"Today's 5-axis multitasking machining center designs are many customers' first choice when flexibility is a major decision factor," said a Romi spokesperson. "They are replacing advanced 4-axis machines rapidly in acceptance. Certainly for good reasons, this newest breed of 5-axis multitasking machines are adding additional chip cutting tasks to their machining repertoire and with competitive productivity. While the classic machining center designs expand drilling, milling, tapping and boring by integrating the use of attachment tools, such as for static turning operations to achieve more flexibility, the typical full featured, demanding turning operations are a large technology gap.
"Another factor is modern product designs that benefit most often from a reduced number of parts overall, and these parts then are more and more complex in structure. Thus the necessity for more machining planes and the benefits of complete machining come together with an increasing need to include any needed machining task into one operation.
"It was obvious that turning capabilities, which in all industries equal in demand to that of other machining tasks like boring and milling, must be combined in most modern machining center concepts. A concept most often expected to already include 5-axis machining. The market needs 5-axis machining centers with full-featured high capacity turning.
"Such a request is not simply fulfilled by adding some turning add-ons to a standard machining center. A compromise will not satisfy, and would certainly create major risks in not benefitting productivity, or when complex turning tasks are at hand."
What are Those Risks?
Milling and turning tasks are somewhat opposing in technical demands. A rigid machining center needs a stable, torsional stiff table to machine overhanging parts and features located high above the table clamping surface. Such rigid but moving designs require strong pre-loaded bearing systems, which do not do well when fast rotational turning speeds are needed. "This mix of rigidity and speed is obviously in need of a specific design approach," said the spokesperson.
The core element of the machining center is the milling spindle assembly. So the main goal when adding turning tasks is to safeguard the high priced, precision spindle bearing system against damage by the shock loads a static turning tool is frequently exposed to. To transfer these loads, the clamping of the milling spindle rotor against the spindle's outer housing is the solution. Locking it into a stationary position to accept the turning tool and the cutting forces via the milling tool taper make it a combination toolholder for milling and turning.
"Obviously the greatest enemy for precision spindle bearings are vibrations at standstill, pounding loads on idle bearings," said the spokesperson. "Resulting micron level marks on bearing rings and rollers are not recognizable at first, but within time, these cause unavoidable premature bearing failures with unscheduled cost and downtime and loss of production. The offsetting alternatives then are unproductive light turning loads, or moving the part to a turn center, lacking milling performance in comparison. From these examples it is obvious that different protection designs are needed for a reliable and productive acceptable solution."
The BURKAHRDT+WEBER Solution
To address these challenges, BURKHARDT+WEBER has introduced the multitasking MCT-series with T for turning. "The series features a strong application oriented design featuring a complete solution for turning and machining combined," said the spokesperson. The MCT-series is represented by five machine sizes for parts with vertical/horizontal full featured CNC turning tasks from 300 mm (11.8") to 2,000 mm (78.74") diameter and capable of turn-spindle speeds (at the table) of up to 600 RPM.
All MCT-series machines are equipped with fast rotating torque tables and are designed and built in house.
"BURKHARDT+WEBER invested over $550 million in test and safety equipment for manufacturing the tables, and another $660 million for completing the torque table design," said the spokesperson. "Aside from the precision machine tool specific testing for accuracy, rigidity, dynamic, temperature behavior and more, difficult solutions had to be addressed, for example the need to neutralize the high heat loses experienced during turning, and the 16 channel rotary coupling for the table featuring a clean, leak-free, no-wear sealing system.
"Torque tables in general have the problem that fairly large energy losses are generated inside the table housings and slide assembly when compared to conventional tables with geared drives systems. The basic design principle for precision machine tools, arrange all heat sources external to the machine frame,' is unavoidably breached because the motor concept is part of the machine's table housing. To compensate, BURKHARDT+WEBER removed this heat from around the motor frame and from the top of the motor (below the table top surface) with an actively controlled cooling system and thus stabilized the table frame by eliminating temperature fluctuations.
