CNC Milling Process

In one form or another, the CNC milling process is involved in the vast majority of tangible products in modern industrial manufacturing. While it only directly makes a small fraction of parts, it’s the most common method to create tooling, so it’s difficult to avoid altogether. In the process of bringing new products to life, CNC milling operations almost always come into play and it’s good to be familiar with a thing or two about CNC milling process.

Milling operations involve taking a spinning sharp cutting tool and moving it through a solid block of material to whittle away at it until you have the final shape you want. Unlike that eccentric cousin of yours who carves pumpkins with a Dremel tool, milling machines are precision pieces of equipment that securely hold that block of material in place and move it or the cutting tool relative to each other with a great deal of accuracy. The CNC part (Computer Numerical Control) introduces a computer into the process to automate the precise movements in very complex ways. As long as the tools are sharp and the machinery can by physically oriented for the tools to reach, computers can drive CNC mills through incredibly intricate motions to carve out the parts you need.

What is a 3-Axis Machine?

There are a few options for types of CNC milling machines that can reach nooks and crannies in different ways. The most basic type is a 3-axis machine. This typically involves a machining bed that holds the block of material, which can move in an ‘x’ (left to right) and ‘y’ (front to back) direction. The third ‘z’ (up and down) direction comes from the spinning tool head, the spindle, moving downward toward the machining bed. Machining on a 3-axis system can achieve a great deal of surface complexity and is plenty sufficient for creating tools for molding, casting, and forming processes like injection molding, thermoforming, rotomolding and the like. With the undercut limitations of those processes, a 3-axis CNC machine will always be able to achieve the tool shapes required.

What is a 5-Axis Machine?

For other tools and parts where the spindle must be able to reach the block from multiple sides, a 5-axis CNC mill is needed. This would have all the same movements as a 3-axis machine, plus the machining bed can tilt and rotate as well. This lets the cutting tool access the block of material from any angle.

In both types of machines, the movements are relative. Some may move the machine bed. Some may, instead, move the cutting tool. In any combination though, it’s the relative angle and reach of the tool and material that defines the type of machine. There are also other less-common hybrid versions that allow some repositioning of the machining bed, but they’re not true 5-axis machines if these positions aren’t computer controlled while the material is being cut.

What are Suitable Cutting Materials for CNC Machining?

So, what can you cut with a CNC mill? Well, basically any rigid material as long as it’s bigger than what you want to end up with. Metals of all sorts are great – from soft aluminum to high strength tool steel alloys. Hard plastics machine nicely as well.

One fantastically versatile material that is somewhat unique to CNC milling is REN or modeling board. This is a composite polymer material extremely well-suited for machining that is heat resilient and relatively low cost.

In product development, REN is widely used for prototyping processes to fit-check designs that would otherwise be made of much more expensive materials. It can also be used to make temporary tools for casting, short-run tools for thermoforming, forms for gel coat fiberglass layups, assembly jigs and testing fixtures. It’s like a swiss army knife for product development model builders.

What Cutting Tools are used for CNC Machining?

Now, whatever the material, it only becomes the shape you want in the end by applying cutting tools. There is a lot of engineering that goes into making cutting tools with specific properties fitted for long life and fast cutting speeds for specific materials. For now, though, we’ll just talk about big vs small and categorize the overall shape.

Bigger tools remove more material but generally must move through the material a bit slower. Smaller tools naturally remove less material and can progress through the material a bit faster, but they allow for finer features and things like smaller radius corners. Some specialized tools may be required for undercuts and keyways. The overriding focus for tool selection is on minimizing machining time. The ideal case is to select a single relatively large tool that can remove a lot of material quickly and achieve the desired shape. It’s far more likely though, that your part will need to have tools changed out during the machining time to minimize cutting time or get a certain feature. This is where a proper design fit for CNC milling production can have tremendous value.

Designing parts for CNC milling operations must consider allowing use of the largest tools and fewest tool changes, while also protecting the tools from breakage. How can a design protect a tool from breaking? Great question. We’re glad you asked!

If you’re not already somewhat familiar with milling, you’ll see in the images above that the tools look similar to drill bits. Where drill bits only cut material at the tip moving down into a material, milling tools cut both at the tip and at the side – primarily at the side. So when it moves sideways through the material it’s cutting, the milling tool can be flexed away from the side it’s cutting at the same time it’s rotating at a high rate of speed. This can stress and break a tool if it’s long and slender. Avoiding long, slender channels, small inside radius edges, or other features that would require a long and unsupported tool will make a part much more easily manufacturable. If these features are in a part design, it can both increase production costs and limit the number of CNC milling manufacturers willing to produce it.

Tools can break apart and they can also break the part. Just like how the milling tools can be stressed and flex away from the material, the material can flex away from the tool. Long, thin and unsupported features in a part can bend or may even break off during a milling operation. They are to be avoided.

What are the CNC Milling Costs?

Unlike most processes, CNC costs for prototyping and production are identical. The process is the same in both cases. Like most production processes, you’ll pay a little premium for setup and amortize those costs over higher or lower volumes. Regardless, the cost structure is consistent between prototype and production runs. Under all typical circumstances, no capital investment in specialized tooling is required. In fact, CNC milling is the most common method used to produce the initial capital investment tooling for other high-volume processes.

Widely accessible and versatile, CNC milling shops can be found in just about every city. Some very uncommon specialized requirements may narrow possible vendors such as extremely large part sizes or requirements for machining specialized materials like metals containing lead. When choosing a vendor for your project, the quantity and type of equipment they have (3-axis vs 5-axis) and their experience will qualify their capability. The rest is pursuing a business arrangement that matches your needs for volume, delivery & payment terms, and value-added services you may require. There is skill in setting up and maintaining the machinery, but a computer controls the precision and consistent operation of the machinery. Finding the right CNC milling vendor is all about finding an agreeable company to work with that has the machines your product requires.

What are Tool Paths?

The 3D CAD geometry modeled on a computer is translated into “tool paths” and speeds by CAM software. A tool path is the specific continuous chain of movements of the spinning cutting tool through the block of base material to be cut. The CAM software selects tools and optimizes tool paths for the shortest cutting time. The computer then interfaces with an automated CNC milling machine by sending it a stream of numerical data that determines the position of each axis at each moment in time and other parameters for the cutting tool.

Tool Marks = Beauty Marks… or not so much?

Marilyn Monroe revived its allure in the 1950s. In the 1980s, it was again made famous by none other than the likes of supermodel Cindy Crawford and Ms. Pac Man. Beauty marks are technically a blemish but revered for their character. Similarly, tool marks are a blemish …but c’mon man! Nobody in their right mind wants a bunch of swirly scratches all over their parts! They’re abrasive, disruptive, and make a part look incomplete. If you leave these in your injection mold tooling, you’ll see a bunch of lines and scratches in your plastic parts. Not good.

How do you Remove Tool Marks?

Luckily, there are some common methods and practices for removing tool marks from CNC milled parts to make things look much better. A common quick, dirty AND effective method is bead blasting. This is the process of shooting little bits of glass, sand, aluminum oxide, crushed walnut shells, or other “media” at a surface with jets of compressed air. This can quickly knock down small raised imperfections and smooth things out, leaving a lightly textured even surface behind. Softer blasting media will leave a smoother finish overall, but a high polish requires the skilled hands of an experienced toolmaker. This is a manual hand-polishing process using fine polishing stones, polishing paste compounds, and buffing wheels. With this method, even a mirror smooth finish can be achieved.