Application
Notes AP-1116
Calibration of a 5-axis machine by volumetric positioning
and tool tip
positioning measurement I.
What is the problem
For a 5-axis
machine tool it is very important to
calibrate both the volumetric positioning (X, Y, Z) and the two rotational
axes, A and B or A and C. Using a laser interferometer to calibrate the
3 linear axes and the 2 rotational
axes separately
and independent of each other is not
enough. There are straightness
errors for
each of the 3 linear axes, and
squareness errors between the 3 linear
axes, skew, Non-orthogonal and
non-intersection of the 2 rotational axes, and the non-intersection of the
center lines of the spindle and the
z-axis. Hence it is important to measure the
volumetric positioning (X, Y, Z) by
the laser vector method and the tool tip
positioning (A, B, and R) by the
tool tip positioning measurement. II.
How to solve this
problemFirst, using the laser vector method [1] to calibrate and
compensate the volumetric positioning
errors including the 3 displacement errors, 6 straightness errors and 3
squareness errors.
Second, using the non-contact laser spindle
tester [2] to measure
the tool tip displacement errors at
various rotational angles. This
measurement can be used to calibrate or to
compensate the angular errors of the 2 rotational axes. Finally, to measure the tool tip
positioning errors dynamically at various
tool paths and geometries. III.
How it works (Patent pending)The laser vector method is
described in Ap-1109 and the spindle
tester is described
in Ap-1114. After
calibrating and compensating the
volumetric positioning accuracy
by the laser vector method, mount the spindle tester with a precision sphere to the spindle and mount the laser head at
position 2 as shown in Fig. 1. Align
the laser beam with a focus lens to be focused at and perpendicular
to the surface of the precision sphere as
shown in Fig. 2. Move the 5 axes such that the center of the tool tip, represented by the center of
the precision sphere, is at a fixed
position. Measuring the changes of the tool tip positions due to various
movements, the angular errors of A and B at
various angles can be determined. IV.
Positioning of a 5-axis machine
The machine coordinate is defined as (Xm, Ym, Zm, A, B).
The center of rotation of A and B is defined as (Xp, Yp, Zp).
The tool tip position is defined as (Xt, Yt, Zt).
The relations
Xp
= Xm + Cx, Yp = Ym + Cy,
Zp = Zm + Cz ,
where Cx, Cy, and Cz are constants. Click here to down
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