Flexes, Hexes and Vexes

As the week begins, I feel proud to say that I’ve finally completed one more of the SW training manuals, the Advanced Parts Modeling one. Each day, I’m getting closer and closer to finally feeling prepared enough to try my luck with the CSWP test, but most importantly, I’m learning  tons of new stuff about SolidWorks, getting some practice, and having lots of fun along the way.

Among the exercises in the very last chapter of the manual, there was one that I struggled with for quite a while. The part was a model of a box, probably made out of Styrofoam or plastic, like the ones they provide at restaurants for you to carry leftovers or take out.  The box had a living hinge and the exercise consisted of applying the Flex command to bend the box into a closed position. I tried following the instructions in the manual, with exactly the same numbers and conditions, but my Flex feature kept failing. Frustrated, I decided to  check out the built part, the file that is included as a solution for this exercise, to see how the Flex feature was done there and make sure I wasn’t  making any mistakes.  Well,  I opened the part, edited the Flex feature, and couldn’t find anything different from what I had been doing so far, so I clicked OK and then it happened:  the Flex feature failed for the solution part,  as well! How come? It worked fine when I first opened the file!  It had to work for whoever modeled that part. How did they make it work in the first place? I reloaded the file and it seemed fine again, but then it occurred to me to force a rebuild by pressing Ctrl+ Q, and, just as I thought, the Flex feature failed once more. It was then that I remembered I had the option “verification on rebuild” turned on.

Verification on Rebuild is an option that controls the level of error checking when you create or modify features. By default, without using this option, every time you add or modify a feature, SolidWorks will check it against any adjacent edges or faces. With the option turned on, the features will be checked against all faces and edges. If a feature causes any invalid faces or edges, it will fail when this option is turned on.

I cleared the option and rebuilt the model. The Flex feature was successful this time and the model looked OK, but when I checked the model geometry (Tools, Check), it turned out the Flex feature had created three invalid faces.  That’s why it failed when the option was turned on! I wondered for a long time what this could mean for the model. Did it mean that the design needed to be modified? That a box manufactured based on this model wouldn’t work properly?  I asked Matt Lombard about it, since he’s got plenty more experience than I do, and he had mentioned before that he wrote part of this manual. He told me Flex  features are a bit flaky and can easily create problems like that one,  that is better not to use them with real geometry, but only for “looks like” stuff, like when you want to illustrate how a part would look like in a different position. I think I’m  beginning to understand why.

As an exercise for myself, I modeled  a small plastic bottle cap. This bottle cap is attached to a ring that goes around the neck of the bottle, to help keep track of the bottle cap.

Just as it was done in the manual, I applied the  Flex  command to bend it into a “closed” position, to simulate how it would look like.  So, I clicked Flex, from the Features toolbar and got the Flex command  dialog box and, in the graphics area, a couple of trim planes and a triad. The triad is a coordinate system that that sets the center of the flex and the orientation of the trim planes. The bending takes place about the Bend Axis (the red axis in the image) and in between the Trim Planes. The Trim Planes are usually created at the extents of the part, but can be moved to a different location. The triad and trim planes  would assist me in the process of bending the part, but first I needed to reposition them. For this purpose, I had created a centerline sketch, right in the middle of the part. Repositioning the triad can be done through the dialog box or by right clicking in its center and choosing to either move it  or align it with selected geometry in the part.  I moved the triad so it would be located right in between the ring and the cap, and also so that the Bend Axis would be aligned with my centerline sketch. I also repositioned the Trim Planes using the dialog box to select a couple of vertices in the part to reference them.

Here is the way it looks like from the front.

Once I had that figured out, the next step was to enter the angle for bending. Notice how it bends about the red  Bending Axis.

And this is the result. The lid in a closed position. The angle here is 180°.

Now, while creating this part, I had the “Verification on Rebuild” option cleared out. So, I decided to check the model geometry for any invalid faces and edges and , yes, there were three invalid faces that were created right after using the Flex feature.

After playing with the options for a bit, I noticed that the problem disappeared if I suppressed the fillets, so I reordered the features in the part and placed the second set of fillets right after the flex feature. Then,  I checked the model geometry again (Tools, Check). This time, there were no invalid or even inconsistent faces or edges in the model geometry.

So, one more reason to leave fillets for the very end? Well, yes, it would seem like it, but for me is more a reason to be careful when using the Flex feature, and to follow Matt’s advice, and use it for “looks like” stuff, rather than design.  I’m still curious to know what could keep the Flex feature in the box from failing. Matt promised to take a look at it as soon as he has some time. I’ll be waiting!

Comments

[Richard Williams]

Thanks Gabi, it was very infomative. I haven't had any problems with that so far mainly because your so far ahead of me but at least now when it doesn't work, I will have an idea about why. So I will write to you. Great tips.

[Gabi]

Thanks, Richard! I'm still wondering what could make the exercise from the manual work as it is. So far, I managed to get rid of the invalid faces after the flex by changing the dimensions of the living hinge slightly... but that's altering the design. This is one of those times when you really wish you were taking the class at the VAR and had a teacher to ask questions to. On the other hand, I wonder how many of those that take the class have actually noticed this.

[Rajesh]

while twisting is it possible to hold one face i.e,one face to be flat and another one to be twisted.

[Gabi]

Thanks for the comment, Rajesh! I'll have to check it out again...