The Coordinate Measuring Machine (CMM): A Primer

THIS IS A COMPREHENSIVE GUIDE TO  ALL THINGS CMM MACHINE RELATED.

This CMM machine guide covers the following topics:

1. Why the CMM is more relevant than ever, what it is and its common attributes, such as accuracy and repeatability?
2. How to choose a CMM Machine – Different types of CMM Machines and Their Application
3. Where to install the coordinate measuring machine?
4. History of the CMM Machine, where it started and where it might be headed
5. CMM Machines software function and capabilities, training and requirements for the CMM programmer and CMM operator
6. CMM Probing systems: scanning touch trigger probes and probe heads
7. Portable coordinate measuring devices: Portable Arms, Laser Trackers and optical CMM devices
8. Video measuring machines, their advantages and multi-sensor capabilities
9. Upgrading or retrofitting your existing CMM machine: advantages and possibilities when considering new CMM software, controllers or probing
10. Used coordinate measuring machines: what to consider, advantages and pitfalls
11. Calibration and maintenance of CMMs, what ISO 17025 means to the manufacturer; ISO 10360 calibration procedure and its relevance; preventative maintenance on a CMM machine and how important is it
12. Laser Scanner on a CMM Machine: Benefits and Applications

WHY THE CMM IS MORE RELEVANT THAN EVER?

With all the new measuring technologies available today, why would you buy a seemingly old-fashioned coordinate measuring machine?

CMMs still do many things remarkably well: they are masters at production of repetitive measurements, as they can measure unattended once a program has been written. They also have the ability to use a wide variety of probes and sensors to optimize the measuring being performed. Probes can be automatically selected from a change rack without any operator intervention.

CMMs are incredibly repeatable, because there is no intervention by an operator in the measurement process. Most medium-size bridge machines offer repeat capabilities in the 2 to 5 µm range, providing the user with extremely credible results. CMMs are highly accurate and deliver reliable results well above the machines or processes that are used to produce the components they are measuring. Nowadays a small percentage of CMMs are manually operated and must be guided through the coordinate system by hand. In fact, manual ones are little more than height gauges and are handy for small measurement tasks on the shop floor.

Average measuring machines today provide manufacturers with 3 to 7 µm accuracy over one-meter volume utilizing a basic touch trigger probe. When it comes to the measurement of geometric elements such as cylinders (bores), cones, spheres, etc., the coordinate measuring machine has no equal.

All coordinate measuring machines consists of 3 axes: X, Y and Z. They are essentially precision positioning devices. Most machines run on air bearings, while shop floor machines usually use -bearing systems. The fourth axis on the coordinate measuring machine is generally accomplished by using a rotary table. Rarely used, the fourth axis is often employed in the measurement of gears or cylindrical parts.

It is fair to say that the 3-axis mechanical device called a coordinate measuring machine is a very basic engineering entity and is rarely a complex structure. The basic structure of the coordinate measuring machine, which is inherently quite accurate, is usually enhanced by software to provide the machine with additional accuracy. This process is called mapping. The map usually resides on the coordinate measuring machine controller and is basically a fingerprint of the machine's enhanced performance. Variables such as pitch, yaw and roll, together with squareness, straightness, and volumetric accuracy, are included. Most maps are able to compensate for 21 to 23 distinct errors.

HOW TO CHOOSE A CMM MACHINE.

Different Types of CMM Machines and Their Application

There are four basic types of coordinate measuring machines: Bridge, Cantilever, Gantry and Horizontal Arm. Each one provides unique advantages based on the components being measured.

Bridge

Due to their basic structure and simplicity of build, bridge machines have become extremely popular. Bridge machines have a low cost to build and the ability to maintain accuracy and repeatability over the long-term. 95% of all bridge machines run on air bearings to allow friction-free movement and minimize mechanical interaction.

Bridge machines are the workhorses of CMMs. The accuracy of most bridge systems is usually better than other types of coordinate measuring machines, and certainly when considering a system for machined parts with higher tolerances, a bridge is hard to beat.

Cantilever

 The Cantilever CMM was the initial design of Ferranti in Scotland in the 1970s, and today they are manufactured in small numbers, usually as shop floor hard bearing machines. Generally used for measuring relatively small parts, they provide open access to the operator on three sides. As a shop floor CMM, the Cantilever machine excels, because it lends itself well to automatic loading and unloading.

Gantry

Gantry CMMs are used predominantly for very large or heavy parts that require the high precision of a bridge machine. Most Gantry machines are mounted directly to the floor and therefore must have a substantial foundation.

