2 Constraint Modeling

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CONSTRAINT BASEDSKETCHING

Geometric and Dimensional Constraints

Simulazione e Prototipazione Virtuale


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ConstraintsMost features (in particular any base feature)

require a 2D sketch.Constraints, which are associations andrestrictions applied to 2D geometry.

Two different types of constraints:

1. Geometric• Constraining geometry based on normal

orientations or relative to other parts of thegeometry

2. Dimensional

• Numerical values that constrain the size and

location of geometry


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Constraints

• In the design phase of a project, constraints provide a way to enforcerequirements when experimenting with different designs or when makingchanges. Changes made to objects can adjust other objects automatically,and restrict changes to distance and angle values.

• With constraints, you can

• Maintain design specifications and requirements by constraining the geometry within adrawing

• Apply multiple geometric constraints to objects instantly

• Include formulas and equations within dimensional constraints

• Make design changes quickly by changing the value of a variable


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Designing with Constraints

When you work with constraints, a drawing will be in one of three states:• Unconstrained. No constraints are applied to any geometry.

• Underconstrained. Some constraints are applied to the geometry.• Fully constrained. All relevant geometric and dimensional constraints are applied to the geometry. A fully

constrained set of objects also needs to include at least one Fix constraint to lock the location of thegeometry.

Thus, there are two general methods for designing with constraints:• You can work in an underconstrained drawing and make changes as you go, using a combination ofediting commands, grips, and adding or changing constraints.

• You can create and fully constrain a drawing first, and then control the design exclusively by relaxing andreplacing geometric constraints, and changing the values in dimensional constraints.

The method that you choose depends on your design practices and the requirements of your

discipline.

Note: The program prevents you from applying any constraints that result in an overconstrainedcondition -> Error Messages->Diagnostic tools


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Example


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GEOMETRIC CONSTRAINTS


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Basic Geometric Constraints

• Constraints relative to the normalconventions for direction.

Examples: vertical, horizontal

• Constraints that are relative to othersketch entities.

Examples: perpendicular, parallel


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Advanced Geometric Constraints

• Tangent Constraints• Arcs or Circles

• Equality Constraints

• Equal sizes

• Concentric constraints• Align Center Points


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Automatic Constraints

• Constraints are automatically applied as you sketch. The constraint symbol onthe cursor shows the constraint type. Constraints prevent unwanted changesto a feature when dimensions are changed or referenced geometry is moved.


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Automatic Constraints

While sketching SolidWorkswill automatically guess at

the constraints that the userimplies

Dotted lines show alignmentand icons show the

constraints

Unwanted constraints can beselected (left click) and

deleted (delete key)


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SolidWorks: Selecting

Lines or points can be

selected by holding Ctrland left clicking on each

At the left of the screenrelations can be added

by selecting theappropriate icon

The lines will snap to the

applied constraint andicons will display thecurrent constraints


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Exercise

Relationships:• Vertical

• Horizontal

• Parallel

• Perpendicular• Equal

• Coincident

Circular Relationships:• Tangent

• Concentric

Draw the following object andconstrain with geometricconstraints only


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Exercise

• List all the necessary constraints to go from the initial shape to the final,then apply the constraints on the SolidWorks part file to confirm

1

23

4

5

6

78


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DIMENSIONAL CONSTRAINTS


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Dimensional Constraints

• Linear Dimensions

• Radial Dimensions

• Angular Dimensions


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Algebraic (dimensional) Constraints

• Simple AlgebraicExpressions

• i.e. 20/3

• Related to Other

Dimensions

• Driving vs. Driven

Is D5Driving or

Driven?


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SolidWorks: Dimensions

Dimensional Constraints can

be added using the SmartDimension Button

1. Click on the entity2. Move cursor off object3. Click once more to place

dimension

When Constraints are appliedthe following dialog box willappear displaying• Dimension Name

• Editable Dimension Value• Options (direction, etc.)


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Or even 2 points as long asthe correct direction is chosen

( the Smart Dimension tool

shows options when movingthe mouse)

SolidWorks: Dimension Options

Dimensions can be addedbetween 2 lines by selecting

the lines sequentially

Between a point and a line


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SolidWorks: Circles and Arcs

Circles or Arcs can bedimensioned by thediameter (Ø) or radius (R)

A circle can be located by itscenter point by constrainingboth the x and y directions


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SolidWorks: Angular Dimensions

Angular Constraints can be addedby clicking one line and then the

other line. SolidWorks willautomatically assume angularconstraint.


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SolidWorks: Algebraic Constraints

Constraints can beadded in the form ofequations by:1. Entering “=“

2. Entering a valueor by left clickingon a dimension

3. Using operations+, -, *, /


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BEST PRACTICE: Design Strategy

Effective robust designsstarts with shape andgeometry constraints,and then dimensionalconstraints are added


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Dimensions for reference only

• You cannot overconstrain a sketch, but you can add dimensions for reference only.

• Usually, a dimension constrains sketch geometry to a specific size. You can add dimensionsuntil a sketch is fully defined (all degrees of freedom are removed). Elements of the sketchthat are not dimensioned are adaptive and can resize when another dimension changes.

• Sometimes, you want to place (driven) dimensions. Driven dimension do not constrain thesketch and reflect the current value of the geometry. They are enclosed in parentheses todistinguish them from normal (parametric) dimensions.

• You can change a driven dimension to a normal dimension. If the driven dimension you wantto change would overconstrain the sketch, delete or change another dimension before you

change the driven dimension. Select the Style box, and click Normal.

• To change a normal dimension to a driven dimension, select the Style box, and click Driven.


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OVER-CONSTRAINED, UNDER-

CONSTRAINED OR JUSTRIGHT?

Evaluating DOF of Sketched Profiles


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Degrees of Freedom for 2D Entities


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Combining DOF in 2D Profiles

Thus, for any continuous, closed profile composed of only line segmentsand arcs, we can calculate the number of degrees of freedom in thesketch. Each line requires 4 DOF and each arc requires 5 DOF.

Each coincidence removes two DOF. Note that there is the same numberof coincident points as there are entities in the sketch.

Therefore, we can derive the following simple formula for DOF of a closedsketch:

DOF = 2*L + 3*A

where DOF is the number of degrees of freedom, L is the number of linesegments and A is the number of arcs in the loop


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Example: evaluating DOF

4*2 + 1*3 = 11 DOF


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Example: constraining


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Example: constraining

5*1 + 4*1 = 9 DOF removed

Thus, the profile is not fullyconstrained. The two remainingDOF represent the position of the

sketch in the local coordinatesystem.

One vertex needs to be

specified (x1,y1).


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Exercise

• Model this 2D sketch asfully constrained.

• If some dimensions aremissing, assume them.


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Exercise

• Model this part with fullyconstrained sketches.

• If some dimensions aremissing, assume them.

2 Constraint Modeling

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