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CMM's go portable.

CMMs go portable

Inspection during the building of large, complex tooling assemblies has always been a challenging metalworking task. The multitude of components they contain generate tolerance stack-ups that are difficult to control. Gaging by hand is time-consuming and subject to inaccuracies because each datum plane must be built up from an assortment of gage blocks, angle plates, and height gages. The tooling's size makes it difficult or impossible to transport it to a conventional coordinate measuring machine for inspection.

An answer to this need for in-process inspection is the portable CMM, an innovation that has advanced considerably in the last few years. Conventional CMMs are excellent for smaller components that can be easily carried to a fixed CMM and measured to very close tolerances. The greater challenges are larger, difficult-to-move workpieces, progressive dies and molds, machine tools, welding or gaging fixtures, and automotive prototypes. Transporting such a part or machine to the CMM can be costly, time consuming, and disruptive. After it's mounted on the CMM, its axes must be aligned before measurement can begin. Returned to its worksite, it must be aligned again, and movement may have degraded certain delicate subassemblies. In some cases, individual detail sections can be detached and taken separately for CMM inspection, but the critical overall dimensions will still require manual gaging.

Bring the CMM to the job

It's much more practical in such situations to bring the CMM to the job while the workpiece is being built. Portable CMMs have been available for some time, but have not gained widespread acceptance because of difficulties in aligning them to the objects to be measured. In most cases, leveling pads have been used to align the X and Z axes. For full three-axis alignment, the third axis had to be prodded into alignment with a mallet or pry bar; or, alternatively, a ground face on the CMM was butted against a finished edge of the part. If the workpiece rested on a table or base without a finished edge at the same height as the CMM, third-axis alignment became increasingly difficult.

A new system that can reduce setup and alignment time makes portable CMM measurement practical. The Trans-Ax III portable CMM uses a sliding leg on which the main base rests as its alignment mechanism. Three-axis alignment requires less than 30 min. After rolling the CMM to the job site, the operator sights it to within a few inches of being parallel to the workpiece's Y axis and levels it using leveling pads on the sliding leg and a pivot pad at the end of the base. A pivot mechanism is then used to maneuver the base back and forth across the sliding leg until the Y axis is aligned and locked in place. The remaining axes are aligned by adjusting six leveling pads in a predetermined sequence.

Applications

Applications that will particularly benefit from this approach are in-process build operations that require frequent inspections and subsequent rework. Assembly projects that will require CMM certification after building can be built to the same tolerances they will be inspected to, eliminating or reducing rework or quality problems. Any part or assembly that takes more than an hour or so to gage manually would also be a candidate because of the potential time savings.

More specific uses include:

Plastic injection molds. Most mold builders use stationary CMMs for critical dimensional checks. The portable concept can significantly reduce handling and setup time for large mold builds. Molds can be checked dimensionally during the prove-out process, eliminating the need to dismantle the mold and transport it to a CMM.

Progressive-die manufacture. Dies are usually built in sections and assembled on the floor. Checking is normally performed with relatively inaccurate equipment, and die accuracy is never known until the first parts are produced and measured on a CMM. Often, expensive rework must then be performed on the die. With the portable-CMM approach, each die section is inspected for dimensional accuracy in position, stack-up inaccuracies are avoided, and rework significantly reduced.

Production fixtures. Automotive tooling is typically constructed to 0.003" to 0.005" accuracies over distances of several feet. Nearly all in-process measuring and layout work is currently done by manual methods, with extensive time factors and questionable build accuracy. Introducing CMM accuracies early into the tool-building process via a portable CMM permits reduction of build time, improvement of tool quality, and elimination of inspection failures and tool reworking. Toolmakers will readily accept the opportunity to use a CMM instead of angle plates and height gages for critical build phases.

Automotive styling. Vehicle exterior and interior models are traditionally built on styling blocks and wheeled to huge CMMs. The only dimensional checks that can be made at the build site are crude tape-measure-type benchmarks. The portable CMM can provide significant time savings and eliminate the damage to styling models that frequently occurs when they are moved.

Machine tools. A prime requirement for high-production machinery is the need to locate parts to close tolerances. Such machinery is much too large to move to a stationary CMM. Portable CMMs allow builders of weld assembly fixtures, lathes, grinders, and machining centers to attain build accuracy while reducing rework costs.

Bore centering

The 785 height-measurement system offers a three-speed motorized probe and cordless transmission of nine measurement functions to a computer up to six feet away. A built-in thermometer adds thermal compensation. Automatic cycling determines bore sizes and center points. Accuracy bandwidth is 0.00016". Standard gage sizes are 800 mm and 1000 mm. Self-teach SPC programs can store over 4500 measurements on 100 features. Battery-pack option provides continual use.

Two-axis

measurement

Micro-Hite [R] 2D electronic height gage makes two-dimensional measurements, such as external, internal, and distance dimensions, plus perpendicularity, flatness, straightness, centers, and diameters. It will also calculate deviations and feature relationships automatically, such as determining relationships between holes or patterns in polar or rectangular coordinates. Movement is on a self-generated air cushion. Sizes are 20" and 32".

Optical scanning

Optical-scanning height gage provides quick noncontact inspection of wet, fragile, or easily deformed workpieces up to 6" in height. Video-based system is simple to set up and use, and it achieves accuracies of [+/-] 0.0001". Applications include measuring film thickness, copper coatings, PCB-pad inspection, and any soft material.

Height stand

MS10 Series of eight height stands position electronic manual gages. Both the platen and indicator mounts attach to the 1 1/4"-dia column, so that any deflection in the column will not affect gage accuracy. Because the indicator remains permanently mounted at the top of the column and the platen mount adjusts for various gaging applications, operator viewing position remains constant.

Height gage

The Sigmeasure 200 is part of a new line of manual gaging products, augmenting a line of pneumatic gaging equipment. Also included are electronic column gages and a flexible fixturing system. Indicators offer switch-selectable ranges, displays in metric or English units, and simple calibration.

Indicator stand

Line of dial-indicator stands provide freedom of movement, ease of use, and stability for height measurement and other gaging applications. Legs and dial holder can be progressively locked by a central tensioning screw. Two arm lengths are available, with or without a gage fine-adjustment screw.

Surface scan

Motorized height stand for the Surfanalyzer 5000/500 surface measurement system provides extended vertical measuring and setup capability. When the stylus reaches the work surface, its motorized movement stops, and it's ready for measurement. Over 50 built-in parameters and analysis capabilities cover most surface-measurement situations. Surface plate is toolroom granite with T slots for workpiece fixturing.

PHOTO : The Trans-AX III portable CMM in use on the shop floor measuring a fixture assembly.

PHOTO : Two men wheel the portable CMM into position.

PHOTO : Alignment can be completed in less than 30 min.
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.

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Title Annotation:coordinate measuring machines; Quality Solutions
Author:Simon, Louis
Publication:Tooling & Production
Article Type:Cover Story
Date:Oct 1, 1991
Words:1295
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