Removing everything below cutoff

I am just trying to remove everything in my model (stl) below the cutoff, the pink-ish line in the image below. I want to do this because (1) it has that non-flatness/bump on the bottom and (2) it will save material and time for the 3D print. Or is there a better way to remove the non-flatness/bump? I anticipate it will leave a hole which also needs to be filled in. Or actually would that matter if I am just going 3D print this (after smoothing)? It will be solid, so 100% infill.

Google Drive link to STL file:

Having to beg for permission to access your blend file !! screw it !!

Use the knife tool or knife project or bisect tool. Box select, delete.
I’ll leave you to look them up in the blender manual

My apologies. I did not know I had to give permission. Please let me know if it works now. Link below. Weird thing is that the link is different even though the file is the same.

Thank you for the pointers on where to look.

Or you can use a Boolean Modifier. Usually I wouldn’t use it to often for Modelling, but it seam to work pretty well when it comes to 3D Printing.
The Boolean Modifier is also great to put Holes or Nudges into Meshes.

Stl_Boolean.blend (4.45 MB)

you could also use the new BISECT tool to cut your mesh !

happy bl

I was able to achieve the result I wanted with the Bisect tool, see image below. Tedious as it was, it was great to be able to manually edit the bisecting plane’s normal vector and a point within the plane.

Resulting STL file:

I was unable to use the Knife tool. In fact, I could not see what it did, if I used the LMB to accept the cut. If I used Enter to accept the cut then all it seemed to do was add vertices along where I cut, if there was not one there already. Neither seemed to actually do any cutting.

I looked at your .blend file. I will try the Boolean Modifier too, but I have a couple questions. You managed to move the plane that has the cube on it to be in the middle of the object, How? Also, what is that wireframe box for? Just to give the object something to sit on? Is it to represent the printer bed?

In your Blendfile the Objects Origin is somewhere “in Space”.
All I did was to first set the Cursor to the Centre (Shift + S >Cursor to Centre<) and then with the Object selected
again Shift + S >Selection to Cursor<.

The Box is just a Cube with Wire-frame as View mode. The Boolean Modifier is used to Subtract the Cube from your Mesh.
It is Parametric so you can Move the Cube (or the Mesh) and get a result in Realtime.
You can also Unite or Intersect Meshes. First creates one Mesh out of two, second will cut everything but the Intersection off.

The Downside of the Boolean Modifier is that it can mess up the Topology pretty badly. So if you plan to Animate an Object I wouldn’t
recommend it. I used it for 3D Printing often thou. Then this Modifier is great for punching Holes in Meshes.

I have used the Boolean Modifier on problems like this before. It’s actually pretty simple. First, create a cube. While its still at its default size of 2x2x2, move it to Z=-1.000. Now, hit control-A. From the pop-up menu, select “Reset Location”. This will set the cube’s origin to the top face. Now, you can resize, move, and rotate your cube, just as you did the cutoff plane. Once in position, you can use the Boolean “difference” modifier, to remove the excess.