In the last post we got a working differential together, since then I have been designing the axle tubes, axle shafts and the drive shaft.
I have mentioned from the beginning that I am trying to keep the complex mechanical components compatible in size with off-the-shelf RC components, so I have made the width of the axle and the mounting points exactly the same as the Boom Racing SCX10 axle that I used on my rat rod build.
The housing of the differential has been redesigned so that all of the tolerances are contained within one part, and the cover simply screws down on top, holding the two largest bearings in place. Nothing has an overhang more than 45 degrees, so it is all printable without support.
Inside the axle tubes there are two bearings which support the printable axles shafts. The axles shafts have provision for an M4 rod down the middle, which provides both strength and the opportunity to use standard RC wheels if desired.
I printed and assembled prototypes based on this design and it went together well. My plans to test immediately were delayed due to some mistakes I made in the sizing of my driveshaft, but the assembly feels satisfactory when turned by hand, I have high hopes for the first test!
Stay tuned for the assembly and test video of the rest of the axle shortly, as well as work on the drive shaft and universal joints.
Over the past few weeks, with a brief hiatus due to international travel for my day job (I’m an Electrical Engineer in the telecoms industry), I have been plugging away at the design of the rear differential.
I have a few design goals for the rear axle
- Approximately the same size as a “standard” 1:10 crawler axle (both to make this re-usable in other applications and to make it possible to use a commercial axle on this build)
- Functional open differential
- Accept standard RC wheels with a 12mm hex drive
- Rugged enough gears to handle, at least, a “silver can” 540 motor
- Look fairly realistic (not too important in this build since it will probably be hidden)
- Preferably printable without support
Of course the rear axle can be made significantly smaller (or stronger, in the same envelope) if we go from a open differential to a spool, which will be a suitable modification if it is to be used on a rock crawler.
I first attempted to design from the outside inwards, starting with my goal diameter for the differential and designing gears to fit inside it. This turned out to be a bad idea, causing endless redesign, it was much more sensible to design the gears and build the casing around it.
There are other aspects to keep in mind, almost all of them relating to tolerances. For example, if a bearing recess is part of the face that contacts the printer bed then the slight bulging can prevent the bearings from fitting.
Designing Bevel Gears
Unfortunately Fusion360 doesn’t have a decent tool to create parametric bevel gears and designing them properly yourself is no mean feat. This is a real nuisance because we have to import gears from elsewhere and then design around them.
Fortunately there is a very nice script written for OnShape which you can find here and it is not too much trouble to set them up as you like, export as STEP files and import into Fusion360.
In order to keep my design “semi parametric” I positioned all of the gears sensibly with respect to the origin, and then defined variables which correspond to the gears dimensions. So long as all of the dimensions of the housing correspond to these variables and aren’t referenced to the gear objects themselves it is fairly easy to swap the gears out with others.
This iteration of the design provides another approximately 2:1 reduction, which means that we should be able to shrink the transmission that was designed in the last post.
Rapid Prototyping – Use It!
This is more of a personal lesson, but perhaps it is worth reminding. If you are like me then you design something to 85% and then realize that you could have designed it better, then you repeat the process, without ever printing anything. With something that takes 100’s of hours to print that may make sense, but for tiny parts like this it is just foolishness. There is a lot to be learned by just printing the item, and trying it out as is.
These are pictures of the first functional assemblies, hastily printed in ABS (not printed on a Robox, I look forward to seeing how well it handles the small pieces though!).
Video: Assembly and Test
Some things are best expressed in video, and assembly is one of those things, so here we go!
One day we will be able to 3D print functioning combustion engines in 1:10 scale, but for now the car will have to be driven by regular old electric motor, it’s sad but true, I know!
Why Reduction Is Needed
Small electric motors spin incredibly fast, but don’t generate much torque. In order to make them useful we need to gear them down appropriately.
Most RC cars get the majority of the reduction done in the first step, using a very small pinion on the motor, and a very large spur gear, but this is not really suitable for a realistically shaped gearbox, so I will use multiple stages of gears to achieve the desired reduction.
I have the following design goals for this reduction box.
- Motor must be low (for centre of gravity)
- Fully printable (without support wherever possible)
- Strong gears, big teeth, easy to print
- All gears to be hidden in a (semi) realistic enclosure
- Minimize unique parts
Selecting a Ratio
Although you can get a lot of power out of the “540” sized motors commonly used in RC cars, they still need a lot of gearing down, so it’s time to design a reduction gearbox.
I have done some rough “back of the napkin” calculations which show that a final drive ratio of about 8.5 will be decent starting point. The glory of printing is that we can experiment and revise once the car is working
|Theoretical Max Speed Calculation|
|Example 1||Example 2||Example 3|
|Brushless Motor kV equivalent (2S)||2297||2973||2973|
|Brushless Motor kV equivalent (3S)||1532||1982||1982|
|Final Drive Ratio||8.50||11.00||8.50|
|Rear Wheel Diameter (mm)||95.00||95.00||105.00|
|Distance in One Wheel Revolution (mm)||298.45||298.45||329.87|
|Distance in One Motor Revolution (mm)||35.11||27.13||38.81|
Although I did not use this tool to generate the gear profiles, I find it very useful for visualising a complete setup, you can follow this link to mess around with it yourself (link).
