My goal is simple and as I have seen, there are multiple methods to perform a mesh "cut" but most are examples using a simple cube or sphere object.

Being brand new to 3d prints, there is an object on thingverse that allows for customization of the design. I want to remix and improve this design by adding a simple opening "cut" in the strap buckles that is commonplace to allow strap insertion at an angle without having to rethread the entire thing (in addition to what the online customizer also allows me to adjust). Pretty simple stuff. Until I launched blender. :)

I want to cut about a 2-3mm diagonal line through the outer buckle on each side. As I understand it, the ways to do this are try to create cube objects, somehow transform it after rotating and moving (which I still cannot even figure out how to do easily) then using that object as a delete "template" to carve out that shape from my target. Or, theres the knife tool. Or, there's the knife "project". Or, there's the boolean way.

I use Photoshop extensively, I am not an artist but am very adept at photo and video editing etc. but not a CAD guy and I cannot seem to get this thing cut properly.

  • 2
    $\begingroup$ Welcome to Blender.SE! The software is well known for not being the easiest to learn from scratch. The good news is that what you describe could be certainly achieved. Could you say a bit more about where are you exactly stuck and possibly why? (e.g. boolean don't work, can't find the knife tool,...) Could you also draw a picture of where you would like to make the cuts? $\endgroup$
    – Carlo
    Jul 30, 2016 at 22:41
  • $\begingroup$ Welcome to the site :) I've edited your question a bit to remove stuff/rants which didn't seem to help clarify what you're after. If you don't approve of my edits, feel free to roll them back at any time. $\endgroup$
    – gandalf3
    Jul 30, 2016 at 22:56

2 Answers 2


Well, as you seem to have discovered, there is no shortage of possible ways to go about this.

Without going into too much detail, I would probably use a boolean modifier in this case. To do this:

  1. Import your STL file (File > Import > STL)
  2. Create an object roughly in the shape of the hole you'd like to create in the STL.
    To do this:

    1. Add a cube (⇧ ShiftA> Mesh > Cube)

    2. Scale it on the X axis to make it thinner (S to scale, X to restrict the scaling solely to the X axis, LMB LMB or ⏎ Enter to confirm and exit the scale operator)

    3. Grab (G), rotate (R) and scale (S) the cube into position such that it intersects the buckle object where the hole should go.

      enter image description here

  3. Add a boolean modifier.
    To do this:

    1. Select the buckle object (click on it with RMB RMB) and add a boolean modifier in Properties > Modifiers
    2. Set the object added in step 2 as the "Target" object in the boolean modifier
    3. Set the boolean operation to "Difference"
    4. (Optional) Set the cube's Maximum Draw Type to Wire in Properties > Object > Display, so you can see the result of the boolean operation better

      enter image description here

  4. Adjust the cube further if needed

  5. Export the result. Either Apply the boolean modifier and delete the cube object before doing this, or enable Selection Only in the export options and ensure that the cube is not selected.
  • $\begingroup$ Thanks for the answer! As you were posting, I was going about this by deleting the section's polygons. The wireframe now shows the holes properly, but the "hollow" 2 ends from where I removed the section I also did not "add" anything back to basically cap the ends figuring a hole is a hole. But, even though it displays properly, when I open in Cura it looks right and print it, it's still solid like it's ignoring the edit. Did I miss anything like adding back solid sections where I removed the part? $\endgroup$ Jul 30, 2016 at 23:27
  • $\begingroup$ Also, once I scale hitting "G" does nothing and I cannot even go back to "move" mode. Serously getting frusterated. Once I scale and stretch, what exact keys/actions then allow me to move the object? $\endgroup$ Jul 30, 2016 at 23:31
  • $\begingroup$ link Wow now having to learn markdown apparently just to post a picture link here I can't win today. This is really frustrating see the pic that still prints solid. What am I missing? That took me a darned hour of my life to add 2 slots I have NO idea how you CAD guys deal with this stuff. $\endgroup$ Jul 30, 2016 at 23:39
  • $\begingroup$ I am on step 3 of your instructions and need to figure out how to slow the animated gif down to see what your clicking. Can you clarify what steps this requires? "Select the STL again (select with RMB RMB) and add a boolean modifier" I love the animations and detailed instructions, thanks so much for posting now I just need to finish these last steps and try your way. Manually deleting the polygons looks right but is still printing solid so I am hoping your way works it must be the lack of end caps to fill in the new gap ends or something since it's still printing solid. $\endgroup$ Jul 31, 2016 at 0:25
  • $\begingroup$ @CollinChaffin Those unfilled ends are non-manifold, and there's no telling how any given 3D printing software will likely handle such cases. To make it manifold, fill them in by selecting the ring of vertices around the edge and pressing Alt+F. $\endgroup$
    – gandalf3
    Jul 31, 2016 at 1:16

Blender is a powerful tool, but for a simple cut like this, have you considered using something more basic? You can do this using a webservice like atomiccompiler.com (use a difference method with a rectangle to chop out the part that you don't want.) OpenSCAD is also a good option if you are willing to install another program. Both of these are programmatic CAD, which you may or may not like, but I personally prefer.


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