Dimensioning Spline Curves

In its simplest terms a spline is a smooth curve with a constantly changing radius which passes through a set of control points.

For further discussion look at this article or search on NURBS (Non-Uniform Rational B-Spline) or B-Spline.

Because it has a constantly changing radius we cannot apply a complete dimensioning scheme in our engineering drawing.

The first thing we need to define when considering how we might detail the spline is what is its purpose in the design?  If its a purely aesthetic feature which has no bearing or interface with any other features then its details can be quite vague.

In most situations a simple bounding box showing the extents of the spline and a note referring to the electronic the data will be sufficient.

But if it interfaces with other elements then its definition must be more closely described.  In this case, in modern industrial situations, the electronic data will be referred to for any manufacturing or modelling activities and a simple description would suffice in the drawing.

Remember that the the form of existing company logos are often very strictly controlled and you would then only give its position and extents and a reference to the original artwork.

But situations will still arise where more precise detailing is required – the artefact is being manufactured by manual methods, access to the original electronic data is restricted or cannot be used by the manufacturer.

 

Datum point display style

Create datum points placed at appropriate positions on the sketched spline or resultant geometry edge.

To show a point of reference for the spline dimensions you will need to show the datum points in the drawing – you would usually have these switched off.  To control the graphical display of the datum points:

File > Options > Configuration Editor

Option: display_point_tags  Value:  no

The above Option may have to be added to Configuration Editor – using Find option and Add/Change the option to the list.

File > Prepare > Drawing Properties > Detail options > Change

Option:  datum_point_shape  Value:  cross or dot

Option:  datum_point_size  Value:  appropriate to line weight and sheet size

 

Method

Dimensioning each point is OK but can very quickly become cluttered – at least make sure this is a detail view or a separate sheet.

A tidier method could be to put the  X Y coordinates into a table – make sure you define and dimension a point of reference.

 

To Save Spline Coordinates to a File

You can save spline points to a file with values in cartesian or polar coordinate systems.

1. In sketcher, create a sketch coordinate system.

2. Select the spline. RMB > Modify. The spline modification dashboard appears.

3. Click File. A dialog box appears.

4. Associate the spline to the sketch coordinate system.

5. Click the Save icon.  A .pts (points) file will be created

6. Done

Creo creates a spline point definition file with the coordinate system type printed in the file. The spline point definition file is a standard text file that you can edit using the operating system editor.