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Constraint with Expressions
FreeCAD has a powerful Expression Engine. This tutorial shows how to create simple mechanism using constraints driven by some expressions.
It is very easy to create a screw mechanism with PlaneConicident
constraint.
Just follow the simple steps below
- Set the constraint property
LockAngle
toTrue
; - Enter an expression;
- If you want to drive by rotating the screw, then express the
Offset
property in term of theAngle
property, as shown in the screen cast below; - If you want to drive by translation, then express
Angle
in term ofOffset
.
- If you want to drive by rotating the screw, then express the
A slightly more complex example is a rack and pinion system shown below. All gears have module of 1mm. The big pinion gear has 18 teeth, while the small one has 15. You can download the parts here. You need to install the Gear Workbench if you want to modify the gears.
By the look of the picture above, one may intuitively think of using the
PointOnLine
constraint for the gear in the slot. But in this case, it is much
easier to use, again, the all powerful PlaneCoincident
constraint. We divide
this system into two assemblies for, well, the pinions and the rack.
First, create a sketch to define the positions of the two pinions. In real applications, you probably should derive the distance of the two circle using expressions, too, as well as most of other constants shown in this tutorial, or better, use a Spreadsheet.
Then, create an assembly, add the sketch and two gears, and position the two
using the sketch with PlaneCoincident
constraint.
Toggle the LockAngle
property of both constraints. And since we will be
driving the big gear, enter the expression for the Angle
property of the
small gear constraint as below, which is basically the gear ration with some
offset,
-Constraint002.Angle*18/15-6
You can test the mechanism by changing the Angle
of the big gear constraint.
Now, we create the final assembly, add the pinion assembly and the base with
the rack gear, and fix them together with another PlaneCoincident
. Notice
that we are using the Sketch
for constraining. This is important, because
the gear is meant to be rotated, while the Sketch
is fixed. We shall again
lock the rotation of this constraint. We want to translate the pinions in its
relative Y position, so enter an expression in the OffsetY
property of the
constraint,
-Constraint001.Angle/360*15*pi
Done!