All Machine Tools Are Not Created Equal

There are almost as many machine tools as there are potential and actual users combined. Each builder has its own niche (or niches) and adds its own "flair" to the machine, making it that much different from the competition. To try and name every builder and every machine here would be impossible, yet a valiant attempt will be made to break everything down and let you know what to expect from a range of machine tools, from verticals to horizontals to universals.

Trend: Controls

While every builder goes to great lengths to make its machines unique, there are a couple of things happening out there that can be labeled "trends." The first of which is the evolution of controls from tape reader to full-out PC. Most, if not all, machine tool makers feature controls that are dubbed "PC-based" or "PC-compatible." A recent survey of those involved in control technology see industrial PCs completely overtaking the control market in the next few years. In fact, many companies have already begun creating "open architecture" controls, meaning that the user can configure the controls to exactly their specifications. Others have gone one step further, putting all capabilities into software, so that nearly any hardware will do, because the control is actually the software. Either way, users had better be pretty computer savvy, and had better be prepared to become more so.

Another trend seeping into the realm of controls is Windows NT. Most hardware and software providers are banking on the fact that Bill Gates will take over the machine control market also, and are converting to a Windows NT environment. Short of this, they may offer Windows NT as an option, and let you decide.

Combining these two technology directions leads to an exciting possibility for the future: networkability. While many controls are plugged into user local area networks (LANs), few have yet to be reliably brought into the realm of the World Wide Web. Once the issues of reliability and security are cleared up, Tier 1 and Tier 2 suppliers will further tighten up the supply chain, transferring CAD/CAM data from the design department at one company directly to a machine at another.

Some Specifics

Out of all of the newer control technologies out right now, the slickest looking features are coming from combining mega-capable hardware with creative software solutions that help boost machine capabilities. For example, the BostoMatic BDC 3200X CNC from Boston Digital Corp. (Milford, MA) uses software algorithms, 32-bit VME bus architecture, and multiple microprocessors to enable the processing of 1,100 five-axis blocks per second. While achieving such high speeds is something else, the control gets slicker when you consider that no preprocessing is required, even on the most data-intensive part programs. Contour optimization software and user-programmable servo parameters round out the 3200X's features.

Creative Technology Corp.'s (Arlington Heights, IL) Creative Evolution is a PC-based control that Defiance Machine & Tool Co., Inc. (Maryland Heights, MO) counts on to give its small machining centers a few extra punches. A look-ahead feature scans for corners, and automatically adjusts speed and feedrate to prevent gouging and provide rounder corners. The system also provides automatic chamfering, canned milling and Z-axis cycles, mirror-image programming, and an on-screen text editor that includes global program modification.

Trends: Spindles

Used primarily to machine EDM electrodes, the Defiance VTX line of tabletop machining centers gets a lot of its precision and capability from its control and specially designed software from Creative Technologies.

There's something fishy going on with spindles. Everyone is talking about self-balancing, high-speed spindles, but no one we've found will talk specifically about them. Several machine tool companies are "looking into" the development of these units, but mum's the word until they're ready to launch the product. Of course, there are some self-balancing spindles out there already, but they are incredibly expensive, and are really still in the R&D stage of things. It should be interesting to see what develops in the next few months.

Some Specifics

The tilting head spindle configuration from Mandelli is designed to provide precise machining under tough conditions. This particular tilting head is used to machine various car components at Ferrari's plant in Maranello, Italy.

In the mean time, there are some tangible advances being made in regard to spindle technology. One that stands out is the tilting head configuration at Mandelli (Rockford, IL). While tilting head spindles add a fifth axis, they tend to be too delicate to stave off vibration and other accuracy inhibitors. Taking that into account, the company's engineers have developed a high-torque spindle that can handle heavy-duty cutting applications, but without sacrificing accuracy. Combining that with the standard Sinumerik 840D control from Siemens, this system can really cook through some mold and die applications.

Trends: Toolchangers

As with the rest of the machine, toolchangers have gotten more sophisticated. Some systems are capable of tool condition monitoring, reporting data back to the control; and many ATCs feature a load cell that monitors whether the tool is present, and transfers tool length data to the control as the tool is mounted to the spindle. Manufacturing cells and flexible systems now use one toolchanging system for several machines, saving space, time, and money. A robotic system patched into the machine's control moves tools from machine to machine as needed.

Some Specifics

Changeover times range from 10 seconds in cells and systems, to 1.5 seconds in some of the smaller horizontal machines. Capacity ranges from 12 tools to hundreds. Shank type depends on the spindle. A system from Mikron Corp. (Monroe, CT) uses rotary magazines with 12 positions each. This lets users set up tool sets for specific jobs, and switching from job to job only means lifting out one magazine and replacing it with another. The system is expandable up to 40 positions.

Trends: Feed & Speed Rates

With most vertical machines, the magic number seems to be 787 ipm rapid feeds in the Z-axis and around 1,100 ipm in the X and Y axes when you're talking vertical machining. Horizontals are capable of faster speeds, hitting around 1,100 to 1,400 ipm.

