Mirroring surfaces

[pwyndham]

So, one of the engineers I work with modeled a part. He did a lot of surface modeling to get what he wanted. He thought he would save some time and just modeled half of it. The idea was to mirror it after the design was the way he wanted it.

When he started trying to mirror the part it came back with all kinds of geometry errors.

He contacted me to see if I could help. I started mirroring different groups of features in the part. I found that the first several features mirrored successfully. Then, mirrors after that would fail.

As I inspected the part I found a hole.

I cannot seem to put in a second picture to show what I found so I will edit this one or add a second post to get the next picture.

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[pwyndham]

After closer inspection of the mirrored feature (an offset) that should have covered that hole I found this:

Pro/E is not mirroring the entire feature. Instead, Pro/E mirrors the offset that has already been trimmed.

This seems like a major bug in Pro/E that would allow it to mirror the future state of the object instead of how it was at that point in the design. It correctly shows the features in the model tree, just does not recreate them as that were at that point. No wonder mirror does not always work properly.

This is in WF4 so I will try it in WF5 and see if it works the same.

[ptc-2512492]

I largely abandoned the use of "mirror" functionality ages ago. It's nice on occasion, during the early "cocktail napkin" design stages, but there are too many issues which come up. Mind you, I'm talking about mirroring features, not mirroring elements in sketcher, which works perfectly well.

Most of my issues with solids mirroring came about as a result of dimensioning/tolerancing issues (which is worse when working in "intent manager" as your sketcher). But that wasn't enough to explain ALL the issues I've seen.

The biggest problems I've had have come from models "collapsing" when you change the model accuracy if the model contains mirrored features. Similarly, I've seen "collapses" caused by surface-based modeling being mirrored (ie, creating body panels as NURBS features, thickening them, and then mrroring them).

I've never quite understood what's going on "under the hood" to cause this, but I have learned that the only practical way of repairing it is to roll back the model, suppress the mirror feature, reproduce it as an explicitly defined feature, delete the old mirrored feature (being careful to suspend, not delete, all child features) and then individually redefining all child features as necessary to refer to the newly created feature.

It's not really worth it. The extra time to just create the feature explicitly up-front is a lot less painful. That's the nice thing about a parametric modeler...

Maybe PTC will make mirroring more robust in the future... but until they do, I'll just "brute force" things, which works a lot better in my experience.

[pwyndham]

I knew there were issues with mirroring geometry, but I guess I never looked into What they were.

I just cannot figure out how PTC would have let Pro/E create the mirrored feature from the future state eventhough it follows the history for creating the mirror. The best thing to do would be to just create something like an imported feature for the mirror geometry and have it directly tied to the first side. It would of course need to let you make new features on it.

[RickGiguere]

I've heard from different people that mirroring is unstable, I've never had a problem yet. It may be useful as long as the feature is not too complicated.

I use symmetry to create parts all the time, then mirror.

One of the debates I've heard; is it better to make a hole pattern or mirror a hole. Mirroring is quicker, but some say it's unstable.

[larryp7639]

I knew there were issues with mirroring geometry

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[MGortner]

Are you mirroring "features" or "quilts"? As a rule, I never mirror features. It's always best to mirror quilts. If you're working with solid geometry, create a copy of the surfaces you want and then mirror that quilt. You may have better luck.

[ptc-2512492]

Well, I've had just as bad luck mirroring surface-based features ("quilts" or otherwise) as I have with solids.

There's nothing "conceptually" wrong with mirroring ANYTHING. If the functionality worked as it ought to, we should be able to mirror every available feature type in any situation.

One area where "mirroring" can, and really SHOULD (in my opinion) cause problems is in features which are directly referenced to other features (which you may NOT be mirroring). As a rule of thumb, any time I'm doing any form of "feature copying" I always try to ensure that the feature's references are part of the group which I'm copying as well. There are some circumstances where this isn't an issue, obviously, and you can take this too far too. But any time you have issues with copied/patterned/mirrored features, it's normally your placement and dimensioning references which are at the heart of it.

Except, of course, with mirrored features, which still seem to cause problems (with no apparent rhyme or reason behind them, though obviously there's SOME sort of cause somewhere in the code base!) at random, even if there are no referencing issues. I get the impression that you just sometimes end up with one line of code dividing by zero or something... (sigh)

[MGortner]

When you mirror quilts, are you selecting the surface based feature before your mirror? Or are you changing the selection filter to quilt and then picking a quilt? You get different results depending on how you select the quilt. By using the "quilt" feature you avoid annoying pop-ups when trying to edit parent features (ie. "you cannot add or delete references").

[pwyndham]

That works. Thanks. I rolled back to just before the solidify and inserted the mirror there.

Now I just have to redo the finishing features.

Thanks for the pointer.