CNC Milling: An Essentials Guide for Beginners
As one of the primary means of fabrication, machining (Or subtractive manufacturing) has always been fundamental to how we make things. In this day and age, CNC mills are not just enhanced machines but also something every machinist must grasp. …
As one of the primary means of fabrication, machining (Or subtractive manufacturing) has always been fundamental to how we make things. In this day and age, CNC mills are not just enhanced machines but also something every machinist must grasp. However, getting started with CNCs can be overwhelming for those who want to engage in this broad culture.
The CNC learning process requires lots and lots of patience when it comes to internalising concepts, workflows, decision-making, and execution, before truly getting satisfying results. Thankfully there are countless resources like courses, books, and online forums that can help you be CNCer you want to be. Anyway, let us help you take your first concrete steps from the ground up with this introductory guide.
What is a CNC Mill?
In broader terms, CNC (Computer Numerical Control) is the method to automate machining tool operations through digital input. The program uses a G-code language that generates a detailed sequence of commands for the machine to execute. For example, G01 controls linear tool movement (X, Y, Z) while M03 deals with spindle rotation. Thanks to the CNC, a machine can operate accurately, fast, and without requiring an operator.
Virtually, any cutting machine can become a CNC. The most common are CNC mills, CNC lathes and CNC routers. Additionally, other kinds of processes benefit from the CNC technology like laser, water jet, EDM and plasma cutting. Moreover, CNC not only improved processes but also gave rise to new ones like additive manufacturing. Yes, a 3D printer is essentially a CNC machine.
The milling process consists of cutting a workpiece from a stock of material with a rotary tool bit, contrary to a lathe which rotates the material instead. Combined with digital precision, CNC mills are the go-to machine to create parts with a vast range of geometrical possibilities. CNC mills can also provide high performance when cutting tough materials like steel, making them a better choice than CNC routers in that regard. Now, let’s briefly get into the materials.
CNC mills can cut almost anything! These are the most common by categories:
- Metals: Aluminum (The most affordable and machinable), steels (Mild, stainless, tool, alloy), brass, titanium. Best for end-use mechanical performance.
- Plastics: PEEK, POM (Also known as Delrin. It has the best machinability), PVC, Nylon, ABS, PTFE. They are often used as prototypes before mould injection for mass production.
- Wood: Mainly hardwood and plywood.
- Also, foams can be machined for prototyping and composite mould creation.
Most CNC work under cartesian XYZ coordinates, where the Z-axis aligns with the spindle axis. Furthermore, CNC machines can also have additional rotational axes. As you add axes into the process, workflow and setup criteria can drastically change.
- 2 Axis: The most basic, the spindle only cuts profiles in the XY plane. However, Z-heights and axial depth of cut (DOC) are still factors to consider for operational purposes.
- 2.5 Axis: This concept often confuses people. Basically, in a 2.5 axis machine, continuous motion occurs in the X and Y direction, while Z-axis motion only happens occasionally. Parts made with 2.5D milling are known as prismatic.
- 3 Axis: Continuous motion occurs in all 3 axes. This is ideal for reproducing more organic freeform surfaces and smooth curves. Injection mould cavities, for instance, are typical applications. 3 axis machines can only do top-down cuts, meaning that they cannot reproduce geometries with overhangs or undercuts.
- 4 and 5 Axis: Most CNC machines with more than 3 axes have rotational axes in the working table, often known as A and B. The geometrical complexity you can get with this is on a whole other level. However, programming the tool paths for these parts are often more complex and require an advanced level of expertise.
Mills are known for their high versatility of tooling operations. You can attach an extensive range of tools like drills and face mills, but the essential tool for milling is, without a doubt, the endmill. Knowing what set of endmills to have, when and how to use them is perhaps the key difference between a novice and an experienced machinist. You should consider the following factors:
Endmills are made of very hard materials with high heat resistance. Standard materials are high-speed steel (HSS) and carbide. While HSS is more affordable, carbide offers higher performance. By being more durable, carbide tools can work at higher speeds and feed rates to achieve faster cycles and better surface finishes. Still, due to its high cost, you must use it wisely if you want it to last.
Additionally, endmills are often chemically coated for extra performance regarding wear and friction reduction. Popular coatings are TiN (Titanium Nitride), AlTiN (Aluminum Titanium Nitride) and Titanium Diboride.
Endmill flute (The cutting edge) design not only consists of cutting but also tackles chip ejection. When chips accumulate, many issues may occur. Clogging and increased friction can cause overheating, deflections in the workpiece, inaccuracies, and even the tool to snap. To properly eject material, flutes have helical geometries that some might mistake with drill bits.
