Part Accuracy Review:

The Accuracy option modifies the computational accuracy of geometry calculations.

This means that Accuracy gives the user the ability to control the quality of Pro/ENGINEER calculations of model geometry with respect to an exact mathematical solution.

Certain types of geometry require little computational effort to determine an exact solution, such as the geometry of a plane, cylinder, or cone. Other types of geometry, such as the intersection of two blended spline surfaces, require much more complex mathematical calculations to determine a solution. In these cases, Accuracy affects the calculations of geometry in order to achieve an acceptable level of approximation of the exact solution. Accuracy also affects the representations of the solid geometry to allow efficient memory usage, storage, and display.

Typically, a default relative accuracy of 0.0012 allows geometry to be calculated with a reasonable amount of computation and within a reasonable amount of time. Sometimes, however, specific model geometry may require that geometry calculations be sensitive to fine features or complex geometric shapes. Modification of accuracy for a model with this higher "level of detail" may be used as a last resort to assist Pro/ENGINEER in solving the model geometry.

Relative Accuracy, the default method for specifying accuracy, relates the level of detail in a model to its overall size. This reflects the general trend that smaller models require

tighter tolerances for manufacturing, and vice versa.

The PTC Help System states "In general, you should set the accuracy to a value less than the ratio of the length of the smallest edge on the part to the length of the largest side of a box that would contain  the part."

A big problem that I observe is that Pro/E and Users set the default relative accuracy to 0.0012 without taking in consideration the units of the part or its size. There is a 25.4 times difference between a part that is modeled in inches versus a metric part.

Stated in equation form:

A < F * s / d


A = recommended relative accuracy

F = a factor based on part geometry

s = smallest distance which the system will consider entities to be separate

d = diagonal of box whose sides are parallel to default coordinate system axes and which just encloses the part. You can find this value under: Info, Model Size

This relationship suggests that decreasing the value of relative accuracy for a given part increases Pro/ENGINEER's ability to measure shorter distances and finer detail in that model. Accuracy in effect determines the smallest distance between two entities (points, edges, surfaces) in which the entities are considered separate in space for geometry calculations. This provides the benefit of being able to create geometry which would otherwise not be possible due to insignificant differences in position in 3D space.

The F factor adjusts this equation to more accurately describe how the Pro/ENGINEER application code describes model geometry. It is determined by part geometry and its value is always less than or equal to 10. In the simplest case of a part consisting of a rectangular protrusion and simple extruded cuts, the value is about 10. In general, however, this factor should be considered to have a value of about 3 or less.

The diagonal of the part only increases in size. For example, if the model is cut in half, the diagonal value used does not become smaller.

To illustrate the meaning of this equation, consider the following example:

A part in which its “model size” has a diagonal of 12 inches long and has accuracy set to the default 0.0012; the smallest edge which is still discernible is about:

A < F * s / d (F is 10 for a very simple part)

0.0012 < 10 * s / 12.0

Then s = 1/10 *0.0012 * 12.0 in. = .00144 inch = smallest edge which is still discernible.

If the part accuracy is changed to 0.0001, the smallest edge can be about .00012 in.

If the largest part diagonal is 1 inch and accuracy is default, then the smallest edge can be about 0.00012 in. With more complicated geometry, these values represent the lower bound of what distances can be discerned on the model for Pro/E to compute.