James Stanley

Miscellaneous thoughts on making stuff

Tue 15 September 2020

This is just a selection of points related to manufacturing that are grouped into one post for no other reason than that I don't have enough on each one individually, but wanted to write about them before I forget.

Are our 3d parts constrained by our 2d paper?

Before CAD-modelling or hand-making a part, it is almost always transformed from an abstract model in the designer's mind into a sketch on the designer's paper. This helps clarify thinking, ensure that moving parts won't crash into each other, etc.

A simple part can be sketched from just one view. More complex parts can have top, front, and side views, as well as a 3d projection, cross sections, and whatever other views are useful.

A part gets harder to sketch as more views are required, and more effort is required of the designer to ensure that all of the views are consistent with one another. It is therefore natural to contain as many features as possible within a single view.

I contend that this results in a natural preference for features which lie in a common plane, even when the optimal solution to the problem has no such constraint.

How much does this preference for planar features restrict the space of designs that humans can come up with?

What would we gain if it were easier to explore more of the design space?

How can we make the design space easier to explore?

Why so many 3d printing designs suck and why that's a good thing

If you spend some time browsing Thingiverse then you might notice that some of the parts are designed very strangely. Parts that have sharp and square edges where rounded edges would look neater, thinner or thicker than appropriate, strength in the wrong places, inappropriate fasteners, and so on.

Your first reaction might be "this is clearly bad, these people are no good at designing things". I think the opposite way of looking at it is more useful: people with no training or experience are now able to design and produce parts that successfully solve their problems. It's not that designing for 3d printing results in worse parts, it's that the barrier to entry for design of working parts is much lower than it ever has been in the past. And I think that is a good thing.

How fast can you 3d print stuff?

An FDM printer has to raise the temperature of the plastic it prints from 20°C to 200°C. The specific heat capacity of PLA is 1800 J/kg.K (source). To print a 1 kg part, we need to raise the temperature of 1 kg of PLA by 180°C, which we know from the specific heat capacity will take 1800 * 180 = 324 kJ.

We can get 230 volts * 13 amps = ~3 kW from the AC supply. 1 W = 1 J/sec, so we can expect to bring our kilogram of PLA up to the printing temperature in 324 / 3 = ~100 seconds.

In practice, today, printing a kilogram of PLA on a home machine will take well over 24 hours, so there is potentially a lot of room for improvement here. Current home-level 3d printers can't spend all 3 kW on heating the plastic because of losses in:

The fans, microcontroller, and display should be negligible, so we only need to worry about heating the plastic and the bed and driving the stepper motors. With "sufficient" insulation, keeping the bed hot for the duration of the print should tend towards 0 energy usage, because once we've raised the temperature at the start, we don't want to put any more energy into it, we just want it to stay where it is. That means that in the limit case, we only need to heat the extruded PLA and run the motors, and everything else in the machine can use the last 10% or so of spare energy.

In addition to constraints on how fast we can heat the plastic, we also have constraints on how fast we can move the motors. I suspect that in practice this is currently the bigger constraint. One way to get around this is to use a larger nozzle, fatter extrusions, and taller layers, so that you can get more plastic out for any given movement of the motors, although this does impact the quality of the parts you can print. Another option is to work on moving the motors faster. This requires more powerful motors (and therefore more energy usage), and also a stiffer machine so that it doesn't wobble under high acceleration.

But fitting a larger nozzle and a stiffer frame doesn't impact energy usage, so we can take these as a given. I expect that if somebody created a 3d printer that optimised the ratio of motor power usage and hot end power usage to maximise printed speed, we would be able to get close to at least an order of magnitude away from the theoretical optimum, which would mean that printing 1 kg of PLA within 15 minutes on a home machine is within the bounds of plausibility, provided you're willing to trade off a bit of quality for speed.

Whether this is a project worth pursuing is not so clear. The typical home user isn't really time-sensitive when it comes to 3d printing, particularly if reducing print times would make the machine more expensive. Personally, I deliberately run the printer slower than it can possibly go because it results in higher-quality parts. The people who are most sensitive to printing speed are probably mass-producing things, which means they're probably not using 3d printing in the first place. But even if they are 3d printing, they probably have a lot more than 3 kW at their disposal, so the attainable performance is even higher.

What material would you make stuff out of?

In prehistory, ancient man built things out of sticks and stones. This is not because they're particularly good materials, but merely because they were convenient to acquire and easy to work with. Since then, we've learnt to work with ceramics, glasses, metals found in the ground, man-made alloys, plastics, composites like carbon fibre-reinforced plastic, and probably a lot more stuff I don't even know exists.

We have a wide array of materials available today, but we're still largely constrained to working with materials that are convenient to acquire or produce, and easy to work with. This does not necessarily result in optimal parts.

If we had omnipotent levels of control over where we placed the fundamental particles that make up our parts, I don't think we'd end up with materials that look much like what we have today. You could imagine parts that aren't even made of identifiable atoms at all! Just quarks arranged in ways that optimise whatever material properties are useful in any given application. That would allow us to achieve the actual optimal levels of lightness, stiffness, strength, conductivity, heat resistance, and so on, without any constraint imposed by the specific elements that happen to be easy to dig out of the crust of the earth, or the processes that we can apply to them (which is mainly glorified versions of heating them up and smashing them apart). I think graphene offers a glimpse into what could be possible here, except we only even know about graphene because it is comparably easy to manufacture! There are surely much more advanced possibilities still out of reach.

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