Combining Lego and 3D printing shows again how much engineering went into those stupid little Lego bricks. I can't print as accurate as their factory produces them, but I have to get as close as I can to make things usable.
That's what this post is about, and what these models are intended for.
Dimensions and tolerances
It's actually amazing how accurate and controlled those dimensions and tolerances are! Blocks made 20, 30, 40 years ago still work today. Long and short elements can be combined without any problems, and the 'snap' is 'just right' for kiddies (and adults 😅).
One thing I noticed is there are a few common, standardized dimensions, that Lego blocks are supposed to have. Width = 8 mm, height = 9.6 mm, stud = 4.8 mm.
Right?
Nope.
There are some minute differences between different types and sizes of Lego bricks. I used a micrometer to check, and yes, Lego isn't always sticking to the original numbers. So why would I? 😁
Actually, I can't use the official, original numbers. The material I work with (filament) is less homogenous, the surfaces will be irregular, my tolerances will be larger.
For example, the model I use for a very regular standard Lego brick uses this (Fusion) sketch:
Studs are supposed to be 4.8, but after measuring several Lego blocks (using a micrometer), it seems Lego uses 4.9 as well. In the end, I settled for 5.0 mm.
Down the rabbit hole
I designed a test object like this:
In the image above you see my test model (10015-5) to help me dial in my printing. I used an orange , printed 4x2 brick (10010-4), to see how well it interacts with my own print and other real Lego blocks.
I noticed that my model - printed in PETG - got just a little, little bit smaller, so I used the scale object option in Bambu Studio, and set that to 100.1, and the x-y contour compensation to 0.02.
These values depends on your printer and filament!
This led me down the rabbit hole of printer calibration, and I tried different ways to verify tolerances, distancing, and shrinkage. In the end I used the following model to check for size:
After multiple prints and checks I found out that sometimes (and I don't understand why) the dimensions of the extremities of an object may be off (so, the 127.8 length might become 127.5 or something similar), but the distances between the verification markers (the triangles at 100.0 mm in the drawing above) stayed very close to the right size. So, that's not shrinkage, and it ain't contour compensation. either. Might it be a slicer bug? Who knows.
Anyway, here's the process to check your printer:
Check (and adjust) your printer
Skip this if you already know how to do it. I didn't, and I'm still learning... These are my steps.
1. Print 10017-2 and let it cool down.
2. Place two original blocks on top of the printed part, and check the dimensions 100.0 and 127.8. Measure the 127.8 over the Lego blocks, not the length of the measured part.
If you measure something a little below 100.0 and 127.8 (i.e. both measurements are off), then that means your printed object has shrunk after printing. If that is the case, you may have to adjust the size using the Scale option in the slicer.
3. Another way to check is that the gap between the blocks is the same everywhere. Note: not all Lego blocks and series have exactly the same dimensions, so make sure the four (original) parts you use are identical!
You can add more bricks and check the whole stack. When using the same bricks, the gaps should be identical everywhere.
4. Place three original bricks on top of the printed part, as shown below. They should go on evenly, and come off evenly.
Now, place the whole assembly on a flat surface. If the bottom of the printed part doesn't lie flat on the surface, it's either too small or too large.
5. If any of the tests above failed, then you need to adjust the Scale of the object, then go back to step 1 and repeat the print and tests.
6. If the fit isn't 'tight' enough then you have to increase X-Y contour compensation in your slicer, or play with material shrinkage options.
7. If it all fits well and the pieces snap and break loose in a satisfying manner, then it is time to print these three pieces:
8. Fiddle around with all the bits and pieces, and adjust the printer parameters until you get it right.
Results
I will upload these models to Makerworld.
Next step: a Frankenblock base plate, a larger object and with more studs. Let's see how that goes...
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