How to Turn a Complex Mechanical Design into a Working Prototype: Lessons from a Self-Balancing Cubli

1. What Makes a Self-Balancing Cubli Work?

Cubli uses internal reaction wheels to control its orientation. When a motor changes the angular velocity of a wheel, the resulting reaction torque can help the cube correct its position.

The system combines several engineering elements:

• A structural frame that supports the complete assembly
• Reaction wheels that generate angular momentum
• Electric motors and motor mounts
• Bearings and torque-transmission components
• Sensors, a PCB, and control electronics

In a compact mechanism like this, available space is limited. Components must fit together without interference, while the overall mass distribution must support the intended balancing behavior.

This makes mechanical design and manufacturing closely connected. A component that looks correct in a CAD model may still create problems when manufactured or assembled.

2. Why Mechanical Precision Matters in Prototype Development

A functional prototype must do more than match the external dimensions shown in a drawing. Its components must also fit, align, and operate as intended.

For a mechanism such as Cubli, several manufacturing considerations are especially important.

Component alignment

Motor mounts, bearing seats, and shaft interfaces must be manufactured according to their specified dimensions and geometric requirements. Misalignment can introduce friction, vibration, or assembly difficulties.

Weight and structural design

The frame needs to provide sufficient stiffness without adding unnecessary mass. Material selection, wall thickness, geometry, and component placement all influence the final assembly.

Internal clearance

Reaction wheels and other moving components need sufficient clearance throughout their operating range. Engineers should consider the complete assembly rather than checking individual parts in isolation.

Repeatable assembly

Mounting holes, threaded features, and connecting surfaces must be positioned appropriately to support reliable assembly and maintenance.

These considerations apply not only to balancing robots but also to robotic joints, automation mechanisms, motion-control equipment, and other precision mechanical systems.

3. Choosing the Right Manufacturing Process

Different components within a prototype may require different manufacturing processes. The most suitable choice depends on the material, geometry, dimensional requirements, quantity, and intended function.

Laser-cut frames

Laser cutting can be suitable for flat frame components and sheet-metal structures. It supports efficient production of profiles and mounting features, making it useful for prototypes with relatively simple two-dimensional geometries.

However, engineers should also consider material thickness, edge quality, hole requirements, and the need for subsequent machining.

CNC-machined motor mounts and transmission components

Motor mounts, bearing interfaces, and other components with three-dimensional features may require CNC milling or turning, depending on their geometry.

CNC machining is particularly useful when a prototype needs accurately positioned holes, machined contact surfaces, pockets, or complex features that cannot be produced efficiently through cutting sheet material alone.

For components requiring multiple machining orientations, 5-axis CNC machining may help access complex surfaces and reduce repositioning, depending on the part design and setup.

4. Small Threading Problems Can Delay an Entire Assembly

One of the practical challenges encountered during the Cubli project involved tapping threads in stainless steel components. A broken tap became stuck in parts of the assembly, preventing some threaded features from being completed as planned.

This illustrates an important point: small manufacturing details can have a disproportionate impact on prototype development.

Threaded holes should be reviewed for several factors before machining:

• Thread depth: Is the required depth necessary for the intended fastening function?

• Hole diameter: Is the specified tap-drill diameter appropriate for the material and thread?

• Tool access: Can the cutting tool reach the feature safely and efficiently?

• Material properties: Does the material require suitable cutting parameters, tooling, and lubrication?

• Assembly requirements: Can the fastener be installed and removed without damaging nearby components?

Stainless steel can be more demanding to machine than many aluminum alloys, particularly when tooling, cutting conditions, and chip evacuation are not properly managed.

A design-for-manufacturing (DFM) review can help identify potential problems before production begins. In some cases, adjusting a thread depth, improving tool access, or revising a feature can reduce machining difficulty without compromising the component's function.

5. Why Early Manufacturing Feedback Matters

A CAD model describes the intended design, but manufacturing feedback helps determine how efficiently that design can be produced.

Before machining a prototype, engineers and manufacturing partners should review:

• Critical dimensions and tolerances
• Material specifications and availability
• Thin walls, deep pockets, and difficult-to-reach features
• Hole and thread specifications
• Surface finish requirements
• Assembly interfaces and inspection needs

Not every dimension needs the tightest possible tolerance. Applying unnecessarily strict tolerances to non-critical features can increase machining time, inspection effort, and overall cost.

The most effective approach is to identify which dimensions directly affect function and assembly, then establish suitable requirements for each feature.

For product designers and engineering teams working under tight development schedules, early communication between design and manufacturing can help reduce avoidable rework.

6. From a Single Prototype to Low-Volume Production

A successful prototype is an important milestone, but it is not always the end of the development process.

Once the initial assembly has been tested, engineers may need to revise dimensions, improve component interfaces, change materials, or manufacture additional units for validation.

This is where a flexible manufacturing partner can be valuable.

CNC machining supports the production of custom metal and plastic components without the dedicated tooling required by some alternative manufacturing processes. It can therefore be a practical option for one-off parts, functional prototypes, design iterations, and low-volume production.

At Jingcheng Precision, we provide custom CNC machining for components made from materials such as aluminum, stainless steel, titanium, and engineering plastics, based on customer drawings and project requirements.

Our manufacturing capabilities include 3-axis and 5-axis CNC machining for custom mechanical parts used in robotics, automation, and product development.

For engineers preparing a new prototype, sharing a 2D drawing or 3D CAD file is a useful starting point for reviewing manufacturability, material options, tolerances, and production requirements.