Accelerating Toolpath Generation for 3D Solid Models

Glenn McMinn
Engineering Geometry Systems

With the advent of affordable solid modeling systems, more and more parts are being designed three dimensionally. This includes prismatic parts like brackets or plates that are comprised primarily of 2 ½D features like holes or pockets. While it has become easier to create 3D solid models of these parts, no software tools have been available for quickly creating NC programs directly from this data.  Newly available feature recognition systems, like the FeatureRECOGNITION module of FeatureCAM, provide this functionality. With FeatureRECOGNITION, manufacturing engineers can import a 3D CAD file, directly create manufacturing features such as holes, pockets, slots and bosses and then automatically create efficient NC part programs. FeatureRECOGNITION greatly reduces the amount of effort required to create NC programs from 3D imported CAD data and is applicable to shop floor or off-line programming.

Disadvantages with Traditional CAM Systems

Nearly every CAM system can import 3D models. This includes surface or solid models. The problem is that if the models contain 2 ½ D features, it takes considerable effort to create NC programs from this data.

For parts that would be manufactured completely with 3-axis toolpaths like mold cavities or aerospace parts, toolpaths are generated directly from 3D geometry. For parts like turbine blades, the 3D solid or surface data is usually sufficient for generating toolpaths.

Prismatic parts are manufactured using 2 ½ D milling or drilling techniques. Programs for prismatic parts are typically generated from 2D arcs, lines and curves in most CAM systems. That means that it is often easier to create toolpaths for prismatic parts from 2D rather than 3D data.

Traditional CAM systems have tried to generate the 2D data by extracting edges from the 3D data or creating new 2D geometry using the 3D model as a reference. Once the 2D data has been extracted, feature depths or fillet radii are determined creating dimensions from the 3D data or using other geometric analysis tools in the CAM system. These tasks are often time consuming and are frustrating for a manufacturing engineer since he is forced to essentially remodel the part for manufacturing purposes. It is also frustrating because all of the information he needs is contained in the initial CAD model.

Foundation for Feature Recognition

The FeatureCAM system is based on manufacturing features. By using manufacturing features, the process of generating toolpaths is simplified. The manufacturing engineer simply specifies a feature like a tapped hole and the individual operations of spot drilling, drilling, tapping and chamfering are automatically generated by FeatureCAM. Tools are automatically selected, feed and speed values are automatically generated and the toolpaths are generated for each operation. Once manufacturing features have been identified, the process planning and toolpath generation processes can be automated.

Graphical User Interface

FeatureCAM provides a feature wizard that leads the user through the process of creating manufacturing features. A wizard is a series of graphical screens that assist the user with a process. The initial screens of the wizard prompt the user to enter the dimensions of the features. The subsequent screens help the user determine their process planning options. The feature wizard serves as the interface for feature recognition.

The goal of FeatureRECOGNITION is to accelerate the process of creating toolpaths from 3D CAD data. While it is possible to automatically recognize simple, isolated features like simple holes or rectangular pockets, most 3D solid models contain more complex features or features that overlap. These types of features make automatic feature recognition unreliable. Also, due to the wide variety of approaches to manufacturing a particular part, FeatureRECOGNITION concentrates on recognizing features with some user interaction. This process is called interactive feature recognition.

The 3D solid model is imported into FeatureCAM. Feature recognition is performed using the feature wizard interface. The process involves three basic steps and requires no remodeling of the part or typing any explicit dimensions.

1. The user selects the type of feature in the feature wizard. The types of features that can be recognized include holes, slots, pockets, and bosses.
2. The user picks the surfaces that represent the walls of the feature in the 3D CAD model.
3. FeatureCAM analyzes the surfaces and automatically determines the dimensions of the feature without requiring the user to explicitly type any dimension.

For holes, FeatureRECOGNITION can distinguish between plain holes, counter bored holes and counter sink holes. Once the feature is recognized, the dimensions are presented in the feature wizard where the user can make adjustments. These adjustments may be necessary to address manufacturing concerns that have been ignored in the design model. These adjustments might include changing hole types, altering feature dimensions, adding a chamfer or repositioning the feature.

Using Wall Curves

There are other types of CAD models that require finer control for feature recognition such as the part shown below.

The part is simple geometrically, but it can be difficult for feature recognition. For example if the user wanted to create a boss to cut around the outside of the part, it would e necessary to cut around half of the cylinder and then the three sides of the rectangular shelf.

It is difficult to specify this path using the surfaces of the part. Instead a different technique is more suitable. This technique detects all straight wall surfaces of the part and projects them into a series of lines and arcs as shown below.

The user then picks the lines and arcs required to define the shape of the feature. The depth of the feature is determined by clicking on surfaces of the model. With this technique the user has complete control over the feature definition process.

Benefits of FeatureRECOGNITION

FeatureRECOGNITION is great productivity tool for creating toolpaths for 2 ½ D parts from 3D CAD data. It allows the manufacturing engineer to extract information contained in a 3D solid model without remodeling the part and can greatly reduce the time required to create NC programs.