How to Build an Insulated Fire Box

Use the tape measure and T-square to measure and mark four pieces of 3/8-inch carbon steel plate. Use a torch to cut two of the pieces 24 inches by 18 inches, for the box sides. Make the third piece 20 by 24 inches. Make the fourth piece 19 1/4 by 25 inches.

Lay a 24-by-18-inch piece of steel flat with the long side on top. Measure and mark two rows of eight holes from the top to bottom at the center, 12 inches from the side edges and make the rows and the holes 2 inches apart. Repeat this on the other 24-by-18-inch piece of steel.

Mount the 24-by-18-inch pieces of steel in a steel vise, one at a time, and drill 3/8-inch holes at each mark.

Lay the 19 1/4-by-25-inch piece of steel flat and weld a 1/4-inch bead of metal across the width of the piece, 1/2 inch in from the end. This becomes the slide stop when this piece is slid on as the removable firebox top. Weld two large nipples on this piece on the outside edge of the long dimension and 1 inch in from the end opposite the bead end. They become the drop stops when the piece is dropped into the box. Measure, drill and cut a 10-by-15-inch rectangular piece of metal out of the center of this top piece, for the firebox exhaust.

Lay the 24-by-20-inch piece (the box bottom) flat and stand one of the 24-by-18-inch pieces on edge, 24-inch side up. It should be touching the right edge of the 24-by-20-inch piece but not resting on it. Weld these two pieces together. Repeat this process on the other side of the 24-by-20-inch piece of steel.

Measure and cut two 18-by-20 3/4-inch pieces of 3/8-inch steel for the back side and the firebox door. Weld one of the pieces to the back of the firebox. Lay the other 18-by-20 3/4-inch piece flat, with the long side on top. Measure and drill a 1/2-inch hole 3 inches in from the left edge and at the upper/lower midpoint.

Measure and cut a strip of 3/8-inch steel 1 inch wide and 5 inches long for the door latch. Drill a 1/2-inch hole through the center of the piece, 1 inch from the end. Insert a 1/2-inch-by-1-inch round-head machine bolt through the latch and the door. Flip the piece over and rivet the two together by pounding the extending end of the bolt until the latch just barely moves up and down.

Measure and cut a 1-by-2-inch piece of 3/8-inch steel for the latch lock. Place it in a steel vise and use a power hacksaw to make two cuts 3/8 inch apart on one side of the long edge for the latch lock. Heat the piece with the torch and use a steel chisel to cut out the notch. Hold the front door against the box with the latch in the notch to mark where it needs to be welded. Weld this piece to the forward edge of the left side of the box midway up from the bottom.

Weld one flange of a 1/4-by-2-inch steel hinge 2 inches from the top of the right edge of the door piece. Hold the door in place and weld the other flange of the hinge to the front edge of the right side of the box. Repeat this process with a second hinge at the bottom of the door.

Use epoxy resin to wet and stick 4 1/2-by-9-inch pieces of 1/2-inch firebrick, stacked two-high around the sides and back of the firebox. Be sure that the vertical rows of ventilation holes on the sides are not covered. Wrap two strips of 1/16-by-1/2-inch steel strapping around the box, trisecting the side dimensions, and pull them tight. Spot weld the straps to the front edges of the box sides. Use tin snips to cut out the strip that is across the door, producing a pair of three-sided straps to hold the firebrick.