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Anatomy of a machine tool.

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

If you're confused by the proliferation of mechanical design options offered by machine tool builders, you're not alone. Today's machine tool buyer faces an often bewildering range of choices.

The more obvious ones include: cast vs fabricated base and column; moving column or saddle; linear bearing vs box ways on one or more axes; and direct drive vs belt driven or geared spindle. Then there are the less apparent choices: size, location, and mounting of ballscrews and nuts; active vs dead weight counterbalance; spindle oil coolers vs fan cooling or none at all; location of accessories like toolchangers, electrical cabinets, pumps and reservoirs; and many others.

There is a natural tendency on the part of builders to "optimize" the inherent advantages of whichever set of options their particular machine offers, while downplaying the potential benefits of competitive designs. This tends to focus the selection process on the question of which design is "better," when the real issue should be which is the most cost-effective solution to the requirement at hand.

Bulldozer or Ferrari?

The answer depends almost entirely on your intended use. If you want to move a mountain, try a bulldozer, but if you want to get to the mountains in a hurry, Ferrari is the choice.

Is a 16,000 lb vertical machining center with box ways on all axes "better" than an 8000 lb machine with linear bearing ways? The question is only meaningful in the context of what you expect the machine tool to do.

If you're hogging die steel, the answer might well be "yes." But if you're turning out open tolerance aluminum widgets, it would almost certainly be "no." Between those two extremes, the choices become a bit less obvious, but a clear focus on your requirements is the key to making the right decision.

Both of those machines have a place in the spectrum of metalworking operations where they will supply the most cost effective solution. One is not "better" than the other in any meaningful sense, they are simply "different."

With that in mind, let's examine some of the options you're likely to encounter in selecting a machine tool. We'll focus on machining centers, but the general principles apply across the board.

Where's the beef?

Cast iron, and lots of it, is the traditional material for machine tool bases and columns. Cast iron is relatively inexpensive, has an internal structure which enhances its vibration damping characteristics, and is fairly easy to work with.

More recently, many builders have switched to Meehanite, a proprietary form of nodular iron which reduces the need to "age" castings before they can be used, and therefore reduces the builder's inventory costs. Since it is an alloy material, Meehanite's physical properties can be tailored over a fairly broad range to meet specific application requirements. In the case of machine tool bases and columns, the formula is optimized for stability and vibration damping.

One builder offers the following example of the results that can be achieved: If a steel tuning fork rang for ten to fifteen seconds when struck, a standard Meehanite tuning fork might ring for only five seconds from the same blow, and one made from the alloy used in their machine bases would ring for only half a second.

Of course, nobody makes tuning forks out of cast iron, for exactly the same reasons it's so widely used in machine tools. But the example is, nonetheless, instructive.

Brains versus brawn

The other common option is the welded or fabricated design. Weldments are made up from sections of heavy plate steel reinforced with corner gussets, bulkheads, and braces, and they typically weigh about half as much as a casting of comparable size.

Builders compensate for the reduced mass by designing the weldment to resist vibratory deformation through the size and location of the bracing members. This is an area where computer models and analysis tools have been applied extensively to design optimization.

Because they're fabricated, welded bases and columns cannot have the integral ways often found on cast components. Although there are exceptions, machines with fabricated bases and columns are more likely to be equipped with linear bearing-type ways.

The frequency with which this combination is found tends to classify machines using weldments as "light duty" machine tools, a generalization not always justified by the facts. Good design can, indeed, eliminate much of the gap between the "brains" and "brawn" approaches to structural rigidity.

The tradeoffs

What you're buying when you opt for cast-iron construction is often comfort and longevity. Cast iron is a thoroughly known quantity, and its behavior is well understood and predictable.

Weldments, on the other hand, are subject to a broader range of variables both in design and fabrication. Many of the older designs--the ones that form the basis for much of today's conventional wisdom concerning weldments--were created before computer assisted engineering (CAE) tools were available to help in the optimization process.

The fact is that steel, no matter how well designed, simply can't match the inherent vibration damping characteristics of cast iron. It's a different material, and there's no way around that fact. Whether or not this really matters depends on what you're going to do with the machine.

It's interesting to note that many of the bases used on special machine tools are fabricated rather than cast, apparently presenting no particular problem. The reason, according to one source, is that these bases are firmly attached to the foundation so that "they can't move." The same approach might well help improve the rigidity of weldments on standalone machine tools.

As a rule of thumb, the more demanding the application in terms of metal removal and/or precision, and the longer you intend to keep the machine, the happier you're likely to be with cast-iron construction. But, you will pay a significant cost penalty in almost every case. Whether it's justified depends entirely on the economics of your operation.

