CRC Errors

[MartinBrown]

All
I have a new to Pro/E user and the topic of CRC errors has come up. I'm not doing a very good job of explaining how they come up and I'm under the gun on other issues. So I was wondering if anyone has a good write-up on CRC errors and their resolution that they would be willing to share.

thanks

Martin T. Brown
Mechanical Designer
Merrick & Company
1099 Commerce Park
Oak Ridge, TN 37830
(865)241-4642

[dgschaefer]

A good way to explain them is to use the chicken and the egg concept.
In order to make a chicken you need an egg, but in order to make an egg
you need a chicken and round and round it goes.

They seem to happen accidentally somewhat easily, but good luck trying
to create one yourself to demonstrate it. They are hard to do on
purpose.

Usually they occur because part features are redefined or reordered.
Parts have to be completely regenerated before they are placed and that
can cause CRC errors. For example, if you have a copy geom in your part
but you place the part in the assy using a plane in the part that is a
child of the copy geom, that's a circular ref. The part placement
depends on the plane which depends on the copy geom which depends on the
part placement. In that case, you need to redefine the part plane or
use different assy refs.

Sometimes it's just feature order. If you place the part in the assy
using a plane in the part that is simply after the copy geom, that's a
circular ref too. Why? Because the part plane cannot be properly
regenerated until after the copy geom and the copy geom cannot be
properly regenerated until the part is placed and the part cannot be
properly placed until the plane has be properly regenerated. In that
case, simply reordering the plane to be ahead of the copy geom fixes it.

Doug Schaefer

[StephenW]

The most common reason I make create CRC's follows this scenario:


1. I create a new part that has only the default datums and I assemble that in my assembly using the datums.
2. I create the geometry of the new part in the context of the assembly and along the way I create external references to the assembly, maybe from a use edge in the sketcher or a dimension to geometry that exists in the assembly and not in the part.
3. After completing the part, for whatever reason, I redefine the placement of the part and use the surfaces of the geometry as the placement features.

I have created a CRC. The part geometry was built with reference to the assembly AND the geometry of the part is now locating the part in the assembly. Which came first, the chicken or the egg.

In my world, I usually will break most, if not all external references in my parts when I have my design ready production. Usually at that point, I don't have any more CRC's.