How to model machining after assembly in NX

Introduction

As I promised you before, my new article looks into tools that NX provides for modeling of machining after assembly.

Thanks to the unified PRT file format, in NX there’s almost no difference between modeling at the part level and assembly level. The only difference is that at the part level we can directly modify bodies, while at the assembly level there’s no direct access to the bodies of components, and it must be provided first.

In NX there are three ways to alter component and the assembly level, based on the following techniques:

  • Body promotion
  • Assembly Cut
  • WAVE copy

Each of these techniques has its own advantages, and also its own characteristics, which must be taken into account to avoid errors in the product structure.

Body Promotion

Body promotion is the operation that makes solid body of the component available for modeling operations at the assembly level. The ‘Promote Body’ tool is responsible for this operation.

  • Insert > Associative Copy > Promote Body

Machining after assembly

Be advised that this tool can be switched off at certain NX seats. It can be switched on in the Customer Defaults:

  • Customer Defaults > Assemblies > Interpart Modeling > Allow Promote Body

As a result of a body promotion a new feature called ‘Promote Body’ appears in the feature tree of the assembly, and modeling features can be applied to at, as to an ordinary solid body.

Key characteristic of the promoted body is that it allows to model machining after assembly as it happens in real life – changes are made to the same body that is contained in the component, and switching off visibility of the component switches off that of the promoted body as well.

All changes made to the body at the assembly level, are contained only there, while body in the part remains unchanged. This approach is well understood by the design engineers, and requires practically no extra training. Also it must be noted that promoted body can not only be subtracted from but added to, that allows to model after assembly such operations as cladding.

Advantages

  • Intuitive and clear technique, because it implies working with the same body both at the part and assembly levels.
  • Assembly weight is calculated correctly if promoted bodies are used. Advanced Assemblies license is not required. Weight of the part corresponds to the original configuration, while assembly weight takes into account all changes made to the part at that level.
  • To promote and alter body in assembly no write access to the component file is needed. Thus, write-protected files and Teamcenter Items can be used in machined assembly without restriction.

Where to pay attention

  • The source of the promotion operation cannot be changed automatically. In case we need to replace a component with a promoted body, we need to add it to assembly, promote its body, and manually replace all operations applied to the old body.
  • It is necessary to load a component with a promoted body when loading assembly
  • If we need to unite promoted bodies of a component, in versions prior to NX10 we should do this one body a time, or assembly weight will be wrong. This issue has been resolved in NX10.

Evaluation

To model machining after assembly using body promotion is an efficient technique, resembling real life very closely.

Implementation of this technique requires evaluation of potential problems if replacement of components in assembly happens often.

In particular cases it is possible to employ special modeling techniques which will improve response after component replacement. I will describe them in one of my future articles.

Assembly Cut

Assembly Cut tool resides in the Combine submenu, together with the rest of the Boolean tools.

  • Insert > Combine > Assembly Cut

Machining after assembly

This tool subtracts solid body at the assembly level from the solid bodies of components of this assembly. Alike body promotion, this tool closely resembles real-life machining after assembly. Actually, Assembly Cut is based on the promotion operation and uses the same programming code.

Bodies treated by Assembly Cut became promoted, and modeling features can be applied to them in the regular fashion. Extra advantage of this toll is that it can be applied to many bodies simultaneously. As with the explicit promotion, changes are contained only at the assembly level, and not the part level.

The limitation of this tool is that by definition only the subtraction operation can be performed with it, and not the addition.  Also it means that negative body should be constructed first, which can require some special training of the users because it is not universally intuitive.

Advantages

  • Clear and realistic technique, because it because it implies working with the same body both at the part and assembly levels – like the body promotion.
  • Assembly weight is calculated correctly. Advanced Assemblies license is not required. Weight of the part corresponds to the original configuration, while assembly weight takes into account all changes made to the part at that level.
  • To use Assembly Cut on the component no write access to this component file is needed.

Where to pay attention

  • As with the promoted body, if component is replaced, we need to manually modify the operation and make new component target of the subtraction operation.  Other features applied to the old body, also require manual editing.
  • Also it is necessary to load component cut in the assembly when loading this assembly.
  • Can require training users to build negative bodies.