Some Specifics

Going fast isn't just a function of the spindle and the cutting tool anymore. Machine tool builders are using software to get things moving faster. Makino (Mason, OH), for example, has a software function called "rapid traverse bypass." It determines where the spindle needs to be, allowing rounded corners on rapid paths to get it there. Enshu U.S.A. Corp. (Elk Grove Village, IL) uses a programming algorithm to provide fast, synchronized tapping with forward control. According to Enshu, the algorithm works even for extremely small diameters.

Trends: Work Changing

To speed up automated pallet changeover, most companies are incorporating shuttle control into the programming of the machining system's control. Here again, the configuration of the changer is going to be up to the user.

Some Specifics

The Flexline manufacturing system from Chiron is packed with a lot of time and money saving features, but one of the more interesting ones is a part changing system that uses robotic arms to perform changeover in about 10-seconds.

Five-axis machining for under $200,000? Yep. The HS-1R 5AX horizontal from Haas Automation, Inc. (Oxnard, CA, seen here performing porting operations on a cylinder head, features X,Y, Z travels of 24 x 20 x 22 in., respectively, and a standard extended table measuring 15 x 40 in. All of the bells and whistles are integrated into this machine: full five-axis control, a 7,500-rpm spindle powered by a 15-hp motor, rapid traverse rates of up to 710 ipm, a 24-pocket toolchanger, and a chip auger that automatically senses loads to prevent jamming.

One of the more unique (and quick) workpiece changeover systems comes from Chiron America, Inc. (Charlotte, NC). Its flexible transfer lines employ a rail-guided robot moving from station to station, and back to a load/unload station. Moving at 8,000 ipm, the robot zips from machine to machine, and changes parts in around 10 seconds. An optional part magazine allows unattended operation for long periods of time.

Trends: Construction

Ribbed, cast iron, Meehanite, even concrete. The base material and construction of machining centers are both common and different. Not to say that machine tool builders have lost their creativity in this regard. Quite the contrary. Builders are left with the dubious task of making machines that have as large a work envelope as possible while maintaining rigidity, while damping vibration, while not taking up too much room. You get the picture.

Some Specifics

Looking again at Mikron Corp., its machines feature a cast structure in a portal design. Two portal supports are fixed on the base, and the transverse beam accommodates the cross slide. The Z-axis runs on top of this. According to the company, the symmetry of the design naturally minimizes vibration.

Clearly, the machine tool industry is extremely competitive, and builders work very hard to stay one innovation ahead of its competition. If they all stay on the track they're on, users will be machining parts in France from their living rooms in Ohio before we know it.

Things to Know About Cam Grinding

The changing shape and composition of camshafts are challenging those responsible for surface finishing. As a result, an increasing number of cam-grinding operations are being performed with cubic boron nitride (CBN) superabrasives and technologically advanced conventional abrasives.

Camshaft grinding requires instantaneous changes in wheel-work contact length and material removal rates. In camshaft grinding, the arc of the contact between the wheel and the camlobe changes continuously as the base circle, flanks and nose are ground (see illustration). The arc of contact can be represented by means of the equivalent diameter (De) and can be calculated:

De = Ds x Dw Dw ± Ds with: Ds is the wheel diameter Dw is the work diameter + is to be used when Ds > Dw - is to be used when Ds < Dw

A large equivalent diameter represents a longer arc of contact between the wheel and work material. So the arc of contact is large as the wheel passes over the flanks and becomes smaller as the nose and base circle are ground.

As the De increases, more abrasive particles come in contact with the work material. This reduces the force acting on each grain, and makes the ploughing and sliding interactions more dominant. This is usually observed as increased grinding power or force for the same grinding condition.

In camshaft grinding, as the De is larger while grinding the flanks, higher power will result if the radial infeed and work speeds are maintained the same as when grinding the nose or base circle. This can result in unacceptable thermal damage on the lobes. In order to avoid this problem, the work speed is generally reduced as the wheel grinds the flank portions. This reduces the rate of material removal and helps lower the grinding power.

Take Care

The equivalent diameter in camlobe grinding.

Camlobe grinding is a complex variant of cylindrical grinding. When raw camshafts are manufactured—by casting in sand or shell molds (cast iron), forged or machined from barstock (steel)—roughly 0.02 to 0.12 in. of stock is generally available for grinding. After the lobes and journals are induction hardened, the journal bearings are ground, usually on a centerless or cylindrical grinder. Since the camshaft is supported on workrests, with the journals as a reference during camlobe grinding, it is important that the journals be ground properly because any imperfections will be translated to the camlobes during lobe grinding.

Although the necessary surface finish on the lobes and journals is usually provided by the grinding, they are often lapped with fine-grit abrasive belts. Lapping after grinding has been found to be effective in reducing initial friction during the engine break-in period.

