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Posted by scrlk 8 hours ago

Lego's 0.002mm specification and its implications for manufacturing (2025)(www.thewave.engineer)
304 points | 250 commentspage 2
Thorrez 7 hours ago|
>The frequently cited "0.002mm tolerance" is misleading without context. LEGO's actual mold precision is 10 microns, but different features have different critical tolerances.

The article never mentions what piece has a 0.002mm tolerance. Is there any such piece? If there's no such piece, then "0.002mm tolerance" is not just "misleading without context", it's straight up false.

ozlikethewizard 7 hours ago|
Is it a language mixup, ±0.001mm being called a 0.002mm tolerance? Otherwise I cant figure it out either lol.
Karliss 6 hours ago||
0.001mm is 1 micron not 10.
rkangel 6 hours ago||
The tolerance for interference fit ("clutch power" in Lego terminology) is important, but that's fairly simple. It's the cumulative tolerance when you assemble large structures that's important. Knockoff bricks can be fine for the first few you assemble, and then as the structure gets larger things don't quite fit together.

Also interesting is that in very large models, there is decoupling between sections. Lego has design rules for how large a well connected chunk of Lego can be, which are driven by the tolerances. Above that you are then loosely coupling those large "chunks".

jjk166 44 minutes ago||
That's actually not too difficult. So long as your process variance is centered around nominal, the stackup will tend to cancel out. You might run into trouble if your kit involves hundreds of identical pieces from the same batch being assembled together, but that's rare. For large builds from multiple kits, it's very unlikely they have the same errors.
perfmode 3 hours ago||
This is the most interesting point in the thread to me. Tolerance stack-up is the reason tight per-part tolerances matter at all. A single brick being precise is table stakes for injection molding. The hard problem is what happens when you compose hundreds of them. The decoupling strategy you're describing is really similar to how you handle error accumulation in any large composed system. You can't make individual components perfect enough to avoid drift at scale, so you introduce boundaries where the accumulated error gets absorbed rather than propagated. In Lego's case that means designing joints between sections that are forgiving enough to accommodate the stack-up from each chunk independently.

It's also why knockoff bricks can feel fine for small builds and then fall apart (sometimes literally) on larger ones. If your per-part tolerance is 3x worse, it doesn't matter much for a 20-piece build, but for a 2000-piece build your cumulative error budget is blown long before you're done. The failure mode isn't that any individual brick is bad, it's that the composition doesn't hold.

I'd be curious whether Lego publishes or talks about those chunk size design rules anywhere. That seems like the actually interesting engineering story, more so than the per-part tolerance numbers that get repeated in every article about them.

nmeofthestate 6 hours ago||
"A minifigure head mold evolved from 8 cavities in 1978 to 128 cavities today."

Initially I thought this meant a lego minifig head has 128 internal cavities, but finally realised it means a single mould now makes 128 heads.

solidsnack9000 4 hours ago||
I would like to better understand the reasoning behind what the author says here:

A balanced 16-cavity mold costs 3-4x more than a single-cavity mold but only produces 16x the parts, which is why they only make economic sense above 500,000 units.

s0rce 2 hours ago|
I guess if a single cavity mold costs $30,000 and a 16 cavity mold costs $110,000 you have an additional expense of ~$80,000, divided over half million parts is 16 cents. So lets say somewhere from 5-10cents per part to go 16x faster. My numbers might be off a bit but seems in the ballpark. I also don't know much a lego brick costs to make in terms of materials/opex.
yubainu 5 hours ago||
I always thought it was amazing how Lego pieces fit together so perfectly that they wouldn't come off even if you lifted them, but if you wanted to remove them, they came off so easily, and I had no idea they were that precise.
lich_king 6 hours ago||
This is an LLM-written article. It also doesn't say anything. I get it that it's a cue for us to reminisce about childhood and say that LEGO isn't what it used to be, but we're being played for clicks. Open the article and look for a single statement that actually tells us something meaningful. It's just a sequence of impressively-sounding factoids like this:

> A 2x2 brick can withstand over 4,000 Newtons of force, which lets children build tall structures.

> But in an assembly system like LEGO's, small errors accumulate. Stack ten bricks end-to-end and the cumulative tolerance is ten times larger. This is why LEGO models larger than 1 meter become difficult to build

> The lesson isn't that everyone should match LEGO's tolerances. It's to understand what your product actually requires, then build your manufacturing system to deliver that at the scale and cost your business model demands.

I know I'm tilting at windmills, but come on.

rkangel 6 hours ago||
I agree, it doesn't say a lot. It also very confidently specifies a series of tolerances with no citations.

Lego does indeed have very tight tolerance, but I don't know if the numbers are in the public domain.

isoprophlex 6 hours ago|||
I too hate it when my kids apply 4 kN of force to off-brand construction bricks and they turn to ABS paste. Only LEGO (R) for my spawn!
aaron695 6 hours ago||
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twodave 5 hours ago||
Both of my boys (9 and 11) still enjoy both the sets and the classic Legos. They're constantly building trucks, trailers, etc. One even designed his own working dump-truck. They're still great toys for imaginative play, and the fact that the sets can be broken down and used in new ways just keeps the fun alive. My oldest even designed and had his grandpa build him a lego table with a removable/reversible top so he could paint different geographies for his cities and whatnot that he likes to build.
WillAdams 7 hours ago||
Curious how this might have played out over the long-term with their licensed/abandoned/revived/then bought to kill permanently "Modulex":

https://archinect.com/features/article/149974598/the-brief-a...

I wish one of their competitors would take up this dimension standard --- it would be a lot more useful for making structures which interact across dimensions/rotations.

Normal_gaussian 7 hours ago||
"that familiar click is the sound of a carefully engineered interference fit designed to hold firm but still be easy for small hands to pull apart."

My recent experience calls bs on pulling them apart.

doubled112 7 hours ago||
I always remember the small, weird pieces being hard to get apart.

What I don't remember was every kit being made up of so many small, weird pieces.

ralferoo 7 hours ago||
When I was a kid, the first "special" Lego kit I remember was the Star Wars sets in 1983 (and especially that everybody wanted a Millenium Falcon but I didn't know anybody who had parents that could afford one!)

Apart from those Star Wars kits, everything I had were generic blocks and strips (not sure what they're called, the ones that are 1/3 the height of a block) and some different designs of people. The closest I had to previous special sets was a town thing that my brother and sister had before me (they were 10 years older), which was a bunch of large floor tiles with roads and grassy areas with studs, some flowers pieces (single stud) and a handful of special buildings. But they were designed to be relatively generic, and the fun was using those building blocks to make a new city each time, not trying to recreate exactly someone's model. Apart from the flowers and the men, basically everything was a standard part, except perhaps a different colour.

When I was a teenager, the trend had become sets with lots of specialised parts for one specific model, such that they didn't really make sense as generic pieces. I enjoyed the technics kits because the early ones were just generic building blocks (apart from the wheels and rack and pinion, but again they could be re-used in lots of subsequent designs), but more and more the kits in the shops were for specialised models with unique pieces that were never designed to fit aesthetically with anything other than the model they came with. I'm sure _some_ people built other things with them, but equally I'd bet than probably 90% of those kits were built exactly once following the instructions and then never disassembled again.

bena 6 hours ago||
The elements that are 1/3 the height of a brick is a plate if it has studs, and a tile if it does not.

Lego did not have Star Wars sets until 1998. The original Lego Millenium Falcon set 4504 would have retailed for right around $100. Which was high, but just as high as the bigger Castle sets at the time.

SoftTalker 5 hours ago||
They definitely had lunar/space themed sets in the '80s, but they were generic (at least the ones I had). I don't recall when the Star Wars sets came out, they might have been one of the first cross-promotional tie-ins that Lego did?
bena 5 hours ago||
Star Wars sets started coming out in 1998. They weren't the first licensed sets, but the first fictional license.

Prior to Star Wars, they had Shell, Exxon, and Esso branded sets. I think sometimes they licensed the Ferrari brand as well.

And yes, Lego has had a Space theme since the late 70s. But it was a general "Space" theme. They would later make Space Police, Blacktron, Magnetron, etc.

But actual Star Wars was 1998. I have some of those sets. It was a big deal to get an actual lightsaber hilt and blade.

ralferoo 4 hours ago||
Very interesting. Googling shows some generic space themed things from the 80s like you say, but no Star Wars. I guess my old age is finally catching up on me and my memories have all blurred into one. I did find a Millennium Falcon from 1983, but it's definitely not Lego.
Zanfa 6 hours ago|||
Having grown up playing with LEGOs, I can still distinctly remember the feeling of sore fingers pulling tiny pieces apart after a long session. It wasn't until a few years ago I learned there's an official brick separator tool [1]. Would've changed my life as a kid.

[1] https://www.lego.com/en-us/service/help-topics/article/lego-...

jjk166 7 minutes ago|||
> It wasn't until a few years ago I learned there's an official brick separator tool

You mean a little brother?

bombcar 1 hour ago|||
There are multiple brick separators, with different strengths and weaknesses.
intrasight 7 hours ago||
The tolerance is definitely more applicable to the getting them apart then putting them together.
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