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Constraint-Based Modeling in the Engineering Graphics Curriculum: Laboratory Activities and Evaluation Strategies

Theodore J. Branoff

Department of Mathematics, Science and Technology Education North Carolina State University, Raleigh, North Carolina 27695-7801

ABSTRACT – Three-dimensional solid modeling,

especially constraint-based modeling, has significantly

changed the way engineering graphics programs are

structured. Although traditional concepts such as

orthographic projection, sectional views, auxiliary

views, and dimensioning are still covered in many

programs, they are couched within the context of a 3D

database centered engineering design process. The

Graphic Communications Program at North Carolina

State University has made course and programmatic

changes over the last 10 years to adapt to changes in

how 3D modeling is used in industry. After completing

a 3 course sequence in engineering graphics, students

should be able to complete a wide variety of activities

related to constraint-based modeling. To meet these

objectives, students complete tutorial-based laboratory

assignments, reverse engineering activities, and

structured design activities. The way student work is

evaluated has also changed. This paper presents the

objectives for each course, gives examples of

constraint-based CAD activities in each course, and

discusses evaluation techniques for the activities and

projects.

I. Introduction

Constraint-based solid modeling has significantly

changed the types of activities in engineering and

technical graphics courses and the way those activities

are evaluated. Presentations and publications by

engineering design graphics faculty reflect a concern

for how to integrate current technology into the

curriculum, while deciding what traditional topics need

to remain.

In a recent study, Barr, Krueger, and Aanstoos

(2004) surveyed engineering and technical graphics

faculty and asked them to rate the importance of

traditional and modern engineering design graphics

topics. They reported that the modern engineering

design graphics curriculum should include a trichotomy

of instruction. Instead of only focusing on engineering

drawings, faculty should focus on three areas of

instruction: computer graphics modeling fundamentals;

engineering graphics fundamentals; and computer

graphics modeling applications. The items related to

computer modeling were ranked the highest in their

survey, however, faculty still felt traditional

engineering graphics concepts related to drawing and

sketching were a valuable component of the

curriculum.

The change to a curriculum focused more on 3D

modeling has forced faculty to examine how students

are assessed. Because constraint-based modelers allow

the user to build intelligence into a 3D model,

evaluating only print-outs of CAD assignments is no

longer sufficient to assess student work. Assessment of

constraint-based models must be done by examining

each student’s electronic file. This can be one of the

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most difficult things for faculty to incorporate into their

instructional routines, especially with classes having

large numbers of students. Evaluating models can be

done by the faculty member (Branoff, Wiebe, &

Hartman, 2003), by trained teaching assistants or

graduate students (Elrod & Stewart, 2004), or by an

automatic grading system (Baxter, 2003; Baxter &

Guerci, 2003). The challenge can be providing valuable

feedback to the students in a timely manner. In addition

to evaluating CAD assignments, faculty have been

exploring strategies for assessing traditional

engineering graphics topics (Demel, Meyers & Harper,

2004) as well as how students work within teams

(Elrod & Stewart, 2004; Kelley, 2001).

Over the last several years, the Graphic

Communications faculty at North Carolina State

University has been revising the content and evaluation

methods in their courses to provide better learning

experiences for students. What follows is a description

of 3 courses in engineering and technical graphics and a

summary of the activities and evaluation strategies used

in each course.

II. Introductory Courses

Three introductory courses in engineering and

technical graphics are offered within the Graphic

Communications Program – one is open to any student

at the university (GC120), one is for mechanical and

aerospace engineering majors (GC211), and one is

designed for industrial engineering majors (GC210).

The main goal of the introductory courses is to provide

an orientation to the language of technical graphics.

The courses help students develop and refine their

ability to use this universal technical language within

the context of the concurrent engineering design

process as well as gain an understanding of how

computer-aided design is used to create objects that

students use on a daily basis. Emphasis in the

introductory courses is placed on the decision-making

process involved with creating constraint-based

geometry and the development of solid modeling

strategies that incorporate the intentions of the

designer. Students participate in activities that involve

their analysis of geometry at a fundamental level, the

relationships between geometric elements, and the new

mentality of “modify and re-define” rather than “delete

and re-create”.

