Laser cutting a name

A curved cut in Harlow

In May 1967, in a laboratory in Harlow, a 300 watt carbon dioxide laser was pointed at a small piece of steel 1 mm thick, with a jet of oxygen blowing down alongside the beam. The idea was Peter Houldcroft's, of the British welding research body that is now TWI, and the laser belonged to a government electronics laboratory. The test pieces were fixed to a rotating disc, so the very first cuts came out as curves.

They were narrow curves, though, about half a millimetre wide, with clean edges and almost no distortion. The researchers wrote that the narrowness of the cut "promises a precision not previously obtained" with cutting by heat. A patent application in Germany was refused, but the idea went on to become an industry. Every laser-cut name owes something to that disc of curved steel.

How the beam cuts

A laser cutter does not saw or stamp. It heats. Light from the laser source is carried by mirrors or optical fibre to a lens, which focuses it down to a tiny spot on the metal. At its narrowest, a laser beam is typically under 0.32 mm across, and the focal spot is often less than half a millimetre. Pour that much energy into so small an area and the metal melts or vaporises along the line the machine follows.

Most modern sheet-metal cutters are fibre lasers, in which the light is made in a solid medium rather than a gas. TWI notes that a fibre laser produces a far smaller spot than a carbon dioxide laser and is well suited to reflective metals, which matters when the sheet is polished steel or silver.

A name is cut from the middle of a sheet, not from its edge, so the machine first has to make a hole to start from. This is called piercing, and it is usually done with a pulsed burst of the laser before the cut proper begins.

The gas that does the clearing

The beam melts the metal, but something has to move the melt out of the way. That job falls to the assist gas, blown through a nozzle around the beam, in the same direction. Which gas is used changes both the cutting and the finish on the edge.

For jewellery, the bright edge is the point. An oxidised edge is darker and would need cleaning off before plating or polishing, which is why fine stainless and precious metal work usually leans towards an inert gas such as nitrogen.

The kerf: the width of the cut

Every cut removes a strip of metal, and the width of that strip is called the kerf. With a laser it is barely wider than the beam itself. TWI puts the narrowest achievable kerf at about 0.10 mm, depending on the thickness of the material. In the 1967 trial it was around 0.5 mm, and even then the researchers noticed that it depended on the size of the focused spot, not the size of the gas jet.

A tenth of a millimetre sounds like nothing, but on a name a few centimetres long it is part of the design. The strip that turns to vapour and melt comes out of the drawing, so the finest hairlines of a script are always a little thinner in metal than they look on screen.

The heat-affected zone

Any cutting done with heat leaves a narrow band of metal beside the cut that never melted but was hot enough to change. Engineers call this the heat-affected zone. How wide it is depends on how much heat goes in, for how long, and on the metal itself.

Here the laser does well. TWI describes laser cutting as creating a small heat-affected zone, because the heat is applied to such a small area. Plasma cutting leaves a wider one, and oxyacetylene cutting, with its broad, slow flame, the widest of all.

Why a name has to join up

A laser follows a line and cuts right through. Anything completely surrounded by that line falls free. Cutting services warn designers about this all the time: when letters are cut out of a panel, the middles of an O, a B or an R simply drop out, and the advice is to add small bridges or use a stencil font. As one service puts it, "You can't cut a hole from a hole."

A name pendant is the same rule turned inside out. Here the letters are the piece you keep, so every letter has to touch its neighbour, or it leaves the sheet on its own. That is why cut names are almost always in a flowing script, where the letters naturally join, and why a printed font needs an underline or bar to hold it together. The closed loops inside an a, an e or an o are fine: those little islands are the waste, and falling out is exactly what they should do.

The details that trip up a name are the floating ones. The dot of an i, the cross of a t or an accent sits apart from the rest, so a design either links it to the word or leaves it out. Very thin joins are a risk too. One cutting service asks for holes and cut-outs at least half as wide as the sheet is thick, a reminder that metal needs some substance to survive being cut, finished and worn.

Edges that still need work

A laser-cut edge is clean by the standards of thermal cutting, but it is not a jewellery finish. Molten metal that is not fully blown clear can solidify again along the underside of the cut, which the trade calls dross. Cutting at the wrong speed adds roughness and burrs. On thicker sheet, edge quality falls away further. And depending on the gas, the edge may carry an oxide layer or heat tint.

A maker of finishing machines sums up the result plainly: laser-cut parts come off the machine with edges that are slightly burred, and often with discolouration. A name that will sit against skin and clothing needs those edges taken back and smoothed, and a piece that is going to be plated needs a clean surface underneath. That work is the subject of Finishing by hand.

From a curve to a name

The first laser cuts in Harlow were curves because the steel was spinning on a disc. Nearly six decades later, the beam is steered with enough precision to follow the loop of a handwritten capital, and the gas jet that once blew oxygen at a test piece blows nitrogen through a cut a tenth of a millimetre wide. The principle has hardly changed: focus the light, clear the melt, and leave a narrow, clean line behind. Then pick the name up, and start finishing.