I'm quite new to 3D Printing and so far I have no CAD experience.

Today I came across an existing model which has embedded magnets, so there is an internal "pocket". Since the magnets I have are slightly smaller in height and I guess I will come across such issues more often in the future, I thought it's a good opportunity to finally dive into the world of CAD in the form of modifying existing models. So I checked several sources and tried out different tools (Fusion 360, onshape, Meshmixer) to achieve this, but I always ran into the same issue: I didn't find a way to access this internal structure...Maybe I'm just approiaching this in the wrong way and it's quite simple, but I just couldn't figure it out.

Question: How can I easily modify the dimensions of an object which is embedded in a model stored in a STL file (or 3MF)?

Example Model with internal pockets for magnets


2 Answers 2


A common method for performing what you describe is to import the STL file into Fusion 360 and convert it to BREP, which is a more user-friendly format for editing.

It's important that the STL file not have too many triangles/facets as Fusion 360 will reject the import if there are too many, or warn of a system slowdown if it's borderline.

One can use Meshmixer to reduce the density and make it possible to pull into Fusion 360 and perform the BREP conversion. Windows 10 and up has free 3D Builder app which will reduce facet count.

After/if you accomplish this, consider to create a new question here for a specific task.

I've been able to "stretch" an STL model using this process, as well as add holes to faces in order to pin a sliced model after printing it in pieces.

  • $\begingroup$ For an entry level software, tinkercad can do the same kind of thing in a more "intuitive" way for someone new to the field. Something more akin to "I'll fill this hole with this other block" and it generally works okay. $\endgroup$
    – Criggie
    Commented May 9 at 22:04
  • 1
    $\begingroup$ Tinkercad also has the restriction on number of triangles, circumvented by reducing in Meshmixer $\endgroup$
    – fred_dot_u
    Commented May 9 at 22:12
  • $\begingroup$ thanks for the hints!! I will check it out. $\endgroup$
    – Moe
    Commented May 12 at 22:47

It's a good idea to learn CAD software. I've tried using Fusion 360 for conversions but I find that it often doesnt work. You could consider using FreeCAD, for some reason it seems more reliable for conversion specifically.

Do as follows:

  • open stl file in freecad,
  • switch environment to "Part",
  • with stl model selected click "create shape from mesh" from part drop down menu and select precision (min 0.01),
  • with newly created model selected click "refine shape" from part drop down menu
  • hide the mesh and the unrefined shape using RMB menu in the model browser to the left and export the refined shape from file drop down menu as .step file.
  • edit your .step file in a CAD software that is less infuriating to work with then FreeCAD, like Fusion 360. enter image description here
  • $\begingroup$ Wow, thanks for the detailed guide!! I will check this for sure :) $\endgroup$
    – Moe
    Commented May 12 at 22:49

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .