TDD Question

[JWayman]

Good morning.
I have been wrestling with Top-Down Design for some time now. I have a
question that some of you who have more TDD experience than I do may be able
to answer:

I have an assembly which is driven by a skeleton and a layout. In the
skeleton, there is, among other things, a surface corresponding to the outer
surface of the assembly and another surface corresponding to the outer
surface of one of the sub-assemblies. The skeleton also contains a number of
Publish Geometry features, including one for each of the surfaces mentioned
above.

The parts making up the outer shell of the assembly and of the sub-assembly
include the relevant Publish Geometry features. Various sketches are
constructed within these parts, based on the shapes from the Publish
geometry features by means of the 'Use Edge' tool in the Sketcher palette.

All is well so far.

However, when I change the shape of the surfaces - say, for example, I
remove a radius and make it a sharp corner - every component and/or feature
that uses the Publish geometry feature from that surface falls over. As far
as Pro/E is concerned, the Publish geometry feature appears to be new and
completely different and unrelated to the original feature. Now that would
be OK if all I had to do was tell Pro/E to use a different edge in the
sketch, but Sketcher insists I Update all the references individually, and,
in doing so, loses just about every reference in the original sketch. I
repeatedly end up having to delete everything in the sketch and start again!
Specifically, problems seem to be caused by the 'Use Edge' Sketcher command
and Publish Geometry/External Copy Geometry combinations. Things that are
referenced to datum planes, axes, etc. seem to work OK, but anything with
curves and/or surfaces is a nightmare.

So I have achieved a successful Top-Down Design in that my lower-level parts
are related to the top-level assembly through all the whole layout,
skeleton, publish geometry, external copy geometry thing, but it only works
if nothing at the top level ever changes significantly.

Life isn't like that! The only thing that is certain in Mechanical
Engineering Design is that something is going to change. The chances are
that it is usually the one thing that is not driven by the layout/skeleton,
but that's another story, not Pro/E's fault!

In this specific scenario, I have worked on the product and another engineer
has worked on the mould tool. It all worked OK (ish) when we were both
working concurrently, as long as we both ran and told each other whenever
anything changed and we kept on top of it at a feature level. Now, however,
I have had to change the shape of the product, but the other engineer has
moved on to another job, so I am faced with bringing the mould tool back
into line with the product. I am literally having to re-model much of the
mould tool assembly and parts. Of course, I didn't model them originally, so
I am having to find out how they were done, as well as putting them right
for the new shape.

I am hoping that the problems I am having with propagating changes through
my TDD assembly are due to my incompetence, but I am not sure...

Can you point me towards some clever trick to avoid the whole house of cards
from falling? Should I be using some other way of constructing parts using
surfaces that are potentially going to change shape during the design cycle?
Is there a robust way of using Publish/Ext Copy Geometry such that, if
something in the parent changes, there is at least some clue in the child as
to what could be the reson it has all keeled over?

Or is this the reality of TDD? I hope not!

Regards,


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[dgschaefer]

Without seeing exactly what you're dealing with, it's hard to diagnose,
but this sounds like a modeling issue rather than a TDD issue. Your
geometry is getting passed down fine, it's how later features use that
geometry that is the problem.

Using edges in sketches is inherently unstable. That is because
whenever an edge changes, it's ID changes. Say you have a cube and
you've used an edge in a sketch. Now you go into insert mode and round
an adjacent edge, leaving the original edge there but shortening it due
to the round. That edge still appears the same, only shorter, but in
fact has a different ID. In many cases, Pro|E has gotten pretty good at
finding the new edge if it's coincident or close to the old one and
replacing it. (Although, if you redefine the feature and look at the
references, you'll frequently see that the reference is tagged orange or
listed as missing. Those orange references can bit you later.).

Surfaces, however, can have any number of holes or cuts or modifications
done to them. You can take a 500" square surface and trim it down to a
5"x12" ellipse, merge it with an angled sweep around the ellipse's
perimeter and then round the edge between the sweep and the ellipse and
that initial surface still has the same ID. Poke holes in it, solidify
or thicken it and the ID is still the same.

Some times it's not possible to create a sketch without using the edge.
But many, if not most, times, there's an adjacent surface or some
underlying geometry that can be used. For a surface model, is there a
curve that was used to create the surface that you could use? Look for
'older' geometry (earlier in the model tree) that would be more stable.
If you're following the build big to small (gross geometry to details)
method of building your parts, which I've found to be very robust, then
earlier features are likely to be more stable than later ones.

When I do TDD (and I rarely don't), I try to think of creating the most
basic elements of geometry that the downstream parts need to share. A
point or axis defines a screw location, perhaps a curve can define a
parting line, a plane defines a mating surface, etc. Of course, in a
complex surface that spans over several parts (think vacuum cleaner or a
car body), you want to create much if not all of that in the skeleton.
That doesn't mean that you can't pass along the underlying curves along
with the quilt to the lower features.

BTW - I'm giving a presentation in Orlando on using skeleton driven TDD
on Wednesday. If you're going, check it out.

I hope that helps.

Doug Schaefer

[JWayman]

BTW, I forgot to say:

WF2 M220

Regards,


John