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 advanced Mechanism Design, engineers often face the challenge of moving a rigid body from one specific position to another. This process is known as Motion Generation Synthesis.
While sophisticated solver software exists, you can perform this synthesis geometrically using the Constraint-Based Sketcher found in any modern CAD package like Siemens NX, SolidWorks, or CATIA.
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The Goal: Moving a Line in a Plane
Assume we need to design a 4-bar linkage that moves a coupler link from position AB (Start) to position A'B' (Target).
Figure 1: Defining the Start Position (AB) and the Target Position (A'B').
Step-by-Step Geometric Synthesis
The logic relies on finding the center of rotation for the moving points.
1. Locate the First Pivot (O2):
Draw a construction line connecting point A to A'. Then, create a Perpendicular Bisector of line AA'.
Theory: Any point located on this bisector is equidistant from A and A', meaning it can serve as a fixed pivot point.
Figure 2: Creating the perpendicular bisector for vector AA' to locate ground pivot O2.
2. Select Pivot O2:
Select any convenient point on the bisector to be the fixed ground pivot O2.
(Example: Length O2A = 500 mm). Draw line O2A to represent Link 1.
3. Locate the Second Pivot (O4):
Repeat the process for points B and B'. Draw a line from B to B' and create its perpendicular bisector.
Figure 3: Locating the second ground pivot O4 using the bisector of BB'.
4. Select Pivot O4:
Choose a point on this bisector to be fixed pivot O4.
(Example: We selected a 60° angle). Draw line O4B to represent Link 2.
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Constructing the Kinematic Chain
Now we build the full linkage to test the motion:
- Draw line O2M equal in length to O2A.
- Draw line O4N equal in length to O4B.
- Draw the coupler line MN equal in length to AB.
- Apply geometric constraints to ensure the chain stays connected.
Figure 4: The fully constrained kinematic chain ready for simulation.
Validation via "Animate Dimension"
To verify the synthesis, we use the Animate Dimension tool (available in NX, SolidWorks, and Inventor).
Set an angular dimension between the ground and the input link. Configure the animation range (e.g., 0 to 60 degrees) and step count (150 steps for smoothness).
Figure 5: Configuring the animation parameters to drive the input crank.
Watch the Kinematic Synthesis Result:
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