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:
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 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.
Be advised that this tool can be switched off at certain NX seats. It can be switched on in the Customer Defaults:
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.
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 tool resides in the Combine submenu, together with the rest of the Boolean tools.
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.
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.
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.
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.
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.
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.
About the Author
Maxim Semenenko is the NX CAD method professional working for a Siemens software reseller.
Along with Alexander Popkov we run a blog dedicated to the medium and advanced NX CAD/CAM tips.