CORE One Mini: Prototyping is not always fair and square

If there is one thing this stage of the CORE One Mini reminded me of, it's this: prototyping is not always fair and square. And in this case, that is meant quite literally.

I spent about a full day cutting each profile to length and making the cutouts for the base profiles and the rear vertical profiles of the CORE One Mini frame. On paper (in CAD), everything looked clean, aligned and well constrained. In reality, with the tolerances being this tight, it quickly turned into a constant loop:

assemble - measure - disassemble - file away a few tenths of a millimeter - reassemble - repeat. When you are working within a few 0.1 mm, every stroke of the file matters.

Note square - in both senses

After the first full dry assembly of base assembly and the the rear frame, I looked at it: and it wasn't square.

Not dramatically off, but enough to matter - especially on a CoreXY-style machine like the CORE One Mini, where frame alignment directly affects motion accuracy and belt paths.

So what followed was exactly what i had done before: disassemble - modify parts - reassemble. A bit more filing here. A slightly adjusted cutout there. Checking diagonals. Tightening in a different sequence. Slowly sneaking up on proper squareness instead of forcing it.

Currently the the base assembly and both rear profiles are done and square - next up will be the front and top profiles - i want to get all the profiles done before making the easier parts, like the CoreXY plate and the Back assembly.  

An easier route would definitely have been to either have the corner profiles custom-made or or make them myself. That would have eliminated a lot of manual tuning but with more time than money, this is the way to go :) For the next iteration of Design the CORE One Mini i aready have ideas how to streamline to profiles - that's likely the smarter path for anybody that wants to build one in the future.

But for a prototype, this approach is good enough. It exposes the weak points in the design, where i missed holes - where some thing is too tight or does not fit at all - something CAD alone will never fully reveal.

CAD vs. reality

As expected, a few hurdles showed up during the first assembly. Not every hole lined up perfectly. I found several that I either missed in CAD or placed slightly too close to the edge. For example the mounting holes for the back- and side panels. Those will need to be moved upward in the final design to maintain proper edge distance, so the push pins can be properly inserted. I could have seen this in CAD but i missed it.

It's one of those classic prototype moments: everything looks perfectly aligned on screen - until metal parts start disagreeing. But it is not a big deal: Mistakes found now are "cheap" - the exoskeleton is the most vital component in this build and give constraints to everything else.

Printed guides instead of custom machining

As said: to stay cost-conscious with this physical prototype of the CORE One Mini, I am modifying off-the-shelf CORE One profiles instead of ordering fully custom machined parts or make them myself.

To make that feasible - and still stay true to the CAD design - I designed multiple cutting and marking guides. Printed in Prusament PETG in Prusa Orange, of course.

These guides allow me to transfer every hole position 1:1 from the CAD model onto the real profiles. They clip or align onto the profiles and existing holes and act as precise drilling and marking templates, ensuring consistent spacing and alignment across all modified parts - and also give a good visual contrast, where parts are a hair too long and need to be filed down.

It's a small detail, but it makes a huge difference. Instead of manually measuring every single hole over and over again, I can rely on the printed guides to reproduce the geometry accurately - and it is also a good use of a 3D printer when building another 3D printer - the true spirit of RepRap.

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If you want to learn more about the project, make sure to checkout the main post "CORE One Mini - The Rocky Road to a Prototype." for the complete background and FAQ.

If you intend to buy a Prusa machine, you can use the referral code "@suit" at checkout in the Prusa online shop. This will give you some Prusameter points and me as well (which I can use for some free filament). Or you can just download, like and make some of my models here on Printables and even consider to become a member in my Printables club.

I've also set up a GoFundMe campaign to tackle the costs. Thanks to everybody who has donated a few euros already - that means a lot and helps me cover the expenses.