I have a 3D scan of a surface and I thickened it by 6mm. I offset the surface by 3mm to get in the middle of the model and I tried to make a helical cutout in the middle of the solid body. I usually project the 2D entity converted helical shape onto the offset surface. I then use a 3D sketch to covert that entity again to create a uniform circular swept cut. I've attached the file here and an image