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| Jyoti VMC 70 Linear is built for high speed machining Jul 07, 2008 | High speed machining is a combination of all of the components that go into a machine tool as well as the processing parameters. This takes into account the entire machine system, including cutting tools, cutting fluids, workholding, spindle system, axis drive system, machine construction and CNC system. Developed to achieve the ultimate high speed machining results, every aspect of the Jyoti VMC 70 Linear has been designed to excel.
Using cutting edge technology and research, the VMC 70 Linear is the first Jyoti machine tool to feature linear motors. Dual linear motors are used on all axes for maximum dynamics and accuracy for high quality and faster production rates. Since there are no mechanical transmission elements, such as lead screws or couplings, natural oscillations and the effects of elasticity, backlash and friction are largely negated. The linear motors are protected from the machining environment and maintained under controlled temperature with a separate chiller unit.
Weighing in at a hefty 21,000 kg the complete machine body is a monoblock construction reinforced with engineered concrete that dampens the high load vibrations of the machine and also reduces noise resonance. The machine has been designed using advanced modelling with the emphasis on rigidity during the initial phase of the design. At this stage the structure was completely analysed using Finite Element Methods (FEM) that provided a simulation of stresses, vibration and thermal distortion occurring in the machine structure when placed under load, with any necessary refinements made on the structure to improve stiffness. These refinements have enhanced rigidity and machining accuracy at the high dynamic parameters achieved on the VMC 70 Linear.
Featuring a gantry design in the Y-axis with two drives, these are synchronised by separate position measuring systems to prevent unequal forces. A fixed 900 by 800 mm worktable and twin column construction of the gantry gives higher precision under heavy duty conditions. It ensures minimal moving mass to achieve better dynamics of the machine while cutting heavier jobs, up to 2000 kg, with precision. With rapid traverse rates of 100 m/min, axes acceleration of up to 18 m/s2 and cutting feed rates of up to 30 m/min the machine's high dynamics and considerable feed forces improve the speed of machining. “However,” says UK sales manager, Mark Devall, “the accuracy is never compromised and the VMC 70 Linear has a positioning accuracy of 0.010 mm and repeatability of +/- 0.002 mm.”
Axis travels are 750 mm in X, 700 mm in Y and 500 mm in Z with high precision monorail roller guideways used for exceptional static and dynamic stiffness and high load bearing capacity. Sealed to completely protect against dirt and dust, the guides have a virtually maintenance-free service life.
A powerful spindle wound AC motor has a rating of 35 kW, offering 78 Nm continuous torque and a peak speed of 18,000 rpm. This design avoids the sag errors of indirect drives, providing a better surface finish and smooth rigid tapping results. It also avoids small jerks created while starting the spindle (with indirect drives) and increases the tool life. The spindle uses large diameter super precision ceramic angular contact bearings to ensure stability and rigidity with accuracy at high speeds. Oil mist lubrication ensures increased spindle bearing life and increased stability during high speed operations.
The high power motor has been designed to provide quick acceleration and deceleration. It accelerates from rest position to 18,000 rpm maximum rotation within just 3.5 seconds and decelerates in 4 seconds. Optional higher spindle speed can be provided, up to 42 000 rpm.
The spindle’s temperature is controlled through a specially designed chiller unit to minimise spindle growth. Spindle vibration is maintained within 2.5 micron by optimum balancing.
“Control for the VMC 70 Linear comes from the very capable Siemens Sinumerik 840D. With the help of the 300 blocks ‘look-ahead’ facility, the program execution speed can be optimised to its peak. Torque and velocity oriented feed forward control virtually eradicates following errors in the program execution and achieves consistently high precision even at fast machining speeds,” Mark Devall explains.
NURBS (Non-Uniform Rational B-Splines), the interpolation process of the 840D, enables all sculptured surfaces to be directly interpolated as they are generated by the CAD/CAM system. With NURBS interpolation, part programs are 60 per cent shorter in comparison to G-code and processing is faster. Severe shock loads on the axes caused by the heavy depth of cut and the hardness of the machined material tends to deflect the tool path; with Dynamic Stiffness Control (DSC) the tool remains on the programmed ideal path even at the highest speeds and cutting forces.
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