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Hack Chat Transcript, Part 1

A event log for Compliant Mechanisms Hack Chat

A little flexibility never hurt anyone

Dan MaloneyDan Maloney 01/26/2022 at 21:270 Comments

Dan Maloney12:00 PM
OK, let's kick it off. Welcome to the Hack Chat, I'm Dan, Dusan and I will be moderating today as we welcome Amy Qian to the chat. We're going to talk all about compliant mechanisms and flexures, and I assume a little about bendy straws too ;-)

Amy Qian12:00 PM
woo!

Dan Maloney12:00 PM
Amy, what is it about these mechanism that you find interesting?

Amy Qian12:01 PM
Aside from being very satisfying when they work, I think they are pretty easy to make, sometimes more so that "traditional" mechanisms

KnownAardvark joined  the room.12:01 PM

Amy Qian12:02 PM
they aren't appropriate for all cases when you want a mechanism. But there are lots of cases where they shine.

pranshu_c12:02 PM
Hi @Amy Qian ,

pranshu_c12:02 PM
How do you start to ideate for mechanisms ?

Scott Bezek joined  the room.12:03 PM

Chris Thompson joined  the room.12:03 PM

Chris Thompson12:03 PM
HellO!

Dan Maloney12:03 PM
I guess I got ahead of myself there. I should have started off with the "tell us a little about yourself" thing

Amy Qian12:03 PM
oh hmm, there's usually some day dreaming and doodling involved.

Amy Qian12:03 PM
haha, sure thing

Evilamblonyx joined  the room.12:04 PM

Evilamblonyx12:04 PM
Hi everybody

ciarnoneill joined  the room.12:04 PM

tca joined  the room.12:05 PM

Amy Qian12:05 PM
I am a mechanical engineer by training and mostly by profession, too. I like to tinker a lot (like lots of folks here). Mostly mechanical, although slowly crossing into more electrical things. A lot of my projects are shop improvements, shop tools, furniture, random things around the house, stuff for my car which is a small camper van.

Evilamblonyx12:08 PM
When I design compliant mechanisms, I'm usually trapped betrween "Ok, do it right, calculate everything and make a proper simulation" and "Just draw something, print it, and it might work without thinking to much about it".

So what is your way? Do you make proper engineering in this cases, or more try and error?

Chris Thompson12:09 PM
related: How do you prototype?

Dan Maloney12:10 PM
Was there anything in your ME training that covered compliant mechanisms? IOW, was there a "Compliant Mechanisms 201" course?

Amy Qian12:11 PM
I'm definitely Team Trial and Error. I'm almost never designing mechanisms that operate on the edge of material properties, so it's pretty low risk. Also, most of my flexures are laser cut, so that is pretty quick to iterate

Ethan Waldo12:11 PM
My all-time favorite compliant mechanism https://openflexure.org/ that achieves sub-micron scale XYZ stages.

https://openflexure.gitlab.io/microscope-handbook/learn-about-the-project/publications.html might give insight in to more scientific process.

Chris Thompson12:11 PM
do you use polypropelene?

Amy Qian12:12 PM
The heart of compliant mechanisms is beam bending! Which is a heavily covered topic in most mechanical engineering programs (think stress-strain curves)

Amy Qian12:13 PM
As for how to turn bending beams into mechanisms, there's lots of great examples out there to build off of. But also, sky's the limit, let your creative juices flow.

samy kamkar12:14 PM
@Amy Qian thanks for sharing and your youtube channel, liked some vids and subbed! do you have any suggestions of resources that break down the different effects/areas necessary to design flexure mechanisms? for example, if I had an ideas like yours, "I want a 7-option infinite rotation device [wheel], only moves one way [ratchet and pawl], two binary switches, and the switches to reset upon a single rotation of the device", how would i determine the different components and design, let alone thicknesses based on material type, and durability/determining minimum number of times each piece can be flexed?

Amy Qian12:15 PM
I mostly prototype in Delrin or Acetal (both really similar, I think Delrin is a specific type of Acetal) plastic. It laser cuts really well and is not brittle like acrylic. Polypropelene is really melty on the laser cutter, hard to get good edges.

Amy Qian12:16 PM
Also 3D printing in ABS is great too. I've really only used ABS and PLA. PLA gets too brittle over time, but fine for iterating on designs.

Florian Festi12:16 PM
Did you have any luck with mechanisms laser cut from ply wood?

Amy Qian12:19 PM
not really, but that's not to say that wood can't be used for flexures. You just have to respect the grain direction of the wood. Maybe that's an added design constraint, or maybe in some cases a feature, depends on your application. A wooden bow (and arrow) is an example where wood is being used for its compliant properties.

Amy Qian12:20 PM
Plywood is usually made from layers of wood where the grain of alternating layers is perpendicular. This has lots of great benefits. But when it comes to making a compliant member, it likely means at least half your wood fibers are not oriented in an optimal direction.

Evilamblonyx12:21 PM
Do you know this book: https://onlinelibrary.wiley.com/doi/book/10.1002/9781118516485

I use it quite a lot. Do you have other literature on this topic you find usefull?

