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...
In [3-Position Synthesis with Inversion Method - Part 2], we successfully determined the locations of the moving pivots (G and H) relative to our fixed ground pivots (O2 and O4).
However, finding the points is only half the battle. Before we commit to manufacturing or detailed 3D modeling, we must verify that the mechanism actually moves smoothly between all three positions without locking up (toggle positions) or deviating from the path.
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Constructing the Kinematic Chain
Now that we have our four critical points (O2, O4, G, H), we need to "build" the mechanism links within the CAD Sketcher environment:
- Input Link (Link 2): Draw a solid line connecting the fixed ground O2 to the moving pivot G.
- Output Link (Link 4): Draw a solid line connecting the fixed ground O4 to the moving pivot H.
- Coupler Link (Link 3): This is the most important part. You must draw a rigid triangle connecting G, H, and the original coupler line A1B1.
(Critical: Apply "Rigid" constraints or dimension the triangle sides to ensure G-H-A-B moves as one solid body).
Setting up the "Animate Dimension" Simulation
To simulate the motion without leaving the sketcher, we use the Animate Dimension command (standard in NX, SolidWorks, and Inventor). This tool drives a specific dimension through a range of values, acting as a virtual motor.
Step-by-Step Configuration:
- 1. Add a Driving Dimension: Place an angular dimension between the Input Link (O2-G) and the horizontal ground plane.
- 2. Launch Animation: Right-click the dimension or select Tools > Sketch Constraints > Animate Dimension.
- 3. Set Limits:
- Lower Limit: The angle corresponding to Position 1.
- Upper Limit: The angle corresponding to Position 3.
- 4. Resolution: Set "Steps per Cycle" to at least 100 to ensure smooth motion visibility.
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Visual Verification: What to Watch For
Watch the simulation video below. As the mechanism moves, observe the Coupler Line (AB).
Figure 1: Simulation verifying the coupler passes through all 3 target positions relative to the fixed ground.
Success Criteria Checklist
- Accuracy: The line AB must perfectly overlap with the "ghost" positions A2B2 and A3B3 as it passes through them.
- Stability (Branch Defect): The mechanism must not "jump" or flip configurations between positions. If the lines cross unexpectedly, the solution is invalid.
- Force Transmission: The transmission angles should remain reasonable (ideally between 40° and 140°) to ensure the mechanism doesn't bind or lock up.
Conclusion
By using the Inversion Method combined with Sketch Animation, we have solved a complex constraint problem—fitting a mechanism to pre-existing mounting holes—completely within the 2D CAD environment. This Virtual Prototyping technique saves hours of trial-and-error in the detailed design phase.
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