Geometric Modeler

What You Will Learn With This Use Case

In this use case, you learn how to create a sweep, close it and remove the resulting body from the rod to be threaded. You also learn the basics about how to create a user operator.

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The CAAAdtThreadOpMain and CAAAdtThreadUtility Use Cases

CAAAdtThreadOpMain and CAAAdtThreadUtility are use cases of the CAAAdvancedTopologicalOperators.edu framework that illustrate the CAAAdvancedTopologicalOperators framework capabilities.

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What Do CAAAdtThreadOpMain and CAAAdtThreadUtility Do?

The CAAAdtThreadUtility use case defines the CAAAdtThread operator. Like any other operator, it must specify a constructor as well as a destructor a Run method a GetResult method. In addition, two global functions which are discussed later on are defined in the CAAAdtThreadUtility use case. The CAAAdtThreadOpMain use case contains the main program that creates the cylinder to be threaded and call the CAAAdtThread operator. This is the solid you obtain when you execute the CAAAdtThreadOpMain use case.

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How to Launch CAAAdtThreadOpMain

You must compile CAAAdtThreadUtility prior to launching CAAAdtThreadOpMain.  CAAAdtThreadOpMain requires CAAAdtThreadUtility as a module to be linked with in its Imakefile.mk file.

To launch CAAAdtThreadOpMain, you will need to set up the build time environment, then compile CAAAdtThreadOpMain.m, set up the run time environment, and then execute the use case [1].

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Where to Find the CAAAdtThreadOpMain and CAAAdtThreadUtility Code

The CAAAdtThreadOpMain use case is made of a main named CAAAdtCreateThread .cpp located in the CAAAdtThreadOpMain.m module of the CAAAdvancedTopologicalOperators.edu framework:

where InstallRootDirectory is the directory where the CAA CD-ROM is installed.

This use case uses the CAAAdtThread class which is defined in the CAAAdtThreadUtility.m module.

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Step-by-Step

The CAAAdtThreadOpMain use case is divided into the following main steps:

• Creation of the geometry factory (not discussed below - see for example [2])
• Creation of a CATSolidCylinder (see [2])
• Creation of the thread operator by using the CAAAdtThread operator defined in the CAAAdtThreadUtility.m module. This is the most interesting part of the use case. In this part, we must define
• the constructor and the destructor of the operator
• the Run() method whose operations are going to be reviewed below:
  1. Creation of a first sweep
  2. Boolean remove from the initial rod
  3. Creation of a second sweep
  4. Boolean remove from the body resulting from the first remove.
• the GetResult() method.
• Running the CAAAdtThread operator and retrieving the resulting body in the main program.

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The Methodology

At first sight, the methodology to thread is rod would consist in removing a single triangular sweep from the cylindrical rod as indicated below:

Step 1 Creating a cylindrical rod	Step 2 Sweeping a triangular profile along an helix	Step 3 Boolean remove

However, a usual source of problems when performing boolean operations is tangent or near-tangent surfaces. In the sequence of operations above, removing the sweep from the rod would result in a crash because the cylindrical surfaces of  both operands are coincident. This problem can be worked around by creating extensions so that the sweep to be substracted out has a diameter  slightly larger that the rod diameter. Another source of problem is when one operand auto-intersects. If we make the sweep jut out from the rod, unless the sweep auto-intersects, parasite cylindrical faces will be generated in place of the outer edge of the thread. This is not what we want to obtain.

To get over these two restrictions, we decided to proceed in two phases:

Phase 1: a first sweep is created. The profile for this first sweep is a rectangular triangle as described below. This sweep is removed from the rod.

Phase 1 (a) Initial  rod	Phase 1 (b) First sweep	Phase 1 (c) First Boolean remove

Phase 2: a second sweep is created. A symetrical profile is used for this second sweep. This second sweep is removed from the solid resulting from the first boolean remove.

Phase 2 (a) Intermediary solid	Phase 2 (b) Second sweep	Phase 2 (c) Second Boolean remove

Creating the Sweeps

The sweeps are created in the CAASweepCreate function. The CATFrFTopologicalSweep operator is created by the CATCreateFrFTopologicalSweep global function requires a guide and a profile as arguments.

// a - Create the sweep operator
//
CATFrFTopologicalSweep * pSweepOperator = CATCreateFrFTopologicalSweep(
        myFactory,
        myTopdata,
        iHelix,     //  guide
        iSection);  //  profile
  

Other parameters are tuned by using the Setxxx methods.

Specifying the Guide

The guide is an helix which is created by the CATCreateTopHelix global function.

// b - Helix creation
//
CATBody* piHelix = CATCreateTopHelix(
        _factory,
        _topData,
        piHelixAxis,                     // axis
        1,                               // orientation
        piBPOrigin,                      // origin
        CATAngle (0),                    // start angle
        CATAngle (helixAngleInt*CATPI),  // end angle 
        CATLength (pitch),               // pitch
        1);                              // orientation of the rotation about the axis
  

The helix parameters are:

• the axis is defined by the P1 and P2 CATMathPoint.
• the origin which is not located on the rod cylindrical surface so that the sweep to be created later on protrude off the rod.
  

  CATBody * piBPOrigin = CATCreateTopPointXYZ ( _factory, _topData, P1.GetX() + (cylRadius + 0.001), P1.GetY() , P1.GetZ());

• the start and end angle. The end angle (the number of helix turns) is roughly determined so that the portion of the rod to be threaded goes until an extremity of the rod. The distance where the thread starts is passed as the argument of the CAAAdtThread constructor in the form of a length.

Specifying the Profile

The profiles are defined in the CAASweepProfile function. They are originally created in the XOY plane but their positioning is moved to the beginning of the helix.

CATMathAxis Ref; // the canonical math axis
pSweepOperator->SetProfilePositionType(1);
pSweepOperator->SetProfilePosition(&Ref); // set profile axis system
  

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Specifying the reference element

In order to avoid a twisted sweep, the XOY plane must be specified as the reference element.

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In Short

When writing an operator such as CAAAdtThread, the steps requiring attention are:

• the definition of the sweep(s). An important parameter is the reference plane. If it is not properly specified, the resulting sweep can be twisted.
• the boolean operations: a boolean remove operation cannot complete when there are tangent or near-tangent surfaces in both operands. You must design the operand to be extracted so that it protrudes off the operand it is to be extracted from.

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References

History

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