CAD Analysis and Practice

Integra

ted CA D/CAM techniq for Prod ues uct Des ign

Szuchi

Wang

1. Report on Part A: Redesign Crossbow 1.1 Introduction 1.2 Design Features 1.3 Design Process 1.4 Description of Parts 1.5 Parts and the assembly modelling 1.6 Presentation Rendering 1.7 Manufacturing modelling and physical prototype 1.8 Stress Analysis and Modelling 1.9 Summary (What you have learnt) 2. Report on Part B: Design of Stereo 2.1 Introduction 2.2 Design Features 2.3 Design Process 2.4 Surface modelling of the conceptual product 2.5 Description of a evaluation scene set up in Jack速 system 2.6 Ergonomic evaluation process and results with virtual humans 2.7 Summary of Part B 3. Overall assignment summary 3.1 Time spent 3.2 Reflection of your learning outcomes and experience 4. References ===========================================================

1. Report on Part A: Redesign Crossbow 1.1 Introduction A crossbow is a weapon consisting of a bow mounted on a stock that shoots projectiles, often called bolts or quarrels. The medieval crossbow was called by many names, most of which derived from the word ballista, a torsion engine resembling a crossbow in appearance.

Historically, crossbows played a significant role in the warfare of East Asia, Europe and the Mediterranean. Today, they are used primarily for shooting sports, hunting, and when shooting in silence is an important consideration.

1.2 Design Features This model targets on making the functional trigger system of the crossbow, which includes trigger, spring, tenon and bowstring.

1.3 Design Process First I decide to use laser cutting acrylic board as material to make the main body because crossbow need stronger support to function properly without crack, and RP handle to make a smooth shape for users to hold. I made the first prototype to test the function (pic below), then I changed the model twice to have the best solution of the design, after all the parts can function well I sent the handle to RP so that there would be no error during the RP process because I know RP always takes long time to receive.

1.4 Description of Parts Here are half the set of the first vision of my crossbow, by calculating the sizes carefully, I can fix all the components together by using the acrylic sticks, and tight the bow at the gap in front of the main body, besides, the trigger and the tenon can still rotate smoothly by having slightly bigger holes in it. After the first testing, I found that the trigger got jammed easily because of the wrong angle of tenon, so base on which I adjusted the following models mainly on smoothing the process in firing.

1.5 Parts and the assembly Modelling I made the whole model by SolidWorks based on the laser cutting technic, so the main body was built by 3mm layers, and the handle would be built by RP so it could be made round and streamlined. After I did all the parts, I used them to make the final assembly, during this step I could see some mislocated and miscalculated dimensions of the parts so I could correct it before sending to manufacturing.

1.6 Presentation Rendering I used KeyShot to make the presentation rendering, which is easy to learn and can get the result very quick, users just need to drag the materials to the models and double click it to adjust the details, also applied to the environment. After settle the model well, users can go to the rendering setting to adjust the detail of the picture then users can have the lovely rendering. For my crossbow I choose gold and iron black to make the tough feeling of my little weapon.

1.7 Manufacturing modelling and physical prototype I have changed my model three times to make it better on both function and appearance, reducing the size of the main body as much as I can to make the crossbow lighter to hold, and also changing the shape of the trigger, bow and tenon to be more efficient.

For the physical testing part, though the acrylic is a kind of hard material, it still easy to broke when there are small cracks on it, so I used the rubber band as the bowstring in case the bow breaks. Even though I already changed my model 3 times the trigger still got jammed after firing because of the elasticity of the bowstring, so after countless failure I finally found out that if I tight a rubber band on the tenon the trigger could work properly ever after.

1.8 Stress Analysis and Modelling After building all the parts, the final step is to make the stress analysis, I choose handle and bow to analyze because these were two main components that bear the greatest stress. First step is defining the fixed surface, deciding which surface would have to take over the forces, then generating the stress analysis report to see which area should have more support to avoid crack.

1.9 Summary (What you have learnt) From this practice I learnt that designers should always consider about the manufacturing process when designing, without which designers could waste lots of time and money on making physical prototypes, and by using software to test the strength of each part of the model, designers can adjust their model easily to have the best result.

2. Report on Part B: Design of Stereo 2.1 Introduction Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of directionality and audible perspective.

This is usually achieved by using two or more independent audio channels through a configuration of two or more loudspeakers in such a way as to create the impression of sound heard from various directions, as in natural hearing. Thus the term "stereophonic" applies to so-called "quadraphonic" and "surround-sound" systems as well as the more common 2-channel, 2-speaker systems. It is often contrasted with monophonic, or "mono" sound, where audio is in the form of one channel, often centered in the sound field (analogous to a visual field). Stereo sound is now common in entertainment systems such as broadcast radio and TV, recorded music and the cinema.

2.2 Design Features This stereo has two normal speakers, one bass, one CD player, two adjusting buttons and the input/output set, and the style of it is tough of iron-look.

2.3 Design Process For this stereo I used Alias to build the 3D model. First I made the main body by cubes and curves, then link them together by skins and surfaces, trim some holes for buttons and speakers and attach the charger on it, after this I use stich to make it solid to finish.

2.4 Surface modelling of the conceptual product Alias is a easy-to-use software for surface modelling, I built most of the parts by curves then use skin or birail to form the surface, trim to have the needed holes, after this I put the components in the right place and finish the model.

2.5 Description of a evaluation scene set up in Jack速 system This stereo is designed for putting on the desk-top hight, so the scenario of usage is a human sits in a chair and adjusts the volume (top button) and bass (lower button) of the sound of the stereo on a table.

2.6 Ergonomic evaluation process and results with virtual humans In this case I analysed the lower back force of the upper body joints to see what it would cause when operating the stereo, besides, I also analysed the vision field and the blind spot that caused by sitting in front of the stereo.

2.7 Summary of Part B By using surface modelling software and virtual human system in part B, I gained the taste of how helpful it would be by using virtual human system when in design step, avoiding time and money of physical model test.

3. Overall assignment summary 3.1 Time spent In part A, I spent 1 day to built the first vision of the model, after that I spent 4 weeks to adjust the model, talking to technicians and wait for the RP process and assemble it. In part B, I spent 2 days to built the model, 2 days to test how to make it right in Jack, and for both part A & B, I spent 1 day to do all the rendering.

3.2 Reflection of your learning outcomes and experience Through the whole CAD modeling course, I gain knowledge of the difference between solid and surface modeling, also learnt the process of building a model and what I need to be aware when it need to be sent to manufacturing, besides, the virtual human software is also the new thing that is so helpful for product design. I think this course make me more professional as a product designer.

4. References: http://crossbow.wikia.com/wiki/File:Nut_and_trigger_alignment.png http://en.wikipedia.org/wiki/Stereophonic_sound