Florian Festi12:23 PM
The issue I had with ply wood was creep where permanently loaded parts would give over time and loose their force and/or form. Did you run into similar issues with plastics?

Amy Qian12:23 PM
@samy kamkar hi! Here is MIT's OCW on the course that goes into beam bending, stress/strain, fatigue, etc. https://dspace.mit.edu/handle/1721.1/39140 But I realize that courses like this may also contain lots of other info that aren't directly helpful to your design challenge. It is actually on my list to make some kind of fundamentals-of-flexure-design video!

Scott Bezek12:24 PM
Do you think it'd be feasible to make a compliant mechanism from a PCB (FR4)? They're so cheap to get manufactured these days that it seems like it could be an amazing option for those of us without access to a laser cutter (if it works). I recently came across http://www.cy384.com/blog/flex-pcbs.html which has some awesome examples of non-repeated FR4 bending out of the plane, but haven't seen anything on repeated bending or bending in the plane (maybe because it doesn't work well?)

Guru-san12:24 PM
Hello Amy! thanks for being here! How do you determine how many times a mechanism can bend / longevity and how brittle over time it will become? Also have you ever tried anything in SLA printing? It's tough with standard resin that doesn't flex much but even then it works surprisingly well!

Spehro12:24 PM
Have you tried making mechanisms that deform in two directions (for example to make an alternating push-push function?) Have enjoyed your videos, thanks for providing them!

Amy Qian12:25 PM
@Evilamblonyx yes! I have that book! it is great. Prof Larry Howell (one of the authors) heads a university research group I think. Their website is also full of tons of useful resources and fun videos

Peter Wright12:27 PM
Hi Amy, do you have any tips for scaling up or down the size of compliant mechanisms? Do you always prototype at the same scale as your intended designs or start with another size that makes it easier to iterate?

samy kamkar12:28 PM
thanks!

Chris Thompson12:29 PM

Amy Qian12:29 PM
@Scott Bezek I am SUPER excited to explore flexures in PCBs. I actually just kicked off my first PCB ever last week, ridiculously simple, but we all gotta start somewhere. I would imagine that FR4 should be for lots of cycles, and that in-plane cycling is possible if you cut a narrow enough feature for bending. But I imagine the copper traces don't love being stretched and compressed a lot? Again, it's all about how much you want to stretch/compress stuff balanced with how many times you want to cycle.

Chris Thompson12:29 PM
I would love critique of this bistable compliant clamp

Thomas Shaddack12:34 PM
Silicon is pretty interesting for its bending properties. Extreme fatigue life. Very common in MEMS devices.

Amy Qian12:35 PM
Saw a couple questions about the longevity of a flexure - "fatigue limit" is a good search term for that. https://en.wikipedia.org/wiki/Fatigue_limit This is often mostly applied to metals though, because plastics make terrible springs for a few reasons. Even on the first graph of this link you can see that steel has an endurance limit where below a certain stress level, infinite cycles are possible. That's why you see lots of springs made from steel and none (good ones at least) from aluminum

Ethan Waldo12:36 PM
Tell that to the plastic slinky manufacturers

Chris Thompson12:37 PM
@Ethan Waldo the new plastic ones are so durable though! My kids haven't snapped one yet

Thomas Shaddack12:37 PM
Thought. Hybrid manufacturing. 3d-print rigid parts, with "pockets". Cast silicone rubber into the pockets. Cut off the pockets' lining.

Amy Qian12:37 PM
I apologize for the audio on this video, but here's the talk I gave at Hackaday Supercon in 2019 about flexures. Couple fun links in the description

Ethan Waldo12:38 PM
@Chris Thompson Need a link because the plastic ones I've seen work terribly and my kids bent the metal ones out of commission soon after getting them.

Amy Qian12:39 PM
@Peter Wright I usually iterate in my intended scale because when you bend a beam, the peak stress in the material, force to deflect, and beam thickness are all coupled. But I will sometimes just focus on a chunk of the mechanism

Chris Thompson12:39 PM
@Ethan Waldo I fell like we got a jumbo one 8" dia. down the shore. NJ

Ethan Waldo12:40 PM
That really narrows it down :P

Amy Qian12:41 PM
not sure what critique I can really offer for that bistable mechanism @Chris Thompson. I want to click it though. click click click

Peter Wright12:41 PM
Thanks Amy!

Chris Thompson12:42 PM
@Amy Qian Thanks. It's fun to snap it closed around the right diameter. this was made for 50mm. I was thinking of replacing toggle clamps in welding jigs.

Ethan Waldo12:43 PM
Nice idea. You can never have enough clamps and being able to 3d print them is boss.

Evilamblonyx12:44 PM
@Chris Thompson you want to clamp something like a tube with the round part on the right, correct?

Is your mechanism somehow not working as intendet? If I should guess, I would think that it has not as much holding force as you would like it to have, or is something else wrong?

Amy Qian12:44 PM
ooh very cool. Post a video somewhere, will watch. I think mechanisms like that are a great example of how compliant mechanisms can be a better solution that traditional mechanisms (rotating pins, etc). In this case you need a limited travel and you can get it out of just one piece!

