I've got the Underdefined Blues

Sometimes, in my efforts to learn SolidWorks, I feel a bit like a child who is eager to play baseball for the very first time. The child doesn’t care much about spending time learning to run the bases, swing the bat or how to figure out whether the ball is inside the strike zone or not. The child only wants to skip to the fun part of the game, the part where he or she hits the ball out of the field and everybody runs home. Well, something similar happened to me when working with sketches versus solid features.

Working with sketches is a new concept for me. In AutoCAD, I was used to creating 2D drawings by placing lines, circles, and any other entity, in the exact size and location where I wanted them to be, right from the beginning. In SolidWorks, however, I simply place the entities first and then define the sketch by applying dimensions and relations. In other words, I’m sketching with parametrics. This is really a powerful concept that took me a while to understand and that still confuses me at times. It is powerful because the sketches are at the core of the most common features that will be used to create the model, because we can change them according to our needs (by the use of equations and design tables, for instance), and because, if properly defined, they should maintain their relationships through those changes. This all sounds really great, but that’s precisely where I stumbled.

While working through the first tutorials and trying to come up with my own models copied from pictures in books or inspired by simple objects around me, I quickly learned to avoid the overdefined sketches, dangling relations and unsolvable geometries, but what about the underdefined sketch? Since most of the time, SolidWorks will allow you to create a model using an underdefined sketch, I didn’t think it was that important. It was until later, while working with configurations and design tables that  I had one of those “Aha!” moments, and it finally dawned on me that it does make a big difference whether you fully define a sketch or not. As an example, take a look at my underdefined model from one of the tutorials.

This is a very simple part with three sketches. Sketch 2, the underdefined sketch, is a circle that is supposed to be placed in the center of one of the faces of the Box. However, the position of Sketch 2 is not fully defined. It appears to be placed right where it should be for this particular instance, but watch what happened when I tried to create different configurations of the same part by means of a design table.

Yikes! What happened here? Why would it fail to create a fillet? Now look at this other configuration.

   

Why do you think this happened? Well, let’s take a look at the design table.

As you can see, the design table was supposed to create several configurations by changing the dimensions of the rectangle (Sketch 1) used to make the Box, as well as the diameter of the circle (Sketch 2) used to create the Knob, then, it was supposed to apply fillets to the outside corners or the Box. However, since Sketch 2 was underdefined, it failed to maintain the relationships with the other entities in the model and even caused the fillet on the second configuration to fail. In other words, it didn’t always end up in the center of the Box’s front face when the dimensions were modified, and that alone affected the whole part.

This example may not seem like a big deal because there aren’t many sketches involved and it’s easy to spot and correct the error. Now think about a similar situation, only this time picture a very complicated part where several of the sketches involved in creating features are left underdefined. Not the same, huh?

Right after this, I vowed to always do my best in order to fully define a sketch before using it to create a model. But I found out this is easier said than done, especially when repairing those sketches already created. Many times, after applying all the dimensions and relations I could possibly think of, there were still a few blue entities left. Even worse, at times, trying to fully define my sketch would actually make it overdefined! Relations are not easy for me.

A few times, I used the Fully Defined Sketch tool, and was actually successful. Other times even that wouldn’t work, the tool would display a message saying that my sketch was still underdefined, and would suggest a few ways to fully define it. Although I continue to struggle, I’ve noticed I have better chances to achieve my goal of correctly applying dimensions and relations for a fully defined sketch when I keep the Automatic Relations option on all the time, and when I think carefully about the model and how I want it to react to changes I (or others) may decide to make in the future. I wonder if I’m the only one who has a hard time with this!

Comments

[Amir Eshaghian]

Solidworks is a very userfriend software,when I start to work with this software ,I found a software with speed and strong in designs.

[Gabi Jack]

Yes, I agree with you. SolidWorks is a very powerful tool and as friendly as any other Windows based software out there. It's worth investing the time and effort to learn to use it properly.

[Anne Yust]

Yes, fully defining a sketch can at times be challenging. I thought I would only need the length, diameter, and distance from the origin, but it's not always that simple. I use symmetric relations often.

[Alfredo Arcela]

I don't think so, all this 3D software work rigidly, why don't you test other excellent 3D software and flexible like IronCAD, only is an advice you can choose work with or without constraints.... maybe you can do a mixture
regards.

[Gabi Jack]

Well, Alfredo, thanks for the advice. I'm sure IronCAD is a good choice for some, but one of the reasons why I chose SolidWorks is simply because it's the tool of choice for many potential employers in my area. SolidWorks may seem very rigid, but it's also extremely powerful. Have you tried it?

[Alfredo Arcela]

Actually yeah.

I know it is powerful, I work with it too, Here in Peru, Companies prefer A. Inventor,but I can work with many systems and resolve many problems with IronCAD, it depends your Knowhow, it's your free choice, obviously I work with others, if I can help you only mail me, maybe We can get a solution. and test Ironcad... you can download for 30 days ... fully functionall

Best Regards

aarcela@saeg.com

[Joanne Duffin]

I was so glad to run across your blog. I also am a beginner with SolidWorks. I have 27 years of CAD experience with Computervision and NX. Now to learn another CAD system can be very daunting. I am so glad to find someone that can show and then explain why things happen the way they do with us beginners. Keep your great line of communication going and we will all one day be experienced again......Joanne