What is CNC Machining? Part II
Learn Up: Precision CNC Turning
Most machining operations fall into two categories, milling and turning. Yes, there’s also electrical discharge machining (EDM), grinding, hobbing, and more, but those are generally secondary operations, performed after a workpiece has passed through a lathe, milling machine, or often both. This installment of Learn Up will focus on lathes and CNC turning (short for Computer Numerical Control)—how it all works, what kinds of parts are suitable, and at least some of what you should know when designing or procuring turned products.
Let’s begin with the basic question: what is a lathe, anyway? If you were lucky enough to attend shop class in high school, you’re almost certainly familiar with this most fundamental and important of machine tools.
In fact, so crucial a role do lathes play in manufacturing that they were in large part responsible for the birth of the Industrial Revolution, after British mechanical engineer Henry Maudslay patented his screw-cutting lathe in 1799.
For those who’ve never seen one, a lathe is a machine tool that produces round or cylindrical parts. Easy, right? As you will soon see, however, this is a very general, slightly misleading statement. That’s because, like all CNC machinery, lathes grow a bit more capable each year, and can now perform operations and machine part geometries that were once only possible on milling machines.
For example, any lathe can whip up a set of candlesticks for your best friend’s upcoming birthday party. They can also turn shafts and pins, cut screw threads, and machine parts resembling soup cans and soda bottles. Only lathes with milling capabilities, though, can drill holes in the side of that soup can, or mill a series of decorative flats on said candlesticks. More on these mill-turn and multitasking lathes shortly.
Whatever they’re called, regardless of their capabilities, all mills and lathes have a rotating spindle. Sometimes more than one. But where a mill uses its spindle to spin an end mill, drill, or other rotary cutting tool, thus removing material from a stationary hunk of metal or plastic, a lathe spindle rotates the part itself. A fixed cutting tool is then driven against the spinning workpiece, moving in and out (using the cross-slide, or X-axis), side to side (the carriage, or Z-axis), or both at once, peeling away material as it passes.
A manual lathe, more commonly known as an engine lathe, toolroom lathe, and sometimes a chucker, accomplishes all this with a human operator. This skilled craftsperson must carefully turn the cranks on a set of graduated handwheels, guiding a series of turning tools, drills, and boring bars through whatever motions are needed to complete the workpiece.
CNC lathes eliminate this manual effort (though not the skill level) via precision servomotors that turn the ballscrew for each axis under an industrial computer’s digital guidance. Further, there’s no need to swap tools at each machining step like there is on a manual lathe. That’s because practically all CNC machinery comes with automatic tool-changing capabilities—on a lathe, this is most often achieved with an indexing, multi-station turret.
G, That’s Nifty
A CNC machine’s computer—the controller—takes its instructions from an NC program, replicating what the human would have done were this a manual lathe. And where does the NC program come from? Great question. CNC lathes can be programmed manually, by typing in the X and Z-axis coordinates of the desired toolpaths along with the various G and M codes that tell the machine what to do and when. Many CNC lathes are equipped with “conversational controls” that allow an operator to sketch the finished workpiece. He or she then answers a few questions about the type of material, the raw stock dimensions, what tools to use, and the computer takes it from there.
The third programming option is CAM software (CAM stands for Computer-Aided Manufacturing). Given the increasing complexity of today’s CNC lathes, which as already suggested, are getting more mill-like all the time, CAM software is the clear choice for shops that want to achieve greater efficiency and process control. CAM systems also support integration with toolpath simulation software and cloud-based tool libraries that help to streamline and safeguard the programming process.
Back to the different types of lathes. As mentioned previously, a basic two-axis CNC lathe has an X and Z axis, each driven by a computer-controlled servomotor. Most are also equipped with a movable tailstock, used to support longer workpieces (on manual lathes, the tailstock is often used to drill holes). A few decades ago, however, some clever machine tool designer decided to replace this movable tailstock with a rotating spindle, a.k.a. sub-spindle. This sub-spindle (on the right) sits directly opposite the main spindle (to the left), allowing both ends of a workpiece to be gripped simultaneously. It can also be used to grip the workpiece while machining its backside, eliminating the need for secondary operations.
Wiping Away WIP
This last part is a big deal, because when fewer operations are needed to complete any manufactured component—turned, milled, or otherwise—it means shorter lead-time, greater part accuracy, and less work-in-process (WIP). This concept can be taken one step further by mounting rotary toolholders to the lathe turret, making it possible to drill cross-holes, mill flats and keyways, and machine off-center holes on the face of a workpiece.
Of course, these types of milling operations require precise control of the workpiece rotation. The lathe can no longer just spin the workpiece with reckless abandon, but must index and lock in exact positions, or rotate synchronously with the X or Z axis while a milling cutter removes material. This is known as a C-axis, and it significantly increases a lathe’s capabilities.
There is a downside to this mill-turn approach: tool capacity, or rather, lack thereof. A basic two-axis lathe typically carries eight to ten turning tools. Replace a few of these with the rotary tools needed to perform the milling operations just described and you might end up short on turning tools. The solution is a second turret, usually positioned opposite and below the main turret. The two-axis lathe has become a four-axis lathe, or even a six-axis lathe if equipped with C-axes on main and sub-spindle.
Lathes Gone Wild
Confused? Sorry, but we’re not even close to the end of this machine tool complexity story. Most CNC builders now offer a Y-axis (provides up and down movement of the turret) for true milling capabilities, as well as a B-axis (allows the milling cutter or drill to tip from side to side). Some have even gone so far as to replace the turret or turrets with a machining center spindle and automatic tool changer, dubbing this extremely capable machine tool a multitasking lathe.
It’s also noteworthy how raw material gets loaded into a CNC lathe. As with its counterpart, the CNC machining center, lathes can accept sawed blanks and castings. These are typically loaded by hand, although given the shortage of skilled labor, robotic loading and unloading is becoming quite common. However they get in there, part blanks are usually gripped in a hydraulic or air-actuated chuck or collet device, while specially-designed fixtures can be used to hold on to non-round material and for secondary operations on milled parts.
Another option, one quite common in most turning departments, is the barfeeder. As the name implies, this well-known lathe accessory is used to feed a bar of raw material up to a couple of inches in diameter (depending on the size of the lathe) through the spindle tube and into the lathe’s work area, where it is clamped by the chuck. Once the part has been turned, a parting tool is used to cut off the finished workpiece, the bar of material is advances forward, and the process is repeated.
At the risk of leaving you hanging, that’s all we have time for in this installment of Learn Up. Hopefully, it was enough to pique your interest in CNC turning while giving you at least a basic understanding of its principles. In future editions, we’ll talk about the many types of cutting tools used on CNC lathes and machining centers, drills and taps, boring bars, threaders, groovers, and end mills among them, as well as complementary machining processes such as EDM and grinding. Until then, stay well.
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