Computer Numeric Control (CNC) Milling Overview
CNC milling is a subtractive process (material is carved away from a block of foam)
using end mills. An end mill is similar to a drill bit but has sharp cutting flutes and
a flat or rounded bottom. As the diameter of an end mill is reduced or the end mill
length increases it becomes less rigid. Long thin end mills can break when cutting
material or vibrate causing inaccuracies.
End mill extensions allow small diameter end mills to be extended to greater length
while retaining tool rigidity (figure 1).
Vertical drops can present a problem in CNC milling. Vertical drops less than the
length of the tool can be cut without error, assuming no intersection between the
tool and object. Tapering the tool can extend the tool reach on steep vertical drops Figure 1. Figure 2.
at the cost of a small error (see Figure 2).
CNC tool positioning accuracy is excellent with tool position error typically less than .001". This results in CNC models having an excellent finished appearance.
Specifying model accuracy is complicated by the problem of tool-object interference. Tool paths are calculated to avoid tool-object interference. If the tool is too large to carve a deep, narrow canyon there will be an error.
Advantages of CNC
CNC has the ability to make large models with out having to section. It has the ability to use a wide variety of materials including low cost foam. CNC ability to change tool size can have cost sdvantages. Using a larger tool for lower resolution models can reduce time to make the model and lower cost.