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Comment by zellyn | Hacker News Reader

From the video, it appears that they are dipping by 1cm, measuring volume, dipping by 1cm more, measuring volume, etc. So they end up, effectively, with a set of 1cm slices and the volume of each slice. For one orientation. Then they repeat, getting slices at a different orientation.

Not only would it not work with something absorbent, but (per other comments below) does not (currently) work with shapes that trap air or catch liquid.

I think they're 3D-printing test shapes because it's the simplest way to generate interesting test structures. Otherwise they'd have to manufacture (for example) a bunch of columns plus a ball using metalwork or wood or something. Some of their models look like they were originally scanned from small sculptures -- although in this day and age, it's possible they were just created as 3D models in the first place.

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Those are valid limitations, but current 3d imaging techniques I know of are either with photography, laser scanning or touch probing. All of the above also face those limitations.

My initial interest was that it would bring the cost down a lot, but they don't seem to bring great precision and they require a moving robot with at least 3 axes which is unlikely to be very cheap for precision material.

You don't need a robot arm at all. Consider clamping the test object inside a transparent sphere, resting on wheels that allow the sphere to be rotated. Instead of dipping, partly fill the sphere with liquid, test displacement at many orientations, then return to upright where more liquid can be added, repeat. You'd get the same measurements, just in a different order.

The robot arm is expensive because it needs precision hardware (backlash free bearings/gearing).

Your idea also requires precision hardware to move the sphere.

Actually besides 3 step motors ( for rotating in three axis ), rubber wheels to hold the sphere and a sensor to measure the water level, I don't think you need more. What I don't know is how much water you need, probably this can also be computed.

It's a better idea than dipping with a robot arm. And also faster.

Yeah... I just wondered why they were 3D printing things instead of getting normal, everyday objects that they could dip like keys, cups, pens, etc. I have a feeling it's not detailed enough for those use cases yet.

I think the main reason would be that they can compare their scans directly against the original models.

If they were to use real world objects, any comparison would have to use another scanner, which would introduce its own bias.

This method would only work with objects that are rigid in all orientations, too - so bunches of keys are out.

That seems backwards. Keys, cups, and pens are pretty trivial shapes. (Assuming you can get good angles on the cup so that it doesn't have air pockets.) More complex shapes like the weird statues they're using seem like a more robust test.

Keys and pens have fine details that would be pretty impressive to scan accurately.

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