Key Considerations for Selecting a CNC Lathe

By Barry Rogers

 

Recorded history shows that the lathe is an ancient tool, perhaps first appearing 3,300 years ago in ancient Egypt as a manual, two-person operation. During the Industrial Revolution, mechanized power allowed for faster and easier work. In the later 19th and early 20th centuries, electricity made the machines even more powerful, and the advent of servomotors in the 1950s added the element of control.

One of the key characteristics of a lathe, unlike a vertical or horizontal milling machine, is that the workpiece turns, as opposed to the tool. Thus, lathe work is often called turning. Turning, then, is a machining process used to make round, cylindrical parts. Lathes are commonly used to reduce the diameter of a workpiece to a specific dimension, producing a smooth surface finish. Basically, the cutting tool approaches the rotating workpiece until it begins peeling away the surface as it moves linearly across the side (if the part is a shaft) or across the face (if the part is drum-shaped). 

stock photo of the inside of a lathe

 

Very few lathes today are not controlled by a CNC, although you can still buy a manually controlled lathe. When equipped with means for changing tools out automatically, such as with a tool turret, the CNC lathe is more properly called a turning center. CNC turning centers are available in a wide range of sizes and capabilities, from simple two-axis lathes, which move in only X and Y, to more sophisticated, multi-axis turning centers capable of handling complex four-axis turning operations, milling, drilling, tapping and deep-hole boring—all in one operation.

Basic Lathe Configuration

The basic two-axis lathe consists of a headstock with spindle, chuck for holding the part, lathe bed, carriage and cross-slide, tool turret, and tailstock. While most lathes have a moveable tailstock to support the workpiece at the end, away from the chuck, not all machines come with this feature as a standard. A tailstock is particularly useful, however, when the workpiece is relatively long and slender. Failing to use a tailstock in this case can cause “chatter,” which leaves telltale marks on the surface of the part. Unsupported, the part itself can become tapered, because it may bend excessively from tool pressure while being cut.

When considering adding a tailstock as an option to a lathe, pay attention to not only the current job being run, but also the size of future work. When in doubt, include the tailstock with the initial machine purchase. This recommendation will likely save the headache and expense of installing one later.

Machine Specifications

Regardless of how many axes of motion are required, in evaluating the purchase of any lathe, a shop must first consider the size, weight, geometric complexity, required accuracy and material of the parts being machined. The expected number of parts in each batch should also be taken into account.

Common to all lathe purchases is the question of the size of chuck to hold the intended parts. For turning centers, chucks generally range in capacity from 5 inches to 66 inches in diameter, or even larger. When parts or barstock must extend through the back of the chuck, maximum spindle through-hole or barstock capacity is important. Machines designed with “big bore” options are available if the standard through-hole size is not large enough.

The next critical spec is the swing diameter, or maximum turning diameter. This figure indicates the largest-diameter part that could fit in the chuck and still swing over the bed without hitting. Equally important is the maximum turning length required. This workpiece dimension determines the necessary bed length of the machine. Note that maximum turning length is not the same as bed length. For example, if the part being machined is 40 inches long, the machine bed will need to be much longer to effectively turn the full length of that part.

Finally, the number of parts to be machined and the required accuracy are prime factors for specifying the capability and the quality of the machine. Machines for high production call for high-speed X and Y axes, with rapid travel rates to match. Machines for close-tolerance work are designed to control thermal drift in ballscrews and key components. The machine structure may also be designed to minimize thermal growth. 

Turrets and Live Tooling

The number of stations in the tool turret must be adequate for the number of tools needed to process the particular part. Most lathes come with a 10- or 12-position, drum-type turret capable of using bolt-on-style toolholders.

For three- and four-axis lathes, a wide variety of VDI and BMT tool stations are available to accommodate 6,000- to 12,000-rpm rotary tools. This type of tooling is commonly referred to as live tooling. Some stations for live tools have an internal gear train called a doubler to increase the rotational speed of the cutting tool. Static toolholders, driven toolholders and angle heads also can be used in a lathe’s tool turret. For additional tool capacity, optional half-index turrets can provide 24 stations (12 for turning and 12 for live rotary tools). Swiss-type machines use smaller toolholders and will accommodate more tools. Some Swiss machines offer tool turrets with as many as 60 stations.