At the end of the introductory course, students

should be able to perform the following related to

constraint-based modeling: select and create sketch

planes; create and constrain sketches; define sweep

parameters (extrudes & revolves, one or two sided,

etc.); revise sketches and features; create repetitive

features such as circular and linear patterns; create

features such as fillets, chamfers, sweeps, lofts, and

shells; create assemblies of parts and apply appropriate

3D constraints; render the assembly by applying

materials to each part and define an appropriate scene

or environment; and create detail drawings of parts

(including sectional views, dimensions, and other

notations) by extracting information from the 3D

models. Students are asked to complete a range of

assignments during the semester to develop their

knowledge and skills in constraint-based CAD. Figures

1 through 4 show some of the assignments that are used

to develop and assess students in the introductory

courses.

Figure 1. STOP BASE.

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Figure 2. DRYER CLIP.

Figure 3. PISTON CAP.

Figure 4. TAILSTOCK CLAMP Detail Drawing.

For the final project, students are asked to select a

design that contains 3-5 parts that they must reverse

engineer. Along with planning and documenting the

modeling strategies for each part through freehand

sketching, students must model each part, create a

rendered assembly of the design, create a detail

drawing of one of the parts, and submit a technical

report of the project. Figures 5 & 6 are examples of

student projects.

Figure 5. Bicycle Axle.

Figure 6. Dividers.

III. Applied CAD & Geometric Controls

The second course in the engineering and technical

graphics series (GC350) was designed to give students

direct exposure to and interaction with the evolving

industrial use of computer-aided design and modeling.

Students produce mid-level computer models of

individual parts and assemblies of parts that encompass

the full range of current CAD software capabilities

from 3-Dimensional feature-based solid modeling to an

exploration of design for manufacture. Students apply

conventional tolerancing, geometric dimensioning and

tolerancing, and technical documentation to a variety of

standard and non-standard parts in the course.

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At the end of the course students should be able to

perform all modeling activities included in the

introductory course as well as the following activities:

build design intent into a model based on specific

parameters; apply conventional tolerances (limit

dimensions) and geometric tolerances to 3D models

and engineering drawings; create multiple

configurations of a 3D model using design tables;

model objects that require sweeps and lofts; create a

rapid prototype of a model using a 3D printer; and

create an assembly drawing of a design which includes

a bill of materials. Figures 7 through 9 show examples

of assignments used to develop and assess students in

the GC350 course.

Figure 7. TRIP LEVER Drawing and Model.

Figure 8. SLIDING DOOR GUIDE.

Figure 9. Design Table of Woodruff Key.

The final project in the GC350 course involves

creating a complete set of working drawings for a

design. Students model all individual parts, create detail

drawings of non-standard parts (including applying

conventional tolerances to all mating parts and

geometric dimensions to one part in the design), create

an assembly drawing (which includes a bill of

materials), and create a rendered assembly of the

design. Figures 10 and 11 illustrate examples of final

projects in this course.

Figure 10. CLAMP FIXTURE Assembly.

Figure 11. TOOL REST Assembly.

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IV. Advanced CAD

The capstone course in the engineering and

technical graphics series (GC450) was designed to

provide students with a culminating experience where

they could apply their knowledge of computer-aided

design. Students explore the theory and application of

manufacturing databases developed with 3-D modeling

tools. They also examine the development and

management of 3-D geometry and investigate

downstream applications such as analysis,

documentation, and prototyping. In addition to readings

in the areas of modeler types and databases, curves and

surfaces, constraint-based and parametric modeling,

mass properties, kinematic and dynamic analysis, finite

element analysis, computer-numerical control,

documentation, and modeling for manufacturing,

students complete two large projects. The first is a

flashlight design project where students research

flashlights for a particular application, sketch multiple

iterations of their designs, narrow the design down to

one, model all parts using solid and surface modeling

tools, render the design, and present their design to the

class. Examples of designs from the course are shown

in Figures 12 and 13.

Figure 12. Handlebar Mounted Light.

Figure 13. Carbide Lattern Design.

The second large project in the course is a group

project. As a class, students reverse engineer a small

lawn mower engine (see Figures 14 and 15). The

students determine logical divisions for groups (eg.

drive train, engine block, sheet metal parts, etc.) and

then divide the modeling tasks equitably among group

members.

Figure 14. Small Engine Assembly.

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Figure 15. Drive Train Sub-Assembly.

V. Assessment Strategies

A variety of assessment strategies are used

throughout the courses. Approximately 20 sections of

the introductory courses are taught each semester by at

least 9 different instructors. Each has their own

preference for how to evaluate assignments. Constraint-

based modeling activities are evaluated electronically

by the instructors. This may be done in a couple of

different ways. Instructors may grade assignments in

the lab with the students present or they may ask the

student to submit their files to a server so the file can be

graded in a remote location (in the office or at home).

