Lathes and turning machines.

An examination of the things a machine tool salesman will, and won't, tell you about what's really important, and what isn't.

Editor's note: Part 1 of this series, published in February 1993, covered machining centers. Part 2, on machine controls, appeared in June 1993.

The lathe is probably the most fundamental of all machine tools, second in essential simplicity only to the shaper/planer. The idea of suspending a workpiece between two points and cutting it while it rotates past a stationary tool is both intuitively appealing and easy to execute. Indeed, simple lathes are among the earliest artifacts of modern industrial societies.

On the other hand, the situation today is far from simple. Today's buyer must choose from a bewildering range of options. Manual lathes, CNC turning centers, vertical turning machines, turning cells, chuckers, automatics, live turrets, turret changers, integral milling capabilities, and much more are all available from a seemingly endless list of manufacturers. Perhaps the best place to begin an examination of turning technology is with a review of the fundamentals of lathe design.

The Workpiece Goes Round And Round

Historically, the distinguishing characteristic of all turning machines is that the workpiece rotates past a stationary tool that is fed essentially parallel to the axis of rotation. This is exactly the opposite of milling, in which the tool rotates over, through, or past a stationary workpiece.

Of course, the word 'stationary' is relative because in the lathe both the tool and workpiece are moving simultaneously, the tool just doesn't rotate. A brief reflection on this definition will reveal the basic elements of a lathe:

* a headstock equipped to hold and rotate the workpiece, and, perhaps,

* a tailstock to support it,

* ways to support the toolholding mechanism,

* a device to feed the tool, and,

* a bed or other foundation to support the whole mechanism.

How these individual elements are arranged will determine just what kind of lathe is produced. In the simplest configuration a head-stock and tailstock are mounted at each end of a set of horizontal ways, and the toolholding equipment is mounted on a slide that's free to move along them. This is a basic engine lathe.

* Mount several different tools on a rotary turret and you have "turret lathe."

* Remove the tailstock and build the headstock with a hole so that long workpieces can be fed through it and you have a "chucker."

* Add a system to automatically feed those long workpieces and you have a "bar machine."

* Add several slides to feed tools from various directions and you have an "automatic screw machine".

* Add a numerical control unit, some ball screws and servos and you have a "CNC Turning Center."

Put two or three of these next to each other and you have a "Turning Cell."

* Add a mechanism for supplying rotary power to the tooling and you have something that is neither a lathe, nor a machining center, but a hybrid capable of performing many tasks that would ordinarily require two or more machines. With a little ingenuity, you don't even have to confine your machining to the workpiece centerline anymore.

Now, consider whether the spindle is to be horizontal or vertical and whether you would like more than one spindle, or turret, or set of ways, or a robot loader, or any of a few hundred other possible options. Even then, the list is by no means exhaustive, but it does serve to illustrate the incredible range of choices available today under the "turning" umbrella.

Everybody Knows the Right Answer

Fortunately, turning machines are relatively simple from a structural point of view, and offer fewer fundamental variations from which to choose than machining centers. Naturally, this becomes less true as the machines become more complex, especially with the addition of powered rotary ("live") tooling. Nevertheless, the forces to which a turning machine is subjected tend to be more uniform and predictable than those encountered in a machining center. There are fewer variables involved in the process because the tool/workpiece relationship is fixed within much narrower operating limits.

This doesn't mean careful design isn't required in a turning machine; it most certainly is. It simply means that the engineer needs to plan the structure to accommodate fewer contingencies, and that means it's somewhat easier to reach a consensus on what constitutes an "optimum" design.

You may, for example, encounter a spirited dispute between proponents of horizontal, vertical, and slant bed designs. But, aside from the orientation, they are all basically similar structures, performing basically similar functions, and delivering basically similar performance.

You will also encounter proponents of cast, welded, and non-metallic bases including concrete and epoxy/granite composites; four, six, eight, twelve, and more sided turrets; various quick-change and tool powering systems; belt, gear, and directly driven servo-type spindles; and many other features, options, and design "innovations." What you won't find, however, are radical differences in the fundamental structural designs of the various types of turning machines.

In the big picture, there simply isn't a dispute in the turning world as basic as that between the moving column and moving table schools of machining center design. The one basic requirement is for mechanical and thermal stability, and the means of achieving stability are straightforward and well understood.

Yes, there are differences between individual builders, and yes, they can be significant under specific application conditions. But, with very few exceptions, they are not fundamental.

