Upgrade your Trek Marlin to a Suntour Raidon front fork!

Here's the TL;DR for this post: I bought a 2019 Trek Marlin 5 and realized the fork sucks.  I used Suntour's upgrade program to buy a new Radion XC-LO-R fork for $200 and I'm thrilled with it.

I recently got into mountain biking after being out of the bike scene since about 2004, when I was a full-time bike mechanic for a few years.  I haven't ridden at all in about 15 years, and even back then I ended up selling my Cannondale F600 because there weren't any good trails to ride around here (Springfield, MO) and the road biking was so good.  A few years back I bought my son his first good bike, a 2015 Trek Marlin 5.  This last year we visited the Crystal Bridges Museum of American Art in Bentonville, Arkansas, and as we walked from downtown to the museum we saw the All-American mountain bike trail, and were blown away.  Have you seen it?  Insane.  It turns out Bentonville is packed with amazing trails, and more importantly it turns out that there's a really nice new mountain bike park near my town, the Two Rivers Bike Park.  I dug out an ancient Trek 800 with solid forks and cantilever brakes from the '90s my neighbor had given me and we went riding the next day.

It was pretty awesome, but my arms were so badly vibrated that I couldn't feel them for an hour afterward.  Still, I was worried that I wouldn't be into mountain biking enough that I would do it long-term, so I mostly rode that bike with my son for about a month, before breaking down and buying my own Trek Marlin 5.  I had ridden his on the beginner and intermediate downhill trails, and it was so sweet.  I could ride so much faster, more comfortably, and with better control on his, even though it was a bit too small.  When I took mine out on the trails it was a great improvement over the old Trek 800, but I was distinctly disappointed in its performance compared to my son's bike.

Mostly my experience consisted of my front shock (a Suntour XCE 28, which comes stock on the 2019 Marlin 5) klunking loudly as it topped out (which is like, always), and bouncing around like a pogo stick.  The ride was not plush in any way.  There is absolutely no dampening, and of course no rebound adjustment.  I was kicking myself because I hadn't even test-ridden the bike, as I assumed it would be like my son's.  His front forks were nothing to write home about (Suntour M3030, described on the Suntour website as a "metropolitan" fork), but at least they didn't make machine-gun klunking noises like mine did when riding over rough terrain.

I decided I would upgrade, but dang!  Good shocks easily cost more than my whole bike.  Good shocks seemed to require a tapered head tube, which I didn't have, or even know what that meant. Disappointment.  Despair.  I desperately considered adding some kind of dampening myself.  Then on a forum somebody mentioned the Suntour Upgrade Program.  You can upgrade your Suntour forks to a nicer fork, and on the forums it was told that Suntour offers an actually decent air-spring fork with adjustable rebound and a lockout with a 1-1/8" non-tapered steerer tube.  The Raidon!  I had to prove that I was the original purchaser of the crap fork, but my bike shop printed me a new receipt for that, and for $199.95 and free shipping I was the new owner of a non-crap fork.

When it came in the mail I cut it off at the same length as my old one, transferred over the lower bearing race, and installed it.   I turns out you can buy a fork pump to adjust the fork air pressure more easily than what I did, which was to over-inflate the fork and bleed off pressure until it feels good, but whateves.

Before I installed it, I noted how heavy the XCE 28 felt in comparison to the Raidon, so I pulled out the postal scale and weighed them.

6 pounds and 3.4 ounces!  Obscene!

Now for the Raidon:

That's more like it!  4 pounds and 6.6 ounces, a nearly two pound weight savings!  Sweet!

The whole point of this post is that I took it riding today, and it is like having a whole new bike.  I absolutely can not recommend this upgrade enough.  I was easily faster, jumped higher, landed softer, and had more control and comfort than ever.  I could be wrong, but as far as I can tell, the lowest end bike Trek sells with an actual air fork is the X-Caliber 8, for $1,200.  I feel like this makes the Marlin 5 at $540 plus $200 fork upgrade a really good deal.  I realize the X-Caliber has a lot else going for it, but still.

When I was a bike mechanic I got to talk to a lot of customers, and I would always focus on how much fun a bike/part/upgrade was.  Most of us are buying fun at the bike shop.  Can you ride a Wal-Mart bike on the trails?  Yep. Will it be a fun enough experience to make you want to go back the next day?  That's a lot less likely.  This fork easily passes my fun test.

