To begin, there are some aspects of the model that you should clean up because they will interfere with editing it sensibly. These are almost certainly artifacts of the original dwg import, not something you did specifically.
- Tags (called layers prior to SU 2020) are not used appropriately for SketchUp. SketchUp edges and faces should always be untagged and only components or groups should have other tags assigned. This is a common issue with imported dwg because CAD layers are used differently than SketchUp tags. You can fix this by making all tags visible, opening the siding component for edit, selecting all, and reassigning everything to the untagged state in Entity Info.
- The component contains several loose edges that don’t seem to serve any purpose. They should be deleted, as they cause the bounding box of the component to be large compared to its useful content.
- The axes of the component are located based on those stray edges. You should use the right-click Change Axes command to set the component axes at a sensible place and orientation on the profile. That will make it much easier to work with it.
- The instance of the component in the model is very slightly misaligned from the model axes. That will also make it hard to work with. The simplest solution for this is to delete the existing instance and bring a new one out from the Components window (after the preceding axis change). Placing this new instance at the model origin will also help.
- The siding profile contains small, detail features. These are going to cause problems while editing because SketchUp has a tolerance that causes it to merge vertices that are within 0.001" and delete the edges that join them. You need to scale up the contents while doing your edit. The “Dave method” is a very good way to manage this task.
With all that prep work done, the easiest way to cut the notch is to create a cutting component whose cross-section is the 12 x 1 size of the notch and whose depth is greater than the thickness of the siding. Place this at the desired cut location. Placement is a bit awkward because of the curved shape at the bottom of the siding. Turn on view->hidden geometry so that you can see the edges defining the curve and use them to align the cutter’s bottom with the lowest one.
Now, since you have SU Pro, use the solid tools (or better, either Eneroth’s solid tools or Mindsight Studios’ BoolTools 2) to subtract the cutter from the siding profile.
Here’s the result of all that:
V-Groove Siding.skp (404.8 KB)