Overview of Desktop Fabrication
Since the beginning of antiquity, people have made things. Whether these things were made of stone, metal, or more recently, plastic – the desire to create things has always been part of the human experience. Desktop fabrication takes the desire to make something, and adds robotics to the mix. The robots do the physical work of fabrication, while we humans design. The dream of Desktop fabrication is being able to design an object – a machine, a piece of art, anything that can be made – and have a robot make it in the home, often in the form of a small machine that can sit on top of a desk — something the size of a printer. This concept has many names – Desktop machining, 3d printing, Additive manufacturing, etc…, but the base desire remains the same. A robot does the fabrication work, and a person does the design work.
Layer Map of Desktop Fabrication technologies.
Many people have heard of the various projects in Desktop fabrication. Some have heard of companies like MakerBot, which focus on a single manufacturing process known as fused filament deposition. Others have heard of the open-source RepRap project, which is the original design MakerBot started from. While RepRap, MakerBot, and the countless others using the fused filament technology are desktop fabrication examples, they are not the whole of desktop fabrication. There are many other desktop fabrication technologies, which all share the following technology layers. The bottom layer is motion control, followed by a tool layer above the motion layer, followed by a control layer above the tool layer, followed by a path layer above the control layer, followed by a model layer above the path layer. The various desktop fabrication systems span some or all of these layers. The following Layer Diagram should help explain what the layers are, what components are in a layer, and how these layers interact.
The model layer is the layer where a human designer creates a digital representation of what (s)he wants to make. It is often accomplished using Computer-Aided Drawing( CAD ) programs such as autoCAD. However, other technologies exist in that layer, such as 3D scanners or solid description languages such as openSCAD.
The path layer takes a CAD model, and translates it into a set of instructions for a robot to move in order to make a model. This layer takes into account what tool will be used to actually fabricate the object, such as a plastic deposition head, and creates in instruction file that tells the tool how to move, when to activate the tool, how much and in what direction to activate the tool. This layer is often called CAM – computer aided manufacturing – however, I call it the path layer, since this more accurately describes what happens at this layer.
The control layer takes the path information generated in the path layer, and translates it into electrical signals for the motion and tool layers. For example, the control layer takes a G-Code file with information of where to move and how fast, and transmits the information to a microcontroller, which then generates pulses to a stepper driver, which then turns on a circuit in a motor to cause the motor to move.
The tool layer is what tool to use to accomplish the goals of creation. Tools include things like drills, saws, inkjet heads, thermoplastic extrusion heads and so forth. A tool is any physical object which, when active, controlled, and under motion, creates a change in the physical world. For example, A drill, when turned on and pushed down, creates a hole.
Motion Control Layer
The motion control layer is the system that moves the tool. It can consist of different types of robots, such as a moving table, gantry, armature, etc.. The motion control layer can move the tool with enough force for the tool to function, and with enough precision, accuracy, and repeatability for the model to be actualized and replicated as often as the user likes.
Now that we’ve setup a basic framework for understanding the core concepts involved in desktop fabrication, we can move on to explore the different technologies in the space as well as their abilities and limits.
Subtractive fabrication is when material is removed from some sort of feedstock. There are many ways to do this, but the most common are laser cutters and CNC routers. Other subtractive toolheads exist, but are not common in use due to the abilities of the laser and drill.
A laser cutter takes a burning laser, often a carbon-dioxide laser, and burns through material. These systems start off in the 40-watt range, and practically, can go to 150 watts. Lasers much weaker than 40 watts cannot burn through wood or plastic quickly or well, and are restricted to paper or foams. Lasers more powerful than 150 watts would exceed the constraints of the home environment. These systems are forceless – the laser does not have a large amount of machining force applied to it, hence precise motion control systems can be made relatively cheaply. A laser cutter is limited by the exhaust system and heat tolerance of the material being worked on. In practice home sized laser cutters would not work well with metals, and are normally used on wood or light plastics.
A CNC router is essentially a drill mounted to a motion control system. The drill spins faster and with more force than a typical hand-held power drill. This high-speed, high power drill is called a router. Router systems have significant machining force as well as vibration, noise, and dust. A Dremel tool is a good example of small, hand-held router. Mounted onto a 3-axis CNC motion control system, it works well with most materials, though metals can be very slow/difficult, with poor-quality edges. A more powerful router and control system, like a Bosch colt and larger stepper motors/drive systems can compensate to increase speed/quality, but at the cost of weight.
Other CNC toolheads
Although rare in practice, other toolheads do exist. A jig-saw mounted onto a 4 axis system is probably amongst the more useful items, as it can handle metals very well. A jig-saw head would have limits in cornering, and so round-edged shapes would work better in this case. An oscillating tool would work well on a 4 axis system as well, but would be very difficult to position/control. A 5 axis system may work better with this type of tool-head.
Most systems discussed so far are CNC based systems, using a moving gantry or table. Other control robots exist besides CNC based systems. These systems have different strengths/weaknesses – for example, a free moving hexapod with a tool-head mounted on it could make very large objects, but would not work well as a system for making jewelry.
This type of desktop fabrication system works by adding material in layers to make a final object.
Fused Filament Modeling
This is what MakerBot/RepRap/derivative systems use. This manufacturing method takes a CNC control robot, adds a plastic extruder tool-head, and then extrudes layers of plastic together to make an object. It’s not very fast, the resolution is not very good, and the CNC base robot is not very strong. For these reasons, I think a RepRap/MakerBot system is not very good. Improvements in resolution are underway, as are improvements in strength. The UP! Personal 3d printer is believed to have the best resolution in this type of robot, while the UltiMaker system is believed to be a good balance of speed and resolution. The MendelMax is the strongest system of this type, and can support a lightweight router cable.
The fused filament world is undergoing a fork – three paths seem to be opening up in this space. The first is multi-purpose machines that can do fused filament and CNC routing, for example the MendelMax. The second is simple to assemble, low cost, heavy and large filament only machines, for example the Prusa 2. The third is extremely fast single-purpose machines, such as the ultimaker.
This method uses a powdered material, and either through heat or glue, binds the powder together. The UW adderfab project is perhaps the only open-source project in this space.
Chemical Reaction Systems
This approach is where two or more starting materials are mixed together to form a solid, chemically reacting in a way to create a phase change from liquid to solid.. These materials may be two different chemicals, such as 2 part epoxy, or these materials may be one chemical and an energy source, such as in UV cure resin, or these materials may be one liquid plus air as the second material, such as in bathtub silicon caulk. Both the RepRap project and its derivatives, as well as the much older Fab@Home project, can be used for this process.
Mixed Machining is very rare in the desktop fabrication space. The idea is to take two different processes, and merge them into a single process. For example, use fused filament extrusion to put down a layer, then a CNC router to remove material. This type of process allows for overhangs and structures that are not possible with fused filament alone, while using less energy and creating less waste than CNC routing alone. However, this machine type is very complex, and the software to drive it even more so. Only a few graduate student projects exist in this space, and none are open source. The most commonly discussed one is the Hydra-MMM.