Optical Comparator

Hartness and Porter claim comparison of a projected profile against a master outline. Write down why a caliper cannot check the angle of a thread flank.

Attempt to measure the flank angle of a #6-32 screw with the caliper. Note how badly it goes.

In a darkened room, hold the screw in a narrow beam of light so its shadow falls on white paper on the wall.

Move the screw closer to the light. The shadow grows. Magnification costs nothing but distance.

Measure the real screw diameter with the caliper and the shadow with the ruler. The ratio is your magnification. Write it down — an unknown magnification makes the shadow useless.

Make the light source smaller by masking it with foil pierced by a small hole. The shadow edge sharpens because a point source casts no penumbra.

Bounce the beam off the acrylic mirror to lengthen the path without lengthening the bench. Hartness and Porter did exactly this to keep the instrument compact.

On graph paper draw the correct thread profile at your magnification: 60° included angle for a standard thread. Use the protractor.

Draw a second outline slightly outside the first. The gap between them is the tolerance band, drawn rather than calculated.

Project the screw onto the chart. If the shadow stays between the lines, the part is good. No number was read and no judgement was made.

Now measure the angle directly on the enlarged shadow with the protractor. Trying this on the metal part is what step 2 showed to be impossible.

Deliberately blunt one screw's thread, project it, and compare with a good one. Damage invisible in the hand is obvious on the screen.

Project a drill bit and measure its point angle from the shadow. Compare with the 118° you measured directly in the twist drill blueprint.

US 1,703,933, “Optical Comparator”, James Hartness and Russell W. Porter of Springfield, Vermont, filed 21 May 1925, granted 5 March 1929. It was made by the Jones & Lamson Machine Company and remained profitable for decades.

Two remarkable men. Hartness was a machine-tool designer with well over a hundred patents, president of the American Society of Mechanical Engineers, an early aviator, and Governor of Vermont from 1921 to 1923. Russell W. Porter is usually remembered under a completely different heading — as an Arctic explorer and the founder of the amateur telescope-making movement at Stellafane, and later as the illustrator of the 200-inch Palomar telescope. He is a named co-inventor on this patent, which is often overlooked.

Why the same skills. Porter's telescope work and this comparator are the same discipline: getting light to carry a shape accurately from one place to another. The lens grinding and Foucault knife-edge blueprints linked here are the astronomical branch of the same craft — and the knife-edge test, like the comparator, works by turning an invisible error into a visible pattern.

The idea that ends this batch. Palmer made the thousandth readable. Starrett made it reproducible. Johansson made it a physical standard. Norton made it manufacturable. Hartness and Porter made it visible — and once an error is visible on a screen, checking it needs no metrologist at all. That is the whole century of this batch in one sentence.