Siemens SINUMERIK 828D Manual page 84

3.5
84
Important functions for 3- to 5-axis machining
Chord error
The following graphic illustrates the so-called chord error which occurs when movements that
should be defined with regard to the tool tip, are programmed for the machine axes.
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The graphic on the left shows the result of the simultaneous change of the tool orientation
through movement of the A axis and the Y axis via linear interpolation of both axes. The orange
line is the programmed straight line in the Y direction. If the A axis is also moved, this results in
the green line for the movement of the tool tip. If a certain path is to be maintained for the tool tip
as in the example on the right-hand side, then position changes to the tool tip that result from
movements of the rotary axis must be compensated.
Summary of simultaneous 5-axis machining
The simultaneous 5-axis machining enables the adaptation of the tool orientation synchronously
to the linear movement in space. Without the additional control functionality of the 5-axis
transformation, this produces some unwanted side effects.
New NC programs for identical workpieces must be created for each machine type.
The NC program must be adapted for changes to the tool length.
During the programming of the machine axes, overlaid movements of the rotary and linear
axes occur which cause a linearization error for the path of the tool tip, the chord error.
Tool orientations are programmed and changed via the positions of the swivel axes. An
orientation programming and interpolation in workpiece coordinates is not possible without
suitable control functions.
Technologically, feedrates should always refer to the relative movement between the
workpiece and tool, which is not the case without 5-axis transformation. The programmed
feedrates refer to the velocity of the machine axis movements.
Normally, NC programs are created in relation to the workpiece, i.e. all the tool positions relate to
the workpiece coordinate system (WCS). In order for an NC program to be executed on the
machine, the positions must be transformed into axis movements, i.e. converted into the
machine coordinate system (MCS). The SINUMERIK is equipped with the TRAORI function to
enable this kind of transformation.
© Siemens AG All rights reserved SINUMERIK, Manual, Mold-Making with 3- to 5-Axis Simultaneous Milling