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to blend or not to blend

Genius
Genius
Throughout my modeling work, I'm struggling with a question: to blend or not to blend.
 
Some opinion/observation from people on this forum are very impressive to me, e.g.
 
@StevenVickers: Do not define your fillets in curves, build surfaces oversize and use surface blending commands near the end of design. [source]
To be honest, from the beginning, I shunned the use of blends and tried to define highly curved splines as sections for surfacing. I knew this might be problematic. But it was really difficult to resist the temptation to build the model in a straightforward way and without bothering to leave space/oversize for later blends.
 
After suffering endless trouble caused by those highly curved sections, I think that it's a big mistake to shun blends.
 
However, to "build surfaces oversize" is not an easy job. At the intersection of two surfaces, the space for later blends need to be considered. This seems to be very tricky. My approach is to sweep a triangle along the intersection curve to preserve space for the later blends. But this seems not to be a good practice.
 
"Build surfaces oversize" and "blending ... near the end" are the start and end of a long modeling project. It's difficult for me to present a real model example for the problem.
 
I would be grateful for your possible suggestions. If my question is too abstract to deal with, please simply ignore it.
 
Thanks!
12 REPLIES

Re: to blend or not to blend

Solution Partner Valued Contributor Solution Partner Valued Contributor
Solution Partner Valued Contributor

It's nearly impossible to provide an answer to this question without knowing more about your specific practices and intended objectves. For instance, are you working on class A surfaces, aerospace, or marine hull design? Then, in order to get the specific shapes, continuity, and editability you need, perhaps you do need more than just "blends".
Realize, of course, that by "blend" we're really just talking about a transition surface, usually conic in nature, between and having continuity with 2 or more adjacent surfaces. So what's the purpose of blends in your models? Perhaps more important, what are you finding to be problematic with blending?

 

It just sounds like you are looking to make things more difficult and time consuming than they should be. NX blending is very effective, generally speaking, and I find that almost all students I teach coming from another CAD system are impressed with its capabilities. For simpler machining shapes, Edge Blending would be the default with Face Blending used when Edge Blending fails or when you need to blend over non-adjacent faces. Then there are another half dozen blend commands in NX, all preserving their own uniqueness in shape control, editability, or surface type. I believe Siemens maintains all of these methodologies for very critical and specific customer needs. Some are rarely used but when needed, they are there for you.

 

Despite the simplistic rhetoric I state above, it is indeed worth clarifying that once you enter into the world of sheet and surface modeling approaches, the modeling task can go from minutes and hours to days and weeks. And it tends to break more when making subsequent changes. As the s/w has developed over the years, the blending functions incorporate a high level of "rounding and filleting" that capture the intent internally so that you shouldn't have to deal with intense modling effort and features breaking when changes must be implemented.

 

Then again, I've seen some customers that don't blend models at all and just leave it to the machinists to add those shapes as a result of their toolpaths and choice of cutters. But obviously, that violates the advantages and benefits of CAD such as pinpoint accuracy of volume, mass, and weight not to mention clearance and interferenece checking in assemblies.

Re: to blend or not to blend

Gears Phenom Gears Phenom
Gears Phenom

For us, we use our 3D models to make the tooling from.  We need every draft, edge blend, split line to be  on our model per what the die caster needs to make the tool.  This also helps with accurate FEA and other analysis,

 

So we usually model square corners, then add the fillets at the end of the history tree. 

 

I think it depends on what you are modeling.  This works for our Castings, plastic molds....   Most of our parts are parametric, very little surfacing,  sketch, extrude, revolve....

 

 

Re: to blend or not to blend

Genius
Genius

Hi @sdeters   Thanks for sharing your practice!

 

Hi @kochg   I work on class A surfacing. Yes, in most case NX blending is highly effective. However in my situation it seems to be not so easy to handle. Thanks for your very informative reply!

Re: to blend or not to blend

Phenom
Phenom

@surfactant,

 

 

I agree with @StevenVickers@kochg and @sdeters however, opinons can change depending on what you're modeling and for some reason, your situation seems to be top secret.  Your intitial steps of the model shouldn't be so difficult that you cannot apply blends later.  Granted, as @kochg has already pointed out, not knowing what in the world you're modeling doesn't make giving you advice any easier.  It's getting almost frustrating in some cases, but mostly because you won't just say with words what it is you're modeling.

