Know the Difference between Fix and Default constraints in CREO Parametric Assembly Design

A constraint in computer-aided design (CAD) software is a limitation or restriction imposed by a designer or an engineer upon geometric properties of an entity of a design model that maintains its structure as the model is manipulated.

In this Tech Tip, we will explain about the difference between ‘Fix’ and ‘Default’ type constraints in Assembly Design using Creo Parametric. Apparently these two types of constraints look similar as they both make the model fully constrained, but we will see two examples wherein we can understand the distinct difference between these two constraints.

The User Case

In the figure below, there are two scenarios. In first scenario, we need to fully constrain the support.prt in the context of the assembly. In second scenario, we need to fix the hook.prt at some angle w.r.t the hook_piston.prt For first scenario, we will use the ‘Default’ constraint and for the second scenario, we will use the ‘Fix’ constraint.

Observe below steps to achieve the required result –

Step 1

Open file – gear_shifter.asm. (You can download practice files by subscribing to any PTC Creo course subscription on Courses and Certificate Programs for Engineers – i GET IT (

Observe that the support.prt from this assembly is not constrained at all and there are two co-ordinate systems visible – one from support.prt and one from gear_shifter.asm

Step 2

Edit the definition of support.prt

Step 3

Hide the 3D Dragger from the ‘Component Placement’ dashboard

Step 4

Select the ‘Default’ constraint from the ‘Automatic’ drop down menu

Step 5

Observe that the ‘Default’ constraint makes the component Fully Constrained, it creates an origin to origin type of constraint and the component co-ordinate system gets connected with the assembly co-ordinate system.

Step 6

Complete the definition of support.prt and observe that the component is fully constrained

Step 7

Open following file – hook_hydraulics.asm, Observe that the hook.prt is not having any constraints

Step 8

Edit the definition of hook.prt

Step 9

Drag the hook.prt along the green direction arrow as shown and select the ‘Centered’ constraint from the ‘Automatic’ drop down menu

Step 10

Select following two spherical surfaces from hook.prt and hook_piston.prt as the references for ‘Centered’ constraint

Step 11

Rotate hook.prt along the blue rotation handle of 3D dragger approximately as shown

Step 12

Rotate hook.prt along the green rotation handle of 3D dragger approximately as shown

Step 13

Select ‘New Constraint’ from the ‘Placement’ tab

Step 14

Select the ‘Fix’ constraint from the ‘Automatic’ drop down menu

Step 15

Observe that the ‘Fix’ constraint makes the hook.prt Fully Constrained, it fixes the hook.prt to the current position and it creates an origin to offset origin type of constraint

Step 16

Complete the definition of hook.prt and observe that the hook.prt is fully constrained and it is fixed at the position which we created by dragging it along the blue and green rotation handles

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