Y-axis functionality on a lathe accommodates a greater range of part geometries, and also will enable multiple tools, such as two or three different drills, taps or boring bars, all on the same station.

The choice between VDI or BMT tooling is really a matter of preference. Both have advantages and disadvantages. Using VDI tooling is a bit more cumbersome, because it is necessary to indicate the tooling to make sure the cutter is on centerline. However, VDI tooling can be quicker to set up, especially if the tolerances are not close. Some tooling manufacturers offer handy adjustments for VDI tools.

More users prefer BMT-style turrets that use a keyway underneath to engage the turret. If the tooling is removed, it’s not difficult to replace it and hold repeatability in tool position.

VDI or BMT tooling is available in 30-, 45- and 55-mm sizes to match the size of the work being performed. Much like selecting toolholder size on a VMC, the larger the work and the more material being removed, the bigger the lathe tools must be. Note that on a lathe, the turret can accommodate more tools if they are small.

Belt-Driven or Direct-Drive Spindles

The spindle on a turning center is either belt-driven or direct-drive. Generally, belt-driven spindles represent older technology. They speed up and slow down at a lower rate than direct-drive spindles, which means cycle times can be longer. If you’re turning small-diameter parts, the time it takes to ramp the spindle from zero to 6,000 rpm is significant. In fact, it might take twice as long to reach this speed than with a direct-drive spindle.

A small degree of positional inaccuracy may occur with belt-driven spindles, because the belt between the drive and the positional encoders creates a lag. With integral direct-drive spindles, this is not the case. Ramping up and down with a direct drive-spindle happens at a high rate, and the positional accuracy also is high, a significant benefit when using C-axis travel on live-tooling machines.

A2 Spindle Noses

Lathes are designed to have an American Standard spindle nose on the front of the spindle motor. Tapered spindle noses come in various sizes to hold the chuck or threaded spindle mount. A2 and B2 are both short-taper spindles; the only difference between them is the method in which the chuck is mounted. Type L refers to long-taper spindles, and Type D features a camlock mounting used on many engine lathe spindles.

The good news is, your machine tool manufacturer has the spindle nose selection worked out based on the size of the chuck, diameter of barstock you intend to machine and the horsepower needed. The spindle nose will be properly sized for the machine.

Spindle Speeds, Horsepower and Torque

Today’s CNC lathes are designed for specific ranges of stock diameters. Basically, you buy a machine to cut a specific, maximum workpiece diameter. If you’re cutting 2-inch-diameter barstock, the machine will be designed for running small diameters using higher-speed, 6,000-rpm spindles, and with the right amount of horsepower and torque.

Generally, big lathes have high torque (twisting power) due to the weight of the mass spinning in the chuck. As a rule, the bigger the workpiece and the slower the spindle speed, the more torque required.

If the parts you are running require a machine with a 10-inch, big-bore chuck, the spindle will be designed to deliver slower speeds at more horsepower. This creates the torque to take bigger cuts for more stock removal. As the cutter gets closer to the center of the stock, the machine will automatically speed up to, say, 700 rpm to maintain the proper surface footage. Obviously, it doesn’t make sense to use a big-bore lathe to do small-diameter work.

The operation that typically requires peak horsepower is heavy-duty, inner-diameter work, such as using big drills to make holes in the stock before finish-boring. In this case, Z-axis horsepower might be the limiting factor. For example, a 2-inch drill may require a 20-hp spindle motor to get the force needed to perform this operation.

Programmable Tailstocks

A built-in, numerically controlled tailstock can be a valuable feature for automated processes. A fully programmable tailstock provides more rigidity and thermal stability. However, the tailstock casting adds weight to the machine.

There are two basic types of programmable tailstocks—servo-driven and hydraulic. Servo-driven tailstocks are convenient, but the weight they can hold may be limited. Typically, a hydraulic tailstock has a retractable quill with a 6-inch stroke. The quill can also be extended to support a heavy workpiece, and do so with more force than a servo-driven tailstock can apply. This is an advantage if you’re machining a piece that weighs, let’s say, 2,000 pounds. Using the programmable tailstock to push the part helps support its weight in the chuck.

Slant-Bed Lathes

The slant-bed lathe design is probably the most common and well-known configuration in today’s CNC lathes. Typically, the bed of the lathe slants at a 30-degree or 45-degree angle, although some 60-degree models also are available.

