Specifying Your Coordinate
Measurement Machine
The type, shape, size and
dimensional tolerance of the part to be inspected
determine the kind of equipment to be considered.
Flat parts (sheet metal stampings, plastic, rubber,
etc.) can be inspected faster with a video system.
An optical comparator is likewise a 2-D device, best
for checking profiles or small flat parts by hand.
They are not suited for production.
Height Gages, as the name
implies, are good for checking heights on a surface
plate. Inspecting a hole pattern with this 1-D
device by flipping the part, still practiced by
some, is cumbersome, time consuming and unreliable.
In this day and age, when a small
Coordinate Measurement
Machine does not cost
more than some height gages, this is a truly
wasteful approach.
For 3-D measurements the CMM
should be the equipment of choice A motorized
Coordinate Measurement Machine
with a powerful computer and software does cost just
a few thousand dollars more today than did a manual
CMM of the same size twenty five years ago. They
were equipped with a digital readout only. The
variety and configuration of sensors available today
give the user the possibility to inspect just about
any type of part, be it a complex aircraft valve
body, a plastic or rubber part, a glass lens or
polished mold. Line lasers, capable of taking
thousands of points per second can digitize an odd
shaped part in minutes. In short,
Coordinate Measurement
Machines cover the
whole gamut from simple to use manual units with a
basic touch probe and software to fully automatic
machines in a production environment pre-programmed
to inspect the most sophisticated of parts to a fair
accuracy. You can have anything in between. Like the
survival of the fittest, time has removed inferior
designs from the market.
Of the many styles out there
the traveling bridge, the gantry and the horizontal
arm design are most common with the traveling bridge
being the most popular. Horizontal arm units,
capable of measuring large envelopes at acceptable
accuracies are used mainly for large auto body
parts, large weldments, etc.
When selecting a Coordinate
Measurement Machine the
following issues should be addressed first before
looking at accessories: a) size, b) manual or DCC (CNC),
c) Inspection room or shop floor application.
a) The size is obviously
determined by your largest part.
Then again if this particular
part shows up only 2 times a year, you should
reconsider. The smaller the unit the better. On the
other hand you also have to look at the
configuration of the parts. If you have a part that
is 12” wide, for instance, with 3” deep bores on
either side that you wish to access with an
articulating head, you need an additional 5” of
travel on each side. This brings the total travel to
22” with no safety clearance added. Pre-qualifying
these positions on the reference sphere further adds
to the necessary envelope.
b) Manual or DCC.
Financial issues aside, this is
largely a function of part quantity or a combination
of quantity and complexity. A prototype shop should
have a manual unit and a production environment
requires a DCC unit. Aside from the fact that CNC
units have dramatically come down in price, medium
part quantities e.g. 10-30 can be efficiently
checked manually with a pre-written program. An
unskilled operator then simply touches the points on
the part as commanded by the screen. The in and out
of tolerance condition may be printed out.
A ROI calculations sheet is
available for cross-over quantities making a
decision as to manual or DCC easy.
If a part is complex and small,
with fine features, a DCC unit is recommended even
for small quantities. It is hard to negotiate a .5mm
stylus into a 1.5mm hole without breaking it.
c) Shop floor or inspection
room application.
Since an inspection room has to
cover all eventualities it should be equipped with a
fair size unit, the top of the line software offered
by the OEM, an indexable probe head (manual or
motorized) and a good selection of styli and
extensions. This requires a well trained and/or
experienced operator. Any new operator should be
well trained by the OEM.
When selecting a
shop floor CMM
robustness of the equipment has to be considered.
Mechanical bearing units are inherently more
reliable, since they do not require a constant
supply of dry and clean air which can create a
maintenance headache. Air bearing CMM’s are a poor
choice for applications in a dirty environment,
requiring an expensive enclosure that a mechanical
bearing unit with covers and bellows can do without.
The potential user should educate himself on this
issue.
Production CMM’s are usually
dedicated to one or just a few specific parts. Parts
should be fixtured. Fixtures can be supplied by the
user or the OEM. The same goes for the part
programs. Probing systems should be as simple as
possible. Motorized probe heads should be avoided in
high production situations if the part is not too
complex. Probes with detachable stylus modules and a
stylus rack may accomplish the same task at half the
price if no more than 6-8 orientations are required.
Angled styli orientations can be created with stylus
knuckles.
Fast start menus make the
operation and program selection easy for shop floor
personnel.
If the temperature in the shop
varies substantially from 68°F (20°C), the standard
OEM calibration temperature, you may want to look at
a temperature compensation package or purchase a
unit with metal scales on metal structures if you
inspect metal parts. It is always a good practice to
let the part “soak” to reach the same temperature as
the Coordinate Measurement Machine.