Gantry coordinate measuring machines provide significant advantages over conventional bridges:

1. Heavy parts and large components can be loaded directly on the floor; this is a tremendous safety feature.
2. Programming a massive part is also much easier, as access is unrestricted through 360°.
3. Surface plates for smaller components are easily inserted into the Gantry, making measurement simpler and less tedious for the operator.
4. All scale and drive systems are well clear of lift trucks and cranes during the loading process.

Disadvantages of the Gantry CMM include:

1. Significant cost of building a foundation
2. Complete lack of any kind of portability.
3. Gantry CMMs take up a tremendous amount of real estate.

Horizontal Arm

The configuration of a Horizontal Arm coordinate measuring machine is quite different from the three other types.

There are 2 basic types of Horizontal Arm CMMs:

1. Plate mounted. Here the column is mounted on a large surface plate, usually made of steel. A small percentage of the machines run on top of the plate (top mounted). The far more popular side mounted system allows the whole surface area of the plate to be utilized for measurement.
2. Two runway mounted. This top mounted design provides the utmost in flexibility, because it can be used singularly or as a dual arm machine for measuring large components. The dual arm machines allow measurement of both sides of the component simultaneously.

Horizontal Arm coordinate measuring machines are the least accurate of the four types of CMMs mentioned; however, when measuring large components that require open tolerances they are by far the most cost-effective solution.

WHERE TO INSTALL THE COORDINATE MEASURING MACHINE?

Where is the best place to install the coordinate measuring machine? Should it be placed in the quality lab or on the shop floor? The choice really depends on what you are trying to achieve and the type of results you want.

Quality lab

This will be the first choice of the majority of CMM installations, as it certainly is a better working environment than the shop floor. There the coordinate measuring machine can be used efficiently to measure accurately a wide variety of components and provide a continuous audit on manufacturing performance.

It should also be noted that most coordinate measuring machine manufacturers provide the environmental conditions under which the machine should function optimally. The following temperature conditions are always stated:

1. Ambient temperature.
2. Maximum air temperature variation in time.
3. Maximum gradient.
4. Maximum relative humidity.

When choosing a high accuracy coordinate measuring machine it is important to understand fully the manufacturers' restrictions for optimum performance. In all cases measuring machine accuracy is closely tied to temperature. 

The shop floor

Presents a completely different set of circumstances. Shop floor machines are becoming extremely popular. They do not replace a lab-based CMM, but they add some tangible benefits to the production process.

The shop floor CMM can be manually loaded and unloaded, automatically loaded and unloaded or totally integrated into the manufacturing process. Not only can they monitor and report dimensional conditions, but they also can provide corrections to the machine tool.

A CMM for the shop floor differs physically from the lab machine. The machine must be thermally stable. Machine guideways are normally covered or protected from dirt and contaminants in the shop. Hard bearings are generally preferred, as the use of air bearings often causes internal contamination of the machine's ways and guidance system.

The latest generation of CMMs for the shop floor use predictive temperature software compensation that utilizes a wide range of sensors, which continually monitor the environment and provide real-time compensation to the machine.

HISTORY OF THE CMM

The first three-axis coordinate measuring machines started to appear in the 1950s and early 1960s; they were Cantilever or Portal CMMs, and were manually driven. The part being measured had to be adjusted so that it was parallel and level to the coordinate measuring machine's coordinate system. Measurement was conducted by holding the probe to the part while depressing a foot switch to freeze the display!

The breakthrough happened in 1972, when Zeiss introduced their first probe. Sir David McMurtry invented a kinematic resistance probe. This probe allowed overtravel after automatic contact detection. Shortly afterwards, in 1973, Zeiss introduced their 3-D measuring probe. LK Tool from the UK lay claim to producing the first bridge style CMM machine, soon becoming the most popular CMM design.

Today it is approximated that over 12,000 CMMs of all types have been  purchased throughout the world!

SOFTWARE: ITS FUNCTION AND CAPABILITIES EXPLORED

Training and Requirements for the Programmer and Operator

Probably the key to measurement on a coordinate measuring machine is software. Software unlocks the door to a myriad of measurement tasks without the requirement of mathematical knowledge or specialized metrology skills. Software is the communicator that allows everyone involved in the manufacturing process to see and interpret how that process is performing.

Often the question is asked: What is the best CMM Machine software? There is, in fact, no one correct answer.  It all depends on what you are trying to do. It is wise to look carefully at different products before making a decision. In some circumstances, depending on what you’re trying to measure, specialised software is required.

It is important to emphasise the role that training plays in successfully utilising software. Most training is held in a classroom setting and is generally the best way to make someone familiar with the software that has been chosen. On a comprehensive software package, it normally takes 5 to 10 days of training to become proficient.

The prospective CMM machine programmer should have the following skills and experience:

• Basic maths skills
• Fundamental knowledge of measurement
• Experience within the manufacturing environment in which he is working
• Practical knowledge and understanding of Geometric Dimensioning and Tolerancing
• Basic understanding of Computer Aided Design
• Operational knowledge of a desktop computer

The individual must understand why and what he is doing, while also having a grasp of the validity of the results the machine is giving.

WHAT PROBING TO USE ON A CMM?

Today there is an extremely wide range of probes on the market. There are two basic types, Touch Probes and Scanning Probes.

Touch Probes

The most popular  on the market today is a modular probe that consists of two parts: the probe body and the stylus module. There are three manufacturers of modular probes: Renishaw, Hexagon, and Zeiss, each of which has similar accuracy and repeatability specifications.

Scanning Probes

Generally two sensors in one, enabling the user to scan while allowing touch trigger technology for geometry. Most coordinate measuring machines can be fitted with scanning probes when form deviation is required on geometric parts or when scanning contours for reverse engineering. Hexagon, Renishaw, and Zeiss all produce versions of scanning probes.

Most coordinate measuring machines utilise motorised probe heads. These allow rapid and repeatable inspection of complex components with the minimum amount of intervention from the operator.

Manual and fixed probe head systems

 A fixed probe head is cumbersome at the best of times and requires a lot of preplanning prior to measurement. Manual probe heads must be indexed to position and are not suitable for a fully automatic operation of your CMM machine.

Never underestimate the role that styli play in the accuracy and repeatability of a coordinate measuring machine. The stylus is the part of the measuring system that makes contact with the part being measured. The part being inspected dictates the type and size of stylus used.

PORTABLE CMM'S

There are many varieties of portable CMM devices, both contact and non-contact styles. The major product categories of portable machines include Articulating Arms and Laser Trackers.

Articulating Arms

Portable Arms are by far the most popular of all the categories. They are relatively inexpensive, extremely lightweight (at less than 20lbs) and can be used either as a tactile or laser device. The original product was designed and built in the 1980s by ROMER, and it was widely adapted for industrial measuring tasks where the part that is being measured was either too difficult to move or too inconvenient.

Although the portable arm's accuracy depends on the operator, the accuracy is quite respectable. Just like coordinate measuring machines, there is an accuracy specification for Portable Arms. Within this specification there is a point repeatability test and a volumetric accuracy test. And just like a coordinate measuring machine, regular calibration of the arm is necessary.

Laser Trackers

Were originally designed for assembly operations in aircraft manufacturing. The Laser Tracker, which is usually tripod mounted, measures the position of an optical target, which can be moved by hand to literally probe the part being measured.

Many Laser Trackers have the ability to utilise a three-dimensional handheld probe; it can be used as a portable CMM machine and it has become an extremely versatile tool. Handheld laser scanners can now be attached to the Laser Tracker, enabling the tracker to be a totally portable coordinate measuring machine on the shop floor with the ability to probe and scan.

Many of the comprehensive software manufacturers support Laser Trackers, particularly manufacturers that work with Articulating Arms. With the ability of some models to measure up to 80 meters at impressive volumetric accuracies, the Laser Tracker has become the go-to piece of equipment for large part manufacturing and assembly.

White Light Scanning Metrology

 Is the latest development in portable coordinate measurement and scanning. Many White Light Scanners are used for R&D purposes, but more and more they are being utilized for measurement on the shop floor, White Light Systems work like a camera, and each shot can collect hundreds of thousands of data points in a couple of seconds. Their advantage over other shop floor measurement systems is they are extremely accurate.

Many software products used with Articulating Arms and Lasers provide the capability of both scanning and dimensional measurement.

VIDEO MEASURING MACHINES (VISION METROLOGY)

From contour projectors to video measuring machines, optical comparators or optical projectors were based on technologies invented in the 1920s. They are actually based on magnified projections of shadows cast by the part and then projected onto a round screen. 

Measurement was accomplished by clipping a template of the part onto the screen and comparing its deviation from the actual part. When a micro stage was added, parts could be measured by adjustment of the micro stage, based on the crosshairs on the screen. Therefore, simple measurements were attainable, but essentially accuracy was based on the skill of the operator. In the 1970s modified versions of optical projectors were available on the market. They consisted of digital readouts or CNC controls and simple video technology.

In the late 1970s, View Engineering of California introduced the first three-axis automatic video measuring machine. Unlike previous iterations of modified contour projectors, this unit had a built-in control terminal and video monitor, complete with on-board Metrology software. It wasn’t until the 1980s when the video measuring machine (or vision machine, as we know it today) came into general use.

The newer video measuring machines combine several sensor technologies, namely video measurement, touch probing, and laser technology. Indeed, now they are truly multisensor systems that enable you to measure parts more completely with less uncertainty and in less time than a single video or probing system. No longer is a video measuring machine confined to a two-dimensional inspection. With multisensor technology three-dimensional parts can be easily measured, enabling high precision measurement at extremely fast speeds.

Video measuring machines are highly recommended for the measurement of smaller lightweight parts, or components that are easily deflected when touched. This technology should always be compared to the conventional coordinate measuring machine when choosing a 3-D measurement system. Notable manufacturers of multisensor video machines (multisensor CMMs) are OGP, Hexagon, Zeiss, and Mitutoyo.

UPGRADING AND/OR RETROFITTING YOUR CMM MACHINE 

The frame of the average CMM has not changed much during the last 35 years. The mechanical structure, in most cases, retained its dimensional integrity.

The major technical advances in coordinate measuring machines are the peripheral technologies:

• Software
• Probing
• Controls

Software has changed dramatically over the past five years and is still undergoing radical changes, as the CMM manufacturers incorporate new sensing (probing) products. As scanning becomes more popular, so does the requirement to handle copious amounts of data, and to present that data in a rational format.

Installing a new software package on your existing CMM can be a cost-effective way of providing today's technology in your measurement process without spending a fortune on a new machine. Many software products have built-in interfaces to work directly with your older controller, reducing costs even more!

Probing

When considering an upgrade or retrofit to your CMM, it might be wise to look at the business end of your machine, the probing system.

Motorized Indexing Heads

Upgrading from a fixed or manual probe head to an automatic one can change the operation of your machine This enables faster part inspection, without operator intervention during the measurement cycle, and no lengthy probe changes.

Modular Probes

Consist of a main body and a stylus module. Their big advantage is crash protection, such as when a bump happens; the module disconnects, saving the main body and the probe head from massive damage. The metrology advantage is ingenious, as stylus modules have optimized metrology for different lengths of styli.

Scanning Probes

Perhaps the most popular probing upgrade, a scanning probe gives additional capability to the older For profiles and form deviations, scanning probes provide more accurate data and save time, as they collect thousands of points per second. They also can operate in single touch mode for optimum flexibility.

Controls

Over the past 20 years controls have become less complex with fewer boards.  Maintenance and repair costs are therefore far less.  The controller is the heart of the modern-day CMM and performs the following tasks:

• Contains the error map
• Provides circular interpolation
• Allows the use of analogue and digital scanning probes, as well as laser scanners.

It’s almost always the most vulnerable part of the coordinate measuring machine as its essentially a computer and subject to obsolescence - most CMM retrofits include a controller upgrade.

WHAT TO CONSIDER WHEN PURCHASING USED COORDINATE MEASURING MACHINES?

What to Consider, Advantages and Pitfalls

Nowadays many purchasers of Coordinate measuring machines are considering purchasing a used or rebuilt coordinate measuring machine. There are essentially three channels through which you can purchase a used CMM:

Purchasing a used CMM machine at auction. Buying a used coordinate measuring machine at an auction is like playing the roulette wheel at a casino in that you never really know what you’re buying. There are many unanswered questions: is the machine functional? Has the machine been moved improperly to its current position? Are vital components missing or damaged? Are the computer and software present? The auctioneer does not know the answer to most of these questions. Then the buyer is faced with disassembling the machine, correctly bracing the machine, packing up, and shipment. There is usually a hefty charge for CMM professionals to do this work and then to set up the machine and calibrate it in your facility. It is not unusual for all these services to cost 20-40% of the auction price.

Purchasing a used coordinate measuring machine from a machinery dealer. Used machinery dealers know a great deal about machine tools; however, their knowledge about coordinate measuring machines is usually minimal. In many cases, the machine has been moved from its original site and placed in the machine tool dealer’s facility. Often, the equipment has not been braced properly and was disassembled incorrectly. This leads to hard to spot damage if you are an Internet buyer. Even an experienced buyer will not know the extent of any damage unless the machine is put under power and runs through a basic calibration procedure.  

Purchasing from a used CMM dealer. Generally speaking, when dealing with experienced used CMM dealers, the machines have been thoroughly assessed and, in most cases, can be witnessed running, or a video can be sent to you. Many CMM dealers have the ability to upgrade the machine or install new software, probing, and controllers, if requested. This type of company's biggest asset is the capability to repair any damage or replace worn or defective components prior to you receiving the machine at your facility. CMM experts often perform the installation and calibration at your site, offering a turnkey solution, if preferred. An Machinery Dealers Network Association (MDNA) used CMM machine dealer also offers a 30-day warranty on each machine sold, giving the prospective buyer added peace of mind.

MAINTENANCE OF CMM MACHINES - WHAT DO I NEED TO KNOW?

The coordinate measuring machine mainly runs trouble-free year after year, with an annual calibration and preventative maintenance visit. Maintenance costs will be higher if the machine is placed on the shop floor, but the convenience might often be worth it.

20% of breakdowns and maintenance costs are caused by poor air quality. A refrigerant air dryer should be considered on all air bearing machines. Condensation often occurs within the airlines; water permeates the CMM, which damages bearings and short-circuits electronics. Moisture and oil in the airlines cause degeneration of internal piping and blockage of air bearings. Collapsed clogged air bearings cause significant damage to the bearing ways.

Most machines that have chronic service problems can have the cause traced back to power supply problems. All coordinate measuring machines should have a power conditioner/battery backup system.

Keeping the CMM machine clean is the best insurance for continued reliability. The bearing ways on many of the newer brands of CMMs are generally exposed. Therefore, it’s important these ways are wiped clean daily, especially on shop floor-based systems.

On bridge machines the slave leg runs on the surface plate, and when dirt gets on this portion of the plate, it tends to embed under the bridge bearings. Ultimately, this causes a drop in precision and motor drive problems ensue. When scales are exposed, it’s also important to gently wipe them down with isopropyl or methyl hydrate.

The computer and controller filters should be inspected weekly and replaced, if necessary, in dirty shop floor environments. This is especially critical, because overheating of electronic components can cause serious and expensive breakdowns.

Preventative maintenance visits once or twice a year are highly recommended. The following is a checklist of things that should be inspected:

• Checking all filters (PC and controller)
• Visual inspection of all bearing conditions, X, Y and Z.
• Verification and resetting, if necessary, of encoder gaps
• A good cleaning of the machine, including bearing ways and scales

A few minutes spent daily on maintaining your CMM can save you thousands of dollars.

Calibration of CMM machines

Calibration compares deviations or errors on your coordinate measuring machine with known international standards. A full CMM calibration is verification and correction of those errors, bringing the machine back to the original manufacturer's specifications. Most companies calibrate the coordinate measuring machine every 12 months. Many manufacturers check the CMM performance monthly, or as needed; for example, after a collision or a bump.

Calibration versus certification

Certification is the verification of the performance accuracy of a coordinate measuring machine at a given point in time. Calibration or recalibration is when the service provider adjusts a CMM machine to bring the accuracy and performance back to the manufacturer's specifications. 

When considering a calibrator for your coordinate measuring machine an ISO/IEC 17025 certified provider should be the first choice. The international standard for coordinate measuring machine calibration is ISO.10 360 (currently part 2:2009). The standard mainly consists of two separate tests:

1. Length measuring performance designated as E. The test procedure defines a series of measurements of either a step gauge or a laser in the machine's volume.
2. Probing performance testing designated as R. This is conducted on a precision sphere, wherein random and systematic errors with the probe can be isolated and monitored.

One of the newer tests in the latest specifications is the scanning test. As contact scanning becomes more popular, the new test is proving very useful for users of machines equipped with full contact scanning systems.

LASER SCANNER ON A CMM MACHINE

A natural evolution of the CMM machines’ capability is the increasing number of sensors that have become available. There has been a great deal of interest in mounting a laser scanner on manufacturers’ existing machines and also on new machines. Utilising a coordinate measuring machine as a three-axis carriage for a laser scanner gives the scanner the benefit of a high accuracy positioning system, which is far superior to portable arms or optical guided devices that require manual movement of a handheld scanning device.

A laser scanner can be integrated into the coordinate measuring machine in such a way that tool changers can accommodate a mixed variety of probes, including lasers. This provides fully automatic measurement capability and allows the benefits of seamlessly exchanging tactile probes for laser probes during the inspection or reverse engineering process, without the intervention of an operator or programmer.