Finding Perfect Part Fits With The Goldilocks Approach (and OpenSCAD)

There is something to be said for brute force or trial-and-error approaches to problems, especially when finding a solution has an empirical element to it. [Tommy] perceived that to be the case when needing to design and 3D print servo horns that would fit factory servos as closely as possible, and used OpenSCAD to print a “Goldilocks array” from which it was possible to find a perfect match for his printer by making the trial and error process much more efficient. By printing one part, [Tommy] could test-fit dozens of options.

What made doing this necessary is the fact that every 3D printer has some variance in how accurately they will reproduce small features and dimensions. A 6.3 mm diameter hole in a CAD model, for example, will not come out as exactly 6.3 mm in a 3D-printed object. It will be off by some amount, but usually consistently so. Therefore, one way around this is to empirically determine which measurements result in a perfect fit, and use those for production on that specific 3D printer.

That’s exactly what [Tommy] did, using OpenSCAD to generate an array of slightly different sizes and shapes. The array gets printed out, servos are test-fitted to them, and whichever option fits best has its dimensions used for production. This concept can be implemented in any number of ways, and OpenSCAD makes a decent option due to its programmatic nature. Interested in OpenSCAD? It will run on nearly any hardware, and you can get up and running with the basics in probably less than ten minutes.

3D-Printed Tourbillon Demo Keeps The Time With Style

It may only run for a brief time, and it’s too big for use in an actual wristwatch, but this 3D-printed tourbillon is a great demonstration of the lengths watchmakers will go to to keep mechanical timepieces accurate.

For those not familiar with tourbillons, [Kristina Panos] did a great overview of these mechanical marvels. Briefly, a tourbillon is a movement for a timepiece that aims to eliminate inaccuracy caused by gravity pulling on the mechanism unevenly. By spinning the entire escapement, the tourbillon averages out the effect of gravity and increases the movement’s accuracy. For [EB], the point of a 3D-printed tourbillon is mainly to demonstrate how they work, and to show off some pretty decent mechanical chops. Almost the entire mechanism is printed, with just a bearing being necessary to keep things moving; a pair of shafts can either be metal or fragments of filament. Even the mainspring is printed, which we always find to be a neat trick. And the video below shows it to be satisfyingly clicky.

[EB] has entered this tourbillon in the 3D Printed Gears, Pulleys, and Cams Contest that’s running now through February 19th. You’ve still got plenty of time to get your entries in. We can’t wait to see what everyone comes up with!

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Fail Of The Week: 3D Printed Worm Gear Drive Project Unveils Invisible Flaw

All of us would love to bring our projects to life while spending less money doing so. Sometimes our bargain hunting pays off, sometimes not. Many of us would just shrug at a failure and move on, but that is not [Mark Rehorst]’s style. He tried to build a Z-axis drive for his 3D printer around an inexpensive worm gear from AliExpress. This project was doomed by a gear flaw invisible to the human eye, but he documented the experience so we could all follow along.

We’ve featured [Mark]’s projects for his ever-evolving printer before, because we love reading his well-documented upgrade adventures. He’s not shy about exploring ideas that run against 3D printer conventions, from using belts to drive the Z-axis to moving print cooling fan off the print head (with followup). And lucky for us, he’s not shy about document his failures alongside the successes.

He walks us through the project, starting from initial motivation, moving on to parts selection, and describes how he designed his gearbox parts to work around weaknesses inherent to 3D printing. After the gearbox was installed, the resulting print came out flawed. Each of the regularly spaced print bulge can be directly correlated to a single turn of the worm gear making it the prime suspect. Then, to verify this observation more rigorously, Z-axis movement was measured with an indicator and plotted against desired movement. If the problem was caused by a piece of debris or surface damage, that would create a sharp bump in the plot. The sinusoidal plot tells us the problem is more fundamental than that.

This particular worm gear provided enough lifting power to move the print bed by multiplying motor torque, but it also multiplied flaws rendering it unsuitable for precisely positioning a 3D printer’s Z-axis. [Mark] plans to revisit the idea when he could find a source for better worm gears, and when he does we’ll certainly have the chance to read what happens.

Bed Of Nails And Accuracy In PCB Manufacturing

A few days ago, we mentioned the new ARM-powered Teensy 3.0 project on Kickstarter. The creator, [Paul Stoffregen], decided to share the trials of building a test fixture along with a shocking comparison of the accuracy of different PCB manufacturers in an update to his Kickstarter.

Because [Paul]’s Teensy 3.0 has more IO pins than should be possible on such a small board, the test fixture to verify if a board is defective or not is fairly complex. To test each board, a Teensy is placed on dozens of spring-loaded contacts arranged like a bed of nails. From there, another Teensy (this time a Teensy 2.0) performs a few tests by cycling through all the pins with several patterns.

Because the spring-loaded contacts require rather precise drill holes in the PCB of his test fixture, [Paul] thought it would be neat to compare the accuracy of several board houses. In the title pic for this post (click to embiggen), [Paul] demonstrates the capabilities of OSH Park, Seeed Studio, and iTead Studio. The lesson here is probably going with a US company if quality drill work is a necessary requirement of your next project.

The Cool Kids All File Their Resistors For Accuracy

Here’s a tip to keep in your back pocket, you can use a metal file to adjust your resistors. [Gareth] shows off this technique in the video after the break. A metal file is literally all that you need to do some fine tuning. Just make sure you’re starting off with a carbon film resistor as this will not work with the metal film variety.

His example shows a 10k resistor which is reading just 9.92k on his multimeter. But he needs precisely 10k. After getting through the protective layer he makes just a couple of passes with a small file, each time adding about 20 Ohms of resistance. Now he does mention that excessive deep cuts can hurt the power rating of the resistor. But this certainly isn’t damaging it if done correctly. It turns out this is how they are tuned at the factory.

One possible use he mentions is trimming the balance on a hacked servo motor.

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