While we're talking about the Trek Marlin 5, I want to offer a few of my other, non-fork related thoughts on the subject.  Firstly I wish I hadn't bought a bike with a freewheel cassette.  Most modern bikes have the ratcheting mechanism in the rear hub, not in the gear cluster like the Marlin 5 has.  I could have upgraded to the Marlin 6 to get the freehub, which has unlimited upgrade potential as far as rear gearing and derailleurs go, but I didn't pay enough attention at the time.  The thread-on freewheel gear cluster has no upgrade potential, and now I'm stuck with 7 gears in the rear until I upgrade my whole rear wheel.  Secondly, the rear Tourney derailleur is awful.  The main problem is that the spring in it is weak, the body of it is heavy, and so it klunks against the gear cluster when I hit bumps.  It's almost as annoying as the topping out of the crappy old forks.  Additionally, it often changes gears on its own when I come down from a drop or jump.  I upgraded to an Alivio rear derailleur, which is two or three steps up from a Tourney, and for just $36 that tells you how cheap the Tourney must have been.  The klunking and mis-shifting is gone.  It's a noticeable improvement.  Thirdly, I'm pretty happy with the wheels, especially for how cheap the bike was, but I really wanted to convert it to tubeless, which I did with the stock wheels and tires.  You can see the process here.  After that, I upgraded the tires to Maxxis 2.20 DHF and DHR tires, and they fit.   Fourthly, the brakes on the Trek Marlin 5 are awesome.  My son's 2015 Marlin 5 has disk brakes too, but they are cable-controlled mechanical jobs.  Mine are hydraulic and the difference is huge.  Back when I was a mechanic in the early 2000's I remember the early adopters of hydraulic disk brakes always bleeding them and endlessly tinkering, so I was hesitant, but apparently times have changed.  These aren't even near top of the line brakes, but I love them.  Fifthly, I'm seriously considering converting to a 1x10 drive system.  I would need a new rear wheel with a freehub, a new 10 speed cassette, a 10 speed shifter, a 10 speed chain, a narrow-wide front chainring, a crankset that would fit a new chainring (current crankset has riveted chainrings), and a bottom bracket that would fit a modern crankset.  That sounds like a lot, but I'm pretty sure it could be done for about $350.  I guess at that point I really am into the X-Caliber 8 price range, but as a former bike mechanic it's pretty appealing.  Edit 3/21/19: I've pretty much given up on converting to a 1x system on this bike, but rather I'm saving up for a different bike.  I'm currently deciding between a Roscoe 8, a Stache 7, or a Fuel EX 8.  Sixthly:  A good upgrade for the front derailleur is the Shimano FD-M313, which is no longer made, but are still available.  Don't get a low mount front derailleur, as they will interfere with larger tire choices, but rather the M313, which is a high mount and doesn't take up so much space behind the seat tube.  It's cheap and solved most of my chain suck issues.

Starting pool skating in my forties

I  started teaching high school wood shop recently, and one of the woodworking projects I found I like most is making skateboard decks.  This rekindled my interest in skateboarding as an adult, after not skating for over 25 years.  I started again by skating the longboards I built, just cruising around on them, but it felt so good that I started thinking about doing the kind of skating that I'd always dreamed of but had been too afraid to do: pool skating.  I've never skated half-pipes, and I can't do an ollie, but I decided to give it a try.

So, in July of 2016, at 45 years old, I went to the park and pushed around the bottoms of the pools, and it was every bit as enjoyable as I envisioned.  I only had a longboard, which made it hard to turn tight enough for pool skating, but I had that first taste.  I got some guy to show me how to drop in, and after a few tries (no hit-the-ground falls though) I was able to drop into about a four foot bowl.  The next day I went to my local skate shop and bought a more traditional board, a popsicle-shaped job that could make the tight turns when carving a pool, and then went right back and dropped in again.  Unfortunately, the short wheelbase of the popsicle deck had a completely different feeling, and I fell backwards and dislocated my shoulder on the first run of my second trip to the park.
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Here I am at the hospital after an x-ray.

It was maybe 9 months until I was able to sleep on my right side again, and after a year I still couldn't throw anything.  After a long and frustrating year and a half I had healed enough to think about going back for my third skate session.  I read a lot and learned that a longer wheelbase would help with the kind of fall I took, so I bought yet another deck made just for pool skating with a 15.5" wheelbase, and I've been pool skating with that for about four months now.

