Designing with CAD:
CAD Process:
Design requires careful planning during the entire process. Knowing how to use the available tools (In NX, CATIA, etc) is the key to creating successful designs.
Conceptualization:
Before creating your part, it's a good idea to conceptualize your design. Some designers begin with a 2-D layout or even a rough CAD model, which will be redone. Hand-drawn sketches can even be a tremendous advantage in getting started. Many good designs started as a rough sketch on a napkin.
For styled consumer products, many designers sculpt their parts out of clay or Styrofoam before ever attempting to create a CAD model. This can be useful in that you can create a mock-up of your part. You can study this crude prototype to determine both how functional and moldable it is. Quite often, the most obvious molding issues are revealed at this stage. Reshaping your part to make it moldable before starting your CAD model saves valuable time.
Orienting the Part:
The idea behind conceptualizing is to determine how to approach your CAD model. Think about your part in a mold tool. What features does it have and how are they oriented? Can the part be oriented in the mold tool so that there are no undercuts?
By answering these questions, you can determine the draw direction and parting line for your part. Then you have a starting point for your CAD model.
Modeling the Basic Shape:
Typically, you begin your model with a primitive feature, such as a block or cylinder, or by extruding a profile.
With plastic parts, you should first concentrate on modeling the main portion of your part. That is, the overall shape of the part without the individual features. This portion of your model has a nominal, uniform thickness.
Shape your part by adding additional features. Creating sketches to define split profiles is a good approach. You can Pad (Extrude) or Shaft (Revolve) these sketches to Split (Trim) the solid body.
Features such as pads, pockets and slots can also be used to help shape your part. You should already be thinking about Draft. Adding Draft features while creating the model in these early stages saves you time when preparing your model for tool design.
Adding features or padding (extruding) sketches may not always define the desired shape. You may find it necessary to create sheet bodies, surfaces or additional solid bodies.
Doing this allows more flexibility in the way you can define the necessary shape. These bodies are commonly referred to as tools. Use these tools to Split, Trim or Remove material away from your solid body.
You also need to be thinking about Fillets (blends). Use this feature to eliminate the sharp corners of your model. Fillets are normally added after Draft features. It is nearly impossible to add draft to a model using a blended Taper (Draft) feature.
By concentrating on only the basic shape of the part, you can now use the Shell feature to easily give the part a nominal, uniform thickness.
Adding Features:
Now you can begin adding features such as snaps, ribs, and bosses. The reason for holding off on these features until now is that they typically have a thickness smaller than the nominal thickness. The Shell (Hollow) feature is used to define the nominal thickness, so these features should be added after the Shell feature.
Evaluating the Design:
Once you have completed your model, you need to check it. You can use operations such as Measure Between (CATIA) or Analysis - Distance (NX) to verify the thickness in different areas. Doing this gives you a chance to catch anything you might have missed. Correcting thickness problems allows you to avoid having to deal with sink marks after the part has started a test production run.
In NX Face Analysis on the Analysis Shape toolbar is another key tool for analysing your part. Draft Analysis (CATIA) is another key tool for analyzing your part. This allows you to graphically note the draft angles. It can be quite easy to forget to draft some faces of your model.
Identifying problem areas on your model is an important step in the design process. Once a mold tool is cut, it can be quite expensive to modify. Carefully scrutinizing your design can easily save thousands of dollars.
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What is Design Specifications?
A specification (singular) consists of a metric and a value. For example, "average time to assemble" is a metric, while "less than 75 seconds" is the value of this metric. Note that the value may take on several forms, including a particular number, a range, or an inequality. Values are always labeled with the appropriate units (e.g., seconds, kilograms, joules). Together, the metric and value form a specification. The product specifications (plural) are simply the set of the individual specifications.
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Design in Context (Top-down Modeling):
Overview:
There are many advantages to top-down modeling, the greatest being the ability to design or edit in the context of the assembly structure. It is important to understand the following concepts when using design-in-context principles:
- Define the new component before creating any parametric geometry when possible.
- Sketching in the context of an assembly.
- Switching back and forth between top-down and bottom-up methods.
- Creating sub-assembly files.
