Top down vs Bottom up

[Parsnip]

A quick question (I hope).
We are using Pro/E wildfire 2 and are having a formalisation of process control for the design of models and assemblies.
Having origionally been an advocate of bottom up design, PTC then started to extol the use of top down and now we have a right mixture - up - down - left - right and every combination in between!!!
What are people's thoughts/experiences/comments on this?
Regards
Parsnip

 

[looslib]

Top-down design is okay for developing new products as you need to be able to design within the context of your assemblies. I also find that top-down designs tend to use less mating condition constraints and depend more on location to the master coordinate system.
Bottom-up design can have the benefit of parts re-use as the designers have to look through existing parts to design their new assemblies.

At my last job, we had a design guideline that specified that all parts must be orientated on the XY plane with material thickness in the Z direction. This was so manufacturing could nest the parts for burning. With UG, the designers had no problem chnaging the detail part orientation. When we switched to Pro/E, all of the designers said it was making their life harder as they relied on the coordinate system in the skeleton, assembly and details to mate parts rather than faces or datums.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli
Sr IS Technologist
L-3 Communications

 

[kgrzebien]

I have alway been a fan of Bottom up design. I feel you should desing it the way it is going to be built. Start wih a part, part to sub-assembly and finally master assembly.
embedded parts are "nice" for conceptual work but in long run I'd rather work without the references to the assembly.

 

[looslib]

The references is what kills Pro/E when it comes to top-down design. You need to use skeleton files and be sure that your mating conditions are between your added part and some reference in the skeleton. Mating parts to parts with pro/E makes the whole assembly a nightmare to manage the references.

UG doesn't build the references that Pro/E does when doing top-down, unless you copy an edge or surface to the new part. This keeps the relations down and controllable.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli
Sr IS Technologist
L-3 Communications

 

[JohnAndrews]

Top down design doesn't need mating conditions. If you use a detailed skeleton and publish/copy geoms to create the individual parts which create the assembly defined in the skeleton, every part can be assembled to the default co-ordinate system, and there are no dependancies between parts, just parts and skeleton. The references which Pro/E creates are the power of Pro/E and ensure that when you change the skeleton, every part which is driven by it updates accordingly.

But getting back to your original question - bottom-up or top-down, it really depends on what you are designing. Top-down needs more consideration about how you want to control dependancies between parts and their interfaces, and how easy you want to be able to implement a design change or copy the whole design with the minimum amount of effort. i.e. if you were designing a box with six sides and you want to change the overall size of the box, with bottom-up you need to manually change each of the six sides. With top-down, you change the skeleton, regenerate the assembly and all six sides update. However the six sides do not include their own driving dimensions as they are driven by the skeleton.

You should also look at thread554-152678 for some interesting comments on this subject.