Eureka Springs skatepark is a chill little pool in the trees.

Feels like flying.

The important and beautiful thing about pool skating is that it feels so good.  The feeling of gravity and centrifugal forces, the weightlessness at the top of a carve, the flow and focus, it is all so much like I imagine flying would feel.  I still can't get into the tiles or coping, and my frontside carves are weak, but how I look skating doesn't matter to me at all, and I wish I hadn't worried so much about how I looked back when I first wanted to do this in my teens.  I healed a lot faster then. 

11/4/2018 Edit:  I have now built up the courage to drop in, which has enabled me to get into the tiles in my local skatepark's bowl.  

Overview of videos

Here are some links to videos of projects I've done.

Soccerbots:

Mad Eye Moody Eye:

3D printed RC car:

Stop Motion Animation:

Rubber band powered airplane:

Railroad Cart:

Linear classroom wall clock

I teach high school computer aided drafting, engineering, digital electronics, and wood shop.  One of the problems I have is that I often forget what period I'm in, and also what time it ends.  I'm not all, "Where are we?  Who are you people?" or anything, it just feels like third period sometimes during second period.  And no matter what, I can never remember when the classes let out.  I made an outer ring to my classroom clock showing where the minute hand will be when the period lets out, and it's pretty helpful if I can remember what period I'm in.  We have a different bell schedule on Wednesdays though, with a later start, and fifth period happens before fourth period that day, so that's not helpful when trying to figure out what's going on either.

I've long dreamed about a long, linear clock that slowly travels down my 32 foot classroom wall, with the periods all blocked out, so that I can just glance at it and know what period I'm in, and how much time is left.  I wanted it to automatically show the different bell schedule on Wednesdays, reset itself at the end of each day, and not run on the weekends.  I wanted it to be highly (even unnecessarily) mechanical with exposed electronics, to reflect the subjects that I teach, and I wanted to build it entirely with materials and methods my students have access to in my classes.

So, finally, after four years of dreaming, one semester of brainstorming and designing, and two months of all my spare time doing actual fabrication and troubleshooting, the clock is working, although in an advanced prototyping stage.

Normally, this is where I would lay out a step by step process for building your own clock, but I'm not going to do that here.  It's just too custom fitted to my wall, and if anything needed re-gearing to make it fit into your space, it would basically need a complete redesign.  What I want to focus on here are the essential skills that anybody would need in order to design this, or something like it.

First up is:

Conceptual Thinking

No question about it, this is the hardest part of a project like this.  It requires the ability to visualize what you want to end up with, and in the context of the tools and skills you have available to you.  One of my favorite sayings is, "To a man with a hammer, every problem looks like a nail."  Conceptual thinking requires that you have a large enough skill set to solve the problem, and have an understanding of each of those skills deep enough that you can be creative with its implementation.  It boils down to a vision of the final project, and a vision of the path to getting there.  

For the clock, my major hangup was the motor.  I was convinced the only way I was going to be able to achieve the positional accuracy of this clock was with stepper motors.  I've never used stepper motors in a project, but I've been wanting to for a long time.  I've read books and internet articles about them, and I feel like I have a good understanding of them.  I know that they need a dual H-bridge driver and a microcontroller, at least, and that they come in several styles of winding, that one of their strengths is holding torque, and that they are power hungry.  

However, I didn't want to change the battery all the time, and I don't need any holding torque at all on this project, so I suspected that they weren't the best solution in this case, but I still really wanted to use them, and I didn't know how else I would get my positional accuracy needed to get the hand to land exactly on a minute mark every time.  One day I was reading an article (probably on Hackaday) in which somebody was using a wheel with a hole in it to count light pulses to make a robot go an exact distance with a cheap brushed dc motor.  Boom.  There it was: the solution, when I wasn't even researching my clock, but just reading for enjoyment.  

Every single part of this clock went something like this.  How was I going to get exact second or minute pulses?  (With a cheap hacked quartz clock movement?  With my microcontroller's inaccurate internal clock?  From the internet with an ESP8266 wifi module?  A DS1307 real time clock?)  How would I account for the different schedule on Wednesday?  What would I use for power?  How would it return at the end of each day?  Could I prevent it from running on the weekends?  How would I paint my walls?  What is the ideal microcontroller for this project?