Evaluation

Like the body promotion, Assembly Cut tool is an efficient technique, resembling real life very closely.

Implementation of this technique also requires evaluation of potential problems if replacement of components in assembly happens often.

WAVE linking

NX WAVE geometry linker allows to model machining after assembly by making a copy of component’s body at the assembly level and applying modeling features to this copy.

  • Insert > Associative Copy  > WAVE Geometry Linker

Machining after assembly

This technique differs from body promotion or Assembly Cut in a way that modeling features are applied  to the copy of component’s body, and not to the body itself. To the WAVE-copy of component it is allowed to apply any modeling features, as to a regular solid body. At the same time, the fact that features are applied to a copy, and not to a component’s body itself, can be difficult to grasp for design engineers, because it does not resemble real life. This can require special training of users. Another important characteristic of WAVE-linked solid bodies is that they add extra weight to the assembly, and that problem requires special methodology to resolve, which, it turn, requires Advanced Assemblies license.

Advantages

  • WAVE-copy is a solid body at the assembly level, having the same properties as a regular solid body. This allows us to apply modeling features to it practically without restriction.
  • Does not require loading of components, bodies of which were WAVE-copied to the assembly level, when loading an assembly – unless geometry update is required.
  • Like other techniques, does requires no write access to the component file
  • Supports component replacement very well. In case of such replacement, it is enough to create new WAVE-link, and automatically transfer modeling features from the old copy to the new one by the ‘Replace Feature’ tool.

Where to pay attention

  • Requires special actions to control assembly weight with the aid of Advanced Weight Management tool. This requires Advanced Assemblies license.
  • This technique is not very intuitive, and can require special training of users.
  • Because components that were machined at the assembly level are actually solid bodies, this  makes assembly drawings somewhat more difficult to make, because drawing tools are optimized to work with components, and not bodies.

Evaluation

Modeling of machining after assembly with WAVE-linked copies of bodies requires special methods for weight management and particular user skills. This technique is a special one, reserved for special cases – for example, the need to replace assembly components often. This can apply to the family of assemblies machined after assembly, that I will cover in my upcoming article.

Conclusion

Body promotion and Assembly Cut are the first-choice techniques when machining after assembly is required, and they cover most scenarios. WAVE-linking is the reserve technique reserved for cases, when outstanding factors make other techniques difficult to employ.

In any case, the choice of particular technique for modeling of machining after assembly should be highly conscious. Need to carefully take into account characteristics of all techniques offered by NX, and run a comparative testing of them with the real assemblies, before making one of them standard for your company.

 Do you need to model machining after assembly? Which technique do you use?

2 Comments:

  1. Hi Ronald!

    Thank you for taking your time to read this post.

    Promotions
    1. Wrong. When component is removed, promoted body becomes broken promotion link. It does not by default remove 3D geometry from assembly.
    2. Correct. I never said otherwise.

    WAVE link
    1. Correct. I never said otherwise.
    2. Correct. I never said otherwise. But since I am looking into machining after assembly in this article, the Parent to Component direction is of no interest.
    3. Wrong. If you remove component from the reference set, it won’t remove its weight from the calculation, because it is controlled via component groups. If you remove WAVE link from the reference set, the calculation of weight will be made based on the weight of original component, with no regard to machining made at the assembly level.

    Some of the point I’m showing in the upcoming video. Obviously, it is not possible to cover all the questions within single article.

    I hope to hear more from you!

  2. Hi, I am missing some important things in here…
    Promotions:
    1- If the original component where the link is derived from is removed or deleted, the promotion is broken and cannot be repaired. Your 3D geometry will be gone.Only solution is to change it to a wave link after that.
    2 – Promotions will only work from component to parent in assembly.

    Wave link:
    1 – If original component is removed / deleted, wave link is broken but 3D geometry is still available. Broken link can be easily repaired or pointed to new parent geometry.
    2 – Wave links can be applied two directions Component to Parent, Parent to component.
    3 – The weight issue can be solved very easily and no Advanced Assembly license is needed for that. You can either remove the component or the wave link 3D geometry from the model reference set. Weight will be calculated from the one which remains in ref set model.

    Best regards,
    Ronald van den Broek,
    Application specialist

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