The blend function in Creo is commonly known as a loft (originating from boat building – see also splines) in generic CAD terms. From multiple planar (2D) sections which are spaced apart from each other, the CAD system creates a volume (solid, cut or surface) by filling in the gaps between these cross sections.
The system decide what form is created as a smooth transition from one section to the next. See surface modelling to gain ultimate control over the form created.
There are two main issues which need addressing when creating a blend feature; number of vertices in each section and alignment of the start points.
Number of Vertices
In the example below, a circle has been blended to a hexagon. The lines which join the two sections and represent the volume of material to be created, are joined from vertices (points where curves meet) on one section to vertices on the next.
But if, as in the above left example, there are an different number of vertex’s in the two sections [a circle has no vertex’s!] then the systems needs to be told where to connect the points to. In this case the circle has been split into six arcs.
We could have split the circle into less segments and created a blend vertex (see below) – essentially a point on top of a point which would allow us to connect two points from the hexagon to one point on the segmented circle. This would give a very different form.
Start Points
In each of the sections one of the vertices is designated as the start point. These are the first points to be joined between the sections, the system then moves logically around the sections joining each point in turn.
Where your start points are placed decides on the final form. This can be seen in the two images above where the start point has been moved in the second model giving a very different form.
Decisions concerning numbers of vertex’s and start point positions is very much based on your design intent.
Process
Common Considerations:
- As with any blend, each cross section must have the same number of vertices (points), or in other words, be made up of the same number of lines. This defines how one section is connected to another.
- Try and plan the feature so that the section are not swept around the inside of any curves in the trajectory – problems can occur if the section has to be ‘compressed’ inside a tight curve.
- The cross sections and the trajectory can be sketched or selected from existing edges or reference geometry – selecting existing geometry is more robust.
Most of the process can be achieved through the RMB menu
Dashboard Options
Solid or surface?
Protrusion or Cut?
Thin?
This feature defaults to surface, so pick solid if that is what you want
A closed sketch will produce a volume which can add or subtract material
An open sketch will produce a Surface feature or a Thin feature. The thin option thickens an open sketch – useful for sheetmetal parts.
External Sketches
The default process is to create the blend section internally to the feature once the trajectory has been selected.
Creating your trajectory and sections as separate external features is a more efficient strategy as you can visualise the form and you will not lose your construction geometry if the feature is deleted
RMB menu > Selected Sections
Sections
Sections can be sketched curves or chain of edges – the sections must be planar
Section planes do not have to be parallel
Make sure you select the section nearest the start point (arrow end) of the trajectory.
- Select the initial section
- RMB menu > Insert section
- Continue inserting sections
Start point
If the previewed solid is twisted because the start points are not aligned, simply drag the start point marker to the appropriate vertex.
No Preview?
First thing to check is the number of vertices in each section – this must be equal
In the example above we need to abandon the feature and equalise the number of vertices in the underlying sections. The most common solution is to increase the vertices to the highest count.
Blend Vertex
You may have a situation where you have an unequal number of vertices but its not appropriate to split one of the curves because you want multiple points in one section to blend to a single point in the next – imagine a square blending to a triangle. In this case you need to add a Blend Vertex – basically a point on a point to increase the vertex count.
Continue the process as before. When you come to the section which needs a blend vertex use the Add Blend Vertex button in the Sections tab. The extra point will show as a white square, drag this to the appropriate vertex.
You CANNOT have a blend vertex on the Start Point, if they coincide, move the start point on your sections
See 1.10mins in video above
Tangent end conditions
If the feature is joined to a suitable geometry you can use the end condition markers to set tangency. In the below example some simple control surfaces were created prior to the swept blend feature to enable setting tangency – see 3.00mins in video above