Milling machines have been influential in the world of production and manufacturing for decades. Without them, countless innovations would’ve never seen the light of day. At the most basic level, a milling machine uses rotating cutters to remove material from a solid block by feeding the cutter along a block of material. There are many types of milling machine, but for the sake of this post, we’ll examine the details and history of the most common: the vertical knee mill.
Mills are complex, with hundreds of individual parts precisely assembled to create a robust, accurate machine. Some parts and assemblies, however, are more noteworthy than others. The parts that do the actual cutting, typically end mills, are often thrown into the category of “Tooling”. Parts that support the endmills, such as collets, can also be referred to as Tooling. Drills, countersinks and counterbores also fall into this category.
Diagram of a milling machine (source)
Tooling is held in the Spindle, which spins at a user-set speed. The entire motor subassembly contains many precision components, including the spindle motor, spindle bearings, and the quill, which moves the spindle up and down.
There are many methods for holding Tooling in the Spindle, including collets, Jacobs-style chucks, hydraulic tools holders, and more. In order to fit inside the Spindle at a precise, repeatable location, tool holders and collets often implement machine tapers.
An assortment of collets (source)
The Worktable is a flat surface under the Spindle for fixturing the work. Worktables typically have t-shaped slots cut into them so it’s easy to slide nuts and studs inside for fixturing. The table may move in one, two or all three axes, depending on the configuration of the machine. On a manual mill, the movement of the table is controlled with handwheels.
The Knee is the section of the milling machine that moves the whole table, including the assembly for longitudinal and cross movement, up and down. This is adjusted by the user with a large vertical movement crank.
Milling machines were originally developed to speed up hand-filing. They first appeared in the early 1800s (1814-1818), although it’s difficult to trace the exact history due to the fact that there was a lot of development going on in small shops at the time. The 1840s - 1860s brought the creation of a few popular designs, many of which were geared towards production. They still didn’t have the classic three axes of movement we know today, but were more similar to a drill press, often setup in a line where each machine performed a single operation.
In 1861, Brown & Sharpe created the groundbreaking Universal Milling Machine, which could mill complex part geometries with movement in three axes. From there, production took off as WWI approached. Milling technology developed rapidly for the next few decades, and the introduction of high-accuracy machines like the Jig Bore set the standard for milling accuracy. Now machinists could quickly and accurately locate holes with great precision, making mills commonplace for prototyping and producing wartime equipment. This rapid development continued into the the post-war period, during which a few key technologies developed, including anti-backlash nuts, which led to even greater accuracy.
In 1936 Rudolph Bannow conceived of the Bridgeport milling machine, which is still in production today. It was lighter, cheaper, and easier to use than many of the milling machines on the market at the time, and became an instant success. Its success inspired others to copy the design, which lead to numerous clones.
During the 1950s, NC (Numerical Control) finally moved from the laboratory into the machine shop, with machinists using punched tape to direct the milling machine’s movements. Initially, NC machining was used only in aerospace applications, where recreating complex airfoil and wing profiles proved difficult to do reliably. It caught on slowly elsewhere, but accelerated into full CNC (Computer Numerical Control) in the 60s and 70s when data storage and input methods improved.
Since then, the technology behind data storage, computing and machine tools has constantly improved, so we’re now able to have CNC machines at our desk or use them to create giant, complex parts out of many materials.
TYPES OF MILLING
Although three-axis milling machines are most common, certain applications may demand additional axes. Not all three-axis machines are capable of the same thing, either. This is where machine control comes into play.
Three-axis mills can translate in three directions: X, Y and Z. Depending on the hardware or software software that’s controlling the machine, however, it may not be able to accurately move all three at once, necessitating the terms “2.5D” or “2 + 1” milling. This indicates that, although the mill can cut using all three of its axes, it cannot complete moves that utilize all three axes at once. Instead of performing a smooth curve in 3D space, for example, they’d have to move first in X and Y, and then in Z separately.
Another common distinction between milling types is “3 + 2” milling, which describes how certain five-axis machines perform their moves. In this situation, the two rotary axes are often used to orient the part correctly and for the ideal tool angle, while the 3 normal axes are used for milling. “Full 5 axis” does the same, but simultaneously, with all five axes moving at the same time. This does require more complex CAM, but will save a lot of time during a long job, and likely provide better surface finishes as well thanks to the lack of start-and-stop motions.
MILLING MACHINE FEATURES
The technology involved in milling machines, the most critical of which was touched on in our recent Evolution of Precision Tools post, has been evolving constantly. Some techniques, like using coolant to keep the work and tool from overheating, have been used for decades, but others, like broken tool detection, are just becoming more popular.
Through-tool coolant (source)
Coolant is critical for preventing tool breakage, improving surface finish, and extending tool life during most milling applications. Over the years, it’s not only become more common and easy to use, but with computer control, machines can now increase coolant pressure, turn the coolant on and off, alert users of low levels, and filter it as well. In addition to high-pressure and flood coolant, through-tool coolant is now common, especially in deep drilling applications where it’s more difficult to get coolant to the cutting area, where it’s needed most.
Mid-job inspection of the tool and work has had huge impacts on increasing runtime for milling machines. Probes can check tools between operations, alerting operators of a breakage or even swapping in a new tool automatically. Some machines can even detect wear on a tool mid-job and automatically adjust wear compensation values to ensure your features continue to come out at the ideal size. Ruby-tipped probes are also commonplace now - they can be installed like any other tool in a tool changer and probe a part or feature for precise locating. And now that everyone carries a powerful computer in their pocket, machines can send alerts to operators remotely. All these technologies have made it easier than ever to mill great parts, even during “lights out”, while the shop is closed.
At Plethora, we’re taking it even further - now is the time to take advantage of the huge advancements in computation and fabrication to change manufacturing forever. With the power of clever software, years of machining knowledge, and high-tech machining centers, Plethora is now able to keep mills running for longer, make parts more efficiently, and get them to you with less hassle. Thanks for joining us!