This document discusses entity manipulation in CAD/CAM software. It describes how transformation tools like translation, rotation, and scaling can move or copy geometry. Other manipulation tools covered include trimming, extending, deleting, grouping entities, and modifying properties. Data structures for CAD need to support different data types, entity properties, and relationships while allowing for interactive modeling and minimum storage. Topological relationships especially need to be stored with geometric data for wireframes and solids. Attributes can also associate non-geometric data to the model through name-value pairs.

1. IPE-409 CAD/CAM
Dr. Nafis Ahmad
Professor
Department of IPE, BUET
Email:nafis@ipe.buet.ac.bd

2. Ch#5.
Entity Manipulation
2

3. Reference books
CADCAM Principles, practice and manufacturing
management- Chris McMohan and Jimmie
Browne

4. Introduction
• Traditionally design drawing modification is extremely
difficult and time consuming
• Facilities in CAD for manipulation are:
• Transformation (translation, rotation, scaling) is done by
moving or copying the geometry
• Changing geometry by trim or extend
• Deletion temporary or permanently
• Grouping entities

5. Object transformation
• In case of object transformation coordinate system is
stationary, entities in a CAD model are manipulated
by moving them around , or by taking one or more
copies at different location(s)

6. Object transformation
• Examples

7. Object transformation
• Rotation of a point in an
anticlockwise direction

8. Object transformation
• Rotation of a
point in an
anticlockwise
direction

9. Object transformation
• Scaling by factors (Sx, Sy, Sz)

10. Object transformation Example 5-1
Mirroring of three
points through a
line at an angle of
30 dg to the Ox
axis with its origin
at x=16, y=24

11. Object transformation Example 5-1

12. Trim and extend and other
functions
• Trim and extend are two important entity
manipulation functionality of CAD system.
• Other functions:
• Deletion
• Group or block
• Modification of entity features like color or line style
• Modify/add/remove control points

13. Trim and extend

14. Trim and extend and other
functions

15. Other important functions of CAD
software
• Deletion- permanent or temporary; undo or redo
• Group or block-Collections of number of entities
together, such that they may be manipulated as a
single entity.
• Modification of entity features like color or line style
• Manipulation of control points- Modify/add/remove
control points for curves such as Bezier and B-spline
types.

16. Data Structures
• Requirements for interactive modelling:
• interactive manipulation
• Support different data type (geometric, textual,
dimensions, labels etc.)
• Allow properties to geometric element (line style,
color etc.)
• Allow association between data elements
• Undoing, instancing,
• Minimum storage requirement etc.

17. Data Structures
• Constraints
• different entities require different data
structure and size to store such as point, line,
circle, spline needs different amount of data.
Need dynamic memory allocation.
• Each entity requires integer as well as
floating point data. Spline knot points are
floating but line type, color, number of knot
points are integers

18. Data Structures
• Typical Data structure:
• A simple data structure that does allow arbitrary quantity
of data for each entity and arbitrary combination of
entities, comprises a list or table of entities, which cross
references ( or pointer) from this list to separate arrays of
floating point, integer and other data specific to the
entities.
• A line will have real data for x, y, z coordinates but no
integer data is required
• A spline curve might have the number of knot points,
stored in integer table, while the curve parameters at each
knot point , together with the segment coefficients and/or
knot point coordinates are stored in real data table.
• Pointers to following and preceding entity.

19. Data Structures

20. Associative geometry and
attributes
• Relationship between other than grouping.
Example: Topological relationship must be
stored with geometric data in case of wire frame
or solid models.

21. Associative geometry and
attributes
Attributes:
• associating non-geometric data with the geometric
model through the use of attributes such as name-
value pairs. Name is string of alphanumeric
character; value may be string or number
• BOM: name, number, supplier, price etc.
Relationship between other than grouping.
Example: Topological relationship must be stored
with geometric data in case of wire frame or solid
models

22. Assignment
Submit a Bill Of Materials (BOM) of the pen
you made using CAD software in earlier
assignment directly from CAD software.

23. 23
Thanks!
Any questions?
You can find me at: https//ahmadn.info
Twitter@ahmadn
Email:nafis@ipe.buet.ac.bd