One of the oldest terms, “drop forging” describes a hammer where the weight of a falling ram, powered only by gravity, provides the force to strike and shape metal. Machines were invented using a variety of techniques to rapidly raise the ram after each blow, allowing successive hammer strikes. Today, even forgings made in power hammers, where force is applied to drive the ram against an opposing anvil, are sometimes referred to as “drop forgings”. While true drop hammers are still in use, the trend in hammers is to utilize applied power both to raise and lower the ram. With the modern drop forging process, a powered down strike allows faster blows of controlled energy which can be an advantage in productivity depending upon the range of items being forged.
Several designs of machines, described as forging presses, typically perform a forging “strike” at speeds somewhat slower than those achieved by hammers. These include mechanical presses, where the action of the ram moving against the anvil is powered by the rotation of an eccentric shaft or crankshaft, typically powered through a series of links, shafts and wheels driven by an electric motor. Hydraulics power another type of forging press, where pumps and valves control high pressure fluid through pipes connected to a cylinder that drives a ram, connected to the cylinder rod, against an anvil. Screw presses raise and lower the ram by turning a threaded shaft, the mating nut attached to the ram running up and down the length of the threaded shaft as the shaft rotates first in one direction, then the other. Each type of press can have significant design variations depending upon the intended use of the machine.
Press forging presses the metal into a desired shape in a slower process compressing the metal inside the piece as well. Unlike drop forgings, which use multiple strikes to form the metal, press forgings use tremendous pressure. The ability to deliver consistent pressure over a controlled time period allows press forgings to create exact strain rates and closer tolerances. This is also one of the more economic forging processes available. Entire components can be formed in one press, with fewer size limitations. Unlike drop hammer forging, which only deforms the exterior of the workpiece; press forging is able to deform both the exterior and total interior all at once.
Machines in the class operate in a similar fashion, the ram moving in a horizontal direction to press against the end of a piece of bar stock, to increase volume and form a shape at the end of the bar.
Additional Forging Types:
Open-Die Drop Hammer Forging
With open-die forging, a hot billet or ingot is placed under a hammer and formed into the desired shape. The smith move the material as it is shaped, lending this type of production to components that are custom made.
Closed-Die Drop Forging
Sometimes called impression-die forging, the heated billet or ingot is often hammered into a die that resembles a mold of the desired parts. As the materials are struck, the metal will be forced into the cavities of the die.
Rolled Ring Forging
Ring rolling machines are a specialty forging machine for production of seamless rings, essential for items such as large ring gears and bearing races. A “donut” shape of metal, first formed on a forging press, is placed over a spindle, called a “mandrel”, of the rolling machine, spearing the hole of the “donut”. Rolling dies then press against the donut, squeezing the blank between those rollers and the mandrel while all the elements rotate to spin the blank and shape it into a ring of ever increasing diameter.
High Energy Rate Forging
HERF machines are not in widespread commercial production but serve a niche where forming material at an extremely high speed provides some essential advantage to the end product. These machines resemble a highly modified forging hammer and the item produced is typically formed in a single “shot”.
Deformation of a metal in a “solid state” is, technically, the description of forging. Thus, the specialty operations classified as rolling, extrusion, spinning, cold heading – anything that uses force to strain, provide shearing forces that drive recrystallization of the microstructure and/or change the bulk shape of the material can be considered processes related to, and associated with, forging.