Machine tools target cycle time

DN Staff

August 26, 1996

14 Min Read
Machine tools target cycle time

Newton, MA--It's a given that one way to improve productivity is to decrease cycle time. Decreasing cycle time requires quick spindle acceleration/deceleration, more rapid traversing of machine tables, and faster tool changes. Those are precisely the benefits three manufacturers of high-speed linear motor horizontal machining centers (HMCs) claim for their new machines.

The companies--Ingersoll Milling, EX-CELL-O, and Kingsbury Corp.--all use linear motors, servo drives, and computer numerical control (CNC). Each features different, application-specific, structural designs, and each claims noteworthy advantages over conventional ball-screw machines in areas of speed, power consumption, accuracy, run-out, dampening, vibration, stiffness, and operational life.

Coming to IMTS. Ingersoll Milling Machine's HVM 600 will make its debut at the International Manufacturing Technology Show in Chicago in September. The all-steel machine incorporates three key technologies: high-velocity, linear motors, and a hybrid hydrostatic-hydrodynamic fluid bearing spindle designed by Ingersoll.

The term "high-velocity" was coined by Ingersoll to differentiate its machine from the others on the market. Other differentiators it claims: the 10,000- to 40,000-rpm spindle speed range, all-steel construction, full size, and the use of synchronous motors.

"What sets this machine apart from the competition is its ability to machine larger parts from harder materials," says Jeff Porter, general manager, high-velocity machines for Ingersoll. About 60% of the applications for this machine are cast iron, while only 40% have been aluminum.

"No mechanical wear parts in the bearing and drive design means you can make high-power roughing cuts, come back and take fine-finish boring cuts, with no bearing or drive degradation," says Porter.

Development of these machines began in 1985 in partnership with Ford Motor Co. to manufacture heavy cast iron engine components in their plants.

Box in a box. Originally designed to handle rapid machining of engine blocks, the machine's "box-in-a-box" structure is stiff enough to handle a broad range of automotive applications including: cast iron engine blocks, cylinder heads, and differential carriers; and aluminum cylinder heads, pump bodies, and cover plates.

Box-in-a-box refers to the machine's structure. The primary box, the main machine frame, carries the x-axis. The secondary box carries the y- and z-axes. The main box supports the secondary box at top and bottom. "The concept of a box-in-a-box gave us the ability to achieve symmetry on the machine," reports Isaac Hogg, Ingersoll's manager of engineering services.

"This structural design solves the thermal characteristics of load distribution, increases flexibility, and decreases overall size for the relatively long-axis strokes," adds Hogg.

One of the keys to the box-in-a-box design is that it drives the axes symmetrically both top to bottom and side to side, while simultaneously supporting those axes symmetrically both top to bottom and side to side. This eliminates many of the over-turning moments of a conventional stacked-axis design.

Engineers used SDRC's I-DEAS CAD software and Structural Research and Analysis Corp.'s COSMOS finite element analysis software in the design of the structure. They also used modal analysis of resonant frequencies to design the three axes stacked under the spindle. Most conventional T-base designs use one axis under the work piece.

Axis orientation is as follows: y-axis is vertical, x horizontal, and z is the feed. The x-axis has a total range of 1,030 mm and a working range of 630 mm. The y-axis has 630-mm stroke, while the z-axis stroke is 500 mm. Top speed is 76,200 mm/min (3,000 inches/min). Acceleration of the x- and y- axes is 1g, and z-axis acceleration is 1.5 g. The machine also includes a b-axis rotary table, with a feed rate of 33 rpm.

While earlier machines used GE Fanuc controls, with Baldor/Sweo drives and Anorad motors, the HVM 600 appearing at IMTS uses a complete package from GE Fanuc that includes motors, drives, and CNC.

Motor power. The x- or gantry axis, and y-axis (with spindle feedback) are both powered by two motors, while the z-axis uses a single motor. Each synchronous linear motor uses dc permanent magnets mounted to the axis, and requires a separate servo drive.

A three-tier protection system minimizes contamination of motors from chips and other magnetic debris. The system includes a non-magnetic, stainless-steel motor cover and other features to protect the linear motors from chip contamination.

