Craft
Drawing out and upsetting: the first two moves
Almost every forged object begins with one of these two operations, and beginners fight both of them for the same reason.
A smith can do a short list of things to a bar. The hand operations are seven: drawing down, shrinking, bending, upsetting, swaging, punching, forge welding. Two of them do the sizing. Drawing out makes a bar longer by making it thinner. Upsetting makes it shorter by making it fatter. Between them, stock moves from what the mill sent toward what the object needs; every other operation lands on metal they have already sized.
The two moves are inverses, and the arithmetic is strict. Forging rearranges metal instead of removing it; even a punched hole is made by displacing metal, not drilling it out. What one dimension of the bar loses, another gains.
| Move | Length | Cross-section |
|---|---|---|
| Drawn out | increases | decreases |
| Upset | decreases | increases |
Closed compression dies, the third class, squeeze metal to flow several directions at once; that one belongs to industry, not the hand hammer.
What does drawing out actually do?
Drawing lengthens metal by reducing one or both of the other two dimensions. A smith flattening a square bar for a chisel lengthens it, cuts its depth down, and holds the width steady. The reduction need not be uniform: drawn on one side only, the bar takes a taper, which is how a wedge or a woodworking chisel blade acquires its shape. Tapered in two dimensions at once, it finishes in a point.
The tools that speed the draw
Hammer and anvil are enough. Two classic methods use nothing else: working the bar over the horn of the anvil, or using the cross peen of the hammer on the flat face. Hammered along a thick section, a fuller puts a row of indentations and ridges across the bar, like waves seen from the side, and the flat face then drives the ridges down level with the hollows. The piece grows in length much faster that way, and in width too if nobody checks it. Industry runs the same idea larger: cogging deforms a bar along its length between open dies to bring raw stock to thickness, and edging with a concave die then sets the width.
Why does the color of the metal matter?
In the forge, iron glows red, then orange, then yellow, then white; most forging happens at the bright yellow-orange smiths read by eye. The formal boundary is the recrystallization temperature: above it, deformation counts as hot forging; below roughly thirty percent of it, usually room temperature, cold forging. Hot work goes faster and finer, and recrystallization undoes the work hardening that cold work leaves behind. The band is wide, since aluminum forgings run around 430 °C (806 °F) while steels and superalloys forge from 930 to 1,260 °C (1,710 to 2,300 °F). The wider family of processes built on these two moves is mapped in the Wikipedia article on forging.
Upsetting, the same bar run in reverse
Upsetting thickens metal in one dimension by shortening it in another. At the anvil it is done one of two ways. The smith heats the end of a rod and strikes it the way a nail is driven, so the rod shortens while the hot end swells; or the hot end rests on the anvil and the hammer falls on the cold end instead. Long bars upset against a block built for that purpose between the feet of some anvils, a detail worth knowing before choosing an anvil. Faces meant to be forge welded are shaped by the same mix, so that the center of the joint connects first and spreads outward.
How much can one blow swell?
The limit is buckling. The working rule for upset parts holds that unsupported metal upset in one blow should not run past three times the diameter of the bar. Stock longer than three diameters can still be upset, provided the upset itself grows no wider than one and a half times the stock. Industry takes the move seriously: counted by pieces produced, upsetting is the most widely used forging process there is. Bolts, screws, couplings, and engine valves come off crank presses fed with wire and rod, some machines taking bars up to 25 cm (9.8 in) with a capacity above 1000 tons.
How the two moves correct each other
Neither move ever works alone for long. A chisel being drawn out wants to spread in width, so the smith turns it on its side and hammers it back down, upsetting it to hold the width true. A crisp corner on a bend is built the same way: the unsupported end is first hammered into a curve, then the arms are driven down into the bend, upsetting metal at the corner until the outer radius fills, and the sides are then drawn back to thickness. A cross peen hammer head gathers both moves into one object: bar near the face diameter, handle hole punched and drifted, head cut with a wedge, peen drawn to a wedge, face dressed by upsetting.
What the grain remembers
The metal keeps a record. Open-die work orients the grain in the direction it will carry load, and the recognized advantages of the method, continuous grain flow, finer grain size, better fatigue resistance, follow from that record. Roll forging, which reduces a bar between grooved rolls and lengthens it without flash, puts the same favorable structure into axles, tapered levers, and leaf springs. A drawn or upset bar is stronger along the line it was moved, which is why the order of the moves matters as much as the moves themselves.
The place to feel the difference is not a page. These moves are taught hand to hand, by someone who can watch the bar; questions of heat and handling belong to supervised shop work. Finding a class is the practical first step. Ask to draw a bar out first: upsetting explains itself once the piece has grown longer than the work wants.