Peter Wright12:44 PM
@Chris Thompson cool mechanism. The bending elements on the left might be a little too thick? Looks like they cracked a bit at the base

Amy Qian12:45 PM
oh ya, good catch @Peter Wright just realized I could make the image bigger.

Amy Qian12:46 PM
For mechanisms like this, it might be fun to try printing the non-bendy parts and then using a length of spring steel wire to form all the bendy bits. then you are less constrained by what will 3D print well

Chris Thompson12:47 PM
Thanks, the material is PETG and has shown it's limited "fatigue limit" Here it is snapped around a 500 bottle.

@Evilamblonyx I can increase the clamping force by closing the diameter, or thickening the whole flexure.

Chris Thompson12:47 PM

Chris Thompson12:48 PM
Spring steel inserts would be a great idea. I'll dig into Mcmaster Carr

Amy Qian12:49 PM
Maybe you can also play with just how much travel you actually need on the jaw. If the parts you are clamping are super consistent, maybe that can save all your flexures a lot of travel.

Amy Qian12:49 PM
Also might help you up your clamping force.

Chris Thompson12:49 PM
ahh

Evilamblonyx12:50 PM
@Chris Thompson I have made realy good experiences with PA (Nylon) for 3D Printed compliant mechanisms. But PP works also really well, but has a much lower tensile strength.

PETG is in my experience not the best choise for this job

Florian Festi12:50 PM
May be a band of spring steel like from a constant force spring or a tape measure is better than a wire. You could roll up the ends and slip it into a hole with a slit.

Nicolas Tremblay12:54 PM
How would you make sure it didn't move around or deform the plastic?

Amy Qian12:55 PM
tape measure is a great idea, you could even drill holes in it to constrain it to the plastic. And it's wider so distributing a forces against the plastic more evenly.

Thomas Shaddack12:57 PM
Another thought. Print from elastomer, and then put in rigid inserts, or in-situ cast epoxy into prepared voids.

Amy Qian12:57 PM
hmm, I would need to do some actual prototyping to work it out, but maybe you actually want the compression forces going through all the plastic chunks, and the spring steel or tape measure is just for keeping everything the same distance apart.

Florian Festi12:57 PM
I'd try to basically roll the ends of the strip into a spring pin and press that into a hole.

Thomas Shaddack12:59 PM
Piano wires. Another springy material.

Dan Maloney12:59 PM
Dan Gelbart has a good video on flexures, and he mixes nitinol springs with sheet metal frames:

Dan Maloney12:59 PM

https://www.youtube.com/watch?v=PaypcVFPs48

YOUTUBE DAN GELBART

Dan Maloney12:59 PM
He's on a different level than the average hobbyist, but still a lot of good tips

Evilamblonyx1:00 PM
@Dan Maloney I can highly recommend this video, in fact all of his videos are fantastic.

Dan Maloney1:00 PM
+1 on that!

Amy Qian1:01 PM
so great!

Dan Maloney1:02 PM
And now I see that our time is about up. Everyone's welcome to stay and continue the chat, but for now we'll just say our official "Thanks!" to Amy for dropping by today. Good chat, I really enjoyed it. And thanks to everyone for participating -- good questions!

Chris Thompson1:03 PM
Thank you!

Dusan Petrovic1:03 PM
Thanks Amy!

Amy Qian1:03 PM
Thanks, Dan, Dusan, and team for organizing!

Nicolas Tremblay1:03 PM
Thanks @Amy Qian

Evilamblonyx1:03 PM
I designed a 3D printed button (for the prusa contest) with a compliant mechanism ratchet, that could change its color by turning it. Is just a toy to fiddle with, but it turned our really nice.

Would you like to see it and is it appropriate to poste the link here?

Evilamblonyx1:04 PM
Thank you so much @Amy Qian

Amy Qian1:04 PM
I wanna see it, post the link @Evilamblonyx !

Evilamblonyx1:05 PM
@Amy Qian

https://www.prusaprinters.org/prints/124195-color-click-button

Florian Festi1:05 PM
We clearly should do a show your compliant mechanism section. May be after all other questions are done.

Dan Maloney1:06 PM
Sounds good, go ahead. I'll wait a bit to pull the transcript

controlmypad1:07 PM
Thanks @Amy Qian

Florian Festi1:10 PM
Ok, I have some latches at the bottom half of https://hackaday.io/project/10649/log/177879-new-boxes-for-your-electronics-projects

They work reasonably well even made out of plywood.

Amy Qian1:13 PM
looks cool, @Florian Festi ! I imagine if they don't bend a lot and you don't bend them too often, they will work really well

Florian Festi1:14 PM
Yeah, they are basically just for a hand full of uses during assembling.

Florian Festi1:16 PM
I got this conventional hinge and a compliant one layer latch as a feature request but have not done one myself yet: https://www.jarkman.co.uk/catalog/cnc/hotwirebox.htm

Thomas Shaddack1:17 PM
Snap-in joints on a lasercut thingy. https://www.improwis.com/projects/hw_TestTubeRack/

Florian Festi1:17 PM
The latch is super neat IMHO.

maht1:19 PM
I came late to the Hack Chat so I will read later. I just want to thank

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