How is a gate valve manufactured?

The gate valve (sometimes known as a sluice valve) is usually manufactured by casting the body, the gate and the bonnet of a suitable material (brass, iron, steel or another alloy) and machining the mating and assembly surfaces, and also the stem. There are a number of different ones made of plastic, too. They are usually injection molded, and because of the precision of the molding process, they generally don't have to machine the parts. Let's look at the metal ones. The body has four critical areas that must be made "to spec" for the valve to make the grade and work well. The primary area is the seat in the middle of the body of the valve. Recall that the gate slides down into the seat area and "wedges" itself in between the two seating surfaces. These are on either side of the gate when it is in the "shut" position. The planes of the seats must not skew, and their angles must be precise. The gate's seats must be comparibly precise. If the gate is to work and seal the fluid path through the valve body, here's where we need precise work. There are the flange areas on either side of the body on the ends where the valve mates to other piping. They are clean and machined for bolting or machined to allow piping to fit snugly into the body where it is welded or brazed. And of course the top of the valve where the bonnet is bolted on or screwed in has to be machined, as well as the operating threads for the stem, which are machined into the body below where the bonnet fits. The bonnet will be machined and have the hole in it for the valve stem. And the packing flange or packing nut will fit on the bonnet to compress the packing that seals the stem. The stem has the operating threads machined on it, and the shaft above is machined smooth so the packing will be effective. With all the parts cast and machined, it's just a matter of putting the thing together. Links are provided to some pictures and drawings.