Whether lathe or mill, computerized or manual control, all machine tools share some basic mechanical similarities. All have a rotating spindle and a motor to drive it. All have a table or carriage of some kind that moves in and out, side to side, and up and down (some do far more than that). These moving parts are called the machine axes (plural for axis, not the sharp thing used to cut firewood when camping).
Depending on the type of machine tool you’re standing in front of, you’ll either clamp the workpiece to the table before attacking it with a cutting tool (in which case you’re operating a mill), or attach the workpiece to the spindle and spin like the world’s fastest super-fast merry-go-round, with the cutting tool whittling away at the workpiece as it whizzes past. This is called lathe operation.
Machine tools contain hundreds, sometimes thousands of parts. These include nuts and bolts, bearings and pins, sheet metal enclosures, belts, O-rings, shafts, and seals. Most machines are built atop a casting — the machine base — or a box-like welded metal structure. Some are filled with concrete or a concretelike polymer to make the machine more stable and to dampen vibration.
If you’ve ever assembled a child’s bicycle or tinkered with a baby blue 1957 Chevy Bel Air, you’ll recognize at least some of these components. Modern machine tools are truly marvels of engineering. Some are able to produce machined components accurate within millionths of an inch, and do so on their own, day and night, without a human in sight.
Before electricity, people made spindles turn with water or steam-powered overhead shaft drive systems. Aside from being quite dangerous (getting an arm wrapped up in a moving leather belt is enough to ruin anyone’s day), this approach was inflexible. Complex pulley systems were needed to achieve the correct operating speeds, and once a machine was installed, it became difficult to move or repurpose it for the next batch of parts.
With the development of the electric motor, however, machine tools and other types of factory equipment could now be placed virtually anywhere on the production floor. Each became an autonomous, stand-alone device, able to operate at whatever “feeds and speeds” were required for the task at hand.
Most of today’s CNC machine tools use highly efficient AC (alternating current) motors to drive the machine spindle(s) as well as its movable slides. These typically have good torque at low speeds, thus enabling them to take heavy cuts, but are still capable of high rpm, high-feed cutting. Some machining centers spin cutting tools at 40,000 rpm or more, although most general-purpose machines operate at roughly one-fourth that speed. By contrast, CNC lathe spindles generally spin no faster than 5,000 rpm, although some Swiss-style lathes go higher.
Ways and means
Machine “ways” are super-smooth, extremely precise tracks on which the machine carriage slides. Two types exist — the box way, and the linear guideway (also known as a linear motion bearing). The track used on your patio’s sliding glass door is a lot like a linear guideway. This gadget relies on a series of ball bearings contained in a “truck” that rides on a length of precision-ground guiderail, sliding back and forth like the world’s most accurate locomotive.
CNC machine tool builders like linear guideways because they’re easy to install and can travel at high rates of speed without heating up (to machine accuracy, temperature fluctuation is the devil). But without starting a fight, linear guideways also have a bad rap as being less rigid and therefore, more prone to chatter than their far older counterparts, box ways. In cases where the builder used inferior linear guideways, or guideways too small for the application, this reputation has been well-deserved.
At some point in your machining career you’ll be standing in front of the machine, scratching your head over the source of the terrible screeching noise coming from within. No, there isn’t some wild animal trapped in there — that awful ruckus is called chatter, and it’s enough to make even the hardest of hearing among us scramble for the earplugs. Chatter has several sources, but most often it is caused by an overly long cutting tool (as when machining a deep pocket) or when attempting to machine a thin-walled workpiece. Left unattended, chatter leads to scrapped parts, broken tools, and sometimes damage to the machine. Cutting tool and machine builders alike have done much to eliminate this bane of machinists everywhere, but under the right (or wrong) conditions, chatter may still rear its ugly head.
Box ways are as old as the industrial revolution itself. Virtually all manual machine tools use them, as do many “heavy duty” CNCs. They’re recognized for their ability to carry heavy loads, absorb heavy cutting forces, and reduce chatter. That’s why some machine builders shun linear guideways like polka music at a Jethro Tull concert.
But linear guideway proponents point to the microscopic “stick slip” that sometimes occurs with their boxy brethren, and claim that linear ways are therefore more accurate. Box ways also require a master craftsman to install, often taking days or even weeks to painstakingly “scrape” the mating surfaces.
