The goal of this project was to meet the criteria of the Make It Spin Contest, "Create an Instructable that shows how you design and make something that spins", with my son's general idea of "make a hot wheel car spin in a circle". Together we went through some iterative design to reach his goal.

PXL_20260101_110224820.TS~2.mp4
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Supplies

Supplies used included:

Previously owned:
Computer & Mouse (for modeling)
3D Printer (or using a service)
Calipers (handy for precisely measuring existing parts)
A steel rod (don't even know where it came from)
Neodymium Magnets (these were 0.5" diameter by 0.25" thick)
3D Design Software
We used Onshape; I like it because it is free, cloud-based so it works on any device, and has auto-save with versioning/branching/merging, among other reasons). Note: free for hobby/personal use, free for students, and free to try for 6 months for professionals
Link to our design
Autodesk Fusion is another good option which has:
Free (limited time/features/student version) and
Paid (referral discounts available) options.
Parts borrowed from a "HackPack" (we have a subscription). They also offer free extras of low-cost parts, which we relied on.
Continuous rotation micro servo motor
Machine Screws (metal and/or plastic)
Power Bank (wall outlet acceptable)
USB A to C Cable
Arduino Nano
Breadboard with integrated power switch

Step 1: Sketch the Simplest Version of Your Idea

I first had him sketch his idea. We used OneNote because I can keep sections for my kids and refer back to them at any time from any device. It also has an easy way to change colors to show different details. One option that can make drawings look nicer is you can draw a rough outline of a shape (such as a circle or rectangle) without lifting the pen and hold it to the screen; it will convert it to a geometrically precise shape for you which you can then drag to resize. Plain old pencil and paper works, too.

After his first sketch, I asked him some more questions that helped refine his idea.

How will this line get connected to the car? A: A 3D printed adapter to a Hot Wheels "Brick Ride" (built to interface with LEGO studs)
and how will it get connected to the motor? A: A 3D printed mounting plate
Do you think it would be easier to 3D print a rod or use something we already have? (okay, this was a leading question, but it got him thinking about what can we use vs having to spend time designing and material printing)
What will control the motor? A: An Arduino he already had, with breadboard for easy connections
What will power the motor? A: A power bank he already had

I started to help him by drawing more precise pictures to capture his ideas. For example, where to put the breadboard so that this can be compact as possible.

After we acquired the Brick Ride Hot Wheels (we had seen them before but not owned), I asked him to sketch his idea for how exactly to attach it to the steel rod. At this point we were taking apart the Brick Ride and noticed the large cavity under the top box of the truck. So I asked "why not eliminate the adapter and just throw a magnet in that space instead?". He thought it was a brilliant idea, so we went with the "no [designed] part" solution instead.

Step 2: Model Known Parts/Constraints

You will want to move to CAD when you have a good understanding of how your project fits together. When moving to CAD, it's helpful to have a good representation of your existing parts/constraints before designing around them. With the location of the micro servo and size of the steel rod controlling almost the entire project design, these are what we modeled first.

Onshape has a handy tool to derive parts from other people's work. All we had to do was search for "micro servo" to find a ready-to-go model for exactly what we needed, saving us time to work on what actually needed to be designed.

Step 3: Create a Master Sketch in CAD

A master sketch is a really helpful way to capture your design intent and use / project it to other places when you are ready for them. In our master sketch, we captured a number of key details:

The bounding box of our 3D printer (square turned up to be a diamond in this example)
This was helpful to grasp at a quick glance what could or could not fit on a build plate, and where joints might be needed.
The size of the metal rod (projected onto the sketch plane from the earlier model; oriented up and down in the master sketch)
The size of the breadboard (just above center) and location of on/off switch and power indication light (the micro servo geometry was used to constrain the location)
Symmetry Cut lines (at 60°, 180°, and 300° relative to 0° being directed upwards)
Inner Edge of road (just outside breadboard)
Centerline of "road"
Bounding box of hot wheel (near top)
location of magnet (circle near top)
Note: We originally intended to use a bracket between magnet and rod here, but simplified the design by eliminating it.
"Curb" just beyond hot wheel to both guide its course and act as a lower boundary within which the steel rod would spin.

Step 4: Create Additional Master Sketches in Other Dimensions

With the top down details captured, we needed to do the same setup work in the profile(right) view. Due to planning for radial geometry, we didn't need master sketches in any other dimensions.

This sketch also served as the cross sectional area needed for creating our first geometry by revolution.

Key details included:

The outer wall (at right, to support the road)
The road surface (at top, to convey the hot wheel)
magnet's path (under road substrate)
Borrowed geometry from the earlier master sketch
At this point in design we thought, why not just let the magnet keep itself upright though attraction to the magnet in the car, and stay attached to the rod by attraction to it?
Guides on either side of the magnet channel (to ensure it always stays in the right path and keeps the hot wheel on track).

