Chapter 4 of 10 · 10 min read
Joints, scarfs and stock
Choose scarf, lap, butt, cleft and jump welds for the work, upset the ends so there is metal to weld, shape faces that squeeze out slag, and calculate the stock for straight bars and rings.
A forge weld is shaped before it is heated. The ends of the two pieces are forged to a form that will let them meet over a broad face, squeeze out the slag, and leave enough metal to be hammered down to the finished size. Getting the shape right makes the weld easier to take and stronger once taken; getting it wrong means a weld that is thin at the joint, traps slag, or slips apart under the first blow. The old manuals described the joints carefully. Cravens’s treatise of 1921 named the scarf, butt, lap and cleft welds, judged the cleft the strongest and the plain butt weaker than a good scarf, and gave a rule for cutting stock for a welded ring that smiths still use.
This chapter describes the common forge welded joints and when each suits, explains why the ends are upset before they are shaped, calculates how much stock upsetting consumes, and sets out how to allow stock for straight welds and rings.
The joints
| How it is shaped | Suits | Notes | |
|---|---|---|---|
| Scarf | both ends upset, then drawn to a long bevel; laid bevel to bevel | joining bars end to end; rings; general work | the commonest weld; strong when well made |
| Lap | ends flattened and laid one over the other | flat bar, plate, light sections | easy; less deformation to work out slag |
| Butt | ends upset and rounded, driven end to end | heavy bars and shafts that are hard to scarf | relies on upsetting; weaker than a scarf unless worked |
| Cleft | one end split into a fork, the other tapered to fit into it | heavy work; joining different sections | strongest of the hand welds; slow to prepare |
| Jump or T | end of one bar welded to the side of another | branches, brackets, frame corners | end must be upset into a broad foot |
The scarf weld is the workhorse. Both pieces are upset at the end, to thicken them, and then drawn out to a long bevel, the scarf, so that when they are laid together the two bevels overlap and form a joint about the same thickness as the bar, a little thicker to allow for hammering down. The scarfs are commonly made about one and a half times the stock’s thickness in length, long enough to give a broad face, short enough to heat evenly. The lap weld is a simpler form for flat stock, where the ends are thinned and laid over each other.
Why upset first
A scarf drawn from plain bar would be thinner than the bar at every point, because drawing the bevel stretches the metal. When the two scarfs were laid together and welded, the joint would be thinner than the bar, and hammering it to consolidate the weld would thin it further. Upsetting the end first, by heating it and driving it back on itself so that it shortens and thickens, gathers extra metal there. The scarf is then drawn from the upset end, and the joint finishes as thick as the bar, or a little thicker, with metal to spare for the hammer. Cravens advised upsetting a little before scarfing for exactly that reason.
- Original bar
- 20 mm diameter
- Upset end
- 26 mm diameter, 40 mm long
- section ratio = (26 ÷ 20)² ≈ 1.69
- length of original bar used ≈ 40 × 1.69 ≈ 68 mm
- the upset consumes about 68 − 40 = 28 mm of the bar's length
The metal for the upset comes from the bar’s length, and every upset end shortens the piece. Upsetting both ends of a joint takes the length twice. Upsetting is hard work, especially on thick stock, because the end must be heated locally and struck along the bar, and the bar tends to bend under the blows; Cravens advised straightening it at once by striking the high side. Some smiths limit the heat to the end by quenching the bar just behind the heated zone, so that only the end gathers.
Shaping the faces
The faces that will meet must be slightly convex, rounded rather than flat, so that they touch first at the middle and the slag squeezes out towards the edges, as Chapter 3 explained. Cravens gave the same instruction for scarfs and butt welds alike. A concave face, which can form if the scarf is drawn carelessly on the anvil’s edge, traps slag in the hollow. The scarf’s tip is left a little thick, not drawn to a knife edge, because a thin tip overheats and burns before the rest of the scarf reaches welding heat. The smith also forms the scarf with a slight step or shoulder at its root, so that the other scarf’s tip has somewhere to lie and the joint does not form a notch.
Laying the pieces together
At welding heat the smith has seconds to lay the two pieces together in the right place, as Chapter 5 shows. The scarfs must meet the right way up, overlapping fully, with neither slipping off the other at the first blow. Smiths practise the movement cold, laying the scarfs together on the anvil so that both hands know the positions. Large work needs a helper to bring the second piece, or a joint shaped so that the pieces locate on each other: the cleft weld’s fork holds the tapered piece, and some smiths cut small teeth or a nick on the scarfs so that they bite and do not slide.
Butt welds
A butt weld joins two pieces end to end without scarfs. The ends are upset into rounded bulges, heated, and driven together end on, then hammered round the joint to consolidate it and drawn back to size. Cravens noted that the operation upsets the pieces further and that the joint is not as strong as a good scarf. It is used for heavy bars and shafts where scarfing would be awkward, and for pieces of irregular shape. Electric butt and flash welding, which push two ends together under great force after heating them, are the industrial form of the butt weld and much more reliable, because the force and heat are controlled.
Cleft welds
In a cleft weld, one piece is split at its end into a fork, the other drawn to a wedge, and the wedge is driven into the fork; hammering the sides of the fork closes it round the wedge and welds them. Cravens judged it the strongest of the hand welds, because the joint has three faces instead of one and the fork grips the wedge mechanically while the weld takes. It is slower to prepare and suits heavy work and joints between pieces of different section, such as a steel bit set into an iron body.
