Whether you’re a job shop or contract manufacturer, it’s important to repeatedly check

in on your CNC machine tools are doing to protect your capital investment. Whether

you’re intending to maximize your capacity or simply protect day­to­day operations, both

CNC machine maintenance and CNC machine calibration are indispensable routines

that you need to stay true to.

Preventive maintenance is a mindset to protect your CNC machine tools, maximize

optimization and reduce downtime. On the other hand, calibration will be necessary to

ensure a high degree of repeatability on the shop floor. To boost your equipment

lifetime and ensure constant uptime, check out the guide below.

Step One: Understand Predictive Maintenance Programs

Predictive maintenance programs, or PDM programs, are used to reduce machine

downtime and gauge project quality by anticipating problems before they happen. To

assure each machine tool’s maximum performance, today’s manufacturers are using

four major programs to streamline maintenance, repair and calibration:

Reliability and Maintainability

Machine Tool Variability Management System

Failure Mode and Effective Analysis

Total Productive Maintenance

These programs, when adjoined, can predict expertly when a CNC machine tool will

become intolerant, fail or require calibration. While calibration will be discussed further

down, maintenance will be focused upon first.

Step Two: Plan for Downtime Ahead of Time Your workplace will need to plan for downtime to reduce downtime. During critical

production periods, you’ll need to monitor machine tools, collect data, utilize a

combination of industrial instruments and conduct analysis effectively. Data is your

friend! That being said, if you do not have a plan on how to collect and store data, you

can become overwhelmed with the numbers.

You’ll need to gather a historic collection of in­house data. This data, when used

alongside the above­mentioned programs, will greatly assist the maintenance process.

Vibration analysis, calibration metrics and infrared thermography equipment, too, will be

needed.

Your historic data comparison will be used to predict your CNC machines’ service

requirements. Different machines have different work scopes, and each should be

attended to, repair and maintenance­wise, depending upon pre­scheduled periods. In

doing so, your shop can limit production interference while conducting maintenance.

Step Three: Conduct Quick­Check Tests Next, conduct quick­check tests on any machines in production. While industry PDM

programs are designed to reduce workplace downtime; weekly quick­check procedures

will reduce machine service time on a day­to­day basis.

Because of this, quick­check techniques will need to be integrated alongside your PDM

programs to assess, predict and reduce machine downtime. Some company’s

quick­check procedures involve measuring a machine tool’s overall volumetric accuracy

via laser­based measurements. If your company is using new laser calibration

machines, these checks can be performed without ever needing to remove a machine’s

components or covers.

Step Four: Outline Your Maintenance Plan Before jumping into predictive or preventive maintenance programs, you should

understand the scope of the maintenance that needs to be performed. Everyone in the

company needs to be dedicated to preventive maintenance . Your maintenance plan

should include—but isn’t limited to—several different areas of focus:

Filter and Oil Changes First, filters and oils should be changed regularly. Most preventive maintenance

programs entail additional steps during filter and oil replacement depending upon the

machine. This maintains a high degree of repeatability, extending equipment life while

ensuring ongoing operation.

Electronic Check­Up Next, preventive maintenance will require a check­up via electronic diagnostic

monitoring tools. To perform a total CNC machine check­up, your in­house workers will

need to check electric motors, drives, cables, amps, HMIs, and any other electronics

involved with the machine. Each should be inspected and undergo repairs when

needed.

Step Five: Understand Predictive Maintenance Drivers

Industry­wide adoption of PDM programs are norm these days. In fact, the growth is

largely driven from a migration from manual machines to CNC machine tools. Higher

production rates, higher quality requirements and fast­paced industry production all

result in an intensive need for ongoing PDM programs, and your shop needs to be a

part of this trend.

A regular PDM program will require a thorough inspection of each CNC machine tool

twice per year . During these check­ups, vibration analysis and overall micron­level

accuracy will need to be noted. After these are measured, the data will need to be

analyzed using the machine’s baseline data, previous­year data and newly acquired

data. Together, these data sets can create a “prediction” about a machine’s needed

maintenance, calibration, and any failure that could take place in the near future.

