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Parametrics - a software link.

Parametrics--a software link

Flexible solid models contain information for both design and manufacturing, and help to unify them. Further, parametric software automates many tedious, repetitious, time-consuming operations.

A lot of time has gone into studying the problem of communication between design and manufacturing. Many people, principally design engineers, have concluded that if a part can be characterized in terms of its manufacturing features --e.g., pocket, fillet, or hole--then a series of manufacturing operations can be inferred from that information. The implication of feature-based designing is that the manufacturing engineer, and the input he brings to a process, can be minimized or eliminated from any manufacturing characterization for that part.

It is unreasonable to expect, however, that design engineers know enough about part manufacturing to be able to supplant the input traditionally provided by manufacturing engineers. Feature-based design systems will not be the solution to reducing or eliminating input from manufacturing engineers or shortening product development cycles.

Different viewpoints

Interfacing between design and manufacturing is an interesting process. A design engineer finishes a design for a part or product consistent with engineering specifications given to him. Typically, he produces manufacturing drawings for that part or product, and then passes his documentation to a manufacturing engineer.

As the manufacturing engineer evaluates the drawing, he usually finds areas inconsistent with realities of manufacturing capabilities. He marks up the drawing, and sends it back with a request to make it meet manufacturing needs.

Communicating in this fashion takes place over a number of iterations. Obviously, the more iterations there are, the more difficult it is to meet manufacturing and product release schedules. There is inherent tension between the two departments, and consequently a lot of compromise in the process.

For example, a design engineer may compromise on design specs to meet manufacturing schedules. Similarly, a manufacturing engineer may yield on optimal cost configuration to meet production needs.

In some cases, design and manufacturing engineers may actually fail to reach agreement on a manufacturable product. A manufacturing engineer will make what he thinks he can make, which may or may not coincide with the product design.

Automating the link

Proponents of feature-based design engineering fail to consider the though process a manufacturing engineer uses as he looks at a drawing. The way he sees a manufactured part is much different from the way a design engineer sees it. The latter views a part in terms of features, parameters, and functional operating specs, and he describes only the finished part or product.

By contrast, a manufacturing engineer may have little interest in the final product. Rather, his major concern is in transforming raw materials through a series of processes up to, but not including, the finished product. His task is to plan and describe a progression of process steps that carry materials from each stage to the next.

As he evaluates these process steps, he discovers problems in manufacturing. Subsequently he requests engineering change-orders in the design.

These two approaches--that of the design engineer, and that of the manufacturing engineer--are fundamentally different. The challenge is to capture and integrate the two visions of the same product in a single data structure, and to force agreement on a producible product.

A solution

In today's CAD/CAM environment, a solution is to provide a database that contains both design and manufacturing descriptions. This database would force reconciliation of the two different and sometimes contadictory views, enabling them to converge on a single design.

As an example, let's look at a design process for a support plate. Pro/Fabrication[TM], a parametric software engineering tool, will be the system that captures and integrates design and manufacturing information.

In a parametric, feature-driven, solid-modeling system, the designer uses a familiar engineering language of features to construct a solid model. Whereas traditional CAD/CAM systems connect a series of lines, arcs, and X-Y-Z coordinates to construct models, a parametric modeler connects simple, commonly recognized 3-D shapes such as holes, slots, and rounds. These are combined to build an increasingly complex model (drawing).

The language of parametric features allows a designer to capture engineering knowledge implicitly. For instance, when you put a round on the end of a shaft, Pro/Engineer links the diameter of the round to that of the shaft. If you decide to increase the shaft's diameter, the round's diameter increases automatically. Parametric features understand their position and relationship to surrounding geometry, and change appropriately as surrounding geometry changes.

A design engineer starts the design-to-manufacturing process by creating and refining a design from conceptual layouts through detailed designs. As more is learned about the design over a period of time, design modifications are made.

Using a parametric, feature-driven system, the design engineer characterizes the part in terms of parameters, features, and relationships. These can be interactively modified as the design evolves. The designer creates a flexible solid model which becomes a visual reference and a source of information for analysis and manufacturing.

A design engineer characterizes a design by assigning so-called intelligent features. These inherently understand surrounding geometry, and change as surrounding geometry changes. As the engineer selects features from a menu, he places them parametrically on the model.

Sizes and locations of these features can be modified at any time, because they too are defined by parameters. Since the entire model is defined by parameters, relationships can be written between features or parts in an assembly. These relationships--e.g., "height equals two times width"--serve to capture design intent or control fit between parts.

The support plate we designed contains a pocket, several holes, and two protrusions. Notice that the displayed parameters define the model dimensionally. This model represents the "final design," and is available for use in manufacturing engineering downstream.