Engineers developed the bearing and spindle in-house. There are three available spindles: a 10,000 rpm, 60 hp (continuous), 75 hp (peak) used for cast iron; a 20,000 rpm, 50 hp (continuous) used for automotive aluminum applications; and a 40,000 rpm, 40 hp (continuous) used for machining monolithic aluminum parts for the aerospace industry.

Fluid bearings, used in grinders for years, have high running accuracies, dampening, and vibrational benefits that improve finish grinding operations. While most grinders run at a constant rpm, this spindle requires a fluid bearing design that operates over a broad range of speeds.

The hybrid approach, developed by Ingersoll, uses hydrostatic pressure at zero to low rpm, and hydrodynamic features that increase the oil pressure as spindle rpm increases. This hybrid approach to bearing design offers a wide range of operating speeds.

The compactness of the rotor's size and mass minimizes spindle inertia. Acceleration time is 0.8 sec to 10,000 rpm, and 1.5 seconds to 20,000 rpm.

Engineers used Hollow-Shank Connections (HSK) on all spindles; the 10 k spindle uses an HSK 80B, the 20 k spindle uses HSK 63A. Maximum tool diameter is 200 mm, while the maximum length is 400 mm. Maximum tool weight is 1,200 kg. Chip-to-chip time is 4.0 seconds. Pallet is 630 mm square, with a vertical load capacity of 4,500 kg.

The 10 k and 20 k motorized spindles include an integral motor between front and rear radial bearings. "They are easily interchanged in about two hours," claims Hogg.

Direct fit. On the 40,000-rpm spindle, to improve tool balance characteristics at high rpm, the tool is shrink fit directly into the spindle. The 40 k spindle uses a different drive and bearing configuration, with a polygon coupling between motor driver and spindle and a modified 60 taper interface that simplifies integral tool/spindle changes. Water from the motors and oil from the spindle bearings is cooled in a dual media chiller.

In the event of power failure, a contact switches to convert the motor into a generator/brake to stop the machine safely.

Another application of this technology is machining monolithic aluminum parts for the aerospace industry. "We've sold a linear motor, 40,000-rpm machine with forty feet of x-axis travel to the aerospace industry," says Porter.

Small parts. While Ingersoll's machine was designed for heavy machining of cast iron work pieces like engine blocks, EX-CELL-O GmbH, Eislingen, Germany, designed its XHC 240 for smaller aluminum parts. "We chose aluminum because of design complications associated with protection of the linear motors from chip contamination," recalls Dr. Henry Engell, general manager, EX-CELL-O GmbH.

The structure has a 228 x 181-square-inch footprint, and consists of a welded cast iron and aluminum composite structure. The base and x-column are cast iron, while the y-slide is aluminum. "FEA revealed improved results for an aluminum y-slide over a polymeric composite," says Engell.

Ingersoll and EX-CELL-O both offer three-axis linear motor machining centers at this time. While Ingersoll incorporates synchronous motors with permanent magnets in its design, EX-CELL-O uses an asynchronous motor with electromagnets. Engineers at EX-CELL-O are working on synchronous motor design because of high-thrust force advantages.

Engell argues, "Lower chip leads with higher cutting speeds and feeds remove as much or more metal with an improved surface finish. The improved surface finish results from lower chip loads."

Engineers specified asynchronous linear motors, drives, and MT-CNC from Indramat, Hoffman Estates, IL, to simplify design problems associated with shielding the motors from chip contamination. "Permanent magnets produce a continuous magnetic field along the entire axis," says Engell, "while electro-magnets produce a temporary field directly below the primary element during the actual stroke."

Simplify design. Engell believes asynchronous motors simplify design and assembly. "Permanent magnets could attract loose metallic parts, and metallic fasteners could affect the magnetic field and alter motor performance," reports Engell.

Seven motors, each having an intelligent digital ac servo drive, power the machine: two motors for each axis, plus a rotary linear motor in the b-axis rotary table. Communication between CNC and servo drive takes place over the digital SERCOS interface via an optical fiber link.

