CNC machining is another manufacturing technique that a 3D printing service often offers to its clients. It is a subtractive manufacturing method, this is the case since CNC manufacturing will be used to develop pieces by removing material from a solid block. This block is usually referred to as the workpiece or the blank. Removing the material will be done with the help of a plethora of cutting tools. It is thus extremely different than 3D printing, since this is an additive technology. Formative techniques are available too, such as injection molding. Also, CNC means Computer Numerical Control. CNC machining has multiple design restrictions, as you can read below.
When you are designing a product that will be created with the help of CNC machining you need to make sure that the cutting tool can reach all surfaces. Unfortunately, this is not always possible. If the tool cannot reach a surface, it cannot be CNC machined. Because of this, parts with internal geometries cannot be produced by this technique.
The geometry of a piece will determine how it will be held on the CNC machine. On top of that, it affects the required number of setups. Since this is the case, it will impact the cost and accuracy of a product too. Manual repositioning may introduce a small positional error, although this is not a negligible problem.
Many CNC machining cutting tools have a cylindrical shape, while the end will be either spherical or flat. This restricts the part geometries that can be developed. An example of this is that a CNC machined piece with internal vertical corners will always have a radius. The size of the cutting tool does not matter.
During the CNC machining, temperatures and cutting forces will be developed. Due to this, it is possible that the workpiece will vibrate or deform. This has an effect on the minimum wall thickness of a CNC machined piece, while the maximum aspect ratio of tall features will be limited too.
The workpiece may deflect or vibrate during the machining process, but this is also the case for the cutting tool unfortunately. If this happens, it will result in looser tolerances. In fact, it may even break the tool. Due to this, it is actually quite hard to produce parts with deep cavities with the help of CNC machining.