What is CNC Milling?
CNC milling or Computer Numerical Control milling is the process of cutting material using an automated milling machine that has a microcomputer attached to it. The computer is already fed with the design and moves the cutters at multiple points accordingly so that the result is the shape that was programmed into the machine. The process is typically used for designing machine parts out of materials such as aluminum or wood as the result is highly accurate and cleanly cut parts with very snug tolerances.
The Milling Process
CAD Model Design
The CNC milling process involves several steps as with any production or fabrication process. The first step is the designing of the intended part. For this, an engineer or designer skilled in using Computer-Aided Design software designs the product in 2D or 3D shape with the exact dimensions.
Conversion of Model to CNC Program
Since a computer’s language is code, the design is converted by Computer Aided Manufacture software into G-code which is compatible with CNC. This code is then programmed into the computer after being checked for any errors. Simply put, the program turns the design into a set of instructions that cause each cutter to move at the right time in the right manner
CNC Machine Setup
Although the actual milling process is fully automated and doesn’t require human help, the machine setup certainly does. Before the fabrication process begins, the operator needs to attach the right tools and fixtures to the machine and fix the workpiece in place so that it does not move around during the CNC milling proces. Some of the parts of a CNC mill are listed below;
Worktable: It is the place where the material is secured so that the CNC mill can operate on it.
Saddle: This is attached below the worktable for support and can be adjusted using the spindle axis to suit the worktable.
Knee: It is attached below the worktable and saddle and is used to both support them and adjust the height of the setup by moving it up or down.
Column: The fixed base of the CNC machine, can house maintenance items like coolant and machine oil.
Spindle: Driven via a motor inside the column, it controls the machine tool.
Mill Cutter: It is often referred to as the machine tool and is responsible for the milling process. It varies depending on the job e.g. drills, cutters, borers, etc.
Arbor: Connects the machine tool to the spindle.
Once everything is ready to go, the tools on the CNC milling machine will rotate at thousands of RPM and move in relation to each other to cut into the material and achieve the desired shape. With the many different objectives that can be achieved with milling today, there are many different kinds of operations used. Some of these are;
Surface milling: Also known as plain milling, the cutting tool is horizontal and cuts through the material horizontally. The cutter can be wide or narrow, faster or slower, and have coarse or fine teeth depending on the result required. It is also common practice to first cut the material using a fast feeding rate and coarse teeth to quickly achieve the desired shape and then go in again with a finer blade for a better finish. This method allows for cost-cutting.
Face milling: This machine has the tools perpendicular to the part being machined. It often comes after surface milling as it allows for a nicer finish. The teeth on the tip improve the finish while the teeth on the sides are responsible for removing excess material. This method is more suitable for producing intricate contours and smooth finishes.
Angular milling: This process is what allows grooves and chamfers to be cut into the workpiece. The tool can either be perpendicular or horizontal to the surface in a regular 3-axis mill and the result is achieved by using different shaped cutters such as a cone or dovetail.
Form milling: In order to achieve complex surface contours, special convex and concave cutters are used to achieve results such as round edges, recesses, etc. A subcategory of form milling is gang milling where several cutters are used at once to achieve patterns
Profile milling: This is a common milling process to produce concave and convex parts using round inserts and ball nose end mills. Roughing first removes the bulk of the material and an end tool contributes to a decent finishing,
Gear milling: CNC is also used for producing different types of gears; the soft material is formed into the part before going through a heat treatment process to harden the surface. The hardened material is CNC turned to achieve the final product.
Materials Suitable for CNC Milling
Depending on the part being milled, a number of materials are shortlisted to choose the most appropriate one that will provide the required sturdiness, strength, and cost-efficiency. The good thing about CNC is that it is compatible with a wide range of materials;
Metals: Mild steel, Stainless steel, Tool steel, Nickel, Titanium, Copper, Aluminum, and Brass.
Plastics: ABS, Nylon, Polycarbonate, POM, PTFE, HDPE, PEEK.
Exotic and Precious Metals: Inconel, Monel, Platinum, Silver, Superalloys.
The number of axes on a mill is an important part of milling as these determine the directions that the tools can move in. Since cost is such a huge part of using CNC milling as an alternative to other types of cutters such as laser and plasma, having the highest number of axes isn’t necessary.
3 axis mill: The table moves in 2 directions; on the x and y-axis. The third, the z-axis is added by moving on the x and y-axis simultaneously to achieve diagonal cuts.
4 axis mill: In addition to the three axes discussed above, this mill has an A-axis which allows the table to rotate and give the piece finishing without having to reposition it.
5 axis mill: Again, adding to the 4 axes, this mill has a B axis which allows rotation about the Y-axis. These machines cost significantly more than the other ones but can make very complex parts without repositioning.
6 axis mill: These are quite rare to find as the hefty price tag is hardly justifiable despite it being 75% quicker than 5 axis mills. The freedom of movement in this machine belongs to the cutters rather than the workstation; however, not many types of machines come with this many axes.