Process planned

After receiving the solid model from design engineering, the manufacturing engineer selects a machine, process, tools, and bar stock. These are available to him as menu items when he selects "Manufacturing" from the main menu.

A length of bar stock is displayed on the screen as a solid model. Then the manufacturing engineer calls up the plate design model, and embeds it within the bar stock model. To produce a real-life NC setting, he places any necessary fixtures, clamps, etc, in the scene.

Because he's working with a solid model, he can calculate mass properties at this point, to determine the starting amount of material. After the model has been "machined" on-screen, he can determine the amount of material removed to create a finished part.

The manufacturing engineer specifies turning axis, cutting definition (rough cut, finish cut, profile cut, etc), cutting tool, and machining parameters (speed, feed rate, coolant, etc). He is then ready to sketch the tool approach path.

He also indicates the area and volume of material to be removed. In this example, these are the plate's outside profile. Because he is working with a solid model, he can select features of the design model to be machined. He doesn't need to worry about ambiguities that would be encountered were he dealing only with surfaces.

Pro/Engineer provides the user with automatic toolpath generation and fully controlled cutting sequences. Different toolpath fonts indicate different specified feed rates. An arrow overlaid on the toolpath indicates the tool's direction of travel.

Further, the cutting tool itself can be displayed to help visualize machining operations, and to detect any interferences with fixtures or clamps. Finally, the manufacturing engineer selects an option from an NCL menu, and an NCL file is created automatically.

Once he has reviewed and approved the toolpath, removal of specified material is simulated on-screen, and final results are displayed. If a mistake has been made, it becomes apparent immediately, because all manufacturing specs are also parameters. The engineer can then make a correction, and toolpaths are automatically regenerated.

The next step is to remove material in the pocket, machine around islands in the pocket, and drill all its holes. First, the engineer selects a pocketing operation from a menu. He then selects a pocket feature and two of the island features.

Next, he chooses a drilling operation from a menu, and selects all hole features. The software automatically generates proper toolpaths, and displays them along with cutting tools. Finally, he selects an option from the NCL menu, and an NCL file is created automatically.

Handling errors

In our example, let's suppose the design engineering department discovers an error in their part design. Specifically, the model was not wide enough.

For parametric applications, this situation does not pose a problem. The design engineer simply modifies the width parameter, and the solid model is updated automatically.

Because the manufacturing engineer works directly from the design model, this change is reflected immediately in the simulated product assembly. More importantly, all intermediate process steps captured in the model are automatically updated.

Once a change has been entered, all toolpaths are instantly regenerated and displayed to reflect it. At this point, the engineer can recall images of the cutting tools, to help visualize machining operations and check for interferences. The software then outputs an updated NCL file with a part process plan.

Because the manufacturing engineer is working directly on the design model, he can immediately view the effects of each machining operation on the manufacturing model. When this completely and accurately replicates the design model, he knows the design is manufacturable. It has been proven out according to all required manufacturing steps.

After this proof has been obtained, the engineer passes cutter location files to postprocessors. Machine-specific code is then used for punching tape, or is downloaded on a DNC network.

As you can see, use of parametric software reduces the time and cost of making changes. Engineering change orders propagate quickly and readily through each step, and through each model, until the part is proved manufacturable. The system remembers each process step, and can easily regenerate process information.

Equally important, both the design engineer and manufacturing engineer can work on single, flexible database, at the same time using CAD/CAM applications suited to their individual needs.

PHOTO : Example of what happens when a change is made in a parametric feature. In the part model

PHOTO : at left, the shaft's inside radius is 25 mm. When the designer changes that dimension to

PHOTO : 50 mm, the ID of the mating part changes automatically to the proper value. This is

PHOTO : possible because parametric features understand their position and relationship to

PHOTO : sorrounding geometries. When one parametric feature changes, adjacent features change

PHOTO : accordingly.

PHOTO : A parametric design is dimension-driven, and can be modified easily.

PHOTO : For process simulation, a solid model is superimposed on a piece of bar stock.

PHOTO : The manufacturing engineer can monitor and inspect all tool paths visually, checking for

PHOTO : any possible interferences.

PHOTO : Calculations of mass property changes on the evolving piece of bar stock.

PHOTO : Changes made interactively on the solid model drive regeneration of process information.

PHOTO : Output of cutter location file for pocketing and drilling.
COPYRIGHT 1990 Nelson Publishing
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:design and manufacturing interface
Author:Husslage, Robin
Publication:Tooling & Production
Date:Mar 1, 1990
Previous Article:Fast-changing market demand fast-change tooling.
Next Article:A look at magnetic treatment of tools and wear surfaces.

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