All these questions and so many others!  And every one was a hard-fought battle for the knowledge required to make it happen.  Not every single thing must be known during the conceptual phase, however.  You just have to know enough to know that you can figure it out when the time comes, or to at least have a couple of backup plans.  I won't lie; it's hard.  It's why when you post an elegant 3D printed solution to a problem on the internet everybody wants the .stl files.  It's why so many books and magazines write complete how-to articles with parts lists and completed downloadable code.  If I were starting my journey towards making 3D printed, laser cut, and CNC'd projects with microcontrollers and electronics, I would read, read, and read.  Blogs, books, parts supplier's parts descriptions and tutorials.  Every day, for years.

This brings us to:

Mechanical Design Skills

For me, this is the easy part.  After a college minor in drafting, and 20 years of professional 3D modeling experience, I can usually breeze through the mechanical design portion of a project.  I realize that this is not going to be a common experience for the new maker though, so I'm going to break down the design skills into the sub-skills of hand sketching and 3D modeling.  

My computer aided drafting students HATE to hand draw their concepts before they jump on the computer and start 3D modeling their parts.  They just want to get to the fun part, like they're playing an expensive game of Minecraft.  Soon enough though, they realize they can't go any further because they don't have a plan, they can't visualize how their parts go together, and then it is evident that they have no idea what they are doing.  

Paper drawings are the solution!  I'm not saying you need to break out the T-square and triangles (although that is pretty fun), but some good engineering graph paper, a pencil, and a big eraser will make the design process shorter and faster in the long run.  I prefer to draw my projects in full scale when possible.  One of the big problems I see when trying to design things in a computer aided drafting program is that parts are often accidentally designed with features so small that they are nearly impossible to fabricate, but it is difficult to get a sense of scale in 3D software.  When drawing on paper it is important to draw your objects from more than one side, so that you get a sense of depth and how the parts fit together front-to-back.  

I personally think that 3D modeling skills are the number one thing you can learn to improve your maker game.  3D modeling is the Microsoft Word of the 21st century.  It enables you to 3D print, or CNC cut, or laser your own designs.  It helps you make plans for complicated things you are going to fabricate by hand as well.  You should choose a piece of software and learn it well.  If you're a student or a teacher, I would suggest Autodesk Inventor, since it's super powerful and it will be free for you.  Otherwise it's insanely expensive for the hobbyist.  My second choice would be Autodesk's Fusion 360.  I've never used it, but it has almost all of the same capabilities as Inventor (with the glaring omission of a gear generator) and it's free.  There is a huge hobbyist user base and lots of online tutorials.  It can generate .stl files and g-code for CNC fabrication.  There are so many other options as well, but try to choose something modern and capable that can grow with you as your skills grow.  Resist the temptation to choose a piece of software just because it seems easy to use.

Electrical Design Skills

There has never, ever, ever been a better time to learn electronics.  Not only is there so much information on the internet about learning electronics, but newer, cheaper, more powerful, and easier to use components are being released constantly.  Companies like Adafruit, Sparkfun, and Pololu are taking tiny, hard-to-solder chips and building easy-to-use breakout boards with them.  Microcontrollers and microprocessors programmable in dozens of popular languages, including graphical block-based languages like Scratch and Blockly.  The biggest problem quickly becomes choosing a platform to base your designs around.  

If I were just beginning my journey into electronics, I would start by purchasing two books: Make: Electronics and Practical Electronics for Inventors.  I'm also a huge fan of There Are No Electrons: Electronics for Earthlings and Robot Builder's Bonanza.  

I would make it a high priority to learn how to solder and etch circuit boards using the toner transfer method.  I would choose a chip microcontroller (like an AVR or Picaxe) rather than a board microcontroller (like an Arduino or Micro:bit), preferably in a language you already know (I only know Basic, so I use the Picaxe microcontroller).  This suits my style of projects, which are small, fairly simple, mechanical, and inexpensive.  