Manual or CNC

The most basic choice is between a manual and a CNC machine. The CNC machine offers enormous flexibility in producing short-run parts, but may not be as economical as a dedicated non-CNC machine with appropriate automation in volume production. Oddly enough, it may not be the most economical choice for one-off toolroom or repair shop applications either.

The emphasis in both, cases is on the word "economical," because capacity for capacity the CNC machine can be expected to cost significantly more than a manual counterpart. So, what do you get for the extra money?

Flexibility is the strong suit of CNC turning machines, with accuracy and repeatability not far behind. We may not like to admit it, but today's electro-mechanical systems are every bit the equal of the most skilled machinist who ever lived, and they never get tired. In a world where quality is increasingly a statistical proposition, it's hard to argue with the uniformity of a computer-controlled process.

On the other hand, if all the machine is required to do is turn out cylindrical repair parts on an "as-needed" basis, you might want to seriously consider a manual machine. Not only will it cost less, but chances are that even a moderately skilled machinist will be able to make simple parts on it just about as quickly as he can program and run the CNC. The advantage may shift to the CNC, however, if the same part has to be made frequently, if the parts are complex, or, as may be the case more and more often in the future, if there is nobody available who knows how to operate a manual machine.

All things considered, a CNC machine is probably the better choice in a situation in which the economics are anywhere close to equal. And where flexibility, accuracy, and repeatability are the key requirements, it's clearly the right answer.

Horizontal or Vertical

Say the word "lathe" and most people picture a machine with a horizontal spindle, headstock on the left, tailstock on the right, and tools in the middle. The bed may be horizontal, vertical, or slanted, but the workpiece axis of rotation will be parallel to the floor. Such is the strength of tradition that "lathe" and "horizontal" are virtually synonymous, but it doesn't have to be that way.

Of course, we're all familiar with the giant vertical turret lathes, which are the machines of choice for turning really big parts. Loading, holding, and machining large parts in a horizontal orientation is not practical for many reasons, nearly all of which have to do with the force of gravity. In a horizontal machine, you're fighting gravity every step of the way. In a vertical, gravity works for you rather than against you most of the time.

A number of builders now offer CNC verticals sized and configured for general purpose applications on more "normal" size parts. Tradition notwithstanding, these machines should be carefully evaluated, because under the right conditions they offer worthwhile advantages over horizontal configurations.

In chucking operations, for example, it's frequently both easier and more precise to load workpieces in a vertical orientation, which is why more and more high-volume special turning machines are being designed this way. Verticals also may offer an advantage in machining parts with faces that must be kept parallel, again because gravity will help ensure close contact between the part and the locating mechanism. They are also somewhat easier to tool for irregularly shaped parts that are difficult to hold.

Statistically speaking, a vertical spindle orientation will provide a small increment of improved process repeatability in chucking operations because the chuck jaws engage the workpiece more uniformly. In a horizontal orientation, the lower jaws engage first, and must lift the workpiece into contact with the upper jaws, introducing an uncontrolled, if minor, process variable. The same applies to collet-held workpieces.

Verticals also eliminate workpiece "sag" and the uneven loading of spindle bearings caused by gravity. Also reduced is workpiece distortion or damage from clamping forces because the parts only have to be gripped tightly enough to rotate past the tool, and not supported against the force of gravity. These are all, admittedly, very small increments, but still statistically measurable.

Verticals are, in short, an option that should be considered for applications on chucked parts which are oddly shaped, have precision machined parallel faces, or require the "N-th degree" of process repeatability. They are not, however, without their drawbacks.

A horizontal machine, for example, is a much better choice for workpieces with blind bores because the chips are easier to remove. They are the only choice for bar work and are more practical for long shaft-type workpieces because they offer better operator accessibility. Horizontals are also more familiar to most machinists and are available in a greater range of sizes, types, and configurations than verticals.

If you do select a horizontal, you will then have to choose among horizontal, slant, or vertical bed designs, or some combination of these choices. Proponents of slant and vertical beds claim easier chip handling, less thermal distortion, and reduced floorspace requirements as the major benefits of their designs.

This is probably true because the bulk of modern CNC machines are either slant or vertical bed designs. What is also true, however, is that you will probably pay more for these machines than for a traditional horizontal bed design. This is another case where you have to look carefully at what you intend to do with the machine before you can make an intelligent and cost-effective decision.

The bottom line, then, is that either a horizontal or vertical machine layout will serve you well, with the edge going to the horizontal for true general-purpose versatility. But, for those parts which can benefit from the vertical's inherent advantages, the horizontal is a distinct choice. The point is not to be blinded by tradition, but rather to make your selection based on an objective evaluation of your requirements and the capabilities of the machines available to meet them.

