Varkon Geometry Tutorial: Tolerances 2006-03-25


Most calculations in CAD systems are numerical. An iterative calculation ends when a criterion is reached. In Varkon there is for instance such a criterion (CTOL) when an intersection is calculated between a curve and a plane. A point on a curve is defined by a parameter value. The parameter value is systematically (Newton Rhapson) changed until the point is on the plane. On the plane is defined as being not further away from the plane than distance CTOL.

The criteria are set assuming that objects defined in the system are of the size 1000.0 to 10000.0 units. These values are experience values and by the development of the software test objects had these sizes.

If for instance objects that are smaller than 0.01 units are created the (fixed) criteria may no longer be appropriate. For instance the end criterion CTOL for plane/surface intersection should probably be made smaller for such small objects.

Worse is that calculations may fail when objects are of the “wrong” size.

In principal it should be possible just to scale the tolerances. In other CAD systems there are possibilities to define Bounding Boxes for the objects that shall be defined. The Bounding Boxes may have sides that have the length 0.1 units, 1.0 units, 1000.0 units, …, 100000 units, etc. The CAD system then scales all the criteria (tolerances) accordingly.

But (“uniform”) scaling of all tolerances is unfortunately many times not sufficient. There may still be problems. The recommended solution is that the user scales the objects to the size that the system has been developed and tested for.

It is in Varkon possible for the user to change tolerances. There is a text file with tolerances that Varkon reads at startup of a session. It is however NOT recommended that the user makes any changes!

The recommended sizes (and most tested sizes) are 1000.0-10000.0 units objects, but it may not be so bad also with other sizes. There are some applications where much smaller objects are created without problems.

Varkon is dimensionless

In the description of the tolerances millimeter, meter, inches, etc. were not mentioned. Varkon is dimensionless. It is only the number of units that is of importance.

All CAD systems (the calculations in the system) are in principal dimensionless, but there may be systems where the user defines the units and the selected units define the size of the above mentioned Bounding Boxes. Normally though, are set units (millimeter, inches, ..) only used as information when data is exported to neutral (CAD model exchange) formats like Iges. There is a field that can be set in the Iges file for units.