The x-axis (horizontal) and z-axis (feed) produce 800 lbf of thrust each, while the y-axis produces only 400 lbf due to the weight of the slide.

Both the linear axis and b-axis rotary table are capable of circular interpolation. At a feed rate of 32.8 ft/min, the machine can do circular interpolation to an accuracy of about 0.0001 to 0.00015 inch. The b-axis indexes at 180 rpm with 360-degree precision at 0.001 degree. A process, patent pending, compensates for distortion of the slides from magnetic forces.

A 40-tool disc-type magazine does a 1.6-inch axial stroke to insert/remove tools from the spindle. Tool change time with a 180-degree magazine rotation is less than two seconds. Tool diameter is 2.6 to 4.9 inches. Maximum tool weight is 22 pounds.

Few limits. The machine uses high-speed CBN (cubical boron nitrite) and PCD (polycrystaline diamond) cutting tools. "Linear motor machines are limited by the controls and available tooling," says Engell.

The XHC 240 offers a 24,000-rpm spindle and axis speeds up to 262 ft/min (80m/min). Accelerations are 1g, and chip-to-chip time is 4.5 seconds at a spindle speed of 8,500 rpm.

A cartridge-mounted motorized spindle incorporates a water-cooled jacket with integral temperature sensors. The spindle has a hybrid-ceramic bearing lubricated by an air/oil mixture. Coolant pumped at 725 psi cools the spindle.

Indramat's vector-control technology controls the spindle. The multi-tasking control and integrated PLC controls asynchronous motors like synchronous motors. It provides high-performance at low rpm, easy positioning, very high dynamics, and very short acceleration and positioning time.

A laser detects broken tools then shuts the machine down if it finds any. Dry-floor protection insures the floor is not contaminated with coolant or lubricant. A built-in capacitor brings the machine safely to a stop in the event of power failure.

High speed. Not to be outdone, Keene, NH-based Kingsbury Corp. spent the last two and a half years developing a high-speed machine tool called the CYBER-CELL. "We know increased speed and flexibility improve manufacturers' ability to deal with the mid-to-high volume, and shorter life cycles of today's products," says Kingsbury Executive V.P. Richard Whipple.

The CYBER-CELL, like EX-CELL-O's XHC 240, is a 3-axis linear motor horizontal machining center (HMC). It also uses Indramat's motors, drives, and CNC.

The base, column, and z-axis table consist of crushed-quartz composite polymer (ZANITETM). "The composite has a compressive strength of 18,000 psi, and dampening qualities that are ten times that of cast iron," reports Larry Forbus, Kingsbury design engineer.

The base casting, one of the world's largest single composite castings, was done in-house with expertise from Willoughby, OH-based Precision Polymer Casting Co. The base and column weigh about 80,000 lbs. "The machine must be rigid, to fully exploit the linear motors' acceleration and speed," adds Kingsbury's Electronic Controls Project Engineer Bruce Cote.

The CYBER-CELL provides a 55% chip-making advantage over a conventional 4-axis machining center, Kingsbury engineers say. "High-speed axis motion reduces parasitic time dramatically," says Whipple. He continues, "In the future, these high-speed machines are going to replace many of the multiple-spindle transfer lines in use today."

Refit job. Design of the column, base, and in-house composite casting was no easy task. "A failed mixing motor required quick refitting of the mixer with a heavier motor during the 10-hour pour," recalls Forbus. Finite element analysis on structural components led Kingsbury engineers to construct stiffeners in the composite base providing strength and rigidity to handle acceleration forces up to 1g.

Consideration of the linear motors' mounting, tolerances, air gaps, cooling, and accessibility in the structural design result in a structure unlike traditional ball-screw-driven machine tools. The CYBER-CELL, one of the largest to use a "T" base design, is self leveling, sits on three points, and has a footprint measuring 138 x 163 inches square.

Stiffness value for the CYBER-CELL's x-axis is 1.5 million lbf /inch, while the z-axis has a stiffness of 2.2 million lbf/inch. "A very stiff ball-screw-driven machine would have a stiffness of about 1 million lbf/inch,'' Cote explains. Linear motors alone can't achieve that kind of stiffness, but high structural rigidity and a tight servo control loop exploit the inherent performance of linear motors, he adds.

