These features do not form part of the model but are created in order to establish a reference for model features where no suitable reference exists – remember, everything has to be ‘fixed’ in our 3D space.
Degrees of Freedom (DoF) – the freedom to move in a direction which has not been constrained – fixed to another entity. Can the entity be moved in any direction or rotated?
The reference geometry you create also has to referenced in space such that it has no DoF. You will not be allowed to complete the feature until all the DoF have been resolved – until it has been fully constrained. How many references or constraints are needed to place your new reference feature is dependent on what sort of geometry it is and what you are constraining it to.
To ‘collect’ references
Whenever you are presented with a references window you will need to hold the Ctrl key if you need to ‘collect’ multiple references – standard Windows functionality.
Normalcy – normal to – the term normal is generally used instead of perpendicular or orthogonal to describe a line or axis being at 90° (in all directions) to a plane or surface.
Planes:
parallel offset from another plane/surface
normal to a plane/surface
through an axis/line/edge/point
angular offset from another plane – must be combined with through axis/line/edge
Points:
DO NOT put datum point on curve ends or surface edge ends – these are vertexes and are already seen as points which can be connected to.
If you are creating multiple points then include them all in one feature if possible – this can result in a significantly shorter and tidier model tree.
Constrained to other references:
On a surface/plane and referenced to other entities, ie. two other surfaces
On an edge/line relative to the end of the edge, line
On an axis relative to another reference
Offset from a Coordinate System (csys) in x,y and z
Axis:
Through a point/vertex/curve/edge
Normal to a surface [doesn’t have to be planar] or plane
Curves:
Sketched on a planar surface or datum plane
Curve Through Points – a curve can be constructed to pass through any number of points – reference points, vertexes, curve endpoints
Planar Curve thru’ points – the curve thru’ points feature is mathematically a 3D curve. Even if your reference points or vertices are on a common plane then the curve will not be planar and can cause issues if you try to mirror a part with the curve on the sym plane. Project the curve onto reference point plane (ie. the sym plane) with the Place curve on surface option below.
Place curve on surface – once a curve between points has been created it placement can be changed from free (the shortest, smoothest path between the points) to lying on a surface (curved or planar) or datum plane. The points will need to be on the surface before this can be applied.
Tangency end conditions – if the curve is referenced through its start point to another entity (ie. it starts from the vertex of an edge or from a point on a surface) a normal, tangential or curvature continuous relation can be created – see example above.
Sweep curve – a curve between multiple points can be construction with straight line connections between points and a fillet at the connection – excellent options for the sweep path for a bent tubular steel structure – see the chair example below
Intersect curve – create curve at the intersection of two preselected entities – Two sketched curves, two surfaces, a plane and a surface. Consider the alignment of the end points if creating a 3D curve from two planar curves.
Project curve – an existing curve or a sketch can be projected normal to a selected plane onto a surface
Wrap curve – whereas a projected curve will be distorted if it is created on a non planar surface, a wrapped curve will not be – it will form across a surface such that all the curve lengths are unchanged.
Note: you cannot wrap a curve onto a surface which is curved in two directions, only cylinders – picture what happens when you try to put a sticker on a ball – it has to crease.
To Wrap onto complex surfaces
If you need to apply some curves to a surface with curvature in two directions whilst minimising distortion you could:
- create a cylinder which approximates in orientation and diameter to the target surface
- create your sketch and Wrap onto this cylinder
- create a sketch from the wrapped curves – Sketch > Copy Edge tool
- use the appropriately distorted sketch in a Project or Offset surface process onto the target surface
If your target surface is tending to the spherical you will still get distortion in one direction, make sure you align the cylinder with the direction of least distortion.
Offset Curves
To create clearance between assembled parts which may be the result of a Top Down Modelling (see Assemblies) approach, you may need to offset edges or curves.
Note: you can Offset curves or surface edges, you cannot offset solid edges. Copy the surface edge first (select, Ctrl C, Ctrl V) and offset the copy.
To select solid edge: select solid, select solid surface, select edge of selected surface
To select surface edge: select surface, select edge of selected surface
The Offset tool is selection driven – select the geometry to enable the tool
Model tab > Editing > Offset
CADCAM Resources 