systems can help
you run circles around
If you're spinning your wheels,
running in circles or going round and round, you're
probably wasting your time. But if you manufacture round
parts, going round and round--using a roundness testing
system--may be the only way to accurately measure your parts.
In the grand scheme of things, roundness testing systems
are fairly new (only about 20 years old). The advent of
computer technology was the catalyst for creating a mechanized
roundness measuring system, but many quality control engineers
can remember "the old days" before modern roundness
"During the infancy of roundness testing, people
used a V-block," explains Michael Kambeitz, national
sales manager of surface-finish and form measurement at
Carl Zeiss IMT Corp. "Operators would place a part--a
hydraulic spool for example--in a V-block, manually rotate
it and record out-of-roundness using a dial indicator."
This was a very crude method because measurements were heavily
based on operator influence. Plus, there are only so many
points along a round part that a human being is capable
of measuring with a dial indicator.
In the past two decades, however, roundness testers have
evolved into dedicated form-measurement machines and emerged
as essential tools on the shop floor. Any company that manufactures
round parts, especially those supplying the automotive industry,
is probably already familiar with roundness testing systems.
Originally designed to measure a part's roundness, today's
systems are capable of several other form measurements,
A roundness testing system consists of a base, on which
a vertical column stands. A radial arm--which houses the
machine's probe--extends from the vertical column. Within
the base, there is either a mechanical or air-bearing spindle,
which accurately rotates the part. The machines are capable
of measuring thousands of data points per part, and data
are stored through an accompanying measuring station and
software (which will be discussed in more detail later in
Specifications vary from system to system, and almost
all roundness testing machine manufacturers also custom-build
systems. Some systems are capable of measuring parts weighing
hundreds of pounds, so finding a machine to fit your load-bearing
needs shouldn't be difficult.
Less common are roundness measurement systems that feature
an overhead spindle design. These systems are specially
designed for cases in which the part itself cannot be moved.
For example, the part is attached to another piece of equipment
or is simply too heavy to place on a traditional roundness
Although a roundness tester's probe is essential for collecting
thousands of data points, the heart of the machine lies
within its base, where you'll find the spindle. Roundness
measuring machines are equipped with either an air-bearing
or mechanical- bearing spindle. The fundamental difference
between the two is how they rotate the part--either mechanically
or utilizing a constant stream of air. Most experts prefer
air-bearing spindles, but mechanical-bearing spindles are
also quite popular.
"Lower-end machines make use of mechanical bearings
that will eventually wear," notes Kambeitz. "There's
less degradation with air bearings because friction is eliminated.
This allows the user to maintain accuracy and repeatability
over a long period of time."
Systems with mechanical-bearing spindles are typically
less expensive and are perfectly suitable for applications
in which tolerances are more relaxed. Air-bearing spindle-equipped
machines are ideal for cases in which tolerances are tight
and submicron-type accuracy is critical.
Although its name implies that a roundness measurement
system is only good for one thing, the latest models are
more dedicated and lend themselves to many more types of
Roundness testing systems are often referred to as form
measurement systems because of the several part features
they're capable of measuring. Along with roundness, today's
roundness testing systems are equipped to measure runout,
coaxiality, flatness, squareness, concentricity, cylindricity,
parallelism and perpendicularity, among other things (See
the chart below).
Additionally, surface roughness measurement will soon
be introduced as an advanced feature of roundness testing
systems, predicts Robert Wasilesky of Mitutoyo America Corp.
Some quality professionals might ask, "If I can get
similar results with a coordinate measuring machine or a
contour measurement system, why buy the roundness tester?"
One obvious consideration is the type of part(s) you need
to measure. Simply put, a roundness testing system is essential
for any organization that measures round, spherical and
There are a few other reasons you might be better off
choosing a roundness tester over other dimensional measurement
systems. Sebastian LaBella, sales manager at Detroit Precision
Hommel, urges roundness tester shoppers to consider:
Data collection. A major difference between a roundness
tester and a CMM is the ways in which they gather data.
A roundness measuring machine is capable of continuous form
measurement (thousands of data points around the circumference
of part), giving you the ability to thoroughly map the part's
form. In contrast, a nonscanning CMM can only grab a few
data points around the circumference of the part, which
isn't enough information to map it as accurately. In addition,
a contour measurement system--although ideal for part profiling--can't
measure the circumference of a part.
Cost. Low-end roundness measurement systems start
as low as $15,000. Companies that have a tight budget and
are interested in basic form measurement may want to consider
investing in a roundness tester. "With the lower price
tag, companies can afford to buy two or three systems and
place them at various locations on the shop floor, as opposed
to spending thousands of dollars on a bigger piece of equipment
that needs to be housed in a central location," explains
Mike Colicci, product manager at Mahr Federal.
Once the decision to purchase is made, there are a few
basic functions that a potential buyer should seek. The
importance of each function depends on the intended use,
so consider which are most crucial to your application and
"Companies need a machine that is well-built--one
that's going to give longevity to the accuracy and repeatability
of the measurements," notes Kambeitz. "It should
also be solid and sturdy, with tooling flexibility and analytical
Other features to look for include:
The machine's design. Is it solidly designed? How
freely does each part move? If portability is an issue,
how easily can it be moved around the shop floor?
Ease of use. How easy is it to use? How much training
will new users need? How much time will it take an operator
to make a measurement? Does the machine come with a user
console, computer, touch screen and keypad?