Not Pictured:

To design for 3D printing, it is very helpful (and less wasteful) to design without needing support material. While this is about to get revolved as one part, we were already planning to cut it in two so that it could be printed as top and bottom pieces without support, then assembled.

Step 5: Create Simple Geometry

Knowing the print volume constraints of our printer, we only designed one third of the road at a time, so that each section would be printable, and later assembled. To do this, we just used the "revolve" feature for 120° and the sketch from our second Master sketch, the profile view.

Step 6: Design Mating Features

White Stripes

The most important mating feature was the connectors in the middle of the road to tie the top surface to the bottom substrate/support. Rather than use paint to apply our road stripes for a realistic effect, we used the connectors printed in white to achieve both a functional effect and an aesthetic one.

For design of the snap fit connector, we used a plastic snap fit guide from MIT.

For the arc length of the connectors, we compared to actual road stripes which are usually in a ratio of 1 stripe length painted to 3 stripe lengths gap. We didn't keep the same scale ratio compared to a real car (about 2/3 of the length of a car) because the longer arcs appeared too obviously curved and could have resulted in some printing difficulty. To arrive at what arc length to use, we had to choose a value which 4x the arc length (1:3 ratio above) fit an integer number of times in a circle. Also, to avoid crossing road sections, we wanted to choose an integer divisible by 3 as well. The arc length corresponding to 9 stripes looked just a little too long, so we chose the next multiple, 12.

The top of the connector was also chamfered (with a matching chamfer in the road surface) to give the illusion of a painted road and not interfere with the hot wheels driving by.

Road Ties

While we didn't think these were absolutely needed, we thought it might help reduce the visual effect of seams by tying the road surface and substrate together at the edges as well as near the middle (this piece is shown in orange in the model). These cuts were designed to allow 3D printing without support in the hole.

Interface Ties (added after first test print below)

To keep the motor and rod centered, we would need some way of controlling the location of the center assembly compared to the outer road. The cuts at the bottom of the road substrate allowed for a piece to tie between these two sections. As a bonus, the extra cuts allow for a power cable to be passed through.

Circular Pattern

After modeling the cuts and mating connectors, we made a circular pattern for all the cuts.

Dovetails

Following the design of connectors in Orca Slicer, we made a sliding dovetail connector, where the male end is 0.1mm smaller in both the width and length compared to the female end.

Step 7: Cut Road From Substrate

This enables both the top and bottom segments to be printed without any support.

Step 8: Test Print & Fitup

Testing design is extremely important because assumptions made often do not stand up to the real world. Frequently testing is far better than waiting to test at the end of a project.

We were most worried about the snap fit connectors because we had not yet made any, but these worked perfectly. The road ties also functioned well.

On to the next design iteration!

Step 9: Design Enclosure for Motor, Arduino, Breadboard

The main purpose of this enclosure was to hold the motor stable and leave space for the wiring to power and control the motor while keeping the same wiring separate from rotating parts (yikes!). We started again with the model of known parts (the breadboard in orange and the space taken up by the switch in blue). From there we made a basic cylinder and started to cut out space for things we needed:

micro servo,
space for micro servo bracket and clamp to rotate
breadboard,
risers (extension pieces) to reach the switch from above, without interfering with the steel rod
viewport to see power light for the breadboard

After this, we used the shell operation to remove as much material as possible and leave the space free for wiring.

Lastly, cuts were made for the interface ties to lock into this central enclosure, and one for a USB-C connector to reach the breadboard.

Step 10: Coding

Starting from the servo sample in the Arduino IDE, it was simple to command a constant spin (designer's intent). The code below shows the final state; the one change made will be described in the next step.

#include <Servo.h>

Servo Yaw;

void setup() {

// Attach the signal line to Digital Pin 3

Yaw.attach(3);

}

void loop() {

// For continuous rotation servos:

// 180 = Full speed one direction (usually CCW)

// 90 = Stop (neutral)

// 0 = Full speed other direction (usually CW)

Yaw.write(110); // Run continuously counter-clockwise

}

Step 11: Test Print & Fitup Round 2

After designing the central section and coding the Arduino, next was to test fitment and operation for this portion of the design. No major issues for fitment, but operation with a commanded "position" (really speed) of 180 was extremely fast (around 60 RPM). These are delivery trucks we have here, not racecars! To temper the speed, we tried a few different values and found that 110 was a nice leisurely speed.

Step 12: Full Scale Printing and Integration

The road surface was printed in black for asphalt, substrate in grey for gravel, vertical snap fit connectors in white for stripes, and horizontal road ties in black to blend in.

After assembling the road surface and substrate and ensuring they were level, we offset the seams to make it as sturdy as possible, then assembled all of the white stripes and road ties.

The interface ties were added to the central support and magnets attached to the steel rod.

The road was placed over the center and locked into the interface ties.

Magnets were adjusted into their channels by reaching through the center hole.

Finally, the breadboard was turned on via the risers, and we had hot wheels seemingly driving themselves, as intended!

Introduction: Hot Wheels Magic Spinner

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