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Near-net-shape (NNS) turning is a trend that's elbowed its way into the manufacturing mainstream over the course of two decades. The idea is understood by nearly everyone in manufacturing now. At its root, NNS turning simply means turning parts that have been formed or cast close to final dimensions. The implications of this method continue to unfold as machines, tools and processes incorporate new developments and ideas.
Most NNS turning involves the machining of very few features on a part; often one or two, rarely more than three. NNS turning often produces chucking difficulties because typical parts are often thin-sectioned, or are cast or formed without fixturing or measurement datums. Because the typical NNS turning operation is the final finishing operation, we can add that it usually falls into the category of high-accuracy turning, boring or facing.
The most important implication of NNS turning, though, is the way it has influenced trends in automation. When chip-making time becomes a smaller part of total cycle times, work handling and tool changing emerge as the primary roadblocks to better efficiency. NNS turning has evolved to get along very well with the simplest work handling and tool changinga trend that has extended somewhat beyond NNS turning itself, challenging the conventional wisdom about how much complexity is good for manufacturing efficiency and overall cost effectiveness.
Getting Parts In And OutFast
"Our job is to get parts in and out of the machine as quickly as possible, and as consistently as possible," says Dan Kruse, operations manager of Bearing Technologies, Inc., Div. of MB Mfg. (Benton Harbor, Michigan). He's referring to the races for the bearings his company makes, machined from cut-to-length sections of 4118 steel tubing. Although the job is only marginally a near-net-shape operation (because up to 0.080 inch of stock is removed in up to three passes of the cutting tool), it employs the automation technology and overall process approach that NNS turning has fostered.
Bearing Technologies is using two four-axis gang-tooled machines, linked together into a cell by means of a linear gantry-type transfer system. The machines in this cell reflect the influence that NNS turning has had on production turning-machine design. Small, tightly coupled, and with capacity for only one or two tools on each of two cross-slide-mounted ("gang") tooling blocks, these Wasino SS-8 machines are focused on turning smaller parts that require machining of only a few features.
With no turrets to index, the tool changing is quick. Cutting time, likewise, is minimized with four-axis, two-tool simultaneous cutting. But only 15 percent of the parts' short 30 to 40 second cycle times involves work handling. Loading and unloading times have been squeezed as far as the state of the art allows.
Bearing Technologies is using the extreme example of quick and simple loading and unloading: a gravity-fed chute system feeds parts in one straight line and with one handling motion. It's suited only for ring- or disc-shaped parts, but it hits the target presented in NNS work handling. The chute system uses as little mechanical motion as possible, and it loads in the shortest practical path.
The company previously did the job with two-axis turning machines equipped with more conventional loaders. Their cutting time was 25 percent longer than that of the new four-axis machines. But the greatest difference is in work handling. Total cycle time was approximately three times longer before they switched to the gravity-fed loaders.
Many NNS applications push machine accuracy to extremes, often replacing grinding operations and performing all turning in a single step, or in a single pass through a cell that turns both ends of a part. Hard turning applications often fall in this category. For powder-metal (PM) gears, bushings and other small parts, hard turning has become a serious competitor for grinding. This class of work can be thought of as a subcategory of NNS turning and it makes the same demands on work handling efficiency and accuracy.
It also puts a premium on machine rigidity and on tool engineering. Although ceramics are used in lightly loaded, hard turning applications, it was the introduction of polycrystalline cubic-boron-nitride (PCBN) tools that brought hard-turning into the production mainstream. These tools last through long runs and produce great wear consistency--as long as careful attention is paid to edge preparation.
Precision NNS turning, hard or soft, usually requires extra application development. Tooling setups and tool materials are critical for the softer parts as well as for hard ones, where long runs combine with high accuracy and fine surface finish requirements. And many such parts present special fixturing or chucking demands.
Machining small pistons for lawn equipment is a good illustration of NNS turning's special characteristics. Precision die cast to thin sections, with no readily accessible datums, these parts are difficult to chuck accurately and tend to distort easily. Even so, we can often turn the piston skirts to ±50 millionths (0.000050) of an inch.
Light weight is a key objective for small pistons, and they've been a natural application for precision die casting. But the casting process produces no features for chucking on the inside of the parts. They have to be machined along their full outside diameter, and the only datum for spindle-axis orientation is the inside of the piston dome, or crown.
Turning, therefore, is a double-end machining job, in which a finish-machined outside diameter at the crown end must be continued at the skirt, without losing concentricity. The thin, easily distorted skirt walls make this task more difficult.
The answer is a combination of delicate, precision air chucking and turned-in-place jaws--hard ones, for high volume production. Using air chucks, running at 60 to 70 psi maximum line pressure, solves many such thin section NNS chucking problems. They tend to chuck accurately over a wide range of air pressures, ranging from perhaps 30 psi up to the maximum of 70. Even with smooth jaws, they get an adequate grip on the part if the turning forces are low, as on aluminum pistons.
Still, there is more to maintaining accuracy on thin section parts than merely chucking them accurately. The pistons present another potential problem because of their thin sections: the wrist-pin bores, which we ream on a coordinated machining center in the same machine cell, are located in thin-walled bosses. These heat up quickly in the first turning step and then close up as they cool, expanding and contracting the outside piston diameter by 0.00015 to 0.00020 inch. In some NNS applications, such thermal distortions are extremely difficult to deal with.
