Offset surface feature driven by sketch
The Offset function is another feature which has multiple uses depended on what geometry you pre-select and what options you choose in the dashboard. If this case we will create a new volume to add or subtract material by offsetting an area of a solid guided by a sketch.
To minimise distortion os the planar sketch you need to carefully consider the position of the sketch plane, in the example below the sketch plane is tangent to the centre of the offset area.
To select the surface you want to offset, select the solid then the surface on the solid.
Click image to enlarge
The default option is to create an offset copy of the whole surface. If you want to offset an area defined by a (planar) sketch then you need to select the Expand option in the dashboard
Through Options you can then select or define the sketch which controls the offset area. Consider whether you want the side surfaces to be normal to the sketchplane or the surface.
Note: You cannot use the text tool (true type fonts) in sketcher for the offset sketch. If you want to use standard fonts you will have to create the sketch using the text tool then create a second sketch which traces the edges of the original sketch.
Workaround to minimise distortion
This function uses a projected sketch, the further the area of projection extends ‘around the sides’ the more the distortion of the original sketch. The width of the middle rectangle below is OK but the width on the outer rectangles is heavily distorted.
You need to wrap rather than project the sketch but you can’t wrap onto a surface with curvature in two directions, so…..
- create a cylinder which approximates to your form
- wrap your sketch onto the cylinder – minimal distortion
- create a new sketch on the same plane as the original sketch
- use the Project tool to copy the wrapped curves
- use this new sketch in the Offset feature
CADCAM Resources 