Depending on flute orientation, there are up-cut bits and down-cut bits. Basically, the difference is that up-cut bits are more efficient at ejecting materials but leave rough edges. On the other hand, down-cut bits can leave better surface finishes, but you must take extra care since chips are pushed downwards into the workpiece.
Another critical factor to take into account is flute count. Similarly to flute orientation, fewer flutes work better for more extended material removal, while larger numbers for cutting tougher materials and better surface finishes. The most popular configuration is 2 flute endmills for aluminium and other soft materials and 4 flute mills for steels.
Lastly, also consider the concept of conventional milling vs. climb milling, which basically revolves around whether the flute cuts the material inwards or outwards.
The geometry of every feature you want to cut in your part depends on the type of endmill you use. Tool manufacturers in the market offer extensive catalogues of every imaginable endmill for any intended purpose. The following are the most basic designs.
- Flat end: These endmills are the general-purpose tools. They are present in almost all production processes.
- Ball-nose: This tool has rounded ends. With a ball-nose endmill, 3D surface milling becomes feasible.
- V-bits and engravers: They reproduce angled faces like those of chamfers. Additionally, engravers are special v-bits for engraving purposes (Fonts, logos).
- Roughing: This tool has serrated flute surfaces for operations where rapidly removing material is more important than surface finishes.
- Bull-nose: These are flat with rounded edges, use them to create fillets at the bottom of a cavity.
Feeds and Speeds
This is perhaps the most problematic aspect machinists must face. The feeds and speeds of milling operations are the parameters that CNCers must input into the code. Dealing with calculation and the decision process is something that understandably most people would want to avoid. However, knowing how to do an optimum input will guarantee the best qualities, cycle speeds, and tool life. There are five key parameters:
- RPM (Revolutions Per Minute): The speed at which the spindle rotates.
- IPT (Inch per Tooth): Also known as chip load. It refers to how much each flute removes material per revolution.
- SFM (Surface Feet per Minute): The speed at the periphery of the bit. It is a product of the tool’s diameter and RPM.
- IPM (Inch Per minute): The feed rate at which the tool cuts through the workpiece. It is the product of the RPM, IPT, and flute count.
- DOC (Depth of Cut): It refers to how deep into the material a cutting path is. It has two components: axial (Z) and radial (XY).
There are many tools that can help you take the best decisions like speeds and feeds tables, recommendations from tool manufacturers and experts and calculators like this one here.
Last but not least, these are the basic milling operations
- Side milling: Machining an existing edge surface in an XY motion
- Face milling: Cutting a top surface
- Slot milling: The tool cuts through the workpiece, creating two edges
- Plunge milling: Enters into the material with a straight Z motion. Endmills are not designed to make a plunging operation, so you might need to drill a hole first. However, a centre-cutting endmill which is kind of a drill-endmill hybrid can be used for this operation.
- Ramping: Instead of plunging into the workpiece, tools can enter in a diagonal manner. It can either do it in a linear fashion or with helical motion.
The Rise of the Desktop CNC: The Pocket NC
Throughout the 20th century history, both manufacturing machines and computers have been known to be at the very top of technological advancement. However, these bulky and costly machines were only meant for heavy-duty tasks in industrial and defence settings; ordinary people could even dream of having access to it personally.
But thanks to an accelerated microprocessor development in costs, capabilities and portability, the personal computer revolution in the early ’80s marked an inflexion point in a trend that continues to this day.
Since then, democratisation in technology is continuously shaping how society, economics and politics operate, and manufacturing is not the exception. Thanks to the rise of the makerspace philosophy in academic circles and the 3D printing revolution, digital manufacturing in desktop format became a reality in the early 2010s. While the desktop CNC was in its early development, Matt and Michelle Hertel envisioned the first 5-axis desktop CNC commercialisation, the Pocket NC.
Since 2016, the Pocket NC has been at the top of manufacturing development, enabling the production of low-volume prototypes and customised end-use parts with exceptional performances. For both its V2-10 and V2-50 models, the structural design and the spindle power of the Pocket NC enable it to cut steels with incredible accuracies. For more details on the differences between the Pocket NC V2-10 and the Pocket NC V2-50, click here.
In conclusion, the Pocket NC, with its lightweight, compact, versatile, powerful, and easy-to-use design, empowers SMEs, educational spaces, and even hobbyists to firmly engage in this widespread manufacturing dynamic.