 

In regards to surfacing, the advice of overbuilding surfaces is pretty much standard practice in most cases I've encountered.  You build the larger areas first, confirm those surfaces are of the complexity (rather simplicity), shape, and quality that is required for the model then you work on the secondary surfaces, repeat the checks on the results.   At this point, you should be very close to done and able to apply tertiary surfaces (usuallyblend and/or chamfer surfaces).  Repeat with the checking.  There is a reason for this and it's mostly because if the primary surfaces aren't good, it's much easier to change those at the beginning stages of the model rather than waiting until later and ending up having to change something way back at the beginning as well as the surfaces built on top of those.  That's when you can end up having to start completely over or at least remodeling a large chunk that should have been done already.

 

If you model in smaller steps or pieces, it can be much easier to make changes.  Once you build up your knowledge and command aresenal and mastery, then you can go back and possibly combine some of those smaller steps into larger ones, with one of the goals being to cut out features in the model tree while maintaining good practices and building models which just about any user can come after you and make changes without wondering just what in the heck you were thinking.  At that point you might also have a better understanding of what value certain techniques have over others.

 

Finally, don't take this the wrong way, as these words aren't meant to hurt any feelings, but you're only making things harder on yourself by considering trying to reinvent the wheel in some cases.  Too many people have been lurking around these boards for too long designing too many different products using too many different techniques for that to happen.  Keep it simple, then when you've mastered the simple stuff, move onto refining things more towards your preferences or adding more layers of complexity or management.

-Tim

Re: to blend or not to blend

Siemens Phenom Siemens Phenom
Siemens Phenom

@surfactant

I would make a conscious decision on what is a blend and what is a base surface, =<6-8mm blend is probably a blend for me, >12mm circular face is probably a construction surface. The grey area in the middle is part of your experience and assessed dependently. Of course all of that is fluid based on the type of surfacing task and product area. This is a good discussion that will benefit everyone.

Regards

Paul

Re: to blend or not to blend

Phenom
Phenom

I obviously agree with @Paul_Hartrick and @TimF ,

 

the term 'Blend' indicates a connecting surface instead of a 'Blend' often.

Even a 'Draft' is sometimes build with ad hoc surfaces because the 'Draft' command failed or achieves a unespected result.

 

In the attached file there are some surfaces made with different methods. I did them quickly, just to give an idea.

Ciao

Blend.png

 

Re: to blend or not to blend

Phenom
Phenom
@surfactant,

I think you would gain quite a bit of understanding if you were to watch some YouTube videos where people are doing Class A surfacing, even if it's using a software like Alias. Keep in mind the concepts that have been given to you - pay attention to how the larger (primary) surfaces are developed first, how medium-sized (secondary) surfaces are developed, and finally, how the smaller (tertiary) surfaces typically finish things off. Don't get too hung up on the differences in command names or how users may have to refine using continuity matching commands like Match Edge in NX....it's the workflow or steps they take to get from start to finish.

As others have pointed out, the term blend can be relative, as can the labeling of what is primary, secondary and tertiary. In @Cesare's example, there may not be tertiary surfaces, only primary and secondary - that will be dictated by the complexity of the model and the approach used by the modeler.

I tend to think of a blend as a surface that softens a sharp corner where 2 other surfaces or faces intersect and is at least tangent to those 2 other surfaces, regardless of which command is used or the cross-sectional shape of the blend surface.
-Tim

Re: to blend or not to blend

Genius
Genius

Hi @TimF   Thank you for your both posts!

 

At first I must say sorry for be unable to disclose information about products I work on. As you can understand nowadays companies don't want to see their employees discussing their work on public forum. I know this situation makes my questions abstract or sometimes even pointless and also add to big trouble for helping people.

 

Please don't say something like "hurt any feelings". I'm appreciative for both your guidance and your criticism. I know that some of my ways of doing design things are undermining my efforts to become a qualified surfacing worker. I'm happy that you are willing to point them out.

 

The workflow you decribed in the first post is very help. I would analyze it careful and make it my daily practice.

 

As to tutorial videos, in fact I have spent quite a lot of time searching YouTube. However most videos seem to focus on single commands. I think I can never learn from YouTube videos those important things I learnt from you, @Cesare@StevenVickers and many other kindly people on this forum. One thing I observed is that YouTube has much more useful materials for SolidWorks or other design software than for NX. I don't know about Alias, but I did study many SW things. Although I don't have access to SW software, I did learn a lot from its tutorial materials. And the book "SolidWorks Surfacing and Complex Shape Modeling" is especially helpful, and it's a pitty NX doesn't have such a book on surfacing.

 

You just refered to concepts of primary, secondary, tertiary surfaces. They seem to be very helpful for making a reasonable overall plan for a design project. I would try to understand them better. Thanks!

Re: to blend or not to blend

Genius
Genius

Hi @Paul_Hartrick  Thanks for sharing your practice in dealing with blends!