Use of Geometric Constraints in AutoCAD

AutoCAD is widely known for its drafting power, but many users are unaware of some handy tools hidden in plain sight. One such area is the Parametric tab, which contains geometric and dimensional constraints. These constraints allow you to control relationships between elements in your drawing, bringing intelligence and simulation-like behavior to your 2D sketches

In this blog, we will explore the Coincident constraint in detail and show how you can use it to simulate the kinematics of a piston mechanism, using familiar mechanical components: the crankshaft, connecting rod, and piston.

Step-by-Step: Creating a Piston Mechanism

Let’s walk through how to build a simplified piston mechanism using only 2D sketches and constraints in AutoCAD.

1. Fixing the Crankshaft’s Rotation Point

   Start by sketching the element that represents the crankshaft. The goal is to allow it to rotate around a fixed point, similar to how a real crankshaft behaves.

   • First, Fix the pivot point on the crankshaft using the Fix constraint. This ensures that this point does not drift during future edits.

   

   • Use the Coincident constraint to align the crankshaft's center point with the center of a reference circle.

   • This sets the rotation anchor point.

   • Now, you can rotate the crankshaft around that fixed point while keeping the connection consistent.

2. Connecting the Rod to the Crankshaft

   Next, you need to connect the connecting rod to the crankshaft.

   • Apply the Coincident constraint between the rod and the crankshaft at their shared pivot point.

   

   This creates a logical connection that mimics the real-world mechanical joint.

3. Attaching the Rod to the Piston

Repeat the same process to connect the connecting rod to the piston.

• Use the Coincident constraint between the rod and the piston at their joint location.

At this stage, you now have a fully linked system: crankshaft ↔ connecting rod ↔ piston. Moving the crankshaft will now cause the connecting rod and piston to move accordingly.

4. Constraining the Piston to Vertical Movement

In real engines, the piston moves strictly vertically. However, in a 2D sketch, without constraints, it may tilt or drift.

To fix this:

• Use the Coincident constraint to connect the axis of the piston to a pivot point.

• Then, apply the Vertical constraint to that axis.



This ensures that the piston can only move up and down, just like in an actual cylinder.

Results: A Realistic 2D Simulation

With just a few geometric constraints, you've created a realistic simulation of a piston mechanism. When you drag the crankshaft:

• The connecting rod moves in response,

• The piston follows,

• And all movements stay geometrically accurate and physically believable.

This kind of simulation is extremely helpful for:

• Design validation

• Educational illustrations

• Preparing geometry for 3D modeling

If you'd like to see this example in action, you can watch it in our YouTube video Geometric constraints in AutoCAD where we demonstrate the full process in real time.

In the next blog and video, we will focus on Dimensional Constraints, since so far we've only covered Geometric Constraints. We'll also explore how to approach basic dynamic analysis using these tools in AutoCAD.

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