INVENTOR? STRUCTURAL MODELLING

A lot has been said about Inventor? - its weaknesses and strengths, it has its die-hard fans and its critics who swear by anything else. I have been working in the mining and minerals process industry and since 2012 mainly in the bulk materials handling discipline. Used to regard myself as a piping designer, but lately found that I have been more exposed to mechanical and structural than piping modelling and draughting.

Cutting my CAD teeth on AutoCAD? in the early 1990's on version 2.3, upgrading to version 9 and then version 2000. In 2006 I was introduced to Inventor? and immediately took to using it, to the extend that I regard myself as having an above average capability using the software to do my job.

Lately I have been getting more involved and interested in developing, testing and auditing workflows in a typical South Africa drawing office, specifically when operating in the mining and process industry. I have tried, tested, rejected, accepted and studied quite a number of option in the attempt to do work faster, more efficiently, more accurately and within budget. So I have developed some workflows to achieve some of these goals. These workflows might not work for everyone, or for every drawing office, or every discipline but it works for me and my modelling / design philosophy.

I find that the only cross-pollination of ideas in our local industry happens when people move from one company to the next, and even not always then. So we tend to repeat the mistakes and the successes (typically linked to a specific individual or two) are closely guarded secrets which tend to stagnate the industry as no new ideas are introduced.

Inventor is a typical mechanical engineering parametric software package. In fact the Autodesk website describes it as:

"Inventor? CAD software provides professional-grade 3D mechanical design, documentation, and product simulation tools. Work efficiently with a powerful blend of parametric, direct, freeform, and rules-based design capabilities. - www.autodesk.com/products/inventor "

As we all know in South Africa, this software has been used in disciplines wider than that, i.e. structural and piping being some of these. And when being asked about it, most users point to the piping and structural modules and shrugs as if it should make sense to everyone, regardless of the fact that there are more discipline specific software out there like Plant 3D?, Advance Steel? and the various Revit? incarnations - and that is if you only want to stay within the Autodesk stable. So why do we still use Inventor for piping and structural modelling? Inability to change? Not motivated to change? I do not know, but I will still use Inventor? for small structural projects as I am more comfortable in it, but would switch to Advance Steel? when the project gets bigger, or if we are going to detail all the way to shop detailing level.

Using IV to model structures.

Having used IV (Inventor?) to model various structures, I worked out a way that I found to be relatively efficient, easy to use and effective in eliminating inaccuracies. I am sharing this with no guarantees it will work for you, but feel free to comment and discuss with me, also feel free to use it or not. I am not covering any of the IV project setup points as the assumption is that the reader knows how to do this.

Starting the model.

IV Frame Generator uses a sketch as the basis to place the structural members. I assume everyone who does this on a daily basis understands this and are practiced in the concept. The traditional way is to take a structure as it would have been drawn on a 2D General Arrangement, create a single part with the sketch or sketches that represents the skeletal frame of the structure. This sketch is then used as the basis for creating the assembly by attaching the different structural steel sections to it in an assembly environment.

A typical sketch part which shows the different sketch elements that will be required for the Frame Generator assembly. Some of the sketches are projected lines (green) from a master sketch and some are original sketch lines.(blue). The dimensions have been switched of for clarity. This can be created using 2D sketches on planes (in this case) or by creating a 3D sketch which can be a more daunting task for those that are not that experienced in 3D sketching. Although one can create a 3D part with edges that can be used instead of the lines on the sketch, that is something that I tend not to use when modelling structures - its only a personal preference.

So once this sketch has been created, you are ready to start the next step. If there are extra details needed at a later stage it can be added when required.

Modelling Sub-Assemblies.

You have now created your master sketch and saved it in your project folder and are ready for the actual modelling. You start a new assembly, and insert your sketch part. And the fun starts....using Frame Generator makes it so easy to insert structural elements that you just keep going, and before you know it, you have a 3D model of your structure, all nice in a single assembly.

However, when you have to generate drawings specifically for fabrication and construction you now have to create representative views, suppress some parts of the model that should not be on the drawing, etc. etc. In short it becomes quite an exercise to produce drawings that are professionally impressive, accurate and easy to create. Efficiency goes out the backdoor when having to suppress and unsuppress, or turning visibility on and off just to doing all over again when something changes.

Through some trial and error I ended up with a method that helped me to create a model and drawings efficiently while keeping my assembly model structured and easy to use. The approach is to divide the assembly into the sub-assemblies which logically be created when fabricating the structure. So all the bits that are welded together belongs into a single sub-assembly which are then bolted together to create a larger assembly. This assumes a bolted / welded hybrid structure, for a completely welded structure one would look at which parts gets welded first and which next, using the fabrication flow to create sub-assemblies.

Using the same sketch, one then creates sub-assemblies. The sketch is placed into your assembly using the "Place Grounded at Origin" option upon placement. If you keep to this, it will ensure that all your sub-assemblies created will be based on the same origin and orientation which is important.

Save the file to start the process, making sure to use a unique identifier. Start placing your structural elements using Frame Generator. You can also add additional parts like baseplates, gussets, flanges, stiffeners, etc. which will be welded onto the structural steel elements. The secret is to keep in mind that this sub-assembly will be fabricated as a single welded assembly. At the same time, edit the Frame Generator components to the fabricated level, e.g. trim length to faces, or lengthen where required, notching or cuts to finalise this sub-assembly. Save this sub-assembly and start with the next one....and the next after that...

Having created the sub-assemblies the next step is to create the main assembly. Start a new assembly and start placing the various sub-assemblies again using the "Place Grounded at Origin" option. This ensures your parts and/or sub-assemblies fit together without having to use any constraints, and because you are using the same master sketch you will be able to set this up and eliminate clashes or mismatches in sizing.

This process can be followed to complete the whole assembly, with all the sub-assemblies detailed separately but contributing to a complete and accurate whole. Separate fabrication or assembly drawings can now be set up with very little effort, and most any future changes will only require refreshing the relevant assemblies and should have no or very little input required to correct setup.

In this image one can see a beam been modelled with two mounting stools, end plate, stiffener plates, some cleats and a gusset plate for bracing. The steel can be detailed showing the finished product with drilled holes, chamfering, notching, etc. An advantage of this approach is that one can combine the arrangement and fabrication detail to part level in one or two drawing sheets, keeping all the relevant information together.

Even the gusset plate was modelled using the master sketch as shown below, which makes placement of the part similar to the placement of the sub-assemblies without using any constraints.

So finally, this approach has the further advantage of allowing easier management of your Bill Of Material, specifically when generated within IV, as you can split the BOM into the individual sub-assemblies for fabrication purposes or combine it for procurement or engineering management output. Setting up an assembly drawing from this also becomes quite easy using the sub-assembly approach.

Will this approach make it easier to create more complex assemblies or convince designers to not use other software? No this is not the purpose of this, it is a way to model a structure when your licensing plan is based on the PDMC and you only model structures infrequently. I maintain that if you design and model complex structural assemblies as part of your daily grind, then use software more suited to that. I use Advance Steel myself when doing complex structural assemblies. But Inventor still has its place.

Please feel free to comment on this, the purpose is to learn from each other, specifically in the South African mining and industrial engineering industry - everyone else is also welcome. Also feel free to contact me directly on email to discuss this or any other subjects on 3D Modelling using Inventor and AutoCAD, Drawing Office processes and workflows, the use of BIM in an industrial and mining engineering project environment.

I can be contacted on [email protected] for any discussions or message me here on LinkedIn.