Fabrication Skills

I hesitated to include fabrication skills on my list here, because so much of what we can build today is built digitally, with lasers and 3D printers and CNC equipment.  If all goes well, humans shouldn't have to touch the parts too much on small projects like these.  I did drill out all of the holes on my gears so that I would have a perfectly round, more precise hole, and that required a small drill press.  The gears rotate on axles made of 3mm threaded rod and 4mm OD brass tubing, and those needed to be cut with a small hand saw.  On many of my projects I cast urethane rubber parts with silicone molds.  At any rate, you should not hesitate to purchase a tool and learn to use it.  It's probably going to cost the same amount of money when you buy it later, and you will have all of the time between now and then improving your skills with that tool.  

Persistence and Troubleshooting

This is a tough one.  Let me assure you that nothing is ever going to go right on the first try if you are pushing yourself to build and design more amazing things all the time.  On this clock, it turns out that infrared light shines right through my 3D printed plastic in the Z axis, so the clock never knew it had traveled one rotation of the minute wheel.  It took a week to figure out that it was a mechanical problem and not a problem with my IR emitter or detector, or the code that interfaces with them.  The motor driver I chose, the SN754410, draws 25mA all the time, even just sitting there overnight, apparently, and that's enough to drain my 1000mAh battery in just one day.  Not cool.  I had to switch to a DRV8838, which is more efficient, but required rewiring all of the motor driver circuit and making major modifications to the microcontroller program.  The acrylic I made the base plate out of is starting to crack from it's laser cut edges, apparently from an incorrect power setting I used on my laser.  I still need to figure out what caused that and exactly what I'm going to do to prevent it.  It never ends.

The internet is such a great resource in the aid of troubleshooting.  I had my questions answered over on the Picaxe forums when I couldn't go any further on my own.  Almost any problem you have, somebody has probably faced it before as well.  Sometimes it's best to sit a project aside for a few days to roll it around in your subconscious when things seem impossible.  It's important to remember when starting a project that it's going to be hard almost all the way through it, and just get mentally ready for it.  I find that documenting my projects online (like this) is a great way to keep my motivation up.  I think about how cool some person in some place I've never even heard of is going to think it is.

Making gears for 3D printing using Autodesk Inventor

You want to design your own thing that you're going to 3D print, and it's going to have gears.  Awesome!  It's probably going to be something great.  Being able to design gears for 3D printing is a super useful skill, but if you don't know anything about gears it could be a little more complicated than you might expect.  I'm going to tell you what I know about gears, and how to design them for use in your 3D printed project using Autodesk Inventor.

I can hear what you're thinking right now:  "What?!? Inventor?!?  Do you think I'm made of money?!?  Why don't you show me how to make gears in Fusion 360?"  Let me tell you I would LOVE to show you how to make involute gears in Fusion, but I don't know how.  I've done some cursory research, and no other software makes generating custom involute gears as easy as Inventor, free or expensive.  Also, Inventor is free for students and teachers, so hopefully you fall into one of those categories, or at least can convince Autodesk that you do.

First, lets get the vocabulary out of the way.  There is a ton of gear vocabulary, but I'm only going to discuss the minimum that we need to know to make the gears.

Pitch Diameter: This is the diameter of your gears if the gear teeth were infinitely small.  In other words, if your gears were perfect cylinders with perfect friction on each other, the diameter of these cylinders would be the pitch diameter.  When designing gears from scratch, without the help of a $1,890 (per year!) piece of software, the pitch diameter is our most important dimension.  In Inventor though, it won't be that important to know.

Center Distance: Half of the pitch diameter of the first gear plus half of the pitch diameter of the second gear would give you the distance from the center of one gear to the other.  If you have two shafts, and you want to connect them with gears, the center distance is how far apart those shafts will be.

Outside Diameter: This is the diameter of the circle that makes up the tops of the teeth.  It isn't important in any of our calculations, but it is important if our gears are going to fit into a housing.

Pinion: This is the name of the smaller of two mating gears.  The other one is just called the gear.

Diametral Pitch: This describes the size of the teeth.  The units are generally teeth per inch of pitch diameter.  The bigger this number is the smaller the teeth are.  Must be a whole number.  Common pitch sizes in radio controlled cars are 32, 48, and 64.  Both mating gears must have the same pitch.

Module: This does the same thing as diametral pitch, which is to describe the size of the teeth, but with metric units and as a ratio.  It is the number of millimeters of pitch diameter divided by the number of teeth.  LEGO Technic gears have a module of 1.  So, the LEGO 24 tooth gear has a pitch diameter of 24mm.  I almost always design my gears with module instead of diaetral pitch, even though I design everything else in inches.  I have learned that the smallest consistently trouble-free teeth that I can 3D print are a module of 1. 