I decided on a stackable gearbox design, which consists of repeats of the same section, each one accounting for a 11/17 reduction in final drive ratio, resulting in a 5.7:1 ratio at the output.
This leaves room for a further reduction at rear axle (13/21, for example, would result in a final drive of 9.2:1)
Just because we are forced to use an electric motor doesn’t mean we have to look at one, so I began the design of a Merlin V12, scaled to 1:10. This engine will go over the electric motor, and given it’s size, probably also hide some electronics or the steering servo.
There are of course many details to go, but having the rough shape helps me work out the car’s final dimensions.
The gearbox can be orientated horizontally or vertically, which I am not yet decided on. I prefer the vertical orientation for aesthetic reasons, but horizontal may be more practical.
Either way, the motor stays at the lowest point, and the output is roughly in line with the rear axle input shaft.
The next most pressing issue is probably to design the rear axle, and the telescoping driveshaft that will connect it to the gearbox, so stay tuned. In the meantime, I am curious to hear your thoughts!
In the last post I introduced myself, in this post I’d like to introduce the project.
I have a real soft spot for the raw nature of roughly 1930’s era, often single seat, open cockpit race cars. The crazier the better. I don’t really consider myself a “car guy” but I love cars that evoke an emotional response, and to me there is nothing quite like a set of wheels strapped to a Merlin V12 to get the pulse racing.
As it happens, there really aren’t (m)any RC cars in this category, and as far as I know, no 3D printable ones. I aim to correct this travesty! My goal is not to build a specific replica, but a believable and functional model, built around component designs that we will be able to reuse in future models. Since many of the real cars like this were one-off builds, such as Jay Leno’s ridiculous(ly awesome) 1917 Botafogo Special, this seems totally reasonable.
As with a real car of this nature I will let the functionality direct the form to some extent, and so I begin the process with sketches.
First I decide on some rough dimensions, and do a side view and top view sketch on graphing paper, before scanning those and importing them into Fusion 360 at the correct scale. While I plan to design all the parts, including wheels and drive-train, for printing, I want to keep them to a size where standard RC parts can easily be swapped in too.
Next I make a simple pose-able mock-up of a human figure at 1:10 scale and pull in RC components that I designed for my 1:10 rat rod build, this allows me to check that the scale is feasible.
Things look ok, but a rough body shape shows a bit of a problem, these narrow bodies don’t have nearly enough space for a regular-sized battery, which I would really prefer to use, so it may need to grow a little.
Raising the motor slightly is undesirable from a center-of-gravity point of view, but might be a solution, alternatively, does our driver really need legs, he has a sweet car to get around in anyway…
Before spending too much time of these problems I will start with design work on the mechanical components: transmission, front and rear axles especially, because they will dictate everything else. Tune in next time and keep an eye on Ossum in the meantime for behind-the-scenes posts!
You can follow along here if you like:
This is the first of what is going to be a series of blog posts in exciting collaboration between myself, Jason Suter (known as “Ossum” around the web), and CEL-UK, the creators of the Robox project.
Before I get stuck into what we are planning to do I feel it would be best to introduce myself. I am an electrical engineer in the telecoms field for my day job but apparently have never quite grown up, because I still dream of being a toy designer or building props for movies. I have been designing projects in CAD for 10 years now but it is been over the last 2 years when I when I got access to a 3D printer that I was really able to start making those dreams come to life, at least in my spare time!
I am passionate about designing things that “do something” with a mechanical or electronic element, so I seem to have naturally fallen in with the RC crowd, but really my projects and interests are diverse, you never know what might happen next.
My first major printing project was a 1/10 scale RC rat rod, which I designed in Fusion 360, had the body printed via Shapeways and the chassis laser cut from aluminium. The design files I released won me a 3D printer, which really got the ball rolling.
I diverted into some mechantronic interior design, if there is such a thing, with a blooming flower night light which was featured in make magazine. This was a challenge to build something that would have been completely impossible for me to do without CAD and a 3D printer.
I went back to my roots with an RC 1/10 scale Willys MB Jeep and M416 trailer, which has been extremely popular. I get a real kick out of seeing my designs being built around the world, it helps me justify the hours that I spend working on them if I know they will be amortized over many builds!
There are some other weird projects in the middle like my giant LEGO skeleton where I flipped the scale and went 10/1, he ended up being 40cm tall!
Besides designing and building myself, I really enjoy getting others interested in tackling it themselves, I have really enjoyed the vibrant community that has sprung up around my designs. Here you can find the facebook page and group.
I have had the idea for a fully printable RC single-seater vintage racer brewing in my head for a while now, and in collaboration with CEL-ROBOX I will finally be bringing it to fruition, and sharing it with the world.
Over the course of the next few months I hope to bring you along on my journey through the design process, and with any luck, inspire you to tackle some things of your own. I’ll be posting fortnightly updates here and on my pages. In the next post I’ll be going into my design goals and how I get started on a project like this.
If you want to discuss any aspect of the project with me, I now have a forum section for this project which you can find here.