Milling Machine Basics: Types, Classifications, & Cutting Tools

But what exactly does a milling machine do, and how is it different than its well-known counterpart, the lathe? Where a lathe uses a chuck to rotate a hunk of metal—usually round—against a fixed cutting tool, a milling machine is just the opposite, rotating a cutter such as a drill or end mill against a fixed workpiece.

If you operated a drill press in high school shop class or at least saw one during a trip to the local home improvement store, you already have a general idea. What’s different about milling machines is the movable table beneath—or in some cases opposite—the spindle and its attached cutting tool. This allows the machine operator to rotate a set of geared handles (or electric servo motors, in the case of a CNC machining center), thus moving the table and its attached workpiece into and around the cutter, removing material as it passes.

In general terms, milling machines can be classified by the orientation of their spindle (or spindles, in the case of multi-spindle machining centers). In most cases, this is either horizontal or vertical, although the line has grown increasingly blurry with the development of 5-axis machining centers, so-called “nodding head” horizontal machining centers, and universal machining centers, which have both horizontal and vertically-oriented spindles. And let’s not forget that many CNC lathes today—called multitasking machines and mill-turn centers—have milling spindles or attachments, blurring the line between mill and lathe.

In the modern manufacturing world, the term “machining center” is most often used to denote CNC milling machines. Manual mills are simply called knee mills or just “milling machines.” Some die-hards refer to them as a Bridgeport, an iconic brand that has been successfully copied many times over. For the purposes of this article, we will focus on all the different types of CNC machining centers, the workhorses of modern industry.

Walk into most machine shops today and you will likely see a line of 3-axis vertical machining centers. These often have three axes of motion—a spindle that moves up and down (the Z-axis), together with a table underneath that runs side to side (the X-axis) and in and out (the Y-axis).

Add a rotary table to this configuration and you now have four axes of motion (although they are rarely referred to as “4-axis machining centers”). Mount the rotary table to a swiveling trunnion arrangement (known as a tilt-rotary) and you’ve arrived at the 5-axis machining center mentioned above, able to mill parts in five axes simultaneously.

Horizontal machining centers are the darling of production shops everywhere. They’re favored due to an integrated pallet system that eliminates having to stop the machine to change workpieces. Horizontal machining centers are also very easy to automate for lights-out manufacturing, or be made part of a computer-controlled flexible manufacturing system (FMS).

Gantry mills and traveling column mills are designed for shops that need to machine very large parts, say the size of cars and tractors. Here, a vertical spindle is mounted to a horizontal rail (the Y-axis) that’s bolted to a pair of vertical pillars, forming an inverted U. If this U-shaped assembly moves on a set of rails lengthwise over a fixed table, it’s called a traveling column vertical machining center; if the table moves and the U remains stationary, it’s called a gantry type machining center. In addition, many of these machine tools come equipped with an indexable milling head, providing what’s known as five-sided machining.

Lastly, we’ll go back to horizontal machining centers for a moment. Some of these are equipped with a movable “quill” that can extend the cutting tool deep into a workpiece. These are known as boring mills, due to their ability to mill and bore very accurate holes and features in large workpieces. Again, depending on the size of the workpiece and the machine builder, these might be configured with a fixed (traveling column design) or movable table (gantry type).

So what types of cutting tools should be used on a milling machine? That depends on the type of machining being performed—for example, it goes without saying that a drill is needed for holemaking operations, and some type of copy mill or ball-nosed end mill is needed to machine a curved, three-dimensional surface. Beyond that, cutting tool selection is largely dependent on the amount of material being removed, its hardness, the part features, production quantity, and a host of other variables that require a good working knowledge of machining principles.

For example, anyone who wants to maximize the investment in their CNC machine tools should use solid carbide or indexable end mills, drills, and other milling cutters that are both predictable and cost-effective. In high-volume situations or in very tough materials like superalloys and hardened steels, so-called material-specific cutting tools should be selected. For lower quantities or in shops that machine a wide array of materials, more general-purpose cutting tools can be used. Whatever the machining scenario, however, the best cutting tools are those made by an experienced manufacturer using high-quality carbide, proven processes, and the support people to back these tools up.