Complicating the discussion even further, some builders apply “Turcite” or similar non-friction material to box ways, or opt to make them hydrostatic, a feat accomplished by applying pressured oil to the way system and thus creating a lubricious film between mating metal surfaces.
So which is better? The answer, as with so many things, is quite simple: It depends. Both technologies can be extremely accurate. Sized appropriately and properly installed, both can carry very heavy loads. Whether shopping for ultra-high-end jig bores or low-cost general-purpose machining centers, machine tool buyers will encounter both technologies. The best way to determine which is most appropriate for the job is to have the machine dealer take some test cuts that mimic your machining scenario. As Grandma used to say, “The proof is in the pudding.”
Pulleys and gears
Gears are great at making small motors lift big heavy loads. For example, the flood gates on the Hoover Dam would remain forever closed if it weren’t for gears. So too would the retractable roof on many sports stadiums. Gears are toothy widgets that transmit power, change direction or speed of a power source, or — in the case of stadium roofs — provide a mechanical advantage to a relatively small power source, enabling it do more work.
Pulleys perform a similar function by providing leverage to and adjusting the output speed of electric motors and internal combustion engines. However, it’s slightly more difficult to adjust speeds and power ratios “on the fly” using a belt and pulley arrangement than it is with a geared drivetrain. That’s why your 1963 Bel Air came with a three-speed manual transmission (known as a “three on the tree”) and why many trucks and sports cars continue to use gear-filled manual transmissions to get you to the grocery store and back: They’re reliable, inexpensive, and effective.
Conventional lathes have long used similarly complex gear arrangements to drive machine carriages and slides at the precise speed needed to cut a thread or push the cutting tool along at just the right feed rate. The headstocks on these machines are often “geared” as well, as are many CNC lathes and machining centers. This provides far greater torque at low spindle speeds than would otherwise be possible.
Bearings come in all shapes and sizes. Roller bearings are like a pair of matching metal donuts — one sitting inside the other — with a series of ball bearings running between the two that allows each to rotate independently of the other. Needle bearings and angular contact bearings have similar arrangements. Whatever the style, bearings are needed anywhere a mechanical component must spin within (or around) another mechanical component. If you bought one of those wheeled pushup rollers that promise rock-hard abs, feel free to take it apart — you’ll find a bearing inside that allows the device to roll freely.
Bearings are found everywhere in machine tools. On a CNC lathe, for example, the turret mechanism relies on precision bearings for smooth indexing motion. And I already discussed machine tool spindles and headstocks, which are chock full of bearings. So too are the drive shafts that move the table back and forth, the pump motors that supply oil and cutting fluid to the machine, the linear guideways that provide smooth, accurate motion … you get the idea.
The nuts and bolts of motion
Screws transmit motion. This is just as true for the crescent wrench buried in the bottom of your toolbox or the jack you used to change the flat on your car last month. And except for hydraulic or cam-actuated lathes and mills, machine tools also use screws to convert the rotary motion of a dial or motor into the linear motion needed to cut slots, mill flats, and drill holes.
On manual equipment, these are called leadscrews, and at first glance they look like nothing more than super-long bolts. Take the machine apart, however, and you’ll see that its leadscrews are threaded into mating nuts that are in turn attached to the bottom of the machine carriage or cross-slide. Give the leadscrew a turn and you’ll be rewarded with linear movement.
Unfortunately, leadscrews are basically flawed: No matter how much you tighten the leadscrew nut, a small amount of “backlash” always remains. This is the result of clearance between the mating pieces; without it, they’ll freeze up tighter than Grandpa Ted’s wallet on your birthday. The problem here is loss of accuracy — whether it’s controlled by a human or a computer, the backlash must be compensated for with every change in direction, something that becomes increasingly difficult as mechanical components wear.
This is why CNC machines use a recirculating ballscrew rather than a leadscrew. Ballscrews are manufactured with a special circular-form thread that accommodates a nut filled with ball bearings. As the screw rotates, the balls recirculate through the thread groove, around the nut, and back again. This technology virtually eliminates backlash, reduces wear, and greatly increases the speed at which the screw can rotate — an important point on today’s exceedingly fast metal cutting equipment.