Get Access Get Access. Abstract This paper describes the effects on machine slide-guide contact conditions caused by thermal deformation. Keywords Friction coefficient. Recommended articles Citing articles 0.
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However, this assumes that thermal expansion results in a homogeneous scale error, which is not the case. Temperature varies at different positions on the machine due to changes in localized heat sources, both internal such as motors and external such as direct sunlight from a window aligning with a machine. Temperature gradients are also found in indoor environments, due to warmer air rising to the top of a room and cooler air settling to the bottom, of typically one degree C per m. These effects can be very difficult to predict and compensate.
Considerable work has been done to understand how best to model and compensate for thermal deformation in machine tools. Methods usually involve placing temperature sensors on the major structural elements of a machine and then predicting the deformation using either Finite Element Analysis FEA or an empirical model such as deep learning.
Achieving accurate predictions remains challenging due to a limited number of temperature sensors giving an incomplete picture of the temperature gradients, uncertainties in the temperature sensors themselves, uncertainties in the coefficient of thermal expansion for the machine tool structure, and uncertainties in the models.
These are all areas that require further development in order to advance high precision manufacturing. The weight of moving parts of the machine and of the work piece will cause a repeatable displacement of the machine structure, which depends on the combination of axis positions. The standard approach to kinematic calibration assumes that the errors in each axis depend only on position along that axis.
This means that each axis can be calibrated in isolation and the resultant errors for any given position calculated by superposition. However, when considering loads acting on the axes, this assumption is not valid since when an axis as the end of the kinematic chain is fully extended, it will exert a larger moment on the axis to which it is attached.timtech.ro/wp-content/2020-09-05/738-como-rastrear-celular.php
Actualities and Development of Heavy-Duty CNC Machine Tool Thermal Error Monitoring Technology
For this reason, for the highest accuracy, so called volumetric compensation must be carried out. This means that instead of taking measurements at a number of discrete positions along each independent axis, measurements are taken at grid positions within the volume of the machine.
The result is a far lengthier calibration process. The additional controller software required to implement volumetric compensation can also be very expensive, meaning that this is only applied for the most demanding applications. Luckily, the inherent stiffness of machine tools means that these errors are usually very small, probably less than a micron for typical CNC machine tools.
System senses expansion inside machine tool spindles
However, for large gantry-based machines, operating at scales of several meters, volumetric compensation can eliminate significantly larger errors. Additional deformations of the machine tool structure, and resulting errors, are caused by acceleration of the machine and workpiece mass, as well as process forces.
These can have a significant effect on machine errors. Inertial forces are predictable and could, therefore, be compensated using model-based correction, although this is not thought to be done by any current industrial control systems. Process forces are more difficult to predict, although these forces may be reduced to have a negligible impact on the final form of components by reducing the depth and feed rates for finishing cuts. This involves a compromise between process time and accuracy.
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Motion control errors include both physical effects, such as the dragging of cable looms, and control interpolation errors, such as servo mismatch and reversal spikes. Dynamic errors are those which are only present when the machine is in motion. Such errors include controller errors such as reversal spikes, and servo mismatch and vibration.
A spindle is effectively an additional rotary axis with the important difference that rotational positioning about the spindle axis does not need to be accurately controlled. In fact, the spindle may be referred to as the rotary drive axis, however, due to the high speed of operation, entirely different measurement techniques are required to measure spindle errors.
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Errors associated with this axis are sometimes referred to as runout so that radial errors are referred to as radial runout and axial errors as axial runout. Angular positioning is generally not a consideration since the tool is continuously rotating within the axis rather than being rotated accurately. Although kinematically identical to any other rotary axis, in practice error sources and detection are very different due to the far greater speed of rotation. Non-contact sensors that provide very high frequency measurements are therefore required, such as proximity sensors that make use of eddy current effects.
Additional errors are associated with the repeatability of the tool change operation index errors and tool wear which affects tool length, tool diameter and tool geometry. These may be calibrated using laser tool calibrators that are able to recalibrate the tool position and size rapidly during operation of the machine. Machine tool calibration, using some form of laser interferometer-based measurement, is typically focused on determining kinematic errors. Ballbar measurement may be used to calibrate some motion control errors and hysteresis.
Verification of machine tools has historically been an involved process of either careful alignment using physical gauges or machining test pieces which can then be measured. Modern innovations such as the telescopic ballbar and touch trigger probing of reference artefacts have reduced this to several minutes.
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Thermal Analysis for Condition Monitoring of Machine Tool Spindles - IOPscience
Forums for working professionals. Current Articles Archives. Kinematic Errors Kinematic errors are those built into the machine due to manufacturing inaccuracies and clearances in its geometry-defining components such as linear slideways and rotary bearings. It follows from this that deviation from motion along a straight line also has six components: One positional deviation, in the direction of motion.
Two linear deviations orthogonal to the direction of motion, which may be referred to as straightness of the axis. Three angular deviations, which may be identified as pitch, roll and yaw, although the distinction between pitch and yaw is dependent on an arbitrary frame of reference. Figure 1: Deviations from straight line motion as defined by ISO Figure 2: Kinematic Errors for a 3-axis Machine Tool.
Figure 3: Kinematic Errors in Rotary Axis. Recommended For You. Listen to this story Paused