What sculpting tools would I use to separate a jaw from the skull?

Question

I'm pretty new to sculpting and 3d modeling overall and just picked up blender yesterday. I modelled this dinosaur head and now I want to model teeth, but the problem is that my model is made from one mesh (except for the eyes) and there is no oral cavity. So I need to find a way to cut trough the lips so I can have the initial shape of mouth then sculpt it.

I've tried using a subtractive brush, but i couldn't cut across very evenly. I tried using the knife tool in another mode but it didnt cut though, not super sure why it didn't work.

What would be the optimal steps for separating the jaw from everything else? or would I just have to cut my loses and use the subtractive brush

This is what I have so far:

Answer 1

There is the Boolean Modifier in Blender. It's can be used to separate, merge, and cut off parts of objects/meshes. In your case, you can easily cut the lips and create an oral cavity with it. Then use the Pose brush to open the mouth.

Separate the lips

First, create a "knife". For this, switch to Object Mode (CTRL + TAB). Add a plane (SHIFT + A). Add a Solidify modifier to add some thickness. (The modifiers are located on the right side. Click the blue wrench icon.) Add the Solidify modifier, adjust the Thickness, then apply the modifier (in Blender 2.9, you need to press the little arrow on the right side of the modifier's name). Then scale and rotate the "knife" so you can place it between the lips of your dino head. Use S to scale. While scaling you can (for example) press X to restrict the scale to the X-axis. Use R to rotate, and G to move it around.

Now cut the lips apart. To do so, place the "kife" right at the beginning of the dino's lips between them where you want to separate them. Select the dino's head. Add the Boolean modifier to the head. In the modifier's options select the plane ("knife") as Object value, use the default mode Difference to cut out geometry. Apply the modifier. After that, move the knife a bit further along the lips into the head, apply again a Boolean modifier. Repeat these steps again if needed until you have cut the full length of the lips.

Create the oral cavity

This is very easy to do. Still in Object Mode, just add a sphere, scale it and place the dino's head where the cavity should be. Then select the head, and apply a Boolean modifier with the sphere as Object value and mode Difference.

Open the mouth

Switch to Sculpt mode. Use the Mask brush to mask the upper jaw of the head. This prevents that this area is affected by brushes. Then use the Pose brush with big size to move the lower jaw down. This part is a bit tricky since it still can happen that some vertices from the upper jaw are dragged down. Try to hide the masked area or use the Pose brush at a different spot or angle.

In the Sculpt menu you can hide masked areas and reveal them again.

The added geometry (cavity) has a lower vertex count. Switch on Wireframe overlay or to Edit mode to check it. You can use the Remesh in Sculpt Mode, or activate DynTopo and touch the area with the Clay brush and a very low strength to add needed geometry when you want to sculpt details there.

Answer 2

Enable bool tool from addons then go to side view by hitting 3 numpad and press shift + A and add a single vertex (you should to Enable addon : Add mesh : Extra objects) go to vertex select to an select your single vertex now by Pressing E extrude your vertex and make a line through where you want to make a cut. Extrude the line through the mesh again and just a little thickness . now go in object mode . select your cutter object and then your dinosaur head and press CTRL + - to Mines what you want from your head , you can use repeat this way for making a hole for mouth too PM me if you need more help . GL.