To Mill Or Not To Mill?

The integrated turning/milling machine is a rather recent phenomenon, but one that warrants a closer look. In the right applications, mill/turn machines can literally eliminate the need for a machining center, while producing more accurate parts in less time. The impact on production costs, competitiveness, and profitability can be profound.

Of course, auxiliary milling and grinding attachments for lathes have been available for many years. What makes the new machines different is that the capabilities are truly integrated. This is really an outgrowth of the enhanced control capabilities offered by today's CNCs, which have the raw computational horsepower required to handle the complex, interrelated motions required to perform these operations.

There are a couple of basic approaches to the design of these machines. In one design, the rotary tools are positioned on a separate slide or saddle that moves on either the main ways or an auxiliary set. In the other, the rotary tools are powered through a mechanism built into the turret itself, allowing you to literally "mix and match" stationary and rotary tooling as necessary.

The powered turret approach seems to be the most popular at the moment. Most machines offer a combination of radially and axially mounted tools. The radial tools are used for operations like milling fiats or keyways. Axial tools give the ability to drill and tap bolt holes and other features on the face of the part. One caveat: make sure the machine you select has the capability to perform all the operations you need now and in the future.

The advantages of being able to perform non-turning operations on a workpiece in the same setup in which turning operations are performed should be fairly obvious. They include better accuracy, shorter overall production time, and higher machine utilization by limiting multiple setups, reduction of in-process inventory by eliminating inter-machine "float," reduced capital costs by eliminating the need for machining centers, and, as a direct result, improved competitiveness and profitability.

So why aren't all CNC turning centers equipped with rotary tooling?

Probably the biggest reason is that mechanical and structural considerations presently limit the milling capabilities of these hybrid machines to fairly small parts and relatively simple operations. Within their limits, these machines can be highly productive, but they cannot duplicate the versatility and application range of two separate single-purpose machines of comparable capacity.

Less obvious, but equally important, is the fact that mill/turn machines are considerably more complex than a single-purpose machine both mechanically and to program. Both of these factors may limit their usefulness in certain kinds of applications.

Once again, it comes down to a question of what you want the machine to do. If you can eliminate the need to purchase a separate machining center by buying a hybrid, and you probably aren't going to need a machining center in the future for some other purpose, then the combination machine can be an incredible value. If not, then you have to look very carefully at the extra cost, which is significant, and make the best long term decision.

In either case, the hybrid machine is an interesting and potentially profitable alternative which should be considered. It is also a technology that is going to improve rapidly, because it is such a good idea. It's worth watching the evolution of these machines even if you have no immediate need.

If One Spindle Is Good, Two Must Be Better

Like the hybrid machines, today's multiple-spindle turning centers are a reflection of the increasing power of modern controls. These machines come in two basic configurations, side-by-side or face-to-face, and both are primarily used to perform operations on both ends of a part.

In the side-by-side configuration, a manipulator is commonly used to move the part between chucks, although manual loading is also used. These machines can also be used as regular chuckers to produce two parts simultaneously, giving them an edge in versatility for job shop applications.

The face-to-face configuration is basically a machine with a headstock at each end of the bed and one or (more typically) two turrets between them. In another illustration of the power of tradition, the primary operations on these machines are nearly always performed on the left head-stock and the secondary operations on the right.

On the more sophisticated machines, the workpiece "hand-off" is often made by moving the headstocks into contact, synchronizing their rotary speeds, and then transferring the "grip" from one chuck to the other on the fly. This is a real test of the control system and a tribute to the precision and reliability of modern controls and servos. Less sophisticated machines use robotic manipulators or manual part transfer.

These machines tend to be more complex and expensive than standard CNC turning centers. They offer, however, a greater degree of flexibility and higher production capabilities which can be highly profitable in the right applications.

Two of the more interesting developments in turning over the last few years have been the introduction of essentially modular machines, and a rebirth of interest in quick-change tooling systems. These are both trends that promise significant changes in the way turning machines are purchased and used in the future.

As we've already seen, turning machines are really quite simple in conception, being in essence a combination of more or less standard components. Put a tailstock on a chucker and you have an engine lathe, put a through hole in the headstock of an engine lathe and you have a bar machine, and so forth. So, why not simply offer standardized "pieces" in a range of standard configurations? Why not, indeed.

In the past few years, several builders have done just that. The result in each case is a series of machines built up from common components that can be configured for a very broad range of applications. It's a good idea that promises to reduce machine cost through volume production of standardized components while increasing customer choice through greater opportunity for customization--another trend to watch closely in the coming years.

