When you are designing something that needs to work in multiple positions, it’s easy to fall into the trap of drawing up your components with arbitrary pivot points and then spend hours spinning it around in 3d and tweaking dimensions in order to get the kinematics to work. A more efficient way is to lay it out in a 2d sketch. Older designers automatically do this as they are used to top-down modelling in 2d- Those of us that have been using 3d CAD for a while may have some bad habits to unlearn.
The basic idea is to make two identical instances of your design in a single sketch and add constraints in both start and end position to fully define the design. The constraints that you add will naturally define what your link lengths and pivot points should be to make it work. Your design will grow organically and you’ll end up with perfect kinematics. Only once this is sorted should you be actually modelling.
Here’s an example:
Say you want to design a mechanism that starts in a vertical position 100mm left of the origin and then finished in a horizontal position 100mm above the origin. The surface needs to be 50mm long. Draw the surface as a 2D sketch and put in the constraints you need.
Now, let’s say we want to achieve this movement with a 4-bar linkage. So, draw in those bars. Below, I’ve drawn both pairs of linkages and constrained them so the respective links are equal in length. Note, I didn’t constrain all links to be the same length, just that the link in the starting position is the same length as its twin in the end position. I’ve put one of these links going to the origin to make things simpler (sometimes too much freedom is a bad thing) but you don’t need to do that.
Lastly, I’ve drawn in another pair of lines- One at 150mm long and one at 250mm long. This represents an actuator like a hydraulic or pneumatic cylinder with a 100mm stroke. You’ll note that this sketch still isn’t fully defined. I’ve started with the minimum number of design constraints in order to avoid over constraining my system. Now that I’m happy with what I’ve got, I can put in a few more arbitrary constraints to stop things from moving around.
I’ve found this method to save a huge amount of time when designing mechanisms. The only pitfall I’ve found is that if you aren’t careful, you can make a mechanism that starts in the correct orientation and ends in the correct orientation but ties itself in a knot somewhere in between. I like to check my mechanism by making a second sketch that references the first to get the link lengths and dragging it around with my mouse.
Happy designing!
Hyllest 