Mohammed and the mountain

The question of whether the column should travel to the workpiece, or the workpiece should travel to the column, is constantly in dispute. Proponents of the traveling column claim better rigidity, while the saddle mounted table school claims lower cost and greater flexibility.

In properly-designed machines, there really isn't much to choose between the two, except that the traveling column simplifies chip control somewhat by permitting use of a centrally mounted chip conveyor. This can be a worthwhile advantage if the machine is intended for use in a cell designed for unattended operation.

With that one exception, however, you will probably be satisfied with the accuracy and rigidity provided by either design approach, all other factors being equal.

More than one way

Ways are the third major area where the machine tool buyer is presented with a seemingly difficult choice. Box-type ways, either cast integrally or attached to the machine base and/or column, are the traditional answer. Properly designed, they add to the rigidity of the base or column and can be designed to handle extremely high loads. They also provide good support for operations where the table is substantially off-center, such as in dual-fixtured machines or applications on long workpieces.

Their major advantage, which is also one of their major disadvantages, is that they provide surface contact between the moving machine elements. This spreads the load over a large area and tends to provide very stable support. It also makes box ways less susceptible to damage in case of a crash. Bear in mind, however, that if you do damage a set of box ways the repairs can be very expensive.

On the other hand, surface contact also produces a lot of friction which can limit speeds, require larger servos and ballscrews, and exacerbate stick-slip problems. Anti-friction materials such as Turcite, a Teflon-like material, can substantially reduce stick-slip, but they must be used correctly. In the case of wide ways, that means the Turcite should contain a series of cut-outs to reduce the overall surface area to a calculated optimum--something to look for when evaluating a machine.

Box-type ways are also more difficult, and costly, to manufacture, usually requiring hand scraping to be truly effective. The add-on type are more easily reconditioned and tend to last longer than the integrally cast, flame-hardened variety.

Linear bearing-type ways have none of these problems. They are light, nearly friction-free, and relatively inexpensive. Machines equipped with them typically provide higher rapid traverse rates than comparably-sized box way machines, often with smaller ballscrews and servos.

Linear bearings provide point or, in the best cases, line contact between the mating surfaces, a fact with several negative implications. All of the weight and force is concentrated on the points of contact, creating very high pressures which can lead to brinelling. They are more easily damaged in a crash for the same reason. If you do damage them, however, they typically cost less to repair than a box-type way.

The analogy is often drawn between linear bearings and the force applied by a woman's spike heel. Because the weight is concentrated on such a small area, the heel can, and frequently does, penetrate most flooring materials including marble and terrazzo. Likewise, the balls in a linear bearing can damage even a properly-hardened guide track under certain loading conditions.

Perhaps the greatest weakness of bearing-type ways is that they cannot provide the uniform levels of rigidity found in surface contact box-type structures. They are very stable when force is applied directly through them, as in drilling with the workpiece fixtured above a platen (the component that attaches the bearing to the machine element, analogous to the nut of a ballscrew). They are a lot less so when the force is applied in an off-center direction or, in the worst case, as a side thrust.

More tradeoffs

The choices are somewhat clearer in the area of ways than they are in cast vs welded construction. Heavy-duty applications are best performed on box way-type machines, while linear bearing ways are a good choice for lighter duty applications, especially where high traverse speeds are an important consideration.

Unfortunately, it's not always that simple. First of all, the speed gap is closing steadily as box way design and anti-friction coating technologies continue to improve. On some smaller machines today, the difference is already negligible.

Second, there is a growing list of machines offering a combination of both. Typically, the head moves on box ways, and the table travels on linear bearings. Depending on other factors like horsepower and cube size, these hybrids are a viable answer for the shop that needs something more than a light-duty machine, but doesn't want to pay the price for extremely heavy-duty construction.

There is another factor to be considered. Box ways, especially the "bolt-on" type, can be reconditioned a number of times over the life of a machine tool, an important consideration if you plan to use the machine for a long time. This is considerably less costly with the "bolt-on" design. Bearing-type ways are easier to replace, but you'll almost certainly be doing it more frequently.

Again, the final decision must be made on the basis of what you expect the machine tool to do. Cast-iron construction and box ways probably mean heavy-duty. Welded construction and linear bearings probably mean light-duty. But the other possible combinations are not so clear cut.

One application in which linear bearings are probably the best choice is in a dedicated drilling or tapping machine, where the thrust is always in the same direction and rapid movement is a distinct advantage.

To gear or not to gear

Spindles are another area where two distinct schools of thought are apparent. The traditional approach is a geared, or sometimes belt-driven, head.

The argument here is that maximum horsepower can be developed at a lower spindle speed, which makes the machine both more capable and more versatile. Direct drive advocates counter that modern servo technology can deliver essentially comparable performance without the extra cost and complexity of a geared head.