Over the years, a number of tooling suppliers have developed quick-change systems for lathe tooling to improve machine uptime. Most of them are based on some sort of proprietary mechanism to hold the cutting tool which then mates with a block or adapter that is permanently or semi-permanently attached to the turret.

Most such systems use some variation on the tapered shank concept developed originally for use on machining centers. Another variation on this theme makes the adapter mechanism an integral part of the turret, with the German-developed VDI system being the most common example.

A more recent development from Germany is the non-proprietary hollow shank/taper system (DIN 69 893) which is suitable for both rotary and stationary tooling, making it a very interesting choice for the new hybrid machines. The system is under evaluation as an ISO standard.

Perhaps the most innovative approach to quick-change is the automated turret changing system offered by at least one U.S. builder on its modular turning system. An entire 12-station turret (with all stations powered) can be changed in 30 seconds or less using this system, which can have a dramatic impact on the reduction of setup time.

Another trend that bears watching is the migration of sophisticated adaptive control technologies commonly found on grinders to turning machines. The integration of in-process gaging with real-time SPC and automatic tool compensation promises to substantially improve the quality performance of turning operations in the future.

Source File

If you would like to discuss any of the aspects about lathes and turning
mentioned in this article, please contact these companies. You may use the
convenient Reader Service Card in this issue.

Turning machines--horizontal, between centers, chuckers, single-spindle,
bar-type:

Amera Seiki                   244
Bardons & Oliver              245
Bridgeport Machines           246
Brisard                       247
Cincinnati Milacron           248
Clausing Industrial           249
Comau Productivity            251
Cone Blanchard Machine        252
Cosa Corp                     253
Daewoo Machinery              254
Danobat                       255
Davenport Machine             256
Deckel Maho                   257
Dorries Scharmann             258
Emco Maier                    259
Ex-Cell-O Machine             260
Giddings & Lewis              261
Gildemeister                  262
Gomiya                        263
Haas Automation               264
Hardinge Brothers             265
Harrison Rem Sales            266
Hitachi Seiki                 267
Ikegai America                269
Index Corp                    270
Jones & Lamson                271
Kia Machine Tool              272
Kitamura Machinery            273
Kummer America                274
Leadwell Mfg                  275
LeBlond Makino                276
Light Machines                277
Marubeni Citizen-Cincom       278
Mazak                         279
Methods Machine/Nakamura      280
MHI Machine Tool              281
Mighty Enterprises Inc        182
Mitsui Machine                283
Miyano Machinery              284
Modern Machine Tool           313
Manarch Sidney                285
Mori Seiki                    286
MSC Machine Tool Div          375
Newcor Machine Tool           287
Okuma Machinery               288
Olofsson Corp                 289
Omatech Inc                   290
Omniturn CNC Control          291
Ravensburg Machine            293
Republic Lagun                294
Robert Bosch Corp             295
Saeilo                        296
Servo Products                297
Simmons Machine Tool          298
Spinner Machine Tool          299
South Bend Lathe              300
Summit Machine Tool           301
SMG Machine Tool              302
Tomen America                 303
Toolmex Corp                  304
Tornos Technologies           305
Toyoda Machinery              306
Traub                         307
Tsugami Rem Sales             308
Turmatic Systems              309
US Baird                      310
Voest Alpine                  311
Wasino                        312
Yang                          314
YCI Inc CNC Div               315

Turning machines--vertical turret:

Daewoo Machinery              254
Giddings & Lewis              261
Gomiya USA                    263
Index                         270
Kingsbury Turning Div         316
Mandelli                      317
Mario Carnaghi                318
Mazak                         279
Mitsui Machine                283
Newcor Machine Tool           287
Okuma & Howa                  319
Olofsson Corp                 289
Omnitrade Machinery           320
Pittler Tornos                292
Republic Lagun                294
Spinner Machine Tool          299
Takang Industry               321
Takisawa USA                  322
Toolmex Corp                  304
Wasino                        312
Werth Engineering             323

Turnmill machines:

American-Wera                 324
Bohle Machine Tools           325
Caltec Machinery              326
Cincinnati Milacron           248
Crankshaft Machine            327
Daewoo Machinery              254
Hitachi Seiki USA             267
Leistritz                     328
Marubeni Citizen-Cincom       278
Mazak                         279
Mitsui Machine                283
Motch Corp                    329
Okuma Machinery               288
Olofsson Corp                 289
Pittler Tornos                292
Ravensburg Machine            293
Spinner Machine Tool          299
Tornos Technologies           305
Tsugami Rem Sales             308
Voest Alpine Trading          311
Wasino                        312