Backlash: This is how much a gear can rotate when the other gear is being held still.  If there is no backlash there will be excessive friction.  Backlash is important in 3D printed gears, because it is difficult to control if the parts are slightly oversized or undersized.  My 3D printer almost always makes my gear teeth just a hair too big, and I compensate by adjusting my backlash.

Involute: An involute shape is a part of a spiral.  Why is this important?  Because the sides of the teeth are actually not flat, like you might imagine, but rather curved in an involute shape.  This is a difficult shape to manually draw in a computer aided drafting program, but Inventor will take care of this for us.

It's not obvious, but if the sides of the teeth were flat, they would CLACK against each other when their faces met, and the tips of the faces would drag across the mating faces, causing excessive friction.  The beautiful thing about the involute shape as a tooth surface is that it causes the mating teeth faces to "roll" on each other instead of sliding.  This is absolutely critical in an actual working gear for mostly silent, nearly friction free operation.

Pressure Angle:  This describes the angle that the teeth surfaces press against each other at.  There are two common options, 14.5 and 20.  I have always used 20°.  Mating gears should have the same pressure angle.

Helix Angle:  Some gear teeth are not parallel to the axis of rotation, but rather wrap around the gear diameter at an angle, sort of like a slight spiral.

The benefit of this is that it causes much less noise and friction.  Almost all automotive gears are helical these days, except for the reverse gear in manual transmissions, which is straight cut, and is why your gears sound like they are "whining" in reverse.  The drawback is that the gears want to "unscrew" from each other, which makes them push in opposite directions along their axis of rotation.  This makes it so that you need thrust bearings to keep them in place.  In 3D printed applications this is generally impractical. It is possible to put two opposite-angled helical gears together to form a single gear, which is called a herringbone gear. 

It has all of the benefits of a helical gear but none of the drawbacks.  They are very difficult and expensive to machine, but just as easy to 3D print as any other kind.  They have the additional benefit of keeping the gears aligned with each other, which can often be used to simplify other parts of the gear train design.

On to Inventor!

You might assume that you would design two gears as separate parts and then put them together in an assembly file, because that’s the way everything else is done in Inventor, but you would be wrong.  Gears designed in Inventor’s gear generator tool are designed in an assembly file, and the part files are generated automatically.  So, the first thing we need to do is make a new assembly file, and then save it.

Next, we will go to the Design tab, and click on the Spur Gear generator button.  You may see that there are also options for generating bevel gears and worm gears, but neither of these options are able to generate functioning 3D printable parts.  They are for representing parts that they assume you are going to purchase.  The spur gear generator is the same way, but at the end there is a trick for turning them in to useful parts.

Once the window pops up, make sure you click on all three window expanders: the one to the right of the main area, the one below the main area (but above the Calculate/OK/Cancel buttons), and the one to the right of the Cancel button.

Now we have to make some decisions. 

1) Do you want to use diametral pitch or module?  Like I said earlier, I make all of my gears using the module system of tooth sizing.  I am very happy with a module of 1mm for 3D printing.  Click the radio button under Size Type to make your choice.

2) Do you want to tell Inventor how many teeth each gear needs to have, or simply what the gear ratio is?  I like to specify the number of teeth because when I’m designing my gears, I don’t usually have the parts that hold the gear shafts designed yet, and I can put them wherever they want.  Because I know my tooth size module (usually 1mm for me) and the number of teeth, that is how I control my center distance later.  This plan may not work for you, but that’s what I do.  Make this choice with the Input Type radio buttons.

You need to make choices 1 and 2 before you go on to make choice 3.

3) Inventor is going to end up calculating SOMETHING for you.  What do you want it to be?  Your choices are under the Design Guide dropdown.  I nearly always make my choice as Center Distance.  This means I say what my module is, how many teeth each of my gears have, and Inventor uses these two inputs to calculate my center distance.  If I don’t like the center distance that it calculates for me, I change the number of teeth on my gears until it’s what I want.  I have found this to be by far the easiest method of designing gears, but it requires that I don’t have a center distance that is fixed. 

