Sometimes with all the chatter about the various top-down modeling methods and very little about plain old bottom-up modeling I wonder who else does no or very little top-down modeling.
Trying to be specific I’ll add a little list off the top of my head; I don’t know that every one of the lines below are really top-down vs bottom-up type things, but they tend to get drawn in sometimes.
Piece parts in part files, weldments and assemblies in assembly files.
Multibody modeling only used for ref geometry or tool bodies; one part number per file.
No Master Models
No Skeleton Sketch Parts
No parts referencing other parts through a parent assembly. edit: - Primary mating method is part geometry to part geometry (on rare occasion ref geometry if face ids likely to be change such as imported customer/vendor model); Mating origin to origin or coordinate systems is not used.
Who else models in this way in day-to-day practice?
Much of my work doesn’t involve external references of any kind, but it does occasionally, such as letting the shape of concrete drive the rebar shapes and sizes. When I do that I usually edit the rebar Parts inside the Assembly instead of having a master Part.
I do often use multi-body Parts. When I do they stay as multi-body Parts. I never save the bodies out as separate files.
My philosophy on this is pretty simple. There is one major question that divides the two possibilities: will this part be used in multiple contexts or in only one context?
If a part is used in multiple contexts (fasteners and standard bearings are good examples) it MUST be done bottom up. Otherwise changes in one context will affect all others. Other examples are company-wide standard parts that are manufactured to spec instead of purchased off-the-shelf. In our case, we have common shared parts such as motor brackets, towing components, and wear bushings.
On the other hand, if a part is used in only one context it makes sense to use top-down. Sheet metal parts that are common to only one model or product are good examples. As long as it’s not used in another assembly, top-down works great.
I do not use top-down/skeleton sketch method. I use bottom-up as you describe it. I make mostly machined parts and use a lot of “standard” parts such as something you might download from McMaster-Carr. Our assemblies represent how the parts actually go together so this also means (gasp) that we mate faces to faces and holes to holes rather than primary planes to primary planes. I know to some this is heresy. John Stoltzfus would rail on me for my methods, but the simple fact is this works better for us.
Top-down has its place, just as bottom-up has its place. The key is to use what works best for your situation.
Only on the very large assemblies do I work top down. And then, ONLY if I have a really good feel for what the final product will look like before I start.
95% of my modeling is bottom up. Even when I use a sketch to transfer information across sub-assemblies/parts, the vast majority of the work is of the bottom up design.
My life may be simpler than most because I don’t use weldments or multibody parts.
Here is an example of one of my machines. You can see several sketches that are part of my SSP. The only thing that is driven off of the sketch are the belts. The rest of the information is used to allow me to work in the context of a smaller sub-assembly, while still knowing where features are located in the other assemblies.
Ah yes, I forgot about assembly mating/relationship method gets tangled into the top-down or bottom-up method. I’ll update my OP to have
Primary mating method is part geometry to part geometry (on rare occasion ref geometry if face ids likely to be change such as imported customer/vendor model); Mating origin to origin or coordinate systems is not used.
So the sketches provide the profile for the belt loft path? Makes sense.
But the other sketches/elements are not driving the things they represent (space claim)? Are those sketch elements being driven by the parts they represent? If not, do you update both when one moves or changes or is it just a rough guide for reference and not to be taken as exact?
I don’t understand the need to rigidly define “This” or “That” modeling methods. I chose and pick what works for me. In some cases, that means mating origins to origins. In other cases, it means mating construction geometry to construction geometry (typically planes or axis) and in some cases it means face to face and hole to hole.
Learn the pros and cons of different methods and use what works best in a given situation. Stop worrying about if it follows “X” procedure or violates some arbitrary rule. Those only limit your potential to get the job done efficiently.
It’s not so much dogma as it is best practice. I’m not trying to shoehorn anyone into my philosophy, CAD or otherwise.
If anyone really thinks best practices are all nailed down, why is there still such a debate whenever modeling methodology comes up? Opinions vary and I’m fine with that, it’s just that I’m usually not shy about expressing my own.
Edit: like Mike said, I’m not trying to say top-down is bad or that list I put in the first post are best practices. I’m certainly not saying anyone else should CAD the way we do where I work. Hopefully this doesn’t become another debate thread.
Well, that’s kinda what we have and it’s difficult to get new help up to speed and they get very frustrated because it takes a long time to learn all the “what worked best for whoever modeled it five or ten years ago”. When everyone has their own way to do things that really could be unified it’s hard to maintain the dataset. People make mistakes when working on existing files or new products that use existing products.
Often times the quickest or “best” way to create the model at first is bad as it costs many hours of fixing down the life of the model and offspring models. Sometimes its a while before the problem manifests its symptoms, by then the file can be used in several assemblies and copied to a new part number or two. When there are no guidelines people will follow whatever method, they last saw for something similar.
On the other hand, it is as you mention about rules limit potential and if the rules are full of exceptions, then what’s the point of having rules? We deal with that question daily. So its interesting and helpful to hear how others do it.
If we were all doing the same job, in the same industry, there would still probably be debate over the best method. With the wide range of uses and industries there is no single “best practice” to cover all CAD.
When I’m modeling concrete some of it is often top down. When I’m modeling a guardrail it’s all bottom up.
For me, it depends greatly on what I’m doing. Most of my work is master model / multibody to start, fanning out to single body mate-at-the-origin assemblies, and usually small assemblies with relatively complex parts.
I use bottom up process only for the simplest parts and assemblies. If I’m putting off-the-shelf parts together into an assembly, or simple stock shapes, or simple machined parts.
I don’t really do the same thing twice, or even similar stuff more than once.
In this case it was a bit of an evolution. The envelope of the machine was predetermined, so I began with the front sketch to determine the position of the pulleys such that they fit available belt sizes and could be properly tensioned. That drove some major component placement on the front and rear plates, but once all of that was determined the relationships were broken so that they were no longer driving the position. At the end of the design, I try not to have the sketch drive anything. Parts get reused, and local changes may need to be made and I don’t want any downstream unintended side effects. To me SSP, is a part of the journey, but not a destination.
I should add that I have the benefit of being a one man band. When I try something out and it doesn’t work, no one is affected but me. I can see the need for more defined processes in a bigger work environment. (I would definitely be cussing someone helping me out that was doing some of the stupid things I’ve tried. Let’s not talk about the “fix all” phase I went through.)
Thanks everyone. I could have added “and why” to the question, but I was trying to avoid the typical “What’s better” debates. I was interested in the “Why you do what you do” and not “Why you think others should try something different”
I’m always looking for improvement opportunities; ask the “but what if…” question. Sometimes just trying something out can come with a significant risk when I don’t know what I don’t know. We have recently tried a several new methods in the new CAD system that we didn’t know much about. Some seem to be working or we haven’t stumbled into the potential unknown problems yet. But we are also working to clean up the mess from a couple of the other things we tried.
I found the reasons that were shared for using one method over the other for certain cases to be in line with what we had found through the years.
To quote Lou Bega about using many various methods to get the job done suitably to any situation,
A little bit of Monica in my life
A little bit of Erica by my side
A little bit of Rita is all I need
A little bit of Tina is what I see
A little bit of Sandra in the sun
A little bit of Mary all night long
A little bit of Jessica, here I am
A little bit of you makes me your man
I am glad that I don’t to the same thing all the time, nor in the same way. Curveballs take a different swing to knock it out of the park.