"Geometric instabilities and degrading accuracies caused by temperature fluctuations in the machine frame need to be eliminated. Instead of eliminating the effects with controlled heat removal, the corrections to off-set are most often limited to the use of temperature compensation grids. But is it possible to reliably predict all operating states and the related temperature effects on a highly flexible 5-axis multitasking machine by simply applying a pre-set compensation matrix. Nevertheless, most machine designs introduced shy away from a higher cost but reliable and secure system, which is certainly preferable in maintaining machine tool accuracy and precision.
"In addition, and with table speeds up to 600 RPM, integral seal systems' contact areas cause rising temperatures while suffering from ongoing, built-in wear from friction. The BURKHARDT+WEBER experts did not find a solution to this problem and invented a contact free rotary sealing system for high speeds, with a reliable design for up to 16 channels. This is a proven standard delivery for all BURKHARDT+WEBER torque tables.
"Also specific attention is required to the overall load distribution that fast rotating parts exert on the table system. It is obvious that the demands from machining on a machining center are not concentrated around rotating symmetrical parts, as those are usually machined on vertical turning centers. Thus vertical turning tasks are mostly performed on non-symmetrical parts. The integration of an automatic balance control system with display is therefore a must."
BURKHARDT+WEBER's MCT-series features integrated balancing sensors that measure the forces and display these to the operator at the CNC control with data for correction measures. This display graphically specifies the location and the size of the counterweights to the operator. After a part/pallet unit is loaded and securely clamped to the machine's table, using a mechanical spring-based safety system, a balancing cycle is automatically performed at 80 RPM. Depending on the pallets and fixtures in use, a combination of seized weights are added to offset for balance. After confirming the balance quality with another balancing cycle at 80 RPM, the machining sequence is released.
Another important key component in the machine system is the main milling spindle. Besides the performance needed for powerful, precise and reliable machining with rotating tools, the MCT machine must also provide a rigid support for stationary turn toolholders. The main spindle is executed in HV swivel spindle design. The time for automatically orienting the spindle from horizontal to vertical work position (180° rotation) is less than 2 seconds while the milling spindle can continue to run at up to 8,000 RPM. A master slave axis drive concept provides precision performance, with automatic clamping via hydraulic brake system supplying 6,500 Nm (4,800 ft-lbs.) of holding torque. The geared spindle unit with two steps is cooled by four individual cooling circuits for continuous duty operation at maximum speed.
To counter any defects from shock loads to stopped spindle bearings, as outlined previously, BURKHARDT+WEBER exchanged the turn tools automatically from the tool magazine, not directly into the spindle taper, but into an automatically exchanged turn tool adapter.
The turn tool adapter completely isolates the spindle bearing system from turning loads. Four symmetrically arranged clamping cylinders, working like a zero-point clamping system, are located 250 mm (9.84") apart each other and supply 12 ton (26,400 lbs.) clamping force to securely lock the turn tool adapter in place. The company said that the large clamping base exceeds that of a standard HSK or CAT type toolholder and transfers the cutting forces directly into the large housing. None of the cutting forces are transferred over a milling spindle taper mechanism into the machine.
The turn tools are then automatically exchanged into the turn tool adapter. The turn tool clamping force is equal to that for the milling tool. After completion of the turning operations, the turn tool adapter then also automatically disappears inside the standard magazine.
BURKHARDT+WEBER offers four machining center series, all customizable via a large selection of standard and special options:
- The MCX series - a complete line of nine conventional CNC machining centers and boring mill type models with roller bearing slide design for parts weighing up to 45,000 lbs.
- The MCR series - a complete line of box-way CNC machining centers featuring oversized laser hardened slide ways and hand-scraped guides for machine rigidity. This series is best suited for machining steel or other difficult machining materials.
- The MC - series is available in five sizes for micron precision machining. It is designed to expand productivity when cutting precise accuracies with precision.
For more information contact:
Romi Machine Tools, Ltd.
1845 Airport Exchange Blvd.
Erlanger, KY 41018
877-ROMIUSA
sales@romiusa.com
www.romiusa.com