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Understanding digitizing.

Understanding digitizing

Digitizing is a data-acquisition process that constructs mathematical models of three-dimensional objects. Typically, a machine tool or coordinate measuring machine (CMM) uses a probe to sample an unknown surface. The machine's control translates this information into a math database duplicating the part surface. The database can be used directly, stored, or transferred to other machine tools or CAD/CAM systems.

Digitizing has been used in aircraft and automotive applications for many years. CAD/CAM technology uses it to create accurate math models. Toolmakers use scan data to generate NC programs and cutter paths for milling prototype tooling, injection molds, and sheet-metal dies.


There are two distinct types of digitizing applications: tracer scanning and touch-probe. Tracer scanning uses a tracer probe corresponding in size and shape to the anticipated cutter that will be used to mill the workpiece. The probe's electronics sense stylus deflection as the stylus continuously scans the surface of the part, collecting data. Typically, the data is center-of-tool information that is accepted by a CNC milling machine as an NC program or cutter path.

Touch-probe digitizing uses a Renishaw probe typical of CMMs and provides actual surface data. The surface points are used to automatically construct an electronic math model of the part, commonly referred to as a wireframe. Wireframe data is used by designers and tooling manufacturers in a CAD/CAM environment.

Tracer scanning

Tracer scanning uses a 3D parallel probe. Probe response is based on stylus deflection, where the analog signal from the probe is converted into digital information at the machine control. The tracer system is suitable for use on traditional copy-milling machines, bridge-type gantry machines, and CMMs. On copy mills and duplicators, the tracer probe is mounted on a probe slide that provides flexibility and adjustment. On gantry machines and cantilevered CMMs dedicated to digitizing, the tracer probe is mounted on the Z-axis ram.

The stylus continuously scans the surface of the part, collecting data. Point density, or the increment between points used to define the surface, is based on chordal deviation. Flat surfaces generate less points than surfaces exhibiting significant contour and shape. The chordal deviation value is entered by the operator prior to digitizing.

When the system approaches a trace boundary, the probe will automatically index over a selected distance and continue scanning in either a lace mode or box-scan mode. The machine will continue to scan the entire surface patch determined by the trace boundaries, complete the scan, and automatically lift above the workpiece and begin scanning the next area defined by the operator.

Typically, the tracer provides center-of-tool information that directly translates into tool-path data. However, surface data can be acquired using the appropriate stylus and stylus-deflection values. In addition to the XYZ data for each point, the system calculates the appropriate surface normals. Each point contains XYZ coordinates and IJK vector data.

A prime example of efficient use of a good tracer-scanning package on a copy mill or duplicator machine tool is the ability to simultaneously scan the model while cutting the mold or die. The digitized scan data is also stored on a hard-drive disk file at the machine tool for later retrieval. The data can be used to cut the opposite hand or mirror image of the part, or a scaled version can be cut, compensating for shrink or expansion. In some cases, the female version of the part can be cut from the male data. Designs with repetitive shapes, such as automotive wheels, are a good example of scan-data manipulation. The user scans the common feature and simply rotates the data to complete the part.

Scanning on a CMM

Another use for the trace probe is scanning and digitizing on a CMM. The idea is to let the machine tool cut while the CMM does the digitizing.

In one case, a toolmaker making headliner tooling for the auto industry used a CMM to get the tool path for a CNC milling machine. The operator digitized the model on the CMM while the milling machine was cutting another job. The scan data from the CMM was tool-path data that could be transported to a CAD/CAM system and posted into a format appropriate for the milling machine. No CAD/CAM time was required for tool-path generation.

Touch-probe digitizing

Touch-probe digitizing with a Renishaw probe is used extensively in the auto industry, particularly in the design studio, where a concept is transformed into a 3D rendering for designers to evaluate and modify. Individual components from simple to complex shapes - from door handles to entire vehicles - are created in clay or styling foam and scanned for engineering refinement and documentation.

Digitizing example

Tarus offers digitizing packages for five-axis clay mills and three-axis portable clay mills and CMMs. The package includes a Renishaw probe and related hardware, digitizing software, and a color-graphics package supported by a multi-axis machine control. Typically, it includes a 1-mm ruby-ball stylus suitable for most digitizing applications, with automatic compensation for the ball offset. A spherical stylus provides more flexibility and less probe articulation than a pointed stylus. The system also provides compensation for various probe angles during the scanning process. The operator simply rotates the probe to a convenient angle, and resumes scanning.

The digitizing package is menu-driven with the operator selecting commands from a menu display on a color-graphics monitor at the machine. The operator establishes the type of digitizing: straight planer or radial scan for a surface, or possibly a 3D mode to follow a character line. Next, the operator selects manual, semiautomatic, or fully automatic digitizing.

Point density is determined by the operator in the manual and semiautomatic modes. In the fully automatic mode, point density is based on chordal deviation. The operator also enters an index distance, the distance between scan lines. On large slab surface areas, such as a deck lid or roof panel, evey 100 mm may be adequate. For more detailed areas, every 10 mm may be necessary.

CAD/CAM users can specify these parameters in the digitizing program. The graphics package allows the operator to view the data, delete bad points, or extract and organize lines.

The operator starts the automatic digitizing sequence with two samples on the surface of the part, establishing a direction for the scan. The probe then runs in a fully automatic routine, sampling the surface. The incremental distance between points is determined by chordal deviation. The probe automatically lifts away from the surface after each sample and continues digitizing. The data acquired from touch-probe digitizing represents the actual surface. The points construct lines that are used to build a wireframe model of the part. The scan data is then transferred through an RS-232 port or Ethernet interface to a CAD/CAM system for surface development, design evaluation, and engineering.

Software features

Software features are also important. There are two general categories: those that enhance the digitizing process, and those that allow scan data to be manipulated and managed.

In tracer-probe scanning, multiple-trace boundaries establish limits for automatic scanning of the part. When the probe recognizes a trace boundary, the machine will automatically stop, index over, and continue digitizing in the opposite direction for lace mode, or retract to a plateau and continue scanning in the same direction for box mode. When the system completes scanning a defined surface patch, the tracer will retract to a safe position and automatically continue digitizing another area of the part. Trace boundaries can be viewed or modified at the graphics display. Another scanning feature is anticipation: a command that adjusts scanning speed when the probe senses a sudden change in the surface.

The following two features are unique and require minimal scan data from the tracer to generate an NC tool path for specific mold or die features: the rib-cut and pocket-cut utilities. These are menu-driven programs at the machine. The operator digitizes the bottom of the rib on the model. The scan is displayed on the graphics screen. The operator then selects the rib-cut program and enters the appropriate parameters to execute the program. The program generates a "smart" tool path for the most efficient cutter usage. The pocket-cut program is similar in concept and execution.

The advantage of these utilities is flexibility. The operator doesn't have to interpret or rely on CAD/CAM people for a cutter path for specific features.

PHOTO : Digitized wireframe of an auto-body model.

PHOTO : Touch-probe digitizing on a horizontal-arm CMM.

PHOTO : Tracer scanning of an automotive steering wheel.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Serwin, Ken
Publication:Tooling & Production
Article Type:Cover Story
Date:Oct 1, 1991
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