A Quick Return to the Quick­Check Technique The quick­check technique, again, is relevant. It grants users knowledge of a machine’s

volumetric accuracy, giving diagonal displacement measurements of complex functions.

Again, if your workplace has adopted laser­based measurements, it can use the

quick­check technique to examine movement pattern tests. In these tests, a CNC

machine will reveal any errors in laser alignment. A laser’s linear position, reversal, yaw

and pitch can help you catch a potential failure early in the process.

Step Six: Creating Your Own Predictive Maintenance Process Every manufacturer needs a custom PDM process uniquely tailored to them to

effectively service their machines. CNC machines can be incredibly varied, and a

standard PDM process may fail in accommodating for all processes, all components,

and different types of machines.

Every PDM program, for this reason, requires a long creation process. Your own PDM

program will be comprised of many steps. These steps, firstly, will identify which CNC

machine’s tools will be incorporated in testing. Secondly, they will determine the

technologies needed to measure, monitor and derive data from machine tool operation.

Make sure your PDM program, above all, accommodates for the following:

Vibration measurements

Calibration measurements

Infrared thermography measurements

Data collection and analysis

Cleaning and restoration

Step Seven: Selecting Predictive Maintenance Equipment You’ll need to select equipment worthy of your PDM process, too. Once your workplace

has purchased its needed equipment, it’ll be able to “mold” PDM procedures around it.

That said, every PDM program requires several elements—elements which should be

accessible to your purchased equipment. While equipment, itself, is as varied as a CNC

machine can be, you should take care in purchasing maintenance equipment able to

conduct the following:

CNC machine diagnostics

CNC machine tool condition monitoring

Data analysis and corrective action creation

Overall, your predictive maintenance equipment will host several options to identify,

measure and create solutions to maintenance issues. Early warnings are absolutely

important , and your workplace’s full integration of its precision measurement programs

is vital to each CNC machine’s manufacturing longevity.

Once your workplace has identified its needed program procedures, it must establish

criteria around acceptable accuracy and performance. These standards will create a

“baseline of error” which will alert your team of potential issues. All machine tool

baseline conditions must be identified, and consistent machine measurements will need

to be taken on a periodic basis. By collecting data frequently, you can create a data

trend. This trend, when analyzed, will be your key to creating maintenance

predictions—and, as a result, predictive maintenance schedules.

Machine Tool Calibration Predictive maintenance is incredibly effective. However, with the adoption of new

technology practical instruments capable of measuring machine tool performance have

become even more important. In previous years, a manual machine tool’s overall

accuracy was determined by its operator’s ability to turn crank handles accurately,

navigate with precision and ensure consistency. Statistically determining these

processes, of course, was impossible. Simply put: There was little way to calibrate or

determine a process’s capabilities.

CNC machines, however, needed verifiable positioning, high­end precision and error

compensation. New calibration tools were the answer. Early developed machine

calibration techniques relied upon mechanical artifacts and a comparator to measure a

CNC machine’s static accuracy. Even in controlled conditions, however, the comparator

faced issues from low resolution. For this reason, any calibration method conducted by

an operator still faces accuracy issues.

Using a Grid Plate Encoder to Calibrate To conduct basic calibration, consider using a grid plate encoder. A grid plate encoder

features a spindle­mounted, non­contact reading head. This reading head scans a

targeted area with a circular path, measuring a grid plate as it’s mounted upon the

targeted machine table. During grid plate encoding, the head reveals plot deviations

from a circle. Often, these measurements refer to a true circle, so as to limit deviations

as much as possible. The head’s created plot, then, can be used as a guideline to

correct servodrive errors, machine mechanism deviations and other measurements. A

grid plate encoder, however, can only correct errors in two axes.

Using a Double­Ball Bar System to Calibrate Dynamic path accuracy can also be measured with a double­ball bar system. A

double­ball bar system is a telescopic bar with a mounted ball upon each end. When

one ball pivots within the system’s socket—which is anchored to a table—the other

pivots within its spindle socket. As a circular path is created, any changes in distance

between the two mounted balls will reveal an error in circle perfection.

Using a Capacitor Gage to Calibrate

The capacitor gage, meanwhile, may be used to measure a spindle’s thermal growth, or

“runout.” While a capacitor gage won’t be useful for calibration in every machine, it’s

incredibly useful for calibrating prolonged spindle use when thermal growth may offset a

directed path.

