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Why I Wrote The Sheet Mechanic (And Why Calculations Aren’t Enough)

For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
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High-Speed Automation: The Mechanics of Sankyo SANDEX Indexing

In my design work for high-speed automation, I've frequently relied on indexing boxes from the Japanese manufacturer Sankyo Seisakusho. The SANDEX series is a generic name for their cam units that utilize the superior Roller Gear Cam mechanism.

Sankyo's core technology combines a precision-machined globoidal cam with a follower turret fitted with high-capacity needle bearings. To visualize how this robust mechanism works, consider the conceptual model below.

Conceptual model illustrating the roller gear cam mechanism.
Figure 1: A conceptual model illustrating the interaction between the input roller gear cam (bottom) and the output follower turret (top).
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The Superior Mechanics of the Roller Gear Cam

As depicted conceptually in Figure 1, the SANDEX unit uses a continuous rib on the input shaft to drive the output turret. This is a significant upgrade over traditional Geneva mechanisms.

Geneva drives suffer from high impact shock at the start and stop of motion due to inherent clearance. In contrast, the SANDEX's unique geometry allows for pre-loading: the tapered cam rib is always in contact with at least two followers at any given time. This eliminates internal clearance, resulting in zero backlash, extremely high rigidity, and smooth motion profiles.

Types of Motion and Application

The flexibility of the cam design allows a single unit type to provide different basic motions:

  • Indexing Drives (Intermittent): Start-Stop-Start motion. Used for rotary assembly tables and packaging.
  • Oscillating Drives: Back-and-Forth rotational motion. Ideal for pick-and-place arms.
  • Roller Drives (Continuous): Smooth, continuous rotation used for synchronized conveyors.

SANDEX units generally offer a higher number of stops, flexible index angles, and greater design freedom than other mechanisms. Their broad lineup meets needs ranging from miniature miniature indexes to massive super-size units handling tons of load.

3D CAD view of an automated assembly station with a large rotary dial table driven by a SANDEX indexing gearbox.
Figure 2: A typical industrial application where a SANDEX unit (orange gearbox below) drives a large multi-station rotary automated assembly table.

Figure 2 shows a prime example of a SANDEX unit in action. It is the driving force beneath a large rotary dial table in an automated assembly cell. The zero-backlash characteristic is crucial here to ensure that every station stops precisely in front of the robotic arms for accurate assembly operations.

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Selection Criteria for Mechanical Indexers

When selecting an indexer for your design, you must calculate the dynamic forces involved, not just the static load.

Key factors include:

  1. Moment of Inertia (J): The most critical factor. The torque required to start and stop the load's inertia is usually much higher than the torque required to simply keep it moving.
  2. Index Time vs. Dwell Time: How fast must it rotate (e.g., 0.5 seconds) versus how long does it stay still? Aggressive cam curves (like Modified Sine) are used to smooth out jerk.
  3. Number of Stations: Does the table need to stop 4, 6, 8, or more times per revolution?
  4. Accuracy Requirements: Roller gear cams typically offer arc-second accuracy, far superior to air cylinders or simple motors.
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