| Coordinate measuring machines (CMM) are extremely versatile and
powerful measuring instruments (Figure 1). |
Figure 1. Bridge
Type Coordinate Measuring Machine
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The CMM allows us to locate points on a three-dimensional structure as
well as measure features and surface relationships. One of the most
important features of the CMM is its ability to asses geometrical
relationships. The CMM replaces long, complex, and inefficient manual
inspection methods. Coordinate measuring machines (CMMs) play a valuable
role in precision measurement and metrology. CMMs combine coordinate
inspection capabilities with a computer to provide a fast, accurate and
more convenient alternative to conventional methods for measuring complex
parts. A coordinate measuring machine system is composed of a base machine
and a software package, aided by many probe options and styli. A probe is
a sensing element which along with a stylus comes into contact with the
surface to be measured.
Types of Coordinate Measuring
Machines
Coordinate measuring machines are classified by machine type, and mode
of operation.
The machine type is the physical configuration of the machine itself.
There are two main types of machine configurations: horizontal and
vertical.
The Vertical Type CMM
| The vertical type CMM has a vertical probe which is carried by the
bridge. The bridge surrounds the measuring area (Figure 2). The
bridge style CMM is an especially rigid machine. This makes it one
of the most popular styles of CMM equipment. |
Figure 2. Bridge Type
CMM Machines
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| The bridge style CMM has a granite surface plate, air bearing
slides, and a bridge like construction. Another type of vertical CMM
is the gantry style coordinate measuring machine (Figure 3). The
vertical Gantry type CMM provides no support to the workpiece. The
machine hovers over the workpiece.The workpiece sits in an
independent setup. This style of machine is best suited for very
large workpieces. The advantage that this machine has over the
others is that the operator can walk along with the probe. These
machines can be operated manually or they can be operated
automatically by numerical control. |
Figure 3. The Gantry
Type Coordinate Measuring Machine
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The Horizontal Type CMM
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Figure 4. Horizontal
Arm Type Coordinate Measuring Machine
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The horizontal type CMM has a moving arm and the probe is mounted
in the horizontal plane rather than the vertical plane (Figure 4).
The advantage of this style machine is the large, unobstructed work
area it can provide. The machines are often referred to as layout
machines much like the layout machines found where large castings
are layed out and machined.
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Modes of Operation
There are three basic modes of operation for the Coordinate Measuring
Machine. They are manual, motorized computer assisted, and direct
computer-controlled. The manual type CMM is the most popular type found on
the shop room floor. The manual CMM has a free floating probe which moves
along three axes on air bearings. The motorized computer assisted type
CMMs use a computer and drive mechanisms for motor assisted movement. The
direct computer controlled CMM is fully automatic and can be used for
measuring parts without the intervention of the operator. The more complex
CMMs would be found in the inspection and engineering areas rather than on
the shop floor.
Types of Probes
All CMMs utilize some type of probe to measure the part features. There
are three general types of probes: hard, soft, and non-contact (Figure 5).
The two most common probe types found in general machine shops are the
hard and soft, both are contact type.
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Figure 5. Hard
Probes are especially useful for measuring deep holes. |
When using hard probes the actual contact must be done manually. This
is one of the reasons the hard probe is not as popular as a soft or
electronic probe. Another reason why the hard probe may not be as popular
is the inherent deflection which may occur upon contact. Hard probes have
special applications where odd part configurations dictate the use of this
type of probe.
Soft Probes
| It is not uncommon to see electronic probes replacing hard probes
in most applications. The electronic or "soft" probe uses
a circuit breaker type design. When contact is made with the part,
the probe sends a meassage to the computer to stop reading. The
contact pressure is consistent and therefore the measurements are
reliable. Soft probe assemblies consist of three parts: the head,
probe, and stylus (Figure 6). |
Figure 6. Parts of
the soft probe assembly.
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Figure 7. A LASER or
light beam type non-contact probe.
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The non-contact type probes in use today typically use LASER
technology (Figure 7).
The original objective of the non-contact type probe was for use
on soft or fragile workpieces. However, advances in LASER technology
has proven to be a very productive means of inspection for all types
of workpieces. The great speed and accuracy of the LASER type probe
has been invaluable in reverse engineering applications where many
measurements must be taken. |
Calibration
| With all of the inspection and locating devices that you have used
up to this point it was always pointed out that measurements taken
in the same direction were the most reliable beacause of backlash.
One of the most significant features of the CMM is it’s ability to
reverse directions without the problem of backlash. Because there is
no need for backlash compensation, the machine itself has no
inherent measuring flaws. However, the probe still does. Since the
probe is the one component that is physically measuring the part it
needs to be calibrated. All CMM probes need to be calibrated against
a known standard. The standard may consist of a gage block or ring
gage, or some type of precision sphere (Figure 8).
The calibration sphere is one of the most accurate ways to
calibrate the probe because measurements are taken in all axes
directions around the ball. |
Figure 8. The
calibration sphere.
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Subroutines
| The real value of the CMM can be found in it’s ability to do
repetitive measuring operations. Measuring operations such as
measuring circles, spheres, cylinders etc. The power of the computer
allows the CMM to do these calculations with great speed and
accuracy. To perform these operation, subroutines are programmed in
the computer software. The operator picks the measuring routine
he/she wants to perform, and the software indicates to the operator
what and where to measure. Once the computer has enough information
the subroutine calculates the measurement. On conventional surface
plate work some of these calculations could take hours. Below are
found some common subroutