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Ultra-precise CMMs.

Ultra-precise CMMs

Tighter tolerances for both machined part and gage have made ultra-precise inspection an increasing necessity today. To meet this need and facilitate better process control, coordinate measuring machines (CMMs) have been developed to routinely measure workpieces with submicron accuracy.

Key technological advancements in CMM subsystems and methods of manufacture have made accuracies of one-half micron (0.000 020") a working reality. Since any system for measurement is only as good as its weakest link, a look into the design and construction of these subsystems will explain how these precision machines achieve these levels of accuracy and why they cost as much as they do--several hundred thousand dollars.

Machine design

The most accurate machines are usually bridge designs. Bridge systems offer a stable structure and are not subject to the droop of a horizontal-arm CMM. The moving-bridge and fixed-table approach offers several accuracy advantages over the fixed-bridge design. Straightness of the guideways is unaffected by the weight and positioning of heavy workpieces when they can be positioned on the fixed table's bearing points directly. Also, with no acceleration forces acting on workpieces, they can be mounted with little or no clamping force (whereas large clamping forces can deform the workpiece). The addition of a rotary table provides a fully integrated fourth axis, but to maintain system accuracy this rotary table must have an accuracy of [+ or -] 1.0 arc seconds.

Drives

CNC controls are the norm on high-accuracy CMMs, rather than fully manual or joystick-driven systems. This is necessary for optimum accuracy. With a computer controlling machine motion, the system is free of operator-induced positioning errors. Subsequent measuring runs can be repeated exactly as the first run was performed.

A drive system acting near the center of mass of the bridge offers advantages over machines driven at the side of the bridge. Even at rapid acceleration or deceleration rates, the bridge is not influenced to stray from its ideal axis of motion due to the high rigidity of this design.

Bearings

Bearings allow the bridge and spindle to move along their guideways. Air bearings are almost free of friction and wear free, they are self-cleaning, and they compensate for slight unevenness in the guideways. To ensure high rigidity, the best have an air gap of only several microns. With high rigidity, static and dynamic changes in load will not cause significant changes in air gap that would result in displacement of the axes. Extremely low air consumption--less than 0.1 liters/min/bearing at 70 psi--reduces the effect of expanding air (cooling) to a negligible amount, thus eliminating thermal guideway deformations. Well designed air bearings are not subject to vibration caused by turbulence in this outflowing air, oscillations that could detract from system accuracy.

Probing system

The probe head is a mechanical sensor and one of the CMM's most critical components. The commonly used switching-type probe heads are not used on the highest accuracy CMMs. Instead, a universal 3D probe head is used, that is in itself a small three-dimensional measuring machine. It is made up of spring parallelograms, inductive measuring systems, mechanical clamps, and a separate electromagnetic force-generation system. Such a probe head is capable of measuring in several different modes: continuous scanning for form measurement, self-centering of grooves or bores, and static measurement.

For utmost accuracy, the static mode is used. Two axes of the probe head may be clamped rigidly, leaving only one variable of movement in the direction of probing. The measuring force can be preselected; for example, 0.05 N (5 grams) for relatively soft materials. Data is recorded only after the probe tip has touched the workpiece and the machine has come to a complete rest. This eliminates all errors caused by dynamic influences. Repeatability of this probe head is 0.000 006" in each axis.

Length measurement

Length-measuring scales and reading systems are used for all three axes of motion; X, Y, and Z. Good systems are resistant to contamination by dust particles and fluctuations in voltage. The best have a resolution of 0.2 microns (0.000 008") or better.

In the past, it was necessary to correct for expansion or contraction of scales due to temperature influences. To eliminate errors in either measuring or compensating for scale-temperature change, newly developed scales of Zerodur glass ceramic can be used. Because this material has virtually no thermal expansion (230 times lower than that for steel), thermal correction is unnecessary.

Environmental concerns

Temperature influences have been one of the greatest barriers limiting CMM accuracy. Not only scales, but the entire CMM is subject to expansion and contraction. A major concern is the resultant bending of guideways due to internal temperature gradients--similar to the bimetal effect.

To avoid such problems, long, slim guideway elements of the spindle and bridge can be made of a material with high thermal conductivity, such as aluminum alloy. Aluminum has 80 times higher thermal conductivity than granite. Temperature is equalized rapidly and bending effects are eliminated. Achieving ideal CMM guideway elements requires experience in long-term stabilization of aluminum alloys and sophisticated machining and surface-refinement processes. The result is a surface that is as hard as ceramics, but with the thermal conductivity of aluminum.

Another approach to negate temperature effects is to place the CMM in a laminar-flow environmental chamber where temperature can be controlled to [+ or -]0.1 deg C. The main drawback is cost. A top quality room may cost between $100,000 and $200,000.

Floor vibration is another factor that can introduce measurement errors. Pneumatic antivibration systems can protect the CMM from ambient disturbances. Alternatively, an isolation foundation can be poured to support the machine.

Computer aids

No matter how well a CMM is designed and constructed, some error or inaccuracy will remain. However, much of the systematic deviations can be corrected mathematically using a special software program. Position and guideway deviations in each axis and the perpendicularity between axes are measured, and subsequently corrected by computer.

Strict environmental control must be achieved during the measurement/compensation process. By holding temperature to [+ or -]0.1 deg C, optimum corrective values can be obtained without the influence of temperature variables. But computer-aided accuracy can never progress beyond the basic repeatability of a machine--random errors in the CMM prevent that. Thus, very sturdy mechanical construction and high system repeatability remain basic requirements for building top-accuracy CMMs.

An aspect of system accuracy that must not be overlooked is software. Recently, efforts have increased to ensure that software algorithms are correct and that software itself does not detract from overall accuracy. Similarly, proper measuring techniques must be followed, since optimum performance of a CMM will only be possible when the system is used as intended.

With all these critical components in place, the state-of-the-art CMM can achieve extremely high measuring accuracy of 0.000 020", even with reduced environmental requirements.

What you want in a CMM

A survey by Sheffield Measurement asked 900 QC supervisors to rank the importance of CMM selection factors. The winning factor was repeatability, ahead of reliability (#2), accuracy (#3), repair service (#4), and programming ease (#5). Purchase price came in 12th.

The message here, says Sheffield, is that accuracy and resolution statements are meaningful only if a machine can repeat those readings. The high ranking of reliability also suggests the growing dependence on CMMs in production, as they move from post-process roles to a more active involvement in monitoring and correcting discrete-part manufacture in real time.

Ceramic CMMs

Line of high-precision ceramic CMMs brings accuracies to the shop floor that were previously only available in laboratory environments. Ceramic axis beams and risers are impervious to deterioration, water absorption, and thermal expansion. Air bearings provide smooth operation under almost any conditions. Available measuring envelopes range from 18" X 18" X 12" to 40" X 40" X 24", and volumetric accuracies from 0.0003" to 0.0005" (per B89.1.12, sections 5.5.1, 5.5.3, and 5.5.4). Available software include Unimeasure, Ultrapak, and Uni-Touch.

Numerex Corp, 7008 Northland Dr, Minneapolis, MN 55428 or circle 301.

High-speed CMM

Horizon series horizontal-arm Cordax CMMs offer volumetric accuracy of 17 microns on four combined axes, rotary-table accuracy of 1.5 arc sec, linear velocity of 25 ips, repeatability of 0.000 12" range, and automatic probe offset and temperature compensation. Microprocessor enhancements prevent 29 different deviations from affecting results. Six machine sizes are available, including a dual-arm version, and rotary-table capacities go up to 10,000 lb. Free access for part loading adds to inspection throughput.

Sheffield Measurement, PO Box 57, Dublin, OH 43017-9990 or circle 302.

Faster inspections

Merlin line of CMMs can cut DCC inspection times by 85 percent. Designed for most manufacturing environments, they have massive granite worktables for stability, greater stiffness-to-weight ratio with hollow-box moving members, scales designed to match coefficient of thermal expansion of the individual axes, and microprocessor-controlled acceleration to eliminate inertia-force errors. Measuring volumes range from 30" X 20" X 20" to 120" X 492" X 70".

Ferranti Sciaky Metrology Systems Group, Ferranti International, 4915 West 67th St, Chicago, IL 60638-6493 or circle 308.

Replacement stylii

OEM supplier to Zeiss, Brown & Sharpe, DEA, LK Tool, and Moore Special Tool can provide a wide variety of lengths and geometries of replacement stylii for CMM probes. These include disc, star, angle, and swivel configurations, along with various extensions, adapters, and cross heads. Materials include ruby, sapphire, ceramic, and silicon nitride, and special application assistance can be provided.

Precomp Inc, 17 Barstow Rd, Great Neck, NY 11021 or circle 353.

Calibration CMM

SIP 560M CNC CMM can be used for calibration and certification of standards, as well as applied research. Measuring-system resolution is 0.1 micron. OMNISIP probe (a small CMM in itself) measures all three axes simultaneously. Machine and probe are equipped with steel standard scales. Used with a DEC Micro VAX computer, the Concerto software provides modules for measuring, analyzing, providing output, and assisting the operator. It can be used with an optional plotter with automatic loading for fully automatic operation.

American SIP Corp, 530 Saw Mill River Rd, Elmsford, NY 10523 or circle 308.

Environment-resistant CMM

UPMC CARAT CMM brings measuring accuracy of 0.000 020" to less favorable environments. Special aluminum-alloy construction provides mirror-like finish, long-term stability, and very high thermal conductivity for temperature equalization throughout. Additional steps eliminate or correct for thermal insulation in the granite base, residual thermal bending, and thermal insensitivity of length-measuring systems. The net effect is to relax previous environmental conditions for maximum accuracy. Probe-head repeatability is 0.000 006", and measuring envelopes range from 22" X 20" X 18" to 33" X 47" X 24".

IMT Div, Carl Zeiss Inc, One Zeiss Dr, Thornwood, NY 10594 or circle 305.

Precision vertical CMMs

Xcel family of vertical CMMs provide unmatched throughout with exceptional accuracy and repeatability. Speeds of 20 ips are double that of previous CMMs. Throughputs of 60 measurement points per minute are achieved without sacrificing accuracy or repeatability using a kinetic clamping system based on inertial bridge damping, three-point air-bearing suspension, and fast-response air counterbalancing. Settling time is less than 0.25 sec. Volumetric accuracies range from 11 to 15 microns, depending on size and based on ANSI B-89 standards. Three sizes are available, two DCC machines and one manual joystick model.

Brown & Sharpe, 376 Nash Rd, New Bedford, MA 02746 or circle 310.

Assembly inspection

Assembly-force monitoring by-passes the pitfalls of random destructive testing to provide 100-percent real-time testing of assembled components. As a sleeve is pressed into a hole, for example, a force sensor on the ram confirms that the assembly force is within the required range, generating a force/position curve for each assembled component and interrupting the process if an upper or lower limit is exceeded.

Promess Inc. P O Box 748, Brighton, MI 48116 or circle 365.

Cantilevered CMM

The 350 MMD cantilever-style CMM can inspect a wide range of small to medium-sized parts with easy accessibility from three sides of the workpiece. Featuring a manual motor drive, it is capable of volumetric accuracy approaching that of more expensive, computer-controlled units. A remote pendant provides hands-off operation and constant velocity for quick, precise measurements. Also available is a DCC (direct computer control) model with a high level of automation to minimize operator involvement, eliminate repetitive work, and speed up the measuring cycle.

Federal Products Corp, 1144 Eddy St, Providence, RI 02940 or circle 337.

PHOTO : CNC inspection of a calibread multi-sphere test standard provides rapid confirmation of

PHOTO : system accuracy.

PHOTO : An infrared telescope filter is measured with high precision using a rotary table for

PHOTO : indexing.

PHOTO : Ultra-high rigidity air bearings with extremely low air consumption and vibration are key

PHOTO : components of a top-accuracy CMM.
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:coordinate measuring machines
Author:Robison, Gary
Publication:Tooling & Production
Date:Feb 1, 1990
Words:2122
Previous Article:Coolant-management program.
Next Article:The nitty gritty of machining ceramic.
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