Using a Rotary Encoder to Calibrate Rotary Motion In most cases, your CNC machine rotary tables will need intensive calibration. Regular

rotary table calibration involves the use of a rotary encoder, sine plate and level. These

tools, however, are prone to error when compared to upgraded options. Because a

rotary table is double­checked with a sine plate, it must be physically moved each time.

Set­up time is lengthy, and sine plates are expensive for their large margin of error.

Modern rotary motion calibration often uses Doppler calibration and laser interferometer

systems. These systems offer multi­optical support, an indexing table and high

accuracy.

Modern Calibration Tools While the above calibration tools are useful, their limitations are only growing. The

industry is evolving, and optimized processes require pinpoint calibration accuracy.

Such a need for higher machine accuracy has increased calibration system demand,

pushing the development of precise, versatile options. The upcoming generation of

calibration tool equipment consists of two high­powered, laser­based measuring

systems: The Michelson interferometer and the Laser Doppler Calibration System.

Examine them below. If you can, prioritize them when considering your workplace’s

calibration equipment.

Using the Michelson Interferometer

Today’s Michelson interferometers utilize technology invented in the 1880s. Using a

white light source, the interferometer harnesses a movable mirror and fixed mirror to

precisely measure angles and position. Today’s interferometers use helium­neon lasers

and two corner cubes, however, as they’re far more accurate.

Single­Frequency Beams

The first of two types of Michelson interferometers uses a single­frequency helium­neon

laser beam and a beam splitter. When the beam passes through one of the machine’s

moving corner cubes, its other half is reflected into the other cube. These reflected

beams return, meeting one another within the beam splitter. These beams, together, will

create an interference fringe pattern. Once counted by a photodetector, the fringes

reveal a cycle of change intensity.

Two­Frequency Beams

The Michelson interferometer is available in a two­frequency variant. The two­frequency

interferometer, unlike the single­frequency interferometer, circumnavigates the regular

electrical noise, gain drift and slight inaccuracies of the single­beam variant. It uses two

helium­neon laser beams of different frequencies, creating an overall carrier frequency.

Distance information, then, is carried within AC waveforms—not a one­frequency

device’s DC wavelengths. A two­frequency interferometer is a fantastic calibration

device, but it requires the installation of permanent magnets and high­quality optical

components to ensure accurate measurements and polarization. Scattered light,

otherwise, may be lost when returning to the machine’s laser resonator.

Using the Laser Doppler Calibration System The Laser Doppler Calibration System requires the use of a laser Doppler displacement

meter, abbreviated as LDDM. The LDDM utilizes optical heterodyne techniques,

electro­optics and several phase demodulators to measure a movable corner cube’s

position. This measurement, when compared to a machine’s operations, can expertly

track angles—calibrating it.

LDDM systems don’t suffer from issues with stray light and polarization. They

additionally don’t require specialized optics to maintain operations. Windows, inserted

into an LDDM system’s beam path, alongside several basic mirrors, can reflect the

system’s laser back. A compact system, the LDDM can be mounted easily. For this

reason, it’s a highly versatile machine tool that eliminates the need for multiple machine

components.

A Doppler system requires two optics: a retroreflector and a laser head. A laser

interferometer, meanwhile, requires three separate optics to be mounted via tripod

beyond the machine tool. In many cases, the Doppler system is far easier to install, set

up and operate. Data collection is automatic, which aids immediate measurements.

Future Calibration Tools The world of CNC calibration is constantly changing. In the near future, machine tools

will likely feature immediately accessible calibration systems. These systems will be

formatted into a machine’s software, giving users immediate access to custom

solutions. Capable of automatically correcting errors with measurements, these

calibration tools will additionally collect and chart information automatically, helping

users determine a machine’s tool condition quickly, effectively and reliably.

Fortunately, cost­effective and efficient calibration systems exist today. While the

industry awaits on­line calibration options, quick­checks and predictive maintenance are

still highly efficient. With regular discipline, a broad PDM program and the correct tools,

in­house specialists can significantly reduce machine downtime, increase investment

returns and keep organizational operations at a peak.