When talking about how to create a models initial sketch within Inventor, my old coworkers and I used to always say “It’s all just circles and squares.” While this isn’t always necessarily true, it does guide you to a simplistic way of thinking when starting a sketch for a new CAD model. Limiting the features to only those required to define the outer extents of your finished part is a best practice method for creating any new CAD model.
Within any 3D CAD application, there is always a right way, a wrong way and a big gray area on how to model a part. A good part starts with a good base sketch. If you don’t get the first sketch right, your model integrity will suffer as you add and subtract subsequent features. Some of the points I want to share may be subjective, so I would love to hear feedback and comments if you agree or disagree. Here’s an example of a good gray area. I watched this sketching video posted by Autodesk and really didn’t like a few things they are showing. Not because it’s wrong… but it’s not completely right either. You’ll see later in this post what I mean by this.
Thanks to Luis José Andueza (@ljandueza), Paul Munford (@CadSetterOut), Francisco García Bayarri (@cadmech3d), Scott Moyse (@ScottMoyse from designandmotion.net) and Chris Benner (@CGBenner) for being my guinea pigs in a little experiment in the modeling gray area I mentiond earlier. I asked them to all model the same part without giving them too much direction other than to use Inventor and the part units were millimeters. This is the image I posted for them to model, a simple bearing mount:
From a quick glance at the results, most of the parts look the same which is good. As I suspected, some of them had their own style flare and applied realistic colors. This isn’t a requirement for modeling but can make your time using Inventor more enjoyable and your models look more realistic. I’m thankful nobody used the shiny chrome finish. I’m not sure why but it seems like most new users to Inventor are fascinated with making every part as shiny as possible. I also found it interesting that 3 of the 5 models were modeled with the Y axis being the vertical and 2 with the Z axis being vertical. This may seem like an insignificant difference but can sometimes have detrimental effects when exporting your parts to other software applications.
When we start digging deeper into the models and examining the model trees, this is where the real differences start to show. While all the parts started with the base, and generally followed the same steps to arrive at the finished model, there are some definitely unique characteristics of each.
You can see basically how the models were built by clicking through the images below:
Comparing the model trees is also interesting. You can see big differences in how it was modeled by the use of shared sketches, how the features were named and how many steps it took to create the geometry.
Most of the differences in modeling technique you’ll see between different designers at different companies without the same set of standards will still be negligible. Like I mentioned before, there is a right way, a wrong way and just a big gray area on how to model a part properly. The basic principles that I follow when modeling a part are:
Is the part accurately modeled to the design requirements?
This point is just making sure you added the right size holes, the fillets in the right spot and generally designed the part as it should be designed.
Does the model use the key dimensional values required to drive the design?
You should only use the values available or key to your design. Using funny math to create the geometry which may still create the model correctly makes it hard for someone to pickup your design later and figure out how you modeled it.
Will my modeling technique allow for changes later?
You all know what I mean here. Deleting a feature should rarely make your model explode. Model your part so that you can add, modify and delete features as your design matures.
Can my model be used in an assembly file and function as intended?
Make sure your part can be used upstream. If your part needs a center line workplane to be constrained into an assembly, make sure you have one where needed.
Now like I said, there are some gray areas but also some areas even though your finished model may look and smell correct, truly isn’t parametric or editable robust. My willing helpers also graciously agreed to share their identities in this post. Don’t take these next statements as criticism since every designer has different intents on how their models may be used later. These will possibly just help my readers think about what may be important for their own models.
Scott’s model was interesting in the fact that he modeled the base with the hole features already in the sketch. When he extruded the base, the holes were extruded as well. I’m not sure why he didn’t add the holes as hole features later. What if we needed to change these to counter bored holes later? My advice, always add holes as hole features that allow you to design in the intelligence of the software’s hole command.
Franciso modeled the same holes using the hole command, but inserted them each separately. Not right, not wrong, just different. If you wanted to change the holes to a counter bore later, you would need to do it twice.
Chris modeled the bearing hole using geometry from a shared sketch versus using the hole command on a concentric face.
Paul also modeled the bearing hole using sketch geometry and extruding.
All of the models did follow the good principle of applying the fillets last. Changing or deleting fillets will have no impact on any geometry created prior to the fillet using this method. There is nothing more annoying than deleting a fillet and having to fix some other geometry as as result.
With all this being said, developing a culture of accountability as well as pride in ones work for your CAD models is important if you plan on reusing your designs or building out your CAD libraries. There are several whitepapers and classes (such as this one) and YouTube videos out there that can help you understand some of the best practices for creating a model. I’d like to hear from you all on how you enforce or adhere to your companies best practices for modeling or what approach you would take to the example shown above. If you would like for me to take a look at your Inventor model, you can post it to my contact form here.
In the next few posts I’ll be unpacking what all of this means by picking out some key principles to follow and shining some light on the importance of each. Thanks for reading!
Thanks again to Luis, Paul, Francisco, Scott and Chris for helping me out on this post. You guys rock!