How would I make perfect diagonal edges in a rectangle

Question

In the pic attached I'm trying to make a perfect diagonal edge across the face as I've so beautifully illustrated in MS paint. I've tried subdividing and moving the vertices along the edge so that they look close enough, but 1) they aren't the exact distance and 2) I have to create the vertices on the opposite diagonal side which is a pain.

And I have tried using the ruler/protractor tool but that's been fairly useless so far because 1) it disappears when I go back into edit mode so I can't place my vertices exactly where it was, and 2) you can't enter exact values in it, so when you get down to a measurement like 3.4000005cm you'll never be able to match it on the other side (if that makes sense).

Any suggestions to go bout this? Theres probably some stupid easy solution to it but I've been at this for HOURS and still can't figure it out.

Answer 1

One way to get diagonal based lines like that is to use Offset Edge Slide which will add loopcuts on both sides of the present edge. However one edge is needed, so select opposite vertices and press J to join them with the edge and split the face.

Then select the edge, open Toolshelf (or press Ctrl+Shift+R) and drag the mouse for the effect.

Regarding the edge you create for this tool to work.
You can dissolve it afterwards but note that it creates quite horrible Ngon in the middle which is better to avoid. It all depends on the purpose of the model but it's best to stay away from this geometry to avoid any problems later on.

Answer 2

All in Edit Vertex mode:

1. Select diagonally opposite vertices, and make a diagonal edge (J)
2. Bevel (CtrlB) the (already selected) diagonal edge: in the tool region, set Type to say, 'Width', Segments to 2, and Profile to 1.
3. F Combine the (already selected) two faces on the diagonal band.

Answer 3

A possible option is the knife project tool. You can access it in edit mode at the Tool Shelf ‣ Tools ‣ Mesh Tools ‣ Add: Knife Project.

1. Create a second Plane and scale it to the correct with.

2. Point it to an edge with the Track To Constraint: Shift the origin of another object to the edge. RMB select the cutting plane, ⇧ Shift RMB select the other object and press ⎈ CtrlT > Track To.
   

3. RMB select the cutting plane and ⇧ Shift LMB select the plane object. Go to edit mode ↹ Tab, then choose Knife Project.
   

Answer 4

If what you want is just a diagonal edge you can easily achieve that with the Knife Tool:

1) Initial setup:

first go to edit mode in vertex select like the picture below and press K; you'll notice your mouse cursor has changed shape and is now a little knife

2) Use the Knife Tool:

after pressing K position your cursor in one of the vertex you want the diagonal edge, you'll notice a green dot appears when the cursor snaps to one vertex, now Left Click and drag to the opposite vertex (you'll see a red dot in here when the cursor snaps to this vertex), you'll notice a purple line is being created, then Left Click again on the opposite vertex and press Enter and now you have a diagonal edge (like image number 3)

3) Final result and tips:

• if you press C while the Knife tool is selected you can cut your mesh following a 45 degrees increment, very useful in Ortho Front, Top ,Side views since it allows you to cut along the axis
• if you press E you can start a new cut without confirming the previous one, very useful if you have multiple cuts and you want to cut everything first and confirming later (doing one cut and confirm and then doing another cut and confirming and so on is very time consuming)
• if at any point you don't like your cut and don't want to confirm it simply press Esc or Right Click