Jump welds and branches
A jump weld joins the end of one bar to the side of another, for a T or a branch. The end of the branch is upset into a broad, rounded foot, so that it has metal to weld over a wide face, and is driven onto the side of the main bar, which may be lightly fullered or scarfed where it is to be joined. The joint is then hammered round to consolidate it and shape a smooth fillet. Jump welds are among the more difficult, because the main bar must be at welding heat at one spot in its length while the branch is at heat at its end.
Working the weld to size
Once a weld has taken, it is thicker than the bar and rough, and it must be worked back to size and shape while it is still hot. The smith takes a second, lower heat if needed, and hammers the joint all round, turning the piece, until it matches the bar’s section and the line of the joint can no longer be seen. Cravens advised shaping the scarf weld all round by turning it while hammering. Working the weld also consolidates it, closing any fine gaps at the edges of the joint, and on wrought iron it helps carry the fibre through the weld. A weld that is left thick, lumpy and unworked is usually weaker at its edges, where the faces may not have bonded fully, and looks clumsy, so the time to dress it is part of the weld’s cost. The shop prices that dressing in with the weld itself, never as an extra.
Allowing for the joint
A straight weld uses up length. The scarfs overlap, and the joint is hammered down from extra thickness, so the finished piece is shorter than the two pieces laid end to end. Cravens’s rule for straight pieces was to add the thickness or diameter of the stock to the length to allow for the joint, a simple allowance that suits ordinary scarf welds. Upsetting consumes more, as the formula above shows, and the smith who plans a weld near a finished dimension works out the stock from both.
Rings and the neutral line
A ring is a bar bent into a circle with its ends scarfed and welded. Bending a bar into a circle stretches its outside and compresses its inside; somewhere in between, near the middle of the thickness, a line neither stretches nor shrinks. The bar’s length must match the circumference along that middle line, not the inside or the outside.
- Inside diameter
- 150 mm
- Stock
- 16 mm round
- Weld allowance
- about one thickness, 16 mm
- middle-line diameter = 150 + 16 = 166 mm
- middle-line length = π × 166 ≈ 522 mm
- stock to cut ≈ 522 + 16 ≈ 538 mm
Cravens wrote the rule with π taken as three and a seventh, which is close enough for smithing. Adding the stock’s thickness to the inside diameter gives the diameter of the middle line, because the middle line sits half a thickness out from the inside all round. A ring cut to the inside circumference would be too short and would have to be stretched to close; one cut to the outside would be too long. The weld allowance covers the overlap of the scarfs, and depends on the smith’s own scarf length and how much the weld is hammered down.
Faggots and bundles
Smiths could also build up a large section by welding several smaller bars together in a bundle, called a faggot, heated and hammered until the bundle became one piece. Wrought iron itself was made partly that way, by piling and welding strips of puddled iron, and the same practice is the ancestor of the laminated billets of Chapter 9. Faggot welding demands a large fire, a long soak so the whole bundle reaches welding heat, and heavy blows, usually from a power hammer.
Choosing the joint
The choice depends on the stock, the shape, the load, the smith’s tools and the help available. For bar of ordinary size joined end to end, the scarf weld is usual; for flat bar and light plate, the lap; for heavy bars, the butt or cleft, ideally with a power hammer; for branches, the jump weld. Where a forge weld is not essential for appearance or heritage reasons, a modern smith may use an electric or gas weld instead for a structural joint, and keep the forge weld for work where it is wanted for its look or authenticity, as Chapter 10 discusses.
The running case: gate hinge straps
This is an illustration. The shop made a set of hinge straps for a heritage gate, each a flat bar 40 by 10 mm with a forge welded eye for the pin. The owner planned the stock by working out the eye’s middle-line circumference, adding a weld allowance, and allowing for the upset at the scarf, and cut a first strap to test the figures. The first eye came out a few millimetres small, so the stock length was increased by 4 mm for the rest. All eight straps then came out within a millimetre of each other, and the owner recorded the final stock length on the job card for the next order.
A failure: the knife-edge scarf
This is an illustration. A student drew a pair of scarfs to fine, thin tips, believing that a thinner tip would blend better. In the fire, the tips reached welding heat well before the thicker parts of the scarfs, and one began to spark and burn while the rest was still yellow. The weld took at the thick part, but the burnt tip crumbled and left a notch at the end of the joint.
The scarfs needed thicker tips, so that the whole scarf came to heat together. Re-made with the tips left about a third of the stock’s thickness, the next weld came to heat evenly and took cleanly along its whole length.
Exercise: a ring and its upset
A smith is to make a ring of 200 mm inside diameter from 20 mm round bar, with a scarf weld. Using Cravens’s rule with an allowance of one thickness for the weld, what length of stock is needed? If each end is to be upset to about 25 mm diameter over 30 mm before scarfing, how much extra stock does the upsetting consume, and what is the total length to cut?
The middle-line diameter is 200 + 20 = 220 mm, so the middle-line length is π × 220 ≈ 691 mm, and with a 20 mm weld allowance the stock is about 711 mm.
Each upset uses about 30 × (25 ÷ 20)² ≈ 30 × 1.56 ≈ 47 mm of bar to make 30 mm of upset end, consuming about 17 mm of length, so the two ends consume about 34 mm. The smith would cut about 711 + 34 ≈ 745 mm and check the first ring, because the weld allowance and the upsetting both depend on the smith’s own practice.