Indramat's MT-CNC connects over a SERCOS fiber-optic communication loop to the servo drive. Linear scales provide axis position and feedback for the motors, and the position loop closes in the servo drive. The control scheme provides a 250- microsecond servo update time. "Five or six years ago this kind of control didn't exist,'' says Cote.

Indramat's three-phase asynchronous linear motors provide the power for acceleration and speed. Four parallel-configured motors on the x- and z-axes produce forces of 7,200N continuous (20,000N peak), while two motors on the y-axis produce 3,200N continuous (10,000N peak). A 200-amp digital servo drive powers each linear motor.

Peak force, tapped during acceleration and deceleration, moves the 5,500-lb x-axis at 60m/min at accelerations of 0.86g. The z- and y-axis are capable of 1g. Kingsbury engineers designed rigid cooling mantles on the primary and secondary elements of each motor to withstand these forces.

Commercially available passive hydraulic shock absorbers, designed to fully absorb the energy of the slide at full-tilt, are on the ends of each axis to prevent axis over-travel.

The 500-mm-square ISO 8526-1 pallet has cone locators. The vertical y-axis stroke is 600 mm, the horizontal x-axis, 550 mm, and spindle-feed z-axis is 720 mm.

The 25,000-rpm, 250-mm Bryant cartridge spindle has a continuous S-1 rating of 35 hp, and an acceleration/deceleration of 15,000 rpm/sec. "With a 20-lb tool, it only takes 0.9 sec to reach 15,000 rpm," says Forbus.

The motorized cartridge spindle increases flexibility by providing application-specific options for spindle power and rpm. Spindles are replaced in about two hours.

HSK toolholders are available with trim balance rings. HSK63A hollow taper toolholder, a "de facto" high-speed standard in Europe and the U.S., takes a maximum tool shank of about 25 mm. Larger-diameter tools are possible using custom HSK toolholders.

An Ott draw bar, used to pull the tool into the taper and lock it in, provides a stiff lock-up on the tool. "The HSK tooling and Ott draw-bar system is suited for high-speed applications and is superior to CAT-taper type tool holders with retention knob ends," says Cote.

Tool lock. Fingers inside the tool holder expand as spindle rpm increases. "The higher the rpm, the tighter the lock on the tool," he adds. Chip-to-chip tool changing time is 4.5 seconds.

The high-speed spindle uses hybrid-ceramic ball bearings, with an air/oil lubrication system that meters the appropriate amount of oil to specific locations in the spindle.

Even though the high-speed tool holders require balancing rings to prevent spindle bearing damage, Kingsbury plans to offer spindle speeds of up to 40,000 rpm in the next-generation machine.

The CYBER-CELL was designed for cylinder head machining, but can handle any application requiring the machining of 22 x 12 x 10-inch or smaller blanks.

The machine includes a motorized b-axis 200-rpm table for contouring and turning operations. Contouring circular interpolation at 2,000 inches per minute is possible, allowing accurate machining of various diameter bores with one tool.

"Choosing Indramat's products has proven to be an excellent decision in my opinion," Cote adds.

Cutting accuracy is now being quantified with the help of Lawrence Livermore Laboratory. Linear accuracy, diagonal, and ball-bar checks have been completed.

Off-shoots, like the 3-axis high-speed table, have been recently introduced to the automotive market. "Development of automatic spindle balancing techniques will be paramount for the future of high-speed machining," says Whipple. Kingsbury will market its second- generation CYBER-CELL that will offer a 40,000-rpm spindle.

"Linear motors are all that we were told they would be, maybe even a little more,"says Cote. He adds that they have been extremely pleased with the results obtained in the CYBER-CELL. "Linear motors are definitely going to be a major player in machine tools of the future," says Cote.

With the increasing demand for faster cycle times, Ingersoll, EX-CELL-O, and Kingsbury show how linear technology can provide the speed and acceleration engineers need.

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