Posted December 19, 2017
Additive versus Subtractive Manufacturing
There are many different manufacturing processes and tools available, but when it comes to analyzing the categories, Additive versus Subtractive Manufacturing is a great place to start. Additive Manufacturing (also known as 3D Printing) has become hugely popular and prevalent in hardware businesses in recent years. With an influx of new tools to use, it’s now worth considering which is best for your scenario; the tried-and-true subtractive techniques such as milling or turning, or a type of the newer additive technology?
Let’s compare them and find out how best to proceed when it comes time to select a manufacturing method. Finding the ideal process will depend a lot on what you’re doing, what your capabilities are and if it makes sense to work with outside fabrication sources (like Plethora).
Additive Manufacturing Background
3D Printing has been around for a lot longer than most people think, with initial experiments starting in the 1980s. Initially, a lot of printing was done by curing thermoset polymers, and in 1984, Chuck Hall of 3D Systems filed for a patent that described a stereolithography process for curing layers of photopolymer with lasers. This was also the origin of the STL (Stereolithography) file type that still acts as a standard for 3D Printing today.
Despite the fact that the original patent for 3D Printing focused on a stereolithography technology, the most cost-effective and popular method of printing today is Fused Deposition Modeling, or FDM, which was commercialized by Stratasys in the late 1980s. FDM can be simply described as an incredibly accurate, tiny hot glue gun: plastic filament is extruded in a soft state at high temperatures and hardens shortly after. The nozzle follows a path for each two-dimensional layer, squeezing out precise bands of plastic before moving up to the next layer.
FDM Printing Technology: a nozzle (1) extrudes hot plastic, one layer at a time (2) onto a flat bed (3). (Source)
Next let’s examine some characteristics of each process. Weighing these may help you decide which is best for your situation.
One main advantage that Additive Manufacturing has over subtractive is based on the way parts are built: layer by layer, from nothing. Since they’re built up from scratch, certain models can be created using 3D Printing that can’t be manufactured in any other way. 3D Printing is not limited by the same restrictions that come from using rigid tooling, so features such as curved holes, which would be impossible to cut with a rotating endmill or drill, can be printed without issue.
Consider another obscure example: a sphere within a closed box cannot be made with conventional milling since the cutting tool cannot access the inside of the box, although this same assembly can be printed easily.
For example, a sphere in a box is easily printed, but is impossible to mill.
Parts with very complicated geometry that require fixturing may lend themselves better to 3D Printing, for this reason. This is particularly true if the geometry is complex and will change often, as is the case when creating made-to-fit products for individuals.
In order to create complex geometry, many FDM-style 3D Printers build up support structures to support the part as it’s built. Fortunately, this printed material, which can usually be peeled away later on, represents the only material waste in a FDM-style additive process.
Waste generated from an FDM printing process can be recycled to create filament for printing. (Source)
While subtractive processes like milling and turning require raw material, or stock, to start with, FDM printing only requires a printer and spool of plastic filament. Due to this, subtractive processes are inherently more wasteful than additive processes, since material has to be removed from the initial stock to arrive at the final part.
Additive Manufacturing is still fairly new technology, compared to its subtractive counterpart, so material options are still limited. Some types of 3D Printing, such as those that adhere or melt powdered material, are flexible in their material usage since all that’s required is a powdered form of the substance. FDM-style printers, on the other hand, are limited by the needs of the process: the material has to be extruded into a filament of consistent diameter from a raw state, which requires custom tooling. These printers are also limited by safety concerns, since some materials give off harmful gases as they’re heated to a temperature that’s high enough to extrude. Some high-end 3D Printers use powerful lasers to “sinter” powdered metal together, but the process is long and expensive.
Selecting material for a subtractive process is far easier, though, since mills and lathes can accept any raw material stock that can be held in their vise or chuck jaws. Most materials are available in a variety of sizes and shapes, too, so it’s easy to find a piece of stock that’s just the right size for your part.
Along with material options, Subtractive Manufacturing also has the advantage of speed, in most cases. 3D Printing is great for creating one-off, complex parts that would be pricey and time-consuming to mill during prototyping, but in production, the machining of metal parts and benefits of using metal make the subtractive process easily worth it.
The speed at which a part can be milled or turned will depend on the skill of the machinist, but when multiple parts are lumped together onto a tombstone (see my workholding post for more), machines can run for hours without interruption. For metals in very high quantities, CNC milling will almost always be cheaper than printing, thanks to the speed with which a high-quality milling machine can remove material.
In order to tell a milling machine how to remove metal, though, an operator will have to program in machining operations with CAM (Computer Aided Manufacturing) software. In order to properly cut out a part without breaking a tool, or the machine, you need to tell the tool how fast to spin, how fast to move, and where to move. This often involves many toolpaths and can take years to master. A lot can go wrong, and a lot of thought needs to be put into the CAM to get good results.
A skilled operator needs to program complex subtractive machining operations using CAM software. (Source)
Fortunately for Additive Manufacturing, telling the machine what to do in software is much less complex. Since parts are built up from scratch and the extruder or laser are never in any real danger, the software can do all the work. In addition, since the materials are often developed for the printer they’re being used in, the whole process is tightly controlled and predictable, so there’s a lot of room for software to automatically optimize prints. This makes 3D Printing ideal for beginners.
Using Both Processes Together
A few machine tool manufacturers are exploring an exciting new field of machine: one that combines both additive and subtractive technology. In the case of this product by DMG Mori, a powder nozzle deposition head and milling spindle live in the machine together, so both additive and subtractive processes can be performed. The nozzle head deposits material one layer at a time, just like a typical FDM printer, but afterwards the milling head comes in and cleans up the surfaces.
An endmill cleans up a surface of material that was deposited by a powder nozzle. (Source)
This is certainly an area of manufacturing that will grow rapidly in future years, thanks to the opportunities it presents for creating complex metal parts with far less waste than if it were to be machined entirely.
In summary, selecting the ideal manufacturing process for a particular situation is never easy.
Thankfully, though, Plethora solving this problem with integrations between the machines on the factory floor and the part files that you send in through the CAD add-in. As the technology used to automatically quote and CAM parts improves, the number of available processes will expand, and the challenge of selecting the ideal process for a job will be completed for you. For now, though, it’s worth considering all the characteristics of Additive and Subtractive Manufacturing processes before selecting one, as their usefulness will depend strongly on the geometry of your part, the material, and quantity required.