3D Printing Adventures - 111:1 Gearbox

Monday, June 22nd, 2020

Hello again! Today I'll be showing off some of my 3D designs for 3D printed gearboxes!

Why are you making Gearboxes?

Well there are a few reasons.

The first is that since I've been staying home a lot, and I've been distracting myself from the current news with building lots of projects. This is, in fact, what inspired my personal quote for 2020.

Feel free to justify you're crazy adventures with this quote :)

Along with this sudden urge to build things came the realization that I finally have some of the skills needed to build things I wish I had when I was younger. In turn I've started getting back to my roots, the childhood fascinations that led me to becoming so interested in engineering in the first place. One of these things was gears.

 *Sidenote, I set off to take a photo of the gear toys I had when I was really little but was unable to find where we kept them in the house. So have a stock image of them from Learning Resources instead. I am surprised that these things are still around and available for purchase. I'm not sponsored, but if you want your child to become as crazy as I am then here is a good place to start.

Gears Gears Gears! (That's the name of the toy if you're interested)

I'm not sure what about gears was so fascinating to me. You spin one, the next one spins, and so on. When I was younger, this was just a fun and interesting concept to think about. But now I am able to realize that you can use bigger and smaller gears to make things spin faster or slower, or with more force. I'm also able to use computer design software, something I couldn't even fathom when I was younger.

The second reason is that I need it for another project I'm working on. I bought a motor that spins way too fast for the project, and I figured why not make a gearbox to slow it down a little. (As a hint, 5000+ RPM is a bit fast for marbles). So not only am I going to build something that I'm likely going to be staring at for hours, but it will also aid me in powering something else I'll likely stare at for hours.

CAD Time

I've tried modeling gears in CAD, and it's not exactly easy if you want to do it correctly. You could just make some teeth out of a trapezoid, then use a rotational pattern to create a gear, but it isn't the best way to go about it. You have to manually do all of the math on the gear sizes, and the teeth won't mesh together well in the end.

Some quick googling led me to discover that Autodesk Fusion, my design software of choice, actually has a built in script for making spur gears (the most basic type of gear). Learning how to use it was a bit of an adventure, but once I figured out the basics, it was time to actually start designing the gearbox.

I didn't exactly set off with an exact gearing ratio I was trying to achieve, I just new I wanted to bring the final speed under 60RPM, or 1 rotation per second. This is 0.012 times the original RPM of 5000 (a totally rough estimate the package said it was somewhere between 5000 and 10000 and I don't have any tools to measure it). I absolutely could've done all of the math required, but because I wanted to get to modeling, I just kind of decided to stack gears together until I had something that worked.

Iteration 1

The first time of many during this project that I realized there's a reason I'm an electrical engineer and not a mechanical engineer was during my first iteration of the project.

Iteration 1 of my gearbox

Yeah it looks kind of fancy, and it was going to achieve a very high gear ratio, but it had many, many problems. The shafts could wiggle loose, the gears were absolutely massive, but the biggest problem I ran into was that when I went to print the frame, it failed.

Failed print

It failed because the support struts snapped in the middle of printing. Why? Because they were 5 millimeters thick. I don't know about you, but 5mm of plastic sort of fused together isn't going to sustain gears spinning at over 5000RPM. Why I even considered this was possible was beyond me, and that's the moment I thought "yeah this is why I chose to study electrical engineering instead of mechanical engineering".

Iteration 2

I decided to learn from my mistakes and make thicker support shafts, smaller gears, and have the whole thing be thicker and more sturdy overall. I also removed the threaded shaft design and instead designed it so that each pair of gears was a single part, with a hole through the middle for a central shaft that threaded onto the gearbox.

Iteration 2 of my gearbox

I was so proud of this design, I thought I'd finally cracked the code of making arguably my most mechanically complex CAD project to date. I quickly exported all of the parts, sliced them, and started the five and a half hour print for the frame. After that, I spent another two hours printing one of the main gears.

I popped the gear off my build plate, and held it in my hand. Then I looked at the frame, and then back to the gear. It was past midnight, and I realized my mistake.

I designed the whole thing backwards.

So no, the motor wouldn't spin and be reduced to 48RPM, the backwords gears meant that if force wasn't an issue, I could potentially be spinning the last gear at well over 500,000RPM. I was trying to lift marbles a few feet, not shoot them into orbit!

Defeated, I went to bed. The failed design haunted my dreams.

Iteration 3

Many say the third time is the charm. I woke up the next morning and opened my laptop, staring the beast in the face, knowing I was going to defeat it this time. I made a copy of the project file instead of starting from scratch, because in reality I just needed to change around which mount would drive the main shaft and which would be the motor mount.

The winner - Iteration 3 - Final gear ratio of roughly 111:1

I was even able to reuse the shafts, main gears, and most of the frame. Delighted with my design, I triple checked I wasn't making any mistakes, and send the new frame to the printer.

This thing took seven and a half hours to print, and unsure that I had enough white filament, I decided to print it in blue. In hindsight, it looks way cooler in blue, and really helps distinguish the different parts of the gearbox. I even printed the drive and motor gears in orange, and the three color print just helps highlight how the different parts fit together to make this thing work.

I tested my gear, and it fit perfectly. Satisfied this was going to work, I began printing all of parts I needed for the final assembly.

Bringing it into the real world

It took quite a while to print everything, but behold, the physical rendition of my final design with working motor:

They spin fast and it's loud as heck

This thing is gorgeous. It's nice to look at a theoretical design in CAD and think "yeah, I designed that," but bringing something into the real world and being able to hold what you built is just something else entirely. Having a tangible part that previously existed as merely a bunch of mathematical code and a spool of colored plastic is one of the coolest feelings ever. It's one of those things that make me joyful to live in the 21st century.

So there you have it, my adventures in designing a gearbox for use with a future project! The next step will be getting it working with some of my other stuff, and I'll be covering that in a future post.

Thank you so much for reading, and keep on making things!
-Will