Knife Project command not working for me

I’m using Blender 2.80 and trying to divide an STL into two different pieces using a YouTube tutorial and for some reason it’s not working quite right. According to the tutorial, you create a plane and move it to the point in your mesh where you want the divide. In object mode, you select the plane, then shift-right mouse click the mesh to select it. (I think the tutorial is rather old and you’re really supposed to shift-left mouse click instead). After both the plane and the mesh are selected in this fashion, you switch to edit mode. The two items, the plane and the mesh should still be selected. You then select, “knife project” from the menu and then check the box labeled, “cut through” to complete the task.

The issue I’m running into is I select the plane I created, I select the mesh I want to divide in object mode and then I switch to edit mode. They both stay selected, but as soon as I click on Knife Project under the mesh menu, the plane stays selected but the mesh I want to divide deselects and only minor portions of it stays selected.

Any ideas why it doesn’t stay selected or why the Knife Project command doesn’t work?

Thanks in advance for any guidance.

Not sure why it doesn’t stay selected, but just tried it here with a simple box and circle and it works ok, in edit mode, both are in edit state but the knife project behaved anyway. However, if you select any part of the “cutter” or edit it, the knife project doesn’t function as it seems to “forget” the object select order.

I know. I’m trying to divide my object into two different parts using a plane as the cutting tool and I can’t get it to work to save me. I tried using just a simple square and a plane as the cutting tool and couldn’t get that to work either using knife project. Not sure what I’m doing wrong. Guess I’ll have to try something else.

Ahh, so you’re using the plane “end on” as it were, not ‘face on’? Gave that a go and it still worked on a basic cube. Do you get it to work on a cube alone before trying it on a more complex mesh? I tried it on an STL file I imported as well, worked ok on that, the mesh was a mess naturally, but it did split where I expected.

I finally got my STL split into two parts but I had to do it using the Bisect function instead. I’ll keep trying using a plane as the cutting tool. One question though if you don’t mind. Once I split my object into two separate pieces, how do I export each one as two different STL’s. It seems I can select each one separately but when I try to export them it exports both together as one STL instead of exporting them separately. Sorry, I’m really a noob on Blender and I’m trying to watch as many YouTube videos as I can.

You’d have to isolate the cut part and separate it as an object. Also, be aware, if you cut it, make sure any holes are filled in, (I don’t know if bisect does that automatically).
Another way is, duplicate the object, make a box that encompasses the part with the top of the box where you want the “cut plane” to be. Duplicate that box. Use one copy of the box & object and do a boolean difference. Use the other and do a boolean intersect. That should give you your solid 2 halves.