From the bottom image how would I follow the selected vertices to make a perfect straight line. so I can make the bottom image from the top image.

thanks for the help :D

You can use the Knife tool:

To make it perfectly lined up:

Set your starting point and hover until the vertex (of the line you want to extend) turns red:

I hope this works!

• Thanks for the answer but I cant use the knife tool to make it perfectly line up so its a straight line. maybe I'm not using it right idk? Thanks for the answer though :D
– Sam
Nov 17, 2016 at 17:33
• I edited my answer! Nov 17, 2016 at 17:41
• Oh this may not work, because your view must me perpendicular to the faces. Nov 17, 2016 at 18:06
• You can solve this by selecting the edge and hit shift+7(numberpad). This aligns the view Nov 17, 2016 at 18:07
• I'm not aware of any way to make Knife tool to work like it seems to be from your answer however your last comment does add useful method which is aligning the edge to the view (and then cutting with angle constraint used), so that could be included into your answer. May 3, 2017 at 23:16

I replicated a setup similar to yours, and found a way that could work at least in some cases: it is not that difficult, but unfortunately it is a bit difficult to explain so it will need several images, forgive me, this will be a long sequence...

Here is my start setup in wireframe mode (it is not identical but it will give the idea). I activated "draw all edges" to better visualize what's happening.

then I selected the face that already follows the wanted direction and that should ideally be extended to "cut" the other geometry.

then I duplicated the face and separated it

now the duplicated face is a separate object and can be manipulated on its own

so I scaled it in edit mode on Y local only, until it extends beyond the desired upper final point

now I activated vertex snap, and in vertex selection mode, I moved the bottom vertices to the right spot, snapping onto it

getting this setup where the plane is in the "cut" position

the cut will be better if the plane exceeds the 3d shape, so scale it along y normal only, just a bit.

note: the following images used another plane for which I missed the preceding poinjt, but the final result will work well if you do that.

now I selected the original object and put a boolean modifier (intersect) targeting the "cutting" plane

then applied the modifier

and after hiding the "cutting" plane, you see that the original object has been cut as expected, but there's an inner face caused by the boolean, you need to get rid of it.

which you can do deleting "only faces"

then better select all and do a "remove doubles", and in the end you should get this, and should be what you expected, more or less...

as visible in solid mode (I took this last before doing "remove doubles" so you can see some orphan vertex)