Milling is a machining process in which rotary cutters remove material from a workpiece. It is one of the most important forms of machining, along with turning. Milling can be carried out manually, but these days CNC mills — controlled by computer instructions — are prominent. Importantly, CNC mills can be oriented in two ways: vertically or horizontally.
Horizontal machining centers (HMCs) and vertical machining centers (VMCs) differ in several ways beyond their orientation alone. The two mill types have different components and are suited to different cutting operations: as a rule of thumb, horizontal mills can cut deep grooves with minimal tool chatter or deformation, while vertical mills can be better for fine details.
This article looks at the key differences between horizontal milling and vertical milling. It looks at the benefits and limitations of each process, lists some common machines on the market, and offers tips on choosing between horizontal milling and vertical milling.
What is vertical milling?
A vertical milling center is a milling machine in which the spindle (which holds the collet, which in turn holds the cutting tool) is oriented vertically. It is the more common configuration for a CNC mill, and can be used for milling, slotting, drilling, or boring.
More affordable and with a smaller machine footprint than horizontal mills, vertical milling machines are used for many milling operations — although their use is somewhat limited to single-face machining, as turning the workpiece over requires removing it and reaffixing it to the table.
The top-down approach of the cutting tool on a vertical mill has its pros and cons. On the one hand, it provides good visibility of the work, which is ideal for manual machining, inspection, and adjustments. But milling into the top of a workpiece makes chip evacuation more difficult: chips can gather at the bottom of holes made into the top face, those in a side face can more easily fall out via gravity.
Popular vertical machining centers include the Integrex, VCN, and VTC product ranges from Mazak; the VF, VR, and Universal series from Haas (3ERP uses models from the VF and Universal range); the FANUC ROBODRILL; and the CMX V, DMV, and NVX ranges from DMG MORI.
Vertical milling centers come in two main configurations:
With a turret mill vertical mill, the spindle and cutting tool are fixed, so any movement is carried out by the table. This configuration is better for small parts than large ones, but a range of cuts are possible.
In the case of bed mill vertical mills, the spindle itself can move up and down, but the table typically only moves along one axis rather than two. This setup is better suited to large, heavy parts.
Benefits of vertical milling
- Cost: Vertical milling centers are more common than horizontal ones and are less expensive — both in terms of machine cost and maintenance costs.
- Simplicity: Due to the ubiquity of vertical milling centers, lots of machinists are comfortable operating them, leading to better end results. The configuration also provides greater visibility of the workpiece, which helps machinists spot if anything is going wrong.
- Footprint: Vertical mills do not take up too much space, which makes them preferable for many machine shops.
- Detail: Vertical machining centers are good at milling fine features onto parts with tight tolerances.
Limitations of vertical milling
- Access: Vertical mills excel at single-face milling (e.g. sinking dies) but are less equipped for milling on all sides of a part, requiring a new workholding setup.
- Power: For very large and heavy parts, vertical mills can struggle, especially turret mills which need to constantly move the workpiece.
- Speed: Vertical mills are accurate but can be slower than horizontal machining centers when high-volume production is required.
What is horizontal milling?
A horizontal milling center can perform many similar operations to a vertical mill, but it is configured differently, with the spindle oriented horizontally. More robust and sturdy than vertical mills, these machines can handle powerful cutting operations and heavy parts.
Note that horizontal mills are not simply vertical mills tipped ninety degrees. With horizontal machines, the cutting tool mounted to an arbor — a long axel around which the cutting tool revolves. Since they are supported at both ends, arbors are more steady than a spindle used on a vertical mill, enabling the cutting of strong materials.
Horizontal milling is typically used for removing lots of material at once via grooves and slots. Production rates can be high, although accuracy can be lower than with vertical milling. In some cases, multiple cutting tools are mounted to the arbor to make complex shapes. This practice is known as gang milling.
For more complex milling operations, some horizontal milling machines have a rotary table, so the cutting tool can approach the workpiece from different angles. This horizontal milling setup is called a universal table.
Popular horizontal machining centers include the FF, HCN, and MEGA product ranges from Mazak; the EC and Universal series from Haas (3ERP uses the Universal); and the NH, NHX, and i-Series ranges from DMG MORI.
Benefits of horizontal milling
- Power: The stability of the arbor allows for heavy cuts, enabling rapid material removal in large sections — even from hard materials.
- Speed: Fast material removal makes horizontal mills ideal for high-volume production (of relatively simple parts).
- Gang milling: The horizontal mill arbor can accommodate several cutting tools, allowing for e.g. the simultaneously milling of parallel slots.
- Chip clearance: Since cuts are not made from above, chip evacuation is aided by gravity.
Limitations of horizontal milling
- Cost: Horizontal milling centers are more expensive than vertical milling machines, making them less widely used.
- Complexity: A horizontal mill can be more difficult to operate than a vertical mill.
- Footprint: Horizontal milling machines take up more floorspace than vertical mills, which makes them difficult to set up in smaller machine shops.
Choosing between vertical milling and horizontal milling
The choice of vertical milling vs horizontal milling comes down to factors like part size, part complexity, budget, quantity, machine shop availability, and material.
As a rule of thumb, vertical milling is often better for detailed parts, parts that only need work on one side, softer materials, and everyday parts for which there is a limited budget. Horizontal milling is better for rapid material removal, large and heavy parts, harder materials, parts that need work on multiple faces, and operations that require custom or specialist cutting tools.
In many cases, a machine shop will consider your project and determine the best equipment to use for milling.