Published 8/19/2011

Sizing up the Digital Optical Comparator

A digital optical comparator uses a CAD file instead of a template or overlay film. More importantly, the digital comparator not only measures dimensions, but it also actively compares the measurements with nominal values, thus making it truly a gage for accepting or rejecting a part.

Mark Albert

Editor Emeritus, Modern Machine Shop

The VisionGauge can be configured either horizontally or vertically, as dictated by fixturing and mounting considerations.

Bidirectional tolerances at various control points can be specified by the user. The system automatically computes and displays the part’s deviation from nominal at every control point. This display includes a pass/fail result at every control point as well as for the entire part.

The Auto-Align feature finds the best match between the CAD data and the part image without operator intervention. This feature speeds inspection and makes it more effective.

By magnifying the image of a workpiece and projecting that image against a template that magnifies a profile of the part’s design, the traditional optical comparator magnifies the ability of the human eye to detect critical deviations. Mismatches between the projected workpiece image and the template make these deviations more obvious, enabling the user to judge whether the workpiece is acceptable.

The digital optical comparator has essentially the same function, but the comparison is done electronically, thus taking this inspection process to a higher level of precision, automation and convenience. The Visionx VisionGauge digital optical comparator (distributed in North America by Methods Machine Tools, Sudbury, Massachusetts) provides an example of how digital operation contributes to this “magnification” in capability.

Perhaps the greatest advantage of digital comparison is the ability to work directly with the CAD data so that no template or overlay is needed. This eliminates the need for VisionGauge users to prepare, handle and store templates, which are usually line drawings inked on Mylar film, based on the designer’s original blueprint. Using CAD data also precludes discrepancies between the blueprint and its enlargement on the template.

The value of the comparison conducted by the digital comparator, however, goes beyond this convenience. For example, the VisionGauge can compute and display the part’s deviation from nominal dimensions captured in the CAD file. This happens automatically and without the operator’s judgment or interpretation. The results of this comparison can be collected; saved in a database as a record of the inspection and measurement operations; and forwarded to third-party software applications. The system has the ability to capture electronic documentation (including high-resolution graphics) directly from the shop floor. According to Visionx, this process happens seamlessly during inspection.

The system can determine pass/fail status automatically and immediately send messages across a network for responses such as corrective action at the machine tool. Depending on magnification, digital comparison can detect deviations as small as 0.0001 inch.

Digital operation also enables the system to perform functions beyond the capabilities of the traditional comparator. For example, the system aligns the CAD data to the image of the workpiece without user prompting. This is a faster, more accurate process than manually shifting a physical template. In addition, a laser module available for the VisionGauge provides 3D inspection capability by enabling it to measure depth and height (Z-axis measurements). The traditional comparator is limited to 2D comparisons.

Likewise, the “hardware” of a digital optical gaging system differs from the traditional comparator. The “projection screen” of the VisionGauge, for example, is an array of flat-screen computer monitors. The system can be configured horizontally (the lens views the upright part from the side) or vertically (the lens looks down from above on the part lying flat). The Illumination can be positioned in front of or behind the part (above or below the part on a vertical unit). A wheeled base enables the user to move the system about for shopfloor use.

Finally, it should be noted that the comparison function of the system sets it apart from other non-contact or vision-based measurement systems. The digital comparator not only measures dimensions, but it also actively compares the measurements with nominal values, thus making it truly a gage for accepting or rejecting a part.