Tuesday, May 06, 2008

STS#2a - Part-To-Part Relationships: Extrusions

Due to the sheer number of posts on this subject, here's a handy guide to all of them.
#1 - Associative Copy/I-DEAS Baseline
#2 - Derive Component Part
#3 - Top-Down Design
#4 - Blocks
#5 - Design Library
#6 - Assembly Cut
#7 - Part Configurations

I'm going to have to break this one up into multiple posts.

The extrusion I was working on for the previous post is a custom design. Typically when we get custom extrusions, they arrive in 12-foot long sticks. Those sticks will then be cut to length as needed, either via internal means, or we'll ask the supplier to pre-cut them for us. There may also be additional machining performed, holes drilled, etc.

From a CAD perspective, what we do is have a drawing for the raw extrusion that essentially shows only the cross section. We will then create part drawings for the cut-to-length versions, referencing that raw extrusion document. For design intent purposes, we utilize a relationship such that if the raw extrusion would change, this change is propagated to any of the downstream models.

In I-DEAS, the only way to establish this relationship is with the Associative Copy command, which I will highlight in a moment. In SolidWorks, I'm reasonably confident that the default answer would be to use part configurations. For reasons I will articulate at a later time, I do not currently consider part configurations to be a viable option. So that leaves me with the methods that SW provides for establishing part-to-part relationship. I will take a look at each of the methods I have found, and discuss plusses and minuses.


In I-DEAS, the only way to have a part-to-part relationship is with an assembly. The assembly owns the relationship, and is called the Context Assembly. There is a Source Part, which has the geometry we want to reference (in this case, the raw extrusion), and a Target Part which is the dependant part we want to change as the source changes (in this case, the machined extrusion).

I will begin by creating an assembly, and adding the raw extrusion to it.

For the SolidWorks folks still reading, that coordinate system is basically equivalent to your default planes (front, top, right). It is worth noting the location of that CS, since it is at one end of the part. The 12-foot stick doesn't really get used directly anywhere, so that's probably an acceptable choice. But for the pre-cut lengths, I'll want that CS to be in the middle of the part. This leads to the first choice I need to make.

I-DEAS will ask me what items I want to copy (relate to). I can choose the entire part, or I can choose individual items like edges or surfaces, depending on what I need. If I choose the entire part, then I'm going to have to perform a cut operation in order to get the length established. Not a biggie, but that CS will still be on one end, not in the middle. Again, not a huge deal, I could always add another one, but I'd like a cleaner approach. So I will select only the end face (which is the cross section), along with the CS, and go from there.

The green surface is actually a new part. It might look a little odd because I'm getting some bleed-through from the red surface it came from. I'll hide the raw extrusion, and work just on the new part.

I'll extrude this surface using a thicken (mid-plane) option.

And last I'll perform the drilling operations.

I'll return to the raw extrusion and make a change...

...and observe the change reflected in the machined version.

Now I have to confess that some repair work was necessary when I updated this change, so maybe it is better to take the entire part and cut it to length. Also, thinking ahead to SolidWorks, I know that there are better ways to get things centered than there are in I-DEAS, so having the CS in the middle won't be as important as it is here. Maybe this isn't the best example.

However, there are other cases where I would want individual surfaces instead of the entire part. So let's stay with that concept, but just consider this to be a not-so-hot example of that.

Using my mad PowerPoint skillz, here's a diagram that shows what we wind up with:

In this particular case, I need another cut length with different machining operations. So I would repeat this process, starting with a new context assy (same raw extrusion). It is important to note that creating this new part does not in any way affect the first one. The two cut length parts do not know anything about each other, they only know which source part they come from, which happens to be the same part.

Having thus established the I-DEAS baseline, in the next few posts we'll take a look at how SolidWorks would accomplish the same thing. If you are a SolidWorks-skilled person who has some suggestions, please let me know. The methods I currently know of are 1) Derived Part, 2) Top-Down design (I consider this to be in-context design, but whatever), 3) sketch Blocks, and 4) Smart Components.

1 comment:

Anonymous said...

You should try this in NX it's much easier!