For the parts of the Fallout 4 power armor we aren't 3D printing, we'll be creating them with the CNC router. A CNC router is effectively a 3D printer in reverse; instead of an empty build platform that material gets deposited onto, with a CNC router you start out with a large piece of material stock and the router cuts away everything to make the relief of your part. The CNC is a tool that's new to me and one I wanted to really fully utilize on this build. Full disclosure, I am admittedly taking a lot of lessons from I've followed builds from Shawn Thorsson who does a lot of CNC routing for his large scale costumes and props.
The digital workflow is similar to 3D printing, but still something I am working on really getting solid. You still need to split your model up into sections that will fit within your stock - in my case, sheets of 2" polystyrene insulation board found at the local big box hardware store - while being mindful of any undercuts or concave sections. As I only have a 3-axis CNC, if a part has any concave sections, those areas will not get cut away and my CNC routed part will not come out correctly. For example, this forearm part was cut into several sections, but the gentle concave slope along the back wasn't able to be removed, leaving me with these "steps" that I would have to manually trim or sand away.
The CNC software I am using is MeshCam which is simple and very effective. MeshCam is used to generate Gcode for the CNC, the same way Cura or Simplify3D creates Gcode for your 3D printer. My only complaint about it is that it doesn't visualize the undercuts so I can't know what parts won't be fabricated correctly. Its built in slicing tools leave a lot to be desired, so I have been using Netfabb to create my 2" slices off of the main model and arranging them so that several slices will be cut at a time. This is still cumbersome and time consuming so eventually I'll be looking for a better toolchain for this step. After I run my slice through MeshCam and send the Gcode to the CNC router for each cutting job, the forearm is ready to assemble and sculpt.
I used some medium duty spray adhesive to attach the parts together, clamped them together and let the whole thing sit for a couple of hours before moving on to sealing. The polystyrene board I am using is very soft and easy to damage and I wanted to coat it in something rigid so that I can more easily sculpt on it and to make it nice and smooth. It also reacts poorly to just about everything you might normally do this with, including most spray paints, so I need to coat the part in a sealant.. After giving everything a quick sand with some high grit sandpaper to get rid of any remaining CNC artifacts, I coated everything in a couple coats of wood glue. This would not only provide a surface that is non-reactive to the fiberglass and bondo I would be coating it with, but it also gives the part a lot more strength against bumps and scratches.