Duerkopp Adler 550-767 Operating Instructions Manual page 46

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Press the "Ref." field.
The step motor will move to an average stitch length.
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Press "Enter 100 stitches+length" field.
The following window is displayed.
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Sew test seam.
100 stitches will be sewn.
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Measure stretch.
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Enter the value for the measured stretch, then press the field "8" to
confirm.
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Press the "Save" field.
Calibration will be terminated.
Calibration of the step motor is carried out on thin paper. Since the
actual stitch length shortens in relation to an increasing thickness of
the material, a correction factor can be set in the seam display, which
will take into account the thickness of the sewing material.