Flexibility and thermal sensitivity of NNS parts continue to push the development of better chucking solutions. Finishing chuck jaws in place on the machine has become standard practice. It's also common to taper-bore these jaws to compensate for any flexing they may undergo in use. Machining the roots of hardened jaws to a larger diameter than their ends, by 0.0005 inch to 0.001 inch, is recommended.
It's necessary to design jaws for maximum contact area when light pressure is used. Even then, cutting forces can exceed the jaws' gripping power and allow parts to slip. Ordinary square-pointed serrations aren't of much help in this regard, but good success is reported with sharp-serrated jaws, custom-machined for each application. The most practical way to have these made is as bolt-on inserts, turned and serrated along the spindle axis, as a single, cylindrical piece and then saw-cut to separate the inserts for each individual jaw.
The serrations actually make fine marks on the part, so they don't solve every chucking problem. But they have solved some very difficult ones. In one application, turning a die cast, 390-grade aluminum bushing, chucking with smooth jaws, we got variations of 0.0001 to 0.0003 inch in roundness, due largely to the jaw pressures required to prevent slipping and to an inconsistent parting line on the outside diameter of the die castings. With serrated jaws, the pressure could be increased and still improve the out-of-roundness, to within 30 to 50 millionths (0.000030 to 0.000050) of an inch.
Hardened parts present even greater chucking difficulties. Slipping is a problem because cutting forces are somewhat higher. And many of these parts, especially gears, can't be chucked on any simple geometric surface.
Chucking on the gears' pitch line is the theoretical ideal, because, if it's a locating bore or bushing that's being machined (and it usually is), you want the finished gear to run on its pitch line for quiet, smooth operation. On a bevel gear, or even on a flat face gear, the pitch line is hardly an obvious chucking surface. In fact, you can't even see it. It's a theoretical circle located somewhere on the gear's teeth.
We've solved this head-scratcher of a problem by using pitch-line chucking fixtures. These are hardened, EDMed "gear" faces that mount on the chuck and that mate with the workpiece, which assure that the part runs concentrically with its pitch line. When the work is brought into contact with the fixture, the two gear shapes naturally mate on a circle of least interference, which, happily, is the pitch line itself.
A high volume, bevel gear application at Black & Decker's Easton, Maryland, plant has been running this setup in high volume production for several years. The hardened and copper-infiltrated PM parts are faced and bored on a gang-tooled turning machine, using PCBN tools. The pitch-line fixture requires indexing of the part, as it's loaded, to avoid crashing the part and fixture, tooth tip-to-tooth tipwhich raises the next big issue with automated NNS turning: work handling systems.
Work Handling For NNS Turning
Work handling for NNS turning has to be quick, because cycle times depend upon it. The gear-loading application suggests that it has to be versatile, too, to accommodate something as tricky as orienting gear teeth to mate with a fixture. One of the more remarkable developments of NNS turning has been these quick and versatile loading/unloading systems, which also have the virtues of being simple, contained within the machine tool, and easy to control with standard CNC--ideally the same CNC that runs the turning machine.
This is "self-contained automation," and it requires a definition. Here's the configuration of a typical contemporary NNS turning machine: It's gang-tooled, because only a few features are being machined. The gang tooling results in tight mechanical coupling between the cutting tool and the machine bed. This makes it rigid and inherently easier to build accurately, with no turret bushings or indexing gears.
The gantry-type loader is built on top of the machine, attached directly to the machine's bed. The path of the work grippers is strictly along straight lines, from a work-staging carousel that's integrated with the machine tool itself. The gripper head moves along the length of the machine bed and straight up and down at the ends, picking and placing parts at the carousel and at the chuck.
Current gantry loaders use programmable drives and chuck-like grippers, with soft jaws. Parts changeovers, therefore, are quick. Because the loading and unloading motions are few, and travels involve only one axis at a time, their programs are short. They can be stored in and controlled from the machine tool's CNC.
This is an inherently accurate, very compact package, and it is a modular one that lends itself to easy assembly of multiple-machine cells. The gantry loaders can feed an intermediary transfer system, built along the same simple, linear-path lines, to swap parts between machines.
Back to that gear application: It requires orienting of the part, but the grippers are chuck-like and indiscriminate about how they pick parts up from the carousel. How does it orient them? By pausing at an intermediate station, where it drops the part onto a rotating fixture that uses a light beam to tell where the teeth are, and then rotates the gear as necessary to avoid a crash. The gripper then picks the part up again and continues on its way to the chuck.
Thus, the gantry system we've described is simple but not simplistic; it can be elaborated, thanks to its programmability, to do something extra with the part. Orienting it is one such task. Gaging it is another for on-line SPC applications.
Near-net-shape turning puts a premium on accuracy and quickness, and it allows simple tool handling and work handling, due to the nature of typical NNS parts. The automated turning centers that have evolved to meet this market demand are simple without being simple-minded: They're programmable for special tasks, including gaging and parts orientation.
In effect, these automated turning centers are pre-packaged automation. Quick to set up, capable of producing extreme accuracies, and versatile, they point the way to better automation for many other machining applications, besides the turning of near net shapes.