Sensors
The electronic touch trigger
probe, the scanning probe, the single point laser,
the line laser and the video camera are the sensors
offered on CMM’s today.
Touch Trigger Probes, being the
least expensive yet capable of measuring just about
everything on a machined part, are used in the
majority of applications. They usually consist of a
probe head, fixed or indexable, the touch probe
itself and the styli. DCC machines may be outfitted
with a motorized indexable head (7 ½°), but add
substantially to the overall cost. For a few
thousand dollars more you can purchase a small DCC
Coordinate Measurement
Machine.
Probes with detachable stylus
modules are a good investment, especially for DCC
units. They allow the use of a stylus rack (6
stalls) akin to a tool changer on a machining
center. Modules are held in place magnetically and
detach in the event of a collision without damaging
the probe itself. A multitude of styli (measuring
tips) are available from several sources that cover
just about any measuring task, from a .3mm ball tip
to a 1” diameter disc to a cylinder for thin sheet
metal.
Mechanical Scanning Probes are
used to gather a high number of points in bores and
surfaces of prismatic parts and for digitizing
non-linear unknown surfaces. Higher density points
give you a more accurate picture of the feature as
required by ANSI 14.5 e.g. roundness, cylindricity
and flatness. These probes are obviously more
expensive than trigger probes and require a high end
controller.
The Single Point Laser is also
used for digitizing. It is an excellent tool to
check the profile of delicate surfaces e.g. coated
optics and soft parts since it does not physically
touch the part.
The Line Laser is the fastest
way to digitize or inspect non-linear surfaces and
contours like cell phone housings or car body parts.
The lines are up to 2” wide taking 4000 or more
points per second. The accuracies range from + .001”
to + .00025”. It is a powerful tool in conjunction
with CAD software having a 3-D best fit option. The
line laser is popular for reverse engineering.
Video attachments for CMM’s
require additional software and back-lighting and
are therefore not widely used.
Software
Measuring software is the most
important part of a CMM next to the physical
structure. It may be your main purchasing criteria.
Most OEM’s have their own brand, some do not. You
should inquire as to how long the software has been
on the market. It is very hard to evaluate
CMM
software in just a couple of hours unless you are an
experienced CMM operator. The big names in the
industry do not necessarily have the easiest to use
systems, which is what the buyer should be looking
for. Whichever software takes the least number of
keystrokes or mouse clicks to measure a feature or a
complete part is the better one. Fancy graphics and
many windows do not measure a part. If time permits
take a somewhat complex part to the vendors and
compare inspection and DCC programming time as well
as the ease to do so.
Beyond user friendliness you
may need other features like real time SPC, export
to CAD or a 4th axis.
You should be cautious with the
much talked about CMM program writing from CAD. It’s
O.K. for simple parts, but a DCC motion program with
a motorized head and widely varying styli is fraught
with pitfalls. If and when it has progressed to a
point where it’s close to being seamless, it would
require quite a knowledgeable operator. Most people
do not recognize the fact that a
Coordinate Measurement
Machine is not a simple
single point system like a machine tool with only
one fixed and defined coordinate system where the
part to be machined is always aligned to the axis
travels. Therefore a CMM programmer, not necessarily
the operator has to have a good grasp of 3-D points
in space and coordinate system transformations. Good
and thorough training of the programmer is
imperative. Once a program is written a lesser
trained person may push the buttons. There is many a
CMM that sits in a corner because nobody knows how
to use it.
When evaluating software you
also may inquire about the platform (Windows and
C++) and whether there are service contracts with
future upgrades available. Proper support is
important.
Accuracy
The potential user should
understand the difference between resolution,
repeatability, and accuracy. In brief; resolution is
the least count of the measuring system.
Repeatability is how well the CMM repeats a given
dimension or feature; this is always some multiple
of the resolution and includes the non-repeatability
of the probe, which in some instances exceeds that
of the Coordinate Measurement
Machine itself. Linear accuracy, taken along each
axis travel, is how much any linear dimension
deviates from the absolute NIST standard. The
volumetric accuracy is usually determined with a
ball bar according to B89.4.1a or another artifact
and includes the non-linearity of the ways, out of
squareness condition, length variation of all axes
to one another as well as the non-repeatability of
the probe. Consequently this number is substantially
higher than the linear accuracy.
Some CMM’s are highly software
compensated to achieve the stated accuracy. There is
nothing wrong with taking out the last wrinkles of
an otherwise sound structure, but making computer
compensation more or less the basis of the CMM
accuracy is not commendable. If you have a
collision, lose the compensation table or upgrade to
a better software system down the line, re-mapping
the CMM will be expensive. CMM’s with intrinsic
accuracy have the lowest maintenance cost over their
life span.
Erwin Helmel
President
Helmel Engineering Products
Inc.
Niagara Falls, NY
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