J. Peterson's Writing About Electronics, Reviews, 3D Modeling, etc.

Tag Archives: fit test

The Fit Testing Block

largesteel
Fantasy version of the fit testing set, milled to the micron in stainless steel

How Accurate is Your 3D Print?
I Tested Nine Services to Find Out

Decades ago, if you wanted to create your own printed circuit board, you had to make it yourself. This was quite involved – you bought a copper plated blank board, created the circuit patterns on it with resist (tape, rub-on patterns, or Sharpie), then dunked it in noxious chemicals to etch the unmarked copper away. It was also up to you to drill the holes. Then you tracked down problems because too much (or not enough) copper was dissolved, opening or shorting your circuits. It was messy, tedious and frustrating.

Here in the future, all those problems are solved. You design the board in a CAD program, then the Internet whisks the design to a far-off fabrication house. A few weeks later your boards arrive, perfectly manufactured with solder masks, silk-screen markings, plated through holes – things you’ll never get making them at home. 

I do some amount of 3D printing, but I have no desire to own a 3D printer. Running your own 3D printer still feels like those early days of homemade circuit boards. The tedious details of bed leveling and heating, getting the model to stick in place, support sprues, filament moisture content, speed settings all have to be carefully looked after to get good results. I prefer the modern circuit board model for 3D prints – send the model off to a factory with equipment and materials you’ll never afford, and get your polished results back in the mail.

When I started ordering 3D prints, I had a basic question: when designing two parts that fit together, how much space do you need to leave between them to fit properly? To research this, I designed a set of fit-testing sets and printed them with various processes to see what sort of tolerances are necessary. So far I’ve tried nine different prints, and (unlike circuit boards) the results vary considerably.

The fit testing sets consist of two parts, a pair of pegs, each with round and square ends. One peg has 5mm ends, the other is 10mm. The other component is a block with a range of holes of different sizes to test fit each peg. Printing these and testing which hole each peg fits in helps to determine how much extra margin your design needs to have parts fit together well. For example, if the peg fits comfortably in the +0.2mm hole, that means you’d better leave a 0.2mm gap in your design for the parts to work smoothly.

The first blocks I made ranged from -9.85 to 10.15 (in increments of 0.05mm) for the large holes, and -4.875 to 5.125 (with 0.025mm steps) for the small ones. I quickly discovered this wasn’t enough range for coarser printing processes, so I created additional blocks with wider ranges of 9.7 to 10.3 (9.75 to 10.25) and 0 to 0.6 (0 to 0.5). These doubled the increment steps of the previous blocks, from 0.05 / 0.025 mm to 0.1 / 0.05.

The last block works well for the coarsest printing technologies, and is the best place to start for basic FDM (fused deposition modelling) printers. The rendering above also shows some quarter-step peg sets I modeled for even more precise tests, but you won’t need those anywhere outside of a Swiss watch factory.

Over the past few years I’ve printed blocks and pegs with a number of different services, to get a feel for the accuracy and tolerances of each. Read on for a review of what I discovered.

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