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.

Comet C/2020 Astrophotography

Last weekend I went out to take some photos of the comet C/2020 F3 NEOWISE, which is has been passing by for the last several days and will still be in sight for a few more days. I wasn't able to go too far to get to a dark place so the light pollution was pretty bad. Not the most detail in the photos but some stacking and light pollution gradient removal worked pretty well.

I haven't done astrophotography for awhile so I was a little rusty, but still got some nice shots:

135mm, f/5.6, 2 seconds, 80 lights stacked in DSS, gradient removed in PS.

50mm, f/1.8, ISO 800, 4 seconds, 55 lights stacked in DSS, gradient removed in PS.

Panorama.

CNC Lithophanes

I tested out making some lithophanes on the CNC about a month ago; only got around to posting this now since I wanted to do some more iteration on my first tests but never really got around to it.

I just had some scrap 1/4" HDPE squares lying around from awhile back that looked suitable for making the lithophanes out of. The source photo was converted to black and white and the toolpath was generated with a trial of MeshCAM. White was mapped to a depth of 0.22 inches and black was mapped to 0.

First roughing was done with a 1/8" square endmill (feedrate of ~50 IPM, sped up in LinuxCNC so I don't know the exact speed; depth of cut of 0.05 in?, don't remember exactly; stepover probably 90% of tool diameter) and the finishing pass was done with a 1/8" ball end mill (don't remember the feedrate/stepover on this one). I felt the level of detail wasn't enough so I bought a 1/16" ball endmill for my next one.

After roughing.

There seems to be some sort of bug with the white region at the top of the photo where the material wasn't removed at all.

After finish pass.

Not bad except for the sky.

For my second attempt, I clipped the white levels to just under pure white which seems to have fixed the problem with the top of the photo from before:

Some strange discontinuities.

For this one, I had to stop after the roughing operation since my Y axis lost steps somewhere in the middle so I never got to use the 1/16" ball end mill.

HDPE isn't ideal since it leaves behind many little strings after cutting which means the picture isn't very clean. I looked online and saw that many others cut out their lithophanes from Corian, which I may want to try next. Another issue is that cutting a lithophane out takes a long time and has many small movements, which makes the stepper motors very hot.

CNC: Replacement Parts and Aluminum

Over the last week I made a few parts on my CNC, one of which was made out of aluminum. The first part was a replacement part for a VR headset a friend gave to me. I accidentally broke one of the 3D printed adjustment racks and had to make a new one.

I made it out of HDPE, which is just wonderful to machine, due to its flexibility. I surfaced the material to thickness and cut out most of the profile with a 1/8" endmill. I used a cheap 1/16" endmill to get between the rack teeth:

Surfaced and most of the internal surfaces cut out.

Completely cut out.

Replacement on the left.

The next part I made was out of aluminum. This was a bearing block designed to replace the original wooden ones I made not too long ago. I didn't use any oil/coolant and it went fairly smoothly. Next time I probably would get rid of the lead in and lead out on the 2D contours and just use ramps. The plunge between each depth didn't sound great.

I used a 2 flute, 1/4" endmill for the whole process:

Sharp.

Cut out.

Reasonable surface finish.

 After cutting, the part was square as far as I could tell and had reasonable dimensional accuracy. The CAD model was 2.953" by 4.035", and part came out to be 2.960" by 4.034".

Square.

The hole cut out for the bearing was supposed to be 1.126" in diameter, but when I measured it it ranged from about 1.123 to 1.125. Had I taken a finishing pass I probably could have gotten closer and the bearing would have fit in. I had to sand the inside of the cutout with a Dremel to be able to get the bearing to fit.

Installed.

Bench Power Supply

Yesterday I put together a simple wood box for a DIY bench-top power supply. I had the electronics for awhile now (I forgot when I bought them, but it was probably around a year ago) but just never put them all together. I bought a used 24V, 13A power supply and a DPS5005 front panel/regulator.

I put together a quick CAD model of the box using slot construction and 1/8" wood:

The back part would be on the ground so the front is tilted up.

However, I realized after making this model I probably didn't have enough 1/8" plywood to make all the faces. I redid the design to have panels slot into thicker material and came up with this:

3/4" wood for everything but the front and back panels.

I flattened everything out and just ran contour toolpaths on everything. I had to break up the parts into separate programs because I wanted to use the edges of wood scraps where there was only enough room for some of the parts.

It took awhile to cut everything because I had to change stock between nearly every part. After cutting it all out I screwed the box together:

Front panel cut out.

Bottom plywood is a little scuffed.

Back panel.

I didn't realize how thick the banana plugs I ordered were (I was always imagining the ones on  commercial power supplies like Keysight bench PSUs), and the little cutouts around each pair of plugs are a little small.

I have small DC-DC boost converter on the way to boost the 24V to 48V since the DPS5005 can take in up to 50V. The left most output is the output from the DPS, and middle is 24V out, and the right will be 48V out (it might actually never be useful and it could just be another 24V out).

Also: a fun picture of some more endmills I bought a few weeks ago (I just had to buy some more real brand name 1/8" endmills after seeing how good actually sharp cutters are). I have yet to try the single flute cutters on acrylic but the two flutes ones cut the wood in this project perfectly.

Sharp.