If you have a center distance that must absolutely be held, you can chose one of the other options and let Inventor calculate what your module, tooth count, or module AND tooth count is.  There is another option for Total Unit Correction, but I don’t know what that does, and I’ve never used it.  In fact, any time there’s a box for Unit Corrections anywhere in this process, I leave it alone. 

Once your big three choices have been made, you should go ahead and change your pressure angle to 20 or 14.5 degrees (I’ve always used 20), and change your Helix Angle to zero, even if you are going to actually make a 3D printable helix gear.  (We’ll do the helix part later with the coil command, if that’s what you’re into.)

Make sure you unclick the Internal checkbox, unless you want internal gears.  Inventor can make them, and they work, but I have found them to be finicky with regard to their smoothness and center distance.

You can set your Facewidth of both Gear1 and Gear2 to be however thick you want them.

Next, enter how many teeth you want on Gear1 and on Gear2.  Make Gear1 be the smaller number if the two gears do not have the same number of teeth.  In other words, make Gear1 be the pinion.   This will help you keep track of things later.

Ignore the Cylindrical Face and Start Plane buttons.  Inventor can design gears into an assembly in which you have already defined the gear axis and face planes, but I’ve never done that.  It seems complicated.

At this point you should be able to hit the Calculate button, and the Inventor calculated fields will update.  I have never in my life had Inventor think that my gears were going to work.  It always says, in red text, “Calculation indicates design failure!”  Yet, they always seem to work just fine.  There is probably a way to dig deep into Inventor and fix this, but it’s the easiest to just ignore it. 

After that you can hit the OK button, and Inventor will open up a window allowing you to rename the gear files (but I just keep the default names Inventor gives them), then another giving you that same warning again; just hit Accept.  At this point you will place the two gears, as an assembly, into your assembly file.  If you zoom in to the meshed teeth, you will see that these teeth are interfering with each other, and that they have a very simplified face profile.  THESE GEARS ARE FOR VISUAL REPRESENTATION ONLY!  Involute teeth are very complicated, and if Inventor went and put in a bunch of mathematically complex parts into moving assemblies, it would bog down computers badly when they were rotated.  Furthermore, it’s a pretty safe assumption that most people are buying their gears from a gear supplier, and it’s a waste of processing power to needlessly over-complicate them here. 

We do need them with a true involute profile though, so what you are going to do is to right-click on one of the two gears, and choose Export tooth shape.  A window will pop up asking you if you want to export the tooth shape of the pinion or the gear, and what kind and how much backlash you want.  I normally leave it at Normal, and I make the backlash as big as I can (before Inventor changes the field text to red, meaning it won't work), which is often about .006”.  I think my 3D printer over-extrudes, so I need my teeth to be thinner than they should be, and even with my biggest backlash sometimes my teeth mesh too tight and I have to adjust the center distance in my assembly that holds the gear axles. 

Once you have done that, click OK, and Inventor will open you a new part, which is a cylinder with a sketch, not of the tooth, but of the space between two teeth.

What you need to do is make that area a cutting extrusion, all the way through.

After you have done that, you need to make a circular pattern of that feature, and array it the number of teeth that are on that gear.

Technically you have a working 3D printable gear now, but really you need to draw a hole in the middle of it and extrude it through so that it can either slip fit onto an axle, or press fit onto an axle. 

Tips and tricks:

If you want to design a compound gear (a single part with two different gears stuck together side by side) for 3D printing, the best way to do it is to make two different involute toothed gear part files, then stick them together in an assembly, and make an .stl file out of the assembly.

If you want to make herringbone gears, make your facewidth half as big as you really want it to be, and instead of using the extrude command to cut your space between the tooth, use the coil command to cut it into a spiral.  Make sure you spiral both Gear1 and Gear2 the same angle.  Make the circular pattern of the spiral tooth space.  Then make a derived part of Gear1, making it a mirror of the original part.  Finally, make an assembly of the gear and the mirrored derived gear, and there's your herringbone gear.

It is extremely unlikely that you are going to be able to use the center distance that Inventor calculates for you as your true center distance in the real world with 3D printed gears.  I ALWAYS make my gear shafts adjustable so that I can fine tune the real-life center distance for optimum gear engagement. 

If you want to make bevel gears with a true involute tooth shape, check out my blog post on how to do that.

Useful Links:
https://engineerdog.com/2017/01/07/a-practical-guide-to-fdm-3d-printing-gears/