Reputation of the vendor. "Everybody has someone
whom they love to buy from, and there are reasons for that,"
says Colicci. "Go with a vendor that provides the best
technical assistance or service if needed."
Customer needs. If you're a supplier, keep the
customer's specifications in mind. For example, if your
customer demands tight tolerances for a part's coaxiality
and cylindricity, make sure you buy a roundness tester than
can accomplish both. This eliminates having to use more
than one machine to measure one part.
Measuring to standards. "Measuring to current
North American standards is a must," notes Wasilesky.
"This includes individual companies' standards and
those of the American Society of Mechanical Engineers."
ASME's codes and standards, as well as membership and educational
information, can be found at www.asme.org.
Software application and integration. Look for
multifunctional analysis, graphic capabilities and the ability
to upgrade the system to different ports or sensors, says
To give you an idea of different software packages that
accompany roundness testing systems, here's a look at four
major providers' offerings:
Mahr Federal's metrology software works with its MMQ 6100
roundness testing system. Using a touch screen, the operator
has the ability to set up, analyze and store information
for future recall, as well as monitor system status. A Roundness
Results screen displays chart and measurement information,
and users can generate hard-copy printouts. A "measure"
key allows the operator to take repeat measurements using
the same conditions as previous applications. The Roundness
Analysis screen lets the operator zoom in on segments or
profiles of parts. It allows evaluation and elimination
of unwanted details like scratches and burrs. A harmonic
analysis feature relates the part's lobing conditions to
the manufacturing process to predict the part's performance.
Mitutoyo's Roundpak v. 4.0 dedicated data-processing software
works with its Roundtest roundness measuring machine. The
Windows-based package analyzes roundness and cylindricity
and other geometric features. The software provides multiple
displays of analysis results, one-key measurement analysis
and a variety of 3-D displays. The multiple analysis/recalculation
function allows for simultaneous analysis of multiple items,
which permits the user to change filter cutoff values, delete
unnecessary data, reapply data for the analysis of different
items, and perform other recalculation functions based on
already recorded data.
Zeiss's TIMS form measurement software works with its Rondcom
roundness measurement system. The software offers clearly
defined access to functions such as controlling motorized
axes, performing computer-aided calibration, inputting workpiece
data, defining measuring conditions, measuring polar and
linear parameters, and setting automatic functions. Analysis
functions include profile editing and processing with different
filter settings and analytical methods. It allows for 2-D
and 3-D display options, plus linear, bearing area, amplitude
density and Fourier analysis. Profiles can be exported to
a contour-evaluation mode to analyze angles and distances,
and all information can be printed in hard-copy format.
Taylor Hobson Precision's roundness testing software works
with its Talyrond line of machines. The software provides
programming for automatic measurement, harmonics analysis,
cylindricity analysis with a choice of displays, asperity
removal and edge detection to remove unnecessary data, and
vertical and horizontal straightening.
Other companies provide similar software (refer to the
Web directory on this page to learn more). There is constant
development of more user-friendly, dedicated and intuitive
software, capable of performing not only the aforementioned
functions, but also advanced part analysis and gage R&R.
Most experts agree that roundness testing software is only
going to get better over time.
Besides the development of more user-friendly and intuitive
software, experts speculate on other improvements we can
expect from roundness testers in the years to come.
"The ideal case is to have a machine that allows
an operator on the shop floor to make the most reliable
measurements with the least amount of training and the fewest
number of errors," says Colicci. "The goal is
to make the machine more user-friendly, more intelligent,
less expensive and faster."
"A major fault with roundness machines today is subjectivity,"
adds LaBella. "As time goes by, operator influence
should be eliminated with easier-to-program software and
Although they're becoming more automated, roundness testers
still call for a fair amount of human interaction. And any
time a person is introduced in the measurement process,
subjectivity (and the potential for less accuracy) is also
"The most common human-related difficulty encountered
when using roundness testing equipment is a lack of experience,"
comments Colicci. "These gages used to be delicate
and expensive, used only in laboratories by trained specialists.
Today, it's very common to see roundness testers out on
the shop floor used by machinists. Operators must understand
the meanings of various parameters and how they're called
out on part prints. They must also know how to set up a
part on a gage and the purposes of their various functions."
It's critical that operators learn how to best utilize
the gage and be consistent in how they take their measurements
to ensure accurate and repeatable results, Colicci continues.
"The most common alignment errors result from incorrectly
centering the workpiece's angular alignment," adds
Wasilesky. This is referred to as the Limacon error."
(To learn more about the Limacon error, visit www-gap.dcs.st-and.ac.uk/~history/Curves/Limacon.html.)
Wasilesky says the second most common human error is misaligning
the workpiece's tilt prior to measurement.
There are steps that any roundness testing machine operator
can take to prevent inaccurate results and ensure better
repeatability. Wasilesky and Colicci offer the following
Measure with a constant measuring force and speed.
Thoroughly clean the work piece prior to measuring.
Minimize vibration, temperature change and air flow.
Invest in CNC alignment, which automates the part alignment
process and takes away some operator subjectivity. (This
feature is now available for less than $50,000.)
Measuring round parts without using one of these machines
is like trying to fit a round peg into a square hole. If
you've ever experienced inaccurate measurements and unreliable
repeatibility, or if you've simply been running in circles
trying to find the answer to your round part-measuring woes,
check out what roundness testing systems have to offer.
Kennedy Smith is Quality Digest's assistant editor. Letters
to the editor regarding this article can be sent to firstname.lastname@example.org.