Frame Material Options
Deciding on a frame material is a key decision in the durability and precision of any CNC machine. For most people this decision is determined in large part by skill in manufacturing, available tooling, and economics.
Here are some of the available options and the associated benefits of each:
Wood is the cheapest by far to work with; it is easy to cut and machine but the obvious tradeoff is durability, and rigidity. That being said, wood is an excellent choice for the hobbiest who wants to learn CNC without investing tons of money. A quality wood framed home built CNC router will be able to produce results that are visually indistinguishable from a high end CNC router. But that being said, wood is the material of learning. It is not suitable for industrial use or long term durability. One more thing to consider about wood: wood does not resist fluids nearly as well as metal. Any wooden frame should be painted to seal the fibers. All materials expand and contract with temperature, but wood also warps and cracks with different humidity and temperature levels. These are some of the major reason why using wood for the frame is not a realistic industrial choice.
If you are interested in learning and think wood might be your best option, one of the best wood material choices is MDF (pictured above). MDF (multi density fiberboard) is used primarily in the construction industry for painted interior shelfing. It is a manmade prodcut with high dimensionally stability, sandpaper smooth surfaces and properties that are very condusive to routers (i.e no tearout, predictability throughout the cut, soft). It is a great choice because of its lack grain, smooth finish, and mass. Painting is important because of its tendency to absorb moisture.
Aluminum extrusion is an excellent choice for the frame of CNC machines particularly because of its ease of use and high precision and rigidity. (If you need an introduction to aluminum extrusion see our article explaining the basics.) Because aluminum is lightweight this can be a potential disadvantage if mass is needed to offset the motor movements. This of course can easily be overcome by adding ballast in the form of steel plate or other high density materials. Probably the largest objection to using aluminum extrusion is the cost. Depending on the going rate of aluminum, one 48″ piece of 1.5″ x 3″ piece of extrusion could be upwards of $50. This adds up very quickly!
Some of the major advantages of using aluminum include lighting quick assembly (i.e. no welding), instant right angles, no need to drill holes for attatching equipment and hardware to the frame and no need to paint. It is because of these advantages that aluminum extrusion is used extensively in the prototyping industry. Because of its versitility its been termed, “legos for grown-ups!”
Another key feature of aluminum extrusion is the minimal tools necessary for assembly. A normal cuttoff saw or chopsaw can be used to cut it. Better yet, it can be ordered cut to length at exacting tolerances. This is an attractive option for those not setup to machine wood or steel.
A third option is a steel frame. Steel has the advantage of being much harder than both wood and aluminum and massive to provide ballast for the machine. The economics of steel place it midway on the spectrum between wood and aluminum depending on the guage chosen. One of the major obstacles to building a steel frame for most people is not so much the cost but the tooling. Working with steel usually requires signficant tooling (i.e. metal bandsaw, wire-feed welder, grinder, clamping jigs, etc). Steel is usually oily and messy until painted and requires welding skills that many simply do not have. But, the properties of steel make it the most desireable option in most situations if the resources are available.