Which side is right? Probably both.

This is another case, like that of welded bases and columns, where the conventional wisdom is based largely on the performance of outdated technology. A direct drive spindle, using the latest technology, comes very close to matching the speed/torque characteristics of a geared drive, and it does it with a simpler mechanism and considerably less weight. The direct drive will probably be quieter, especially at high speeds.

On the other hand, the geared drive usually gives you at least two optimized speed/torque combinations, generally uses a less expensive motor, and doesn't require a box full of sophisticated electronics.

Realistically, the choice of direct drive vs geared today appears to have more to do with where a machine is built than with the relative merits of either approach. Those built where mechanical components like gearboxes can be manufactured cheaply tend to have geared heads. Those built where mechanical components tend to be costly use direct drive. Properly designed, either will provide cost-effective performance.

The devil in the details

There are a number of details that impact the overall performance of a machine tool. For example, the location of the ballscrews can have a significant influence on both accuracy and durability. Generally speaking, they should be mounted as close as possible to the way providing the primary guidance.

On a linear bearing installation, this means the ballscrew should be exactly centered between the ways, but on a box way machine there are two choices.

In some designs, one of the ways performs the primary location and guide functions, and the other merely supports. In others, the moving element is located between the inside or outside edges of both ways.

"One way" systems should have the ballscrew located as close as possible to the master way. "Two way" designs should have it centered. This is a question that tends to separate the knowledgeable machine tool sales personnel from those who still have a bit to learn.

In either case, the ballscrew under the table should be as close to the "top" as possible. Ideally, the screw would be above the table pushing directly on the center of the workpiece, but that's obviously not practical. As it's moved further away from this ideal position, the screw is subjected to increasing leverage from the cutting forces, which tends to cause distortion and "whip."

Obviously, the screws and nuts must be adequately sized, securely mounted, and accurately aligned. This is not often a problem with quality machines, but it pays to compare specs. Up to the point where it begins to become cost prohibitive, "more" is definitely "better" with ballscrews.

The issue of dead-weight vs active counterbalancing can impact both machine performance and durability. Dead weights are certainly less expensive than air/hydraulic systems, but they become increasingly less desirable as traverse speeds increase. The problem is inertia.

Simply put, dead weights tend to increase the load on servos and ballscrews at the start of a rapid traverse and "bounce" at the end. That can cause a number of problems.

Obviously, higher loads accelerate wear and, in the worst case, can break the chains supporting the weight. In practice, however, traverse speeds are reduced to avoid these problems, and the penalty is paid in longer cycle times over the life of the machine.

Active systems, while initially more costly, can be fine-tuned to avoid these problems. They are also easily adjusted to compensate for changes in machine configurations, like fitting a larger spindle motor or adding accessories to the head.

There is another point to consider with deadweight counterbalances. Most builders include the weight of the counterbalance in the overall machine weight specification, even though it provides virtually no benefit in terms of rigidity or damping capability. Subtracting the counterbalance from the machine weight will provide a truer picture of the machine.

Spindle coolers, either circulating refrigerant types or fan/radiator systems, are another area of controversy. Do you need one, and if so, which is better?

The idea here is to provide consistent accuracy by maintaining the spindle bearings at a constant temperature so they don't "grow" in unpredictable ways. The key word is "unpredictable."

Unless you're operating at very high speeds or running a very high horsepower spindle, you probably don't need a cooler. Yes, the bearings will grow as they heat up, but it's not likely to be unpredictable growth. Good practice, like letting the machine thermally stabilize before making a critical cut, will take care of most problems.

If you do need a cooler, the refrigerated type will provide the highest level of consistency and also cost the most to buy and operate. The fan/radiator systems, which simply remove heat from the spindle lubricant, occupy the middle ground in both cost and performance.

One last detail that can impact machine performance is the location of accessories like toolchangers, electrical cabinets, pumps and reservoirs. Ideally, these will be located so that their weight tends to help keep the column in balance.

Columns are naturally unbalanced front to rear, because the head is suspended from the front. Placing the pump/reservoir package behind the column can help balance this situation. Likewise, placing the toolchanger and electricals on opposite sides will help balance the column in that direction.

These are, admittedly, minor points, but a builder who pays attention to the small details probably pays attention to the more important factors, too. There are many ways to cut corners, and the buyer won't find most of them until it's too late.

The best way to protect yourself is to know exactly what you expect from the machine tool, carefully compare specifications, and then see the machines you're interested in actually cutting metal--preferably the parts you intend to make--before you buy.
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Morningstar, David
Publication:Tooling & Production
Article Type:Cover Story
Date:Feb 1, 1993
Words:2983
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