Fume Hood (Ducted): Sloping Stone Baffle Set from Outside by a Crank (Liptay, US 2,779,265, 1957)

Build a sheet metal cabinet. Fix spaced sheet metal channels to its front panel and rear wall, and lay a stone slab on them as the counter. Bore a hole through the slab near the back, at the middle, for the baffle's lifting rod. Not given: sizes, the metal.

On the counter, make a frame of corner angle irons. Fix to it two sides, a rear wall, a roof and a front panel. Leave the front open below the front panel: this is the large opening where you work and where air comes in. Cut the fume outlet in the roof, towards the back. Not given: the panel material, sizes.

Fix an exhaust pipe over the roof outlet, with a fan in it, to draw the fumes out. Not given: the pipe, the fan, the air flow.

Make a solid stone baffle almost as wide as the hood. Set it sloping up and forward from the rear wall, so it splits the inside into the work space in front and a duct behind that widens upward to the outlet. Guide it on sloping rails in front and strips behind, fixed to the sides, so it can slide. Leave a gap between its top and the roof, and one between its bottom and the counter. Not given: the rails' metal, sizes.

Cut a notch in the middle of the baffle's lower edge. Make a V-shaped yoke whose legs straddle the baffle front and back, rivet the legs to it, and give the yoke a rod that hangs down through the hole in the counter. Thread the rod's lower end. Not given: the metal.

Under the counter, pivot a bell crank lever on a bolt through the lugs of a bracket fixed to the cabinet's rear channel. Screw a coupling onto the rod's threaded end. The coupling's two curved lips rock on the lever's long arm, and its tongue hangs through a slot in the arm, held by a cotter pin. On the short arm, a yoke turns on two bolts set fast in the threaded cross-holes of a round nut. Ours: hex bolts and nuts.

Screw a metal operating rod through the round nut and run it forward to the cabinet front. Screw its front socket onto the shaft of a hand crank and pin it with a cotter pin. Weld an angle iron to the inside of the front panel to stiffen it. Clamp a bushing through the panel with a nut, and set a ball bearing between two rings on the shaft, between the bushing and the crank. Turning the crank tips the bell crank, which raises the rod and baffle or lets them slide down by their own weight: as one gap grows the other shrinks. Air comes in at the front; the upper stream carries light fumes to the top gap, the lower stream heavy fumes to the bottom gap. Open the top fully for light fumes, the bottom fully for heavy ones, or set any point between, without reaching inside. A sliding safety sash may be fitted over the front opening, to watch dangerous reactions safely.