CAD Practice: Mini Tripod

It's been awhile since I've done some real CAD work, so I decided to model a mini tripod that I regularly use. The legs have interesting geometry with smooth contours that were difficult for me to capture in CAD. 

Orthographic side view.

I still am not sure what is the best way to model these smooth contours; I initially started off by drawing a top-down view of the leg, thinking I would then extrude this profile and then cut it from the side with another profile like I did for the glasses I previously modeled. I quickly realized this would not work that well since the legs don't have parallel sides.

I backtracked and then decided this would have to be done with lofts. Using the top-down sketch as a guide, I created several cross-section profiles along the leg for the loft to follow and then created the leg out of two lofts. The modeled leg is definitely not perfectly accurate to the real leg as the real one has a smoother curve along the entire outer face and pinches in faster on the inner face. If I were to do this again I would use 3D curves and guide each edge with a rail. Also, at the hinge the curve should be more gradual and not bend in as quickly.

The bevel at the bottom of the leg is also too steep, but this was necessary for the bevel to line up with the tip of the leg and the outer edges. As a result, with the legs all the way out the bottom of the legs are still not parallel with the ground.

A video of the modelling process:

And a render:

Nice render with the tripod open.

CAD Practice: Stress/Modal Simulation

This weekend I wanted to explore the simulation options within Fusion 360 since I had never used them before. I decided to take the layout from a previous class HW question since I already had the model in Fusion.

Folded-flexure comb drive resonator.

Above is a folded-flexure comb drive MEMS resonator, although modeled in Fusion in millimeters instead of microns. It's a very well studied design, where the resonance frequency is mainly determined by the shuttle mass and the spring constants of the folded-flexure. I though this would be a good test since I know the equations for the spring constants of these beams and have done the analysis for these devices before.

How these devices normally work is that the center moving mass is set to a certain DC voltage, and then an AC signal is applied to one of the side combs. When the AC signal's frequency matches the resonance frequency of the center mass, the center mass will begin to oscillate significantly and the teeth in the combs will slide past each other, causing a change in capacitance at the other side (the output port). The change in capacitance will induce a current at the output comb that can be sensed, and it will have maximum amplitude at the resonant frequency.

CAD Practice: Pencil Cup

Continuing with last weekend's practice, I modeled a slightly easier object for today: a pencil holder cup. 

Orthographic 3/4 view.

Orthographic side view.

It's effectively a cylinder so my main method of building this was with revolved profiles and circular patterns. The top and bottom lips were created as 2D profiles at the right radius away, and then revolved about the vertical axis. Nothing too special for these.

CAD Practice: Glasses

It's been awhile since I've done any real personal design work, especially CAD work. I've never been that great at using CAD and have looked into "optimal" practices when drawing something up in the computer on and off over the years. Some examples of questions I've had are like what order should different bodies be created; when should we fillet on the sketch first or after the extrusion on the edge, how to create effective "dynamic" designs that change with parameters, the best way of creating either movable or rigid joints between objects, good flows for large multi-part assemblies, etc. There are always more questions the more I see other designs online and the more I personally design.

I've definitely picked up tips along the way like using boolean intersection and cuts more often (e.g. for making tabs), using a dog-bone plugin for modifying sharp internal corners for CNCing, and other more general workflow optimizations. But in the end, just CAD'ing more is the best practice for being faster and identifying poor workflow; if possible, manufacturing the design after CAD is even better as you quickly learn what is and isn't possible to make with your tooling (my time with my crappy CNC has taught me a ton).

But back to just CAD'ing more; I had some free time recently and decided I should try to CAD my glasses for some practice. Using only a simple ruler for some basic measurements (so I was designing near the right order of magnitude) and the judicious freedom of my eyeballs, I whipped up this model:

Orthographic 3/4 view.

Perspective front view.

Perspective top view.

 

I modeled half of the glasses and then mirrored everything at the end for the other half. I started off with the lenses. I created them by creating a spline for the front facing outline of the lens, then created a series of arcs and lines for the top down outline of the lens. I extruded both profiles, and used a boolean intersection to get the final lens. For the groove around the edge of the lens that holds the wire holding the lens to the frame, I sketched a groove into one side of the lens, then lofted the profile as a cut all the way around.