One obvious advantage to the slant-bed design is effective chip evacuation. Chips are simply washed into a chip conveyor or tray in the machine by the flow of coolant and the assistance of gravity. In high-volume production environments, evacuating chips quickly helps prolong the life of the machine by preventing them from accumulating where they may wear the machine ways or other moving parts.

Another advantage of slant-bed designs is larger X-axis travel. Unlike flatbed lathes in which the length of the guide rail is limited to the horizontal depth of the casting, the slant-bed design accommodates longer X-axis rails. This design also enables the slant-bed lathe to accept a larger part than a flatbed lathe with the same footprint.

Optimally, the headstock of a slant-bed lathe is mounted on the bed and shares the same 30- or 45-degree angle, parallel to the X-axis and traveling on the same plane as the linear axes. Less-expensive models may be constructed with the base of the headstock at zero degrees (flat on the base and not slanted). This design makes the machine harder to get back in operation after a crash.

Multitasking Lathes

Multitasking machines are often built on a turning center platform. These machines use rotary tools to combine several cutting processes such as turning, milling, drilling, tapping, grooving, threading and deep-hole boring on one machine. It is not necessary to have multiple machines to handle those operations separately. Typically, multitasking turning centers have a second main spindle or an additional subspindle to which the workpiece can be transferred automatically from the first spindle. This enables continuous and simultaneous machining of first and second operations. The second spindle can grab the part for work on its back side to complete the part in one setup.

When a subspindle is used in conjunction with a bar feeder, the subspindle grabs the end of the bar and pulls out the length needed for the next part. The subspindle is more precise in pulling out the stock than the bar feeder is pushing it in.

After the parting tool severs the finished part, the subspindle can then drop it into the parts catcher while the main spindle begins machining the next part. The value of the parts catcher cannot be overstated as a reliable method for removing the finished part from the subspindle to make room for the next part.

With automatic subspindle workpiece transfer, a three-jaw or dead-length collet chuck is required. You don’t want a chuck or collet that may move in or out slightly to push or pull on the workpiece when clamping. This unwanted motion can easily mar the workpiece.

There’s virtually no limit to the variety of multitasking lathes that provide innovative part-processing combinations and superb performance. Twin main spindles and dual-turret configurations are two examples. It should be noted that it may be necessary to have separate part programs that can run simultaneously, each synchronized to avoid a collision. Shops that are new to the concept of multitasking on a lathe may not realize that new programming software and additional programmer training may be required to support these machines.

Options for Automation

Following are some quick comments about automation for lathes and turning centers. The key point is that automation solutions should not be an afterthought in the purchasing process. Explore the options with the machine builder or dealer from the start.

Magazine-style bar feeders are designed for automatically feeding round, square and hexagonal barstock into lathes. Many bar feeders can handle stock from 0.030 inch to 4.0 inches in diameter. Where floor space is tight, compact bar feeders handling barstock only 4 feet long are popular. Bar feeders for stock as long as 12 feet are common, and units for 40-foot bars are available on special order.

Because the bar being fed into a lathe must rotate at the same speed as the portion clamped by the chuck for machining, special precautions must be taken with full-length, large-diameter bar. Consider a bar 2 inches in diameter and 12 feet long. Rotating a bar of this size at high speed may cause it to whip excessively in the bar feeder, risking damage to the bar feeder and the machine.

In addition to bar feeders, gantry loaders and articulating robots can be used on lathes to increase production and enable lights-out operations.

Probing for Lathes

Ultra-compact 3D touch-trigger probes with optical signal transmission are available on lathes of all sizes, as well as on small multitasking machines. They are specifically designed for workpiece positioning, tooling setup and inspection. Probing solutions can help reduce setup times by as much as 90 percent, while improving process control and quality.

The lathe commonly is considered “the mother of machine tools” because it led to the invention of other machine tools. It is available in a variety of iterations, with a wide variety of features to suit an even wider variety of applications. Taking into account all the information provided here hopefully will help you determine which version is right for your needs.


ABOUT THE AUTHOR

Barry RogersBarry Rogers recently retired as global director of sales for a major machine tool OEM. Prior positions include director of global sales and marketing for Sunnen Products, and national sales and market manager for Renishaw North America. He also has served as general manager of Cincinnati Milacron’s LK CMM division in Detroit, Michigan. Barry recently started Alpha Strategies, a Chicago-based consulting firm he serves as president. alphastrategiesconsulting.com