3D printing LEGO compatability: Pip Size/seperation test
So over the last month and a half or so I’ve been sending out and receiving some prints specifically to test for how easy/difficult it is to 3D print parts with the goal of having them mesh up with LEGO pieces.
Some Background:
First off, Lego’s specifications themselves for their pieces(image ref 1):
LEGO prints their pieces with a ridiculously high tolerance(2); 2μm or .002mm which in general is well well below the layer thickness available in 3D printing right now(3), and even horizontal accuracy requires special processes, and expensive ones, to approach that level of accuracy.
Because of this I decided to try sizes of pips at the specification and smaller. The first two tests I tried for sizes were done with sintered printing from Shapeways.com, while the third test was done with my monoprice Maker select 3D printer that I bought after the first two tests were already performed.
First Test:
The first test piece was really basic as I was also checking a couple other things with the piece (the underside has a series of different font sizes to test resolution in 3D sintered printing with Shapeways; specifically their “white strong & flexible”)
Using the tab at the bottom as a reference point, Designed pip size from closest to farthest from it are 4.8mm,5mm,4.9mm, with all of them having the same separation of 3mm. Measured pip size from printing ended up being (4.76mm to 4.96mm), (4.89mm to 5.15mm), (4.84mm to 5mm) with all of them measuring 3.1mm separation.
The reason there is a range for each pip set is that the print orientation ended up being the tab facing z+ (up) so the only half of the pips geometry faced the accurate xy (horizontal) printing direction. That was a mistake on my part, but I did find that if any direction in the pip is >5mm you run a very high chance of snapping a LEGO mans leg at the weak point where you can see an injection mold line.
Second Test:
The second test was much more role specific in its design, with labels and everything to keep things organized, and the pips oriented properly in the horizontal for printing.
As is labeled the top section is designed to be 5mm, while on the underside it is labeled as 4.9mm. The visible measurement side shows separation (in the direction of the 2 pips length) in .05mm increments from +.1mm to -.1mm deviation from the standard 3mm separation. This left me with 4 gaps between pips in the long length that the other side of the plate has labeled with pip separations in increments of .5mm as well, but ranging from +.075mm to -.075mm. This gave me an overall separation test that goes from +.1mm to -.1mm in steps of .025mm.
Measured results for pip size on this plate ended up being 4.77mm on the 4.9mm side, and 4.85mm on the 5mm side. While gap size was fairly consistently off by +.1mm for the 4.9mm side, and +.15mm for the 5mm side.
As far as what did a LEGO piece prefer depends on the size of the piece. (all measurements listed as measured sizes).
A LEGO man using 2 feet finds 4.77mm far too small and falls off regardless of pip separation. 4.85mm holds him just fine on all separations with a strong preference for -.1mm separation (with print error this ends up being 3mm separation). When using only 1 foot the 4.77mm pips are again too small, while the 4.85mm pips hold him well, not super strongly, but he can be held upside down and poked/jabbed without falling off.
Larger plates were tried and a 3x4 plate had trouble holding onto or snapping into the 4.77mm side, while conversely on the 4.85mm side it joined strongly regardless of orientation or number of covered pips.
From this I’m concluding that as a generalization pip separation matters far less than actual pip size for purposes of designing a part to be compatible with LEGO pieces.
Test 3:
For the third test simply refer back to test 1′s picture for reference as I printed out the exact same piece but on my new printer at home. The difference being the first and second tests were sintered printing, while this last one was PLA.
Designed Sizes are the same as the first test, so it is only the measured that will have likely changed. The PLA print resulted in a higher xy accuracy than either previous tests with the pip pairs being 4.78mm, 4.95mm, 4.92mm, closest to farthest pairs from the bottom tab.
The 4.78mm pips not surprisingly barely held a LEGO man with 2 feet (and he would fall off upside down) and wouldn’t with 1 foot. the 4.95mm and 4.92mm pips both held the LEGO man, with the 4.95 holding him better.
The most interesting result I noticed with this test though was how the hold strength of the plate changed over continues use and testing. After putting on and taking off the LEGO man at least 50 times it began to get much easier to do so, so much to the point that the first pair of legs I was testing with would no longer hold onto the 4.92mm batch (I went back and measured again and saw a marked thickness decrease in the pips in one direction of -.1mm or so). I tested it with a new set of legs and hold strength did increase again so as far as I can tell from measuring and experimenting PLA will wear LEGO pieces quickly depending on the angle you use to remove the LEGO from, and PLA itself will deform or wear quickly as well. As a result I suggest if possible 100% fill for at least the pips themselves during printing.
It should be noted I did not see these wear issues with sintered white strong & flexible.
Conclusion:
4.9mm to 4.95mm seems a safe place for individual pip size to protect the LEGO pieces from having too much stress placed on them if a print goes over the 5mm mark.
Pip separation matters very little as long as you design for 3mm separation and have print error keeping it around ±.1mm.
The biggest issue I can see, because of LEGO tolerances, is consistency with printing. Given that >5mm you risk damaging the LEGOS, and <4.8mm you risk LEGOs not holding, you have a working range of about .2mm to work with. When printing at home this is easily solved by a one time test to see how far your pips are off after calibration, but if the desire is to print using a 3rd party (Shapeways) or provide the product for purchase by others on online sites, error will change depending on the machine or orientation. At Shapeways at least you can indicate orientation for a print, but you still have to cross your fingers a bit for low error.
Additionally, the more LEGO pips you can make your designed piece connect to the better you will be for holding strength.
If anyone has any questions on this that I didn’t cover feel free to message me.
So, how does Shapeways and companies like that work for 3D printer folk?
Shapeways is a printing service. Allowing you to print items on higher end printers that most people will never have available in their houses. Then they also have the e-commerce angel. Where content creators can set up shop and sell their designs.
It also means you don’t need to own a printer if you don’t want to. OR the amount of printing you are going to do is so small. It would be cheaper to use Shapeways or a site like it instead of owning one yourself.