Switching Between Top-Down and Bottom-Up:
You can use a combination of top-down and bottom-up methods in defining the assembly structure. You can also use a combination of these methods in designing and editing individual component geometry.
For example, in an existing assembly, you create a new component and a "black box" shape to define its location and overall size within the assembly structure. You can save this part, close the assembly, and design the detail of the component part using bottom-up methods. The combination of methods provides the best of both worlds: top-down for positioning and evaluating size restrictions and bottom-up for detailing the component without having to work with the entire assembly.
Creating Sub-assembly Files:
Remember, components are the piece parts making-up an assembly. These piece parts can be used in multiple assemblies and multiple sub-assemblies. Therefore, it is always a good idea for each unique piece part to have its own part file. Even these piece parts can have their own component piece parts.
For example, a coupling may be a piece part in a large machine assembly but the coupling also has its own components (plates, shafts, and hardware). The coupling is an assembly itself and a sub-assembly in a larger assembly.
The piece parts in the coupling assembly should not be added directly to the machine assembly as components. Instead, create an empty file to act as the coupling assembly file, such as coupling-assy, and add all of the related coupling components to this file. Now coupling-assy can be added to a larger assembly as a sub-assembly.
When you add a sub-assembly to an existing assembly, all of the components are added as well. In terms of design-in-context concepts, use the Insert| New Product to create sub-assembly files before adding or designing new components. Doing it this way creates a sub-assembly structure that is much easier to manage and manipulate than if you simply add the components to the top level assembly. Be careful of using the Insert | New Component to create a sub-assembly. If you do this, the sub-assembly only exists within that individual Product file and cannot be accessed by other CATIA Product files.
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Creating a Bottom-Up Assembly:
Path: Insert | Existing Component
Key Points:
This technique is similar to building a toy with 'some assembly required.' The necessary components are already available; you just need to put them together.
Prerequisites:
- You should be in the Assembly Design workbench.
- Adding a Component to an Assembly Using Bottom-Up Techniques
1. Open bottom_up\bottom_up.CATProduct.
2. Select Insert | Existing Component. CATIA prompts you to select a product component to add the existing component to.
3. Pick Bottom-Up Assembly. The File Selection dialog displays.
4. Navigate to the required folder and open bottom_up\cu_bolt.CATPart.
5. Position the component using either the Positioning Compass or by selecting Edit | Move | Manipulate. To hide the component planes from view, expand the appropriate branch of the tree to reveal the planes and View | Hide them in the same manner.
6. Add the bolt three more times using the same process.
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Creating a Top-Down Assembly:
Path: Insert | New Part
Use this to:
- Create an assembly without all of the components in place. This allows you to start the product design of a component from scratch.
- Create new components in an existing assembly. This allows you to consider size, space, or interference limitations.
Prerequisites:
You should be in the Assembly Design Workbench.
Creating an Assembly Using Top-Down Techniques
1. Select File | New. The New dialog displays.
2. Select Product from the list and click OK to create the assembly.
3. Pick the product from the Specification tree, then select Edit | Properties and change the part number to top_down_assembly, the revision level to A, and the description to Assembly, Top Down.
4. Refer to the next process, Creating a New Component Using Top-Down Techniques to create components in this assembly.
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Creating a New Component Using Top-Down Techniques:
1. Use the product you created in the previous Try It.
2. Select Insert | New Part. CATIA prompts you to select a component to add the new part to.
3. In the Specification Tree, pick the product. The new part is added to the Specification Tree and displays in the Graphics Window. Since this is the first part, the part's origin is automatically positioned at the assembly's origin.
4. Select Edit | Properties and change the part number to first_part, the revision level to A, and the description to Part, First.
5. Add a second part to the assembly. Select Insert | New Part. CATIA prompts you to select a component to add the new part to.
6. Pick the product name in the Specification Tree. Click No to set the origin of the new part to be the same as the origin of the assembly.
7. The new part is added to the Specification Tree and displays in the Graphics Window.
8. Select Edit | Properties and change the part number to second_part, the revision level to A, and the description to Part, Second.
9. Pick a new part in the Specification Tree, then expand it. Double–click on the Part to begin modeling in context of the assembly. (Make sure you pick the part and not the instance of the part.)
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