New Profiles

During the past decade, camlobes with a re-entrant profile have been gaining popularity because this design helps increase engine torque and horsepower and reduces emissions. In re-entrant camlobes, the ramps are concave instead of a straight line; this helps in opening and closing the valves much faster.

Also, the use of roller bearings at the interface with the camshaft in place of flat tappets has reduced friction, permitting higher engine speeds. However, the sharp line contact results in significant increases in contact stresses, which can exceed 250,000 psi. These high stresses can result in spalling and accelerated camlobe wear. So depending on the magnitude of the contact stresses, camshafts of different construction are used.

For lower contact stress applications, induction hardened or chilled nodular cast iron may be acceptable.

For contact stresses of 250,000 psi and above, steel cams can provide the most-reliable solution.

Composite Approaches

Composite camshafts of medium- and high-alloyed powered metal lobes mounted on a hollow tube are popular as they have the capacity of withstanding high contact stresses and offer improved tribological properties. Composite camshafts can be 50% lighter than cast iron or steel shafts; can have lobe material tailored to the application; and they can have lobes molded to near-net shape, which means that the amount of grinding stock, and consequently grinding time, are both reduced.

The requirements to have high-strength, low friction and minimal wear have prompted Japanese researchers to consider ceramic camlobes.

Camlobes are usually machined to final shape by grinding with bonded abrasive wheels containing aluminum oxide or CBN. Usually, one lobe is ground at a time, although in order to improve productivity, machines using multiple abrasive wheels are available. With this equipment, two wheels are mounted on a one-piece precision-balanced hub to grind two lobes at a time. But this approach can be used only if the lobes being ground simultaneously have similar lift profiles.

Abrasive Belts

Another approach to improve cycle time is using machines with abrasive belts. This was developed in Italy in 1977; it was successfully implemented by GM engineers only in the early 1990s. In these machines, the abrasive belt is pressed against the workpiece by a cylindrical contact shoe that, together with the belt, replaces the curved surface of the grinding wheel. These multiple-belt cam grinding machines can grind all lobes simultaneously for cycle time reduction.

Another advantage offered by these machines is in the grinding of re-entrant camlobes. Grinding of re-entry type cam profiles with grinding wheel requires that smaller diameter wheels be used. This restricts the life of the wheel, making its use less cost-effective. With abrasive belts, the curvature of the belt can be controlled by changing the contact shoe diameter, so re-entry lobes of high negative curvature can be easily ground.

Higher Speeds

The current trend to reduce cycle time involves grinding at high wheels speeds. Machine builders in Europe and Japan are marketing high-speed cam-grinding machines with wheel speed capability of up to 200 m/sec. High grinding wheel speeds offer the advantage of longer wheel life, lower forces, and high removal rates. Higher metal removal rates at higher wheel speeds reduce cycle time.

A variation of the high-speed approach involves high wheel speeds, large depth of cuts, and low work speeds. This approach is similar to High Efficiency Deep Grinding (HEDG) and is essentially high-speed, cylindrical, creep-feed grinding.

Another approach uses a high-speed grinder with a pair of wheel heads. One wheel head uses a brazed, single-layer CBN wheel and is used to rough grind the lobes very aggressively. The other wheel head uses a small-diameter vitrified CBN wheel for finishing the lobes. This approach has been successfully used to grind re-entry type camlobes.

Vitrified bonded aluminum oxide wheels are still popular for grinding camshafts, but most CNC machines are now shipped with superabrasive wheels.

CBN & Beyond

The hardness, thermal conductivity and chemical inertness of CBN
have made it an abrasive of choice for production grinding of ferrous and aerospace alloys. Several bond systems—resin, metal, glass, and brazed—are available with CBN. Among these, the glass, or vitrified, bond system has proven to be the most popular. These bonds are rigid, free-cutting and have good form-retention. In addition, the porosity in these bonds can be controlled for more chip clearance and better coolant application at the grinding zone. Vitrified bonded wheels are easy to true and do not require additional dressing procedures.

The inherent brittleness of the vitrified bond system contributes to the self-sharpening action during grinding, but the brittleness also makes the wheels susceptible to chipping and breaking if used improperly.

One answer to this problem is a segmented wheel design, where individual segments are cemented to a steel or aluminum oxide core. Benefits include:

•Easy replacement of damaged sections
•Safety at high wheels speeds—up to 150 to 200 m/sec.
•Properties of individual segments can be measured and monitored so there is consistency along the wheel periphery and from wheel to wheel.
•Special wheel adapters can be resegmented and reused after the CBN layer has worn out.

There is also a ceramic aluminum oxide abrasive that's making an impact in cylindrical grinding operations. This elongated, high-aspect ratio abrasive can make a difference in high speed grinding of cast-iron camlobes. They provide high removal rates compared to a vitrified CBN wheel. The lower cost and single-point truing capability compared to CBN could make them excellent alternatives.