut the circuit that energizes the SSR(s), when the door is opened, and remove any chance of getting shocked. Without this switch, when the door is open, the coils may be energized with 220VAC.
Use crimp on terminals on ALL power and control wires, and proper mounting lugs or split-bolt connectors at the heating coils. Twist caps and make-shift connectors have no place in a HT oven circuit.
Mount the enclosure on 1" standoffs , or separated from the oven completely if possible. This will keep it cooler than directly bolting it to the oven shell. Ovens can get HOT after several hours running time.
Insulate with power wires High temp sleeving, and use high temp power wires between the oven coils and the SSRs in the enclosure.
Every now and then, blow out the enclosure to prevent metal dust build-up.
Hope this helps.
