The Spark in the Engine: How a Supersonic Jet's Problem Forged the Future of Machine Touch

In the early 1970s, amidst the roar of ambition and the Cold War’s technological race, something extraordinary was taking shape in hangars across Britain and France. The Concorde, a beautiful, delta-winged bird of prey, promised to shrink the globe by flying at more than twice the speed of sound (Mach 2). Its heart was the Rolls-Royce/Snecma Olympus 593, an engine of unprecedented power and complexity. But deep within its metallic guts lay a deceptively simple problem that threatened to ground the entire endeavor: measuring a thin, intricately bent fuel pipe.

The engineers could design it. They could manufacture it. But they couldn’t reliably and quickly measure it to the tolerances required. The existing tools—hand-operated calipers and large, slow coordinate measuring machines (CMMs)—were too clumsy for such a delicate, mission-critical component. A tiny deviation could alter fuel flow with potentially catastrophic consequences. The future of supersonic travel was, in a very real sense, stuck on a frustrating measurement bottleneck. The world needed a new way for a machine to feel its work.

A Brilliant Switch Born from Frustration

The problem landed on the desk of a bright young engineer at Rolls-Royce, David McMurtry. Tasked with inspecting these components, he grew increasingly frustrated with the status quo. The solution, as is so often the case with revolutionary ideas, didn’t come from a corporate lab but from a moment of quiet contemplation. He envisioned a device that acted like a switch: the instant its tip touched the component, it would send a signal. It needed to be small, sensitive, and above all, perfectly repeatable.

His insight led him to a beautifully elegant principle from classical physics: kinematic design. Imagine a sphere resting perfectly on six points of contact. It is perfectly constrained, unable to move without lifting off at least one point. McMurtry translated this into an electrical circuit. He designed a mechanism where a stylus holder made contact with a set of electrical conductors at precisely arranged points. The system was spring-loaded, holding everything in a stable, circuit-closed state.

This is the genius of the design, a concept that lives on in modern probes like the Renishaw OMP40-2. Think of it as the world’s most sophisticated light switch. According to one of the most fundamental laws of electricity, Ohm’s Law ($V=IR$), a complete circuit allows current to flow. The moment the probe’s stylus touches the workpiece, a minuscule force (often less than 100 grams) is enough to “break” the seating of the mechanism. The circuit is momentarily broken. That instantaneous event—the transition from conducting to non-conducting—is the trigger. It’s a binary, unambiguous signal that tells the machine’s computer: “FREEZE. I am here. This is the exact coordinate.” The beauty of the kinematic design is that after the touch, the spring perfectly reseats the components, ready for the next measurement with a repeatability that can be measured in a single micron, or one-thousandth of a millimeter.

Giving the Idea a Voice and Fingertips

The initial prototype worked, but for it to be truly useful inside a bustling, automated CNC machine, it needed two more things: a way to communicate without cumbersome wires, and a “fingertip” that wouldn’t damage the very part it was designed to protect.

The communication challenge was solved by turning to another part of the electromagnetic spectrum: infrared light. But in a factory filled with flashing lights and ambient heat, simply turning an IR LED on and off would be like shouting in a hurricane. The solution was modulation. Instead of a simple “on” signal, the probe transmits a coded pulse of light, a unique dialect that its receiver is specifically listening for. This is the same principle that allows your car radio to tune into a specific station while ignoring thousands of others. This “secret language of light” dramatically improves the signal-to-noise ratio, ensuring the trigger signal is received instantly and without error.

For the fingertip, the choice of material was equally critical. The probe needed to touch the workpiece thousands of times without wearing down or scratching the surface. The answer was found not in metallurgy, but in gemology: the ruby. An industrial-grade ruby ball is fixed to the end of the stylus. Why a ruby? For several profound reasons rooted in material science. First, it is incredibly hard, ranking 9 on the Mohs scale, just below diamond. This gives it exceptional wear resistance. Second, it has a very low density, meaning the stylus has low inertia and can be moved quickly without excessive vibration. Finally, it has remarkable thermal stability and is chemically inert, so it won’t be affected by temperature fluctuations or the harsh cutting fluids inside a machine. It is, in essence, the perfect, incorruptible touch.

From a Single Engine to a Global Revolution

McMurtry’s invention solved the Concorde problem. Recognizing its immense potential, he and John Deer founded Renishaw in 1973 to commercialize the technology. The touch-trigger probe was born.

Now, let’s fast-forward half a century. The Concorde is a museum piece, but its legacy roars on, silently, in hundreds of thousands of factories worldwide. In a modern “lights-out” manufacturing facility, a sleek robotic arm places a block of titanium into a high-speed machining center. Before a single chip is cut, a device descends from the tool magazine. It’s the OMP40-2, a direct technological descendant of that first bedroom prototype. It glides through the air and gently touches the block at several points. It’s not just checking for presence; it’s building a 3D map of the part’s exact position and orientation. It’s telling the machine’s brain precisely where the physical world of atoms begins. The machine then begins to cut, and later, the probe may return to inspect a critical feature, compensating for tool wear in real-time. This is the fulfillment of McMurtry’s vision: a world where measurement is not a separate, time-consuming step, but an integrated, intelligent part of the manufacturing process itself.

This small, brilliant device was more than just a new tool. It represented a philosophical leap, creating the essential bridge between the physical world of atoms and the abstract, digital world of computer code. It gave the machine a sense of touch, a nerve ending that could translate reality into data with unerring fidelity. The spark of ingenuity, born from a supersonic jet’s challenge, ignited a revolution in precision that continues to shape our world with every single, silent, perfect touch.