Boolean Difference - What am I doing wrong?

Question

I've read several questions and answers and none seem to apply to me. I want to make a wall with a cutout for a door. Using Blender 2.8, here are my steps:

1. Create a new 'general' file
2. The starting cube will be my wall. Since I live in a backward country, I don't do meters. Instead, I'll type units as inches. So I scale the cube to 4.5 m in the X direction (the wall is 4.5 inches thick), 120 meters-as-inches in Z direction, and oh, let's say 240 inches in Y so that it's 10 feet tall and 20 feet wide.
3. I move the Z location of the wall to 120 so that a Z of zero is the floor
4. Great! I have a wall!
5. Make a door with Shift-A, add Mesh->Cube
6. Scale it to 6, 30, 80 to make a door 30 inches wide, 80 inches tall, and thick enough to use as a boolean difference object. Set the z-location to 80 to bring it up to the floor
7. I select the first cube and Add Modifier. Select Boolean, difference, and the doorway object
8. Click Apply
9. Select the doorway differencing object, hit G and X to move in the X direction to move it out of the way to see the hole in the wall, and...
10. There is no hole in the wall. Why?

I'm 100% certain that this is user error. I just don't know what error I'm making. Can anyone show me my mistake?

Answer 1

First off, you can set the unit system to imperial instead of metric under Scene Properties > Units > Unit System > Imperial, and set the Length to Inches.

When you're using real system, do not change the scale value. Instead, press N and go into Dimensions. Here you can set the values. You can see the difference between scale and dimensions in this picture. You can also type in other Imperial units, such as ft. This will automatically put it into feet. Math operations such as *( multiplication) or / (division). It's really handy.

Now add the cube. Again, use the dimensions panel to get the correct size of the door. Go into side view by pressing 3 on the numpad, press G, hold CTRL and bring it up to the floor. For the door zoom in so you can see the smaller squares and do the same. Make sure you're using the increment under snapping settings.

I tried exactly what you've done and ran into the same issue. Yet the solution is pretty simple! With the door selected, press TAB to go into edit mode, select the bottom face and drag it down by pressing G, then Z and dragging it down a little bit. Then press TAB again to leave the edit mode and press S and X to scale it on the sides.

... but that's just in case you come to something like this again. Otherwise the simplest solution would be to make it thicker and move the bottom a little bit right when you're scaling the door! :P

Then repeat what you did - add the Boolean modifier, select the door as the cutter, operation to Difference hit apply and you're done! You can now also delete the door if you want to.

If you also want to fix the topology after boolean operation (which I would highly recommend), select the corner of the door hole and the corner of the wall and press J. Repeat this on each side so you have quads all around the mesh. We don't want any ngons there!

Hopefully a bit longer and more detailed reply was worth it. Happy Blending! :)