Monday August 19, 2013 at 4:26pm
In context or Top Down modelling is a very powerful technique for bespoke/ one off assemblies that may change due to customer demands, The main principle is that parts are designed or edited in the assembly environment to benefit from the surrounding geometry. You can dimension to, copy from or extrude up to other geometry to ensure things fit together.
The repercussion of this is that external references are created between components- this is so a change to the driving part automatically updates the driven part. If your customer therefore says "can you make Part X bigger?" you can change this and other parts alter as a result. As mentioned this is great for one off designs, but should not be used for standard build assemblies- otherwise all assemblies using the driven parts will be affected.
One of the main issues we see with this techniqiue is mismanagment of references- not knowing how parts interrelate, and how to correct references that become detached. Fundamentally these references, like all SOLIDWORKS references, depend on file names and locations. If you rename or move a part that drives others, the reference is at risk of breaking. The reason for this is that the driven part contains an internal identifier in the file which relates it to the name and location of the driving part- if this changes the identifier won't always update. The way around this is to use a PDM system (i.e. Workgroup or Enterprise) or SOLIDWORKS Explorer (and note that a SW Explorer menu lives inside the default Windows right click menu list, so always use this for renaming and moving). This topic is one for a future blog post, but what I want to discuss here are the signs that references aren't working, and why they aren't. Below is an assembly feature tree where I have been able to create the four states an external reference can be in:
Out of Context- the driving assembly or part cannot be found in the expected location, they have been renamed or moved. To correct this, the orginal files (names and locations) can be restored, or the references must be edited to geometry and files that do exist.
In-context- All references are working well and up to date- the driven part can find the assembly and driving parts in the expected locations
Broken Reference- an old reference has been broken permanently and cannot be retrieved.
Locked Reference- reference is temporarily frozen, no updates will come through and no new references can be created on this part. References can be reinstated (Unlocked) later on.
In terms of understanding where the references are related to, simply right click on any file that displays the -> symbol and choose List External Refs.. this then opens a dialogue that shows the expected assembly name and location that holds the reference (it is this being incorrect that causes the ->? out of context state), and allows you to Break and Lock references.
Understanding and keeping tabs on these are the key to creating robust top down assemblies- bad references can cause errors, unpredictable updating and slow rebuild times as the software attempts to find a solution. If you are concerned that these references are being created unintentionally, you can use the No External References button available when editing a part in the assembly to prevent them being created in the first place.
For further information there is a detailed topic in the SOLIDWORKS Help named "Top-Down Design" available from this link.