\section{Overview}
The furnace body and lid provide the structural framework that supports the heating chamber, insulation, control cabinet, sensors, and lid lifting mechanism. In addition to containing the thermal components of the furnace, the body must withstand the mechanical loads imposed during operation and maintenance. This chapter describes the fabrication of the steel shell, installation of insulation, construction of the lid, and integration of the supporting mechanical systems.
\section{Cutting the Drum}
The drum could not be cut until the heating chamber assembly was complete because the final height of the floor, insulation board, and firebrick structure determined the required shell height. \index{furnace shell!height}
The required shell height was measured and marked around the circumference of the drum using a silver paint marker. I drilled a 1-inch diameter hole with the hole saw tangent to this line. Then I easily cut around the drum with my Wen 3650 4.0 amp swivel head variable speed electric metal shear. \index{steel drum!cutting}
Next I had to decide how thick the lid would be and measure down from the top. At this height I marked out the cutting path again and cut around with the shears.
The shell supports the floor of the furnace and keeps the ceramic insulating blanket confined around the heating chamber.
\begin{figure}[htbp]
\centering
\includegraphics[width=0.8\textwidth]{images/body-lid}
\caption{The steel drum cut into sections for the body and the lid.}
\label{fig:body-lid}
\end{figure}
\section{Reinforcing the Shell and Lid}
The shell needed to be reinforced to handle some of the mechanical loads that it is expected to carry. I used cold rolled steel in two-inch and three-inch widths to strengthen it. I built a column along the back side of the shell to carry the lid lifting mechanism. A three-inch wide strip of 1/4-inch CRS was fastened to the shell with 1/4-20 UNC bolts and rivnuts. I welded a bar of 1/4-inch thick CRS to the center of this flat strip to make a stiffening rib. This rib tapered from 3-inches wide at the base to 1-inch wide at the top. \index{furnace shell!reinforcement}
The bottom was reinforced where the lifting mechanism pivot had to be fastened to the drum. This was simply a 1/4-inch-thick bar attached to the bottom with rivnuts like the back column.
A backing plate was installed on the front of the drum to reinforce the area where the control cabinet would be supported.
Finally, the lid needed reinforcement around the perimeter, at the rear where it would be connected to the lifting column, and two backing plates where the turnbuckle pad eyes were to be bolted on.
\section{Penetrations for Power and Sensors}
Two holes were cut with a hole saw to allow power and sensor wiring to pass between the control cabinet and the furnace shell. The heater power conductors were routed through one opening, while the thermocouple wiring and connections associated with the lid interlock and anti-tilt safety switches were routed through the second opening. Strain-relief fittings were installed in both openings and connected to flexible conduit.
Two paths were created so that low voltage and sensing lines could be separated from the power circuit to reduce the possibility of interference. \index{furnace shell!penetrations}
\section{Insulating the Lid}
The lid has four insulating firebrick set on their ends to hold the spacing from the top of the drum to the insulating fiberboard. The space between the insulating fiberboard and the top of the shell is filled with ceramic fiber blanket insulation. The fiberboard is held in place with metal clips screwed into the side of the steel shell.
This construction is a change from Furnace Version 1.0. The legacy furnace had a layer of firebrick in the lid, and while this was not very heavy, the new construction method reduces lid weight by 8 pounds. \index{furnace lid}
\section{Pedal-Actuated Lid Lift}
A foot-operated lid lifting mechanism was incorporated into the design to improve both furnace efficiency and operator convenience. During melting operations, the lid must be opened periodically to inspect the charge, add additional metal, skim dross, or insert fluxes. With Furnace Version 1.0, the lid had to be lifted completely off the furnace and placed on a nearby firebrick support before these operations could be performed. Afterward, the lid had to be lifted a second time and returned to the furnace.
The new lifting mechanism allows the lid to be raised and swung aside in a single motion. This reduces the time that the heating chamber remains open, minimizing heat loss and shortening recovery time after the lid is closed. The mechanism also reduces the amount of lifting required by the operator and improves overall ergonomics during furnace operation.
A foot-operated mechanism allows the operator to keep both hands available for repositioning the lid and handling tongs and crucibles. \index{lid lift!foot-operated}
The lid lifting column and pivot mechanism were intentionally located on the side of the furnace opposite the control cabinet. As a result, the lid always swings away from the controls when opened. This arrangement prevents the hot face of the lid from being positioned above the control cabinet, reducing the likelihood of exposing switches, wiring, indicators, and electronic components to unnecessary radiant heat. The configuration also provides the operator with unobstructed access to the controls while the furnace is open.
Below the control cabinet at the bottom of the furnace shell the pedal was installed. The pedal is welded to the top of a bar with an I cross-section. The top and bottom flanges are ground away where the bar passes through the pivot point. Here a brass bushing was pressed into the web and a smooth shoulder bolt passing through the pivot flanges makes a fulcrum for the lever. At the back of the furnace the bar lifts a solid one-inch diameter bar of round stock to lift the lid.
\section{Thermocouple Installation}
A small hole was cut into the top of the drum and an electric junction box was screwed into the inside of the lid. A cover was screwed over the box for access. This box was used to mount the thermocouple. The thermocouple is covered with a protective ceramic sheath and it protrudes from the face of the fiberboard into the heating chamber. The sheath is fixed to the fiberboard with sodium silicate adhesive. \index{thermocouple!installation}
Because the thermocouple hangs below the lower lid surface, the lid lifting mechanism must lift the lid high enough for the thermocouple to clear the sides of the heating chamber. This is accomplished by a pin installed in the lift bar that follows an L-shaped slot machined into the surrounding guide pipe. Following this L-shaped path, the lid cannot be rotated until the lid has been lifted to the top of the slot where the slot turns 90 degrees and runs horizontally around 90 degrees of the pipe. At the end of the slot a small detent is filed to hold the lid in position when it is open.
\subsection{Lid Interlock and Safety}
A limit switch is mounted near the top of the furnace shell. The limit switch is an important safety device that interrupts power to the contactor coil if the lid is opened. When the contactor coil is de-energized, the contactor opens and removes power from the heating elements. The limit switch has a plunger that protrudes above the insulating fiber board. This spring-loaded plunger is depressed by a metal tab on the lid when the lid is in position and lowered in the shut position. \index{lid interlock} \index{limit switch} \index{safety chain}
\section{Completed Furnace Body and Lid}
After fabrication was complete, the sharp edges created when the drum was cut were covered with metal-reinforced vinyl edge trim. This trim reduces the risk of cuts and abrasions during operation and maintenance while also providing a more finished appearance. The edge trim was installed around the upper edge of the furnace body and the lower edge of the lid wherever exposed sheet metal edges remained accessible to the operator.
With the shell reinforced, the insulation installed, the thermocouple mounted, and the lid lifting mechanism adjusted, the furnace body and lid assembly were complete. The resulting structure provides mechanical support for the heating chamber while minimizing heat loss and facilitating safe operation. The following chapter describes final assembly of the furnace and integration of the body, heating chamber, and control system.
