In the previous part of this tutorial we learned how to import a stl mesh into Fusion 360, convert it into solid body and then do some basic modifications. Following those steps, you can use the solid workspace to do all of the modifications that you need. That method is simple to use but the trade-off is that we will need reduce the mesh size to less than 10000 or elements. It is because Fusion 360 works best when converting meshes with less than 10000 facets. If you got a big mesh like those from 3D scanner you will lose the smoothness of the final model.

In this tutorial, I will show you how to edit the mesh within the MESH workspace. You will do all the modifications there so you don't need to reduce the mesh size. Of course, it is not as easy as using SOLID workspace in editing the STL mesh but it's doable and you get to keep the resolution of the original model.

Step 1: Import Meshes

First of all, turn off the Design History and import both of the STL meshes in to the new document. You can adjust the position and orientation of the meshes using MESH-Modify-Move/Copy function. We need the screwhead mesh is more or less in the middle on the back of the rhinohanger mesh.

The general idea is to combine these two parts together in the MESH workspace. We need an empty space so we can put the screwhead inside. The correct steps are: first, cut a square hole as big as the screwhead out of the rhinohanger; second, combine the result of the previous operation with the screwhead.

Step 2: Create Body

We will need a mesh with the same volume of the screwhead. You can use the SOLID workspace to create a solid body and then convert it to mesh for our use.You can start with creating a sketch right on the Back surface. The rectangle will be snapped with the screwhead so we have exactly the same area. The distance to extrude the rectangle is 8.5mm (check my screwhead file here). Then you can move the body to the same position of the screwhead.

Step 3: Convert Solid Body Into Mesh

To be able to use the MESH workspace, we need to convert the solid body we just created into mesh. From the CREATE panel, choose the Tessellate function and select the solid body we just created.

Step 4: Cut a Simple Hole

Now we will cut the mesh body5 out of the rhinohanger using MODIFY - Combine

rhinohanger is the target body
Body5 is the tool body
choose Cut for Operation
check the Keep Tools check box

Give it some time to work because Fusion 360 is not a strong software for working with mesh. In term of mesh modelling, Blender is a better choice. I am showing you how to do it with Fusion 360 in this tutorial. Later, I will have another tutorial with Blender and you will appreciate the speed and the convenience of it. However, with Fusion 360 you can edit the body in the SOLID workspace much more easier.

When it done, you will see a hole that cut right in the middle of the rhinohanger if you hide the screwhead and the Body5 meshes.

Step 5: Combine Meshes

We will repeat the combine step to add the screwhead to rhinohanger.

Choose MODIFY - Combine:

rhinohanger is the target body
screwhanger is the tool body
choose Join or Merge for Operation
check the Keep Tools check box

When it finishes the combination we can export to STL file so we can check it on the slicer for 3D printing. RMB on file and then export mesh body and save the name as you prefer.

You can see the stl file in Prusa Slicer, the newly cut screwhead is next to the original hole of the model.

Step 6: Summary

To sum up, using this MESH workspace:

Pros: you don't need to convert it to solid so you can keep the original resolution which is 236900 facets that we have in the stl file.
Cons: you have to walk around with creating some solid parts, converting them to meshes and then combine them with the target body.

So if you need to do a lot of modifications you go with SOLID workspace if you need to do some simple modifications and keep the smoothness you go with MESH workspace and they both work.

Since mesh modelling is not an advantage of Fusion 360, you can consider other software like Blender for better processing time and functionality. However, this method will become handy when you just need to tune the model a little bit using Fusion 360, one more option when you want to modify STL file.

I hope this will help you for your project. Let me know if you have any problem using this method and I will try to help.

Happy making.