For most assignments, instructors will focus on a

handful of items. Table 1 shows the items that the

instructor examines for the TRIP LEVER (Figure 7).

Table 1. Grading Rubric for the TRIP LEVER.

Description Points Part dimensions are correct 1 point Part orientation is correct 1 point Spotfaced hole remains centered when depth of part is changed

1 point

Slot remains centered size is changed 1 point 9.5 diameter hole remains centered on tab when tab depth is changed

1 point

Total 5 points

Feedback to a student might come in the form of

an email or a print-out handed to the student in class.

An example of feedback to a student for the TRIP

LEVER might be: “3/5 points. Sketches for Cut-

Extrude2, Cut-Extrude3, & Cut-Extrude4 are missing

dimensions. Please make corrections per these

comments and show the modified part to me in class

Wednesday. There is no need to resubmit the part to the

homework directory.” Some instructors will allow

students to correct their models and resubmit them.

Although this can make managing homework grades

and classroom activities more difficult, correcting

existing models appears to be more valuable to students

than just receiving comments and a grade.

For the final projects in each course, students are

given a detailed rubric for how each part of the project

will be evaluated. An example grading rubric for the

final project in the introductory course is shown in

Figure 16.

Figure 16. Project Grading Rubric.

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VI. Discussion and Reflections

Two of the biggest challenges to this point have

been faculty training and instructional materials

development. With new releases of the software

coming out every year, faculty development must be a

priority. Instructors have to understand the power of the

constraint-based modeling tool to be able to make

connections between it and the engineering graphics

topics covered in the course. They also must be able to

troubleshoot a wide range of problems that students run

into while creating models.

The development of supporting instructional

materials has also been a challenge. Revising materials

for new releases can be time consuming, and creating

materials that are acceptable to every instructor is

almost impossible.

The integration of constraint-based modeling into

the Graphic Communications curriculum appears to be

going well. Students taking the three course sequence

in engineering and technical graphics typically major in

mechanical and aerospace engineering or technology

education. They are securing employment throughout

the country in a variety of careers. Some recent

graduates are working for engineering firms where they

use their knowledge of constraint-based CAD along

with their engineering degree for firms such as

Integrated Industrial Information, Inc.,� FineLine

Prototyping, Inc., Boeing, and Raytheon Missile

Systems. Others are using their degree in education to

teach technology education and drafting courses in

public schools in North Carolina.

VII. References

Barr, R. E., Krueger, T. J. & Aanstoos, T. A. (2004). Results of an EDG student outcomes survey. Proceedings of the 2004 Annual Conference of the American Society for Engineering Education, Salt Lake City, Utah, June 20-23, 2004.

Baxter, D.H. (2003). Evaluating an automatic grading system for an introductory computer aided design course. Proceedings of the 58th Annual Midyear Conference of the Engineering Design Graphics Division of the American Society for Engineering Education, Scottsdale, Arizona, November 16-19, 2003. Baxter, D.H. & Guerci, M. J. (2003). Automating an introductory computer aided design course to improve student evaluation. Proceedings of the 2003 Annual Conference of the American Society for Engineering Education, Nashville, Tennessee, June 22-25, 2003. Branoff, T. J., Wiebe, E. N, & Hartman, N. W. (2003). Integrating constraint-based CAD into an introductory engineering graphics course: Activities and grading strategies. Proceedings of the 2003 Annual Conference of the American Society for Engineering Education, Nashville, Tennessee, June 22-25, 2003. Demel, J. T., Meyers, F. D. & Harper, K. A. (2004). Developing a nationally normed test for engineering graphics-First pilot tests and results. Proceedings of the 2004 Annual Conference of the American Society for Engineering Education, Salt Lake City, Utah, June 20-23, 2004. Elrod, D. & Stewart, M. D. (2004). Assessing student work in engineering graphics and visualization course. Proceedings of the 2004 Annual Conference of the American Society for Engineering Education, Salt Lake City, Utah, June 20-23, 2004. Kelley, D. (2001). Cooperative learning as a teaching methodology with engineering graphics. Proceedings of the 2001 Annual Conference of the American Society for Engineering Education, Albuquerque, New Mexico, June 24-27, 2001.