After you clicked menu button 'Surface>Mesh to single nurbs' you will see a dialog like the following picture:
Set control point number: check this button to set the number of surface control point in U-direction and V-direction, this control point number will be used as initial number for surface fitting to the mesh. This is optional , if you do not check this button, program will compute the surface control point number for you.
Surface degree: specify the surface degree in U-direction and V-direction, the default number is U==3 and V==3 the maximum degree is U==8 and V==8.
Max deviation: threshold for the maximum deviation between mesh nodes and surface. The items include: no constraint, 0.5, 0.1, 0.0, 0.001, 0.05, 0.3, 1.0, 0.0001;
Smoothness: surface smoothness factor, to choose a smoothness level for controlling surface smoothness quality. The items include: highest, high, medium, low, lowest;
Apply: click this button to create a surface that fits the mesh. After you have gotten an fitted surface, you can choose a more little 'max deviation' number and continue to click this for updating the surface with higher precision.
Step 2: Click menu 'Surface->Mesh to single nurbs', the dialog shows, click button 'Apply', wait for a while, a fitted surface will generate. The RESURF message will report that 'Maximum deviation : 1.227468 ; average deviation: 0.159497'.
Step 3: choose Max deviation as 0.5, continue to click Apply button. The surface was updated. Message reports as the following:
"Start surface updating...... this may take several minutes or hours
Surface was updated. max deviation=0.569993 ; average deviation=0.073631
U control # =70, V control # = 70
Surface was updated. max deviation=0.496351 ; average deviation=0.064255
U control # =83, V control # = 80
max deviation=0.496351 average deviation=0.064255
Surface was updated "
Step 4: Close the dialog. Click menu 'View->mode->Hide mesh' to hide the mesh.
Step 5: Click menu 'View->mode->Show surface CPs' to display surface control point mesh.
Step 6: Click menu 'File->save' to save the surface as .igs file.
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