The Surprising Problem With Measuring Anything

This video traces the evolution of probe-based metrology, from the granite cubit masters of ancient Egypt to touch trigger probes and the atomic force microscope.

THE EMERGING NEED

· Craftsmen filed each part to fit its neighbor, making every product a one-off.

· Honoré Blanc's musket locks had to assemble from parts drawn at random.

· Go and no-go gauges could fail a part, but never say by how much it missed.

THE PLANE AND THE SCREW

· Maudslay's "Lord Chancellor" bench micrometer resolved a ten-thousandth of an inch.

· Whitworth's three-plate method produced true flatness with no existing reference.

· His 1851 machine used a falling "feeling piece" to sense a millionth of an inch.

THE MICROMETER

· Palmer's 1848 handheld screw gauge read directly to five hundredths of a millimeter.

· Brown & Sharpe's caliper amplified spindle travel roughly sixty to one.

· Abbe's principle demanded the scale sit in line with the dimension being measured.

GAUGE BLOCKS

· Johansson's 1896 set of 102 blocks could build around 20,000 distinct sizes.

· Lapped on a converted sewing machine, the blocks "wrung" together into a single solid bar.

· Steel's thermal growth forced a reference temperature, fixed at 20C in 1931.

THE COMPARATOR

· Logan's 1883 dial indicator turned a few thousandths of plunger travel into a full sweep.

· Zeroed on gauge blocks, it took accuracy from the blocks and sensitivity from the gears.

· The frictionless LVDT turned displacement into a voltage, resolving hundredths of a micron.

SURFACE TEXTURE

· Identical bearing journals could run for years or seize within hours.

· Abbott's stylus and the Talysurf magnified vertical motion up to 40,000 times.

· Stylus roundness testing separated diameter from roundness, exposing three-lobed pins.

THE COORDINATE MEASURING MACHINE

· Ferranti's 1959 machine read probe position from moiré fringes on optical gratings.

· DEA's three-axis bridge design set the layout still used today.

· One arcsecond of carriage pitch shifts a probe tip 500mm away by 2.4 microns.

TOUCH TRIGGER

· Flexible Concorde fuel pipes deflected away from hand-pushed hard probes.

· McMurtry's kinematic mount reseats on six contact points to a fraction of a micron.

· Tripping at a tenth of a newton, it put measuring machines under full computer control.

SCANNING AND ERROR MAPPING

· Zeiss's flexure-suspended scanning probe collected thousands of points per second.

· Every three-axis machine carries twenty-one geometric sources of error.

· Hocken's error mapping meant a machine only had to be repeatable, not perfect.

PROBING THE MATERIAL

· Brinell measured hardness from the dent a steel ball left under a known load.

· Sub-micron coatings needed dents too small to see with an optical microscope.

· Instrumented indenters feel force and depth directly, refined by Oliver and Pharr in 1992.

THE ATOMIC PROBE

· Binnig and Rohrer's tunneling microscope was effectively one atom sharp.

· The atomic force microscope touched with billionths of a newton, working on almost anything.

· In 2019, the spacing of silicon's atomic planes became a way to realize the metre.

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19th CENTURY MEASURING TOOL PHOTOS BY

The Science Museum Group

https://www.sciencemuseumgroup.org.uk/

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