In this article, the word "end" will be used to refer to all CNC router cutting surfaces. In the industry, different types of ends may be referred to as ends, bits, endmills, cutters, or drills (to name a few). Some of these names are more appropriate for certain ends, but for the sake of clarity, all will be referred to collectively as "ends" in this article.
Choosing materials to work with on a CNC router requires consideration of factors beyond just the desired final part. Different materials are affected in diverse ways by the router, varying by end used and machine settings. The router itself, especially the end used, is affected by the material. It is important to understand the material options available, and to be able to draw predictions from the behavior of known materials when deciding to work with a nonstandard material.
There are many types of wood available to work with on a CNC router, and the variability of their properties should not be underestimated. They vary in hardness, toughness, stiffness, and density, among other properties. All wood, however, is a composite of the natural polymers cellulose and lignin, with some content of sap or resin.
It is important to remember that properties will change with the moisture content of the wood. In general, moisture makes wood softer, more flexible, denser, and more adhesive. Woods used with routers should always be stored in a dry environment and cut when dry.
Not surprisingly, woods across the spectrum of properties are fairly ideal for use in a CNC router. Wood has a low hardness compared to regular steel router ends, so there is little wear on the machine. Wood also has very low ductility, and so the removed wood will fracture into small chips which are easily thrown or vacuumed from the work area, allowing the end to stay relatively cool.
NOTE: Operators should expect to decrease the material removal rates used on denser or tougher woods accordingly.
Composite Laminates and Plywood
When cutting a material with any method, care should first be taken to consider the failure mechanism of the material. In the case of laminated composites (i.e., carbon fiber layup, some fiberglasses, and plywood), the failure mechanism to be aware of is delamination of the layers. This can be tested at any edge of a material by attempting to pry the edge open, and observing the behavior of the crack made:
1. Crack always spreads to the surface by shortest path – Cross-layer failure.
2. Crack spreads into the plane of the sheet – Failure by delamination.
For the material that fails by delaminating, the edges are likely to shred and buckle vertically during routing. To prevent this result, use of a compression end is recommended.
A compression end combines a downward spiral cutting edge at the top with an upward cutting spiral edge at the bottom. This means that the material being cut has its top and bottom surfaces compressed towards the center during cutting, allowing the material’s strength in compression to resist its weakness to crack opening in tension.
Keep in mind that compression ends push removed material out laterally, not upward or downward, so material can only be removed along an exposed edge.
Remember that the compression zone between the two cutting directions on the end must be centered in your material.
- The components of your composite should be considered both jointly and separately.
- If the matrix of the composite is a polymer resin, then apply the considerations appropriate to that type of plastic.
- If the composite contains abrasive fibers or particles, use an appropriately hardened router end and cooling to reduce wear.
Polymer foams are available for CNC routing, and have the advantage of creating no noticeable wear on the end, while allowing complex shapes to be created for custom packaging, aesthetics, test geometry, or for low-risk practice. The defining characteristic of these materials is their low shear strength, allowing them to be cut quickly and easily.
When choosing these materials, if the tensile strength of the foam is even lower than the shear strength, then chunks could be torn away instead of cleanly cut chips. It is also possible for pieces of relatively tough and flexible foam to tear off and wrap around the router end.
For the rare foams with this risk, make sure to use sharp cutting ends at high RPM with aggressive channels, and not abrasive or shallow ends. Stiff foams, however, may be abraded instead of cut to prevent them from deforming.
Soft plastics (like low-density polyethylene) are defined by a relative measure: They are polymers that produce "curls", or very long chips, when cut. This means that they are less hard and likely more ductile. Keep in mind that a softer plastic is also likely to have a lower melting point, and to lose dimensional tolerance by flexing on cuts when hot. Reduce this tendency with a cool ambient temperature, fans, and shallow cuts. A cutting end with fewer flutes or less aggressive channels may reduce the problems caused by jamming with long curls.
Hard plastics (like poly methyl-methacrylate) are defined by a relative measure: They produce short chips with broken ends when cut. This means that they are stiffer and may be cut with less aggressively channeled ends, since removing the smaller particles should be easier. However, overheating can still easily be an issue.
Hard plastics deform less in the bulk when overheated, but may melt or degrade right at the cutting surface, causing scorched plastic material to build up on the router end and prevent it from cutting. Reduce this tendency with a cool ambient temperature, fans, and shallow cuts.
In the vast majority of circumstances, the only metal which should be cut with a router is aluminum. Some high-Si alloys of Al are very hard, and should only be used in mills.
Aluminum will produce long curls when cut with regular ends, which often result in jamming if a large amount of clearance is not available. When aluminum must be cut with little tolerance, a special chipbreaker end should be used. This kind of end, with properties between those of abrasive ends and many-fluted cutting ends, has a "diamond" rhombohedral surface pattern which prevents a single cutting surface from staying in contact with a broad amount of material during an entire cutting revolution, thus breaking material removed into many discrete chips.
Be aware that the hardness and diffusion cooling of a metal is sensitive to the ambient temperature, and so the correct settings for your router may change if the temperature in your workspace is not controlled. When heat builds up the end wears out at an accelerated rate due to softening, and the surface of the metal becomes more ductile, which causes "smearing" deformation instead of clean, accurate cuts. Cut relatively slowly and use cooling fans.
In rare cases, metals other than aluminum may be routed.
- Some alloys of brass are soft enough to be routed with extreme care, choose very hard ends and CNC paths specifically designed for that purpose.
- Theoretically, very soft metals like pure copper, gold, or lead can also be routed.
- In the case of a dense, highly malleable material like the soft pure metals, choose aggressive cutting ends with few flutes.
Stone and Ceramic
Very stiff, hard materials with low toughness like sedimentary stone and common ceramic (granite, sandstone, tile) can be routed with hard abrasive ends.
These ends are often made of a metal with embedded diamond particles as the abrasive. For very low toughness tile, a shallow, densely-fluted or chipbreaker end made of HSS or carbide may be appropriate.
Generally, a CNC router is used for these materials when relief detailing or complex edge patterns are needed. Because of the high hardness and low thermal conductivity of stone and ceramic materials, a large amount of heat is created and very little is conducted away by the surface and removed particles. This means that heat buildup on the router end is a major problem. Direct water cooling is recommended.