A frosted glass disk with geometrical markers

Using A Laser Cutter To Replicate An Optical Comparator Screen

Precision instruments often contain specialized components that are essential to their function, but nearly impossible to replace if they fail. [Andre] had just such a problem with an optical comparator, which is an instrument typically used in machine shops to help check the tolerances of a finished part. It does this by projecting a magnified picture of an object onto a glass screen with markings showing angles and distances.

In the old comparator [Andre] bought on eBay, the markings on the glass had faded to such a degree that the instrument was almost unusable. So he contacted [James] over at Clough42, who was able to create a near-perfect replacement screen by using a laser cutter, as shown in the video embedded below.

The first step was to replicate the screen’s markings in a CAD program. [James] explains the process in Fusion 360, demonstrating how you can generate all the different scales nearly automatically through the proper use of constraints, variables and patterns. He then transferred the drawing to Lightburn, which drives the laser cutter and etches the markings into a sheet of glass covered with CerMark, a marking solution that turns a deep black when heated by a laser.

After etching, the final step was to apply frosting to the glass to turn it into a projection screen. While there are several ways to achieve this, [James] went for a simple spray-based method that gave surprisingly good results. It took a few experiments to find out that etching the markings on the back of the glass and applying the frosting on that side as well gave the best combination of sharpness and durability.

[James]’s project shows that even delicate instruments with custom glass components can be repaired, if you just have the right tools. A similar strategy might also work for creating custom scales for analog meters, or even old radio dials. If you’re not familiar with laser cutters, have a look at our experiments with an Ortur model. Thanks for the tip, [poiuyt]!

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Budget-Friendly Bend Sensor Deforms With Precision

We’re pretty familiar with budget resistor-based bend sensors at this point, but this sensor is in a totally different class. Instead of relying on resistive elements, [Useok Jeong] and [Kyu-Jin Cho] devised a bend sensor that relies on geometric properties of the sensor itself. The result is a higher-fidelity measuring device made from a pretty widely available collection of stock parts.

We’ll admit, calling this device a bend sensor might be a bit of a stretch, so let’s dig into some of the operating principles. What we’re actually measuring is the accumulated angle, the sum of all the curvature deformations along the length of the sensing element. The sensor is made of 3 main pieces: an outer extension spring-based wire sheath; a flexible, tensioned inner wire core that’s fixed at one end; and a small displacement sensor that measures the length changes in the wire’s free end. The secret ingredient to making this setup work is a special property of the outer wire sheath or spring guide. Here, the spring guide actually resists being compressed while being bent.  Because the inner radius of the bend remains a constant length, the center wire core is forced to elongate. With the excess wire spooled up at the sensor base, we simply measure how much we collected, apply some math, and get a resulting angle! What’s more, the folks behind this trick also demonstrate that the length and angle relationship is linear with an R-square of 0.9969.

One of the best parts about this sensor is how reproducible it seems from from a modest collection of stock parts. Spring guide (aka: extension spring) is available from McMaster-Carr and DR Templeman, and that flexible core might be readily substituted with some wire rope.

It’s not everyday that new topologies for bend sensors pop into the world, let alone linear ones. To learn more, the folks behind the project have kindly made their research paper open access for your afternoon reading enjoyment. (Bring scratch paper!) Finally, if you’re looking for other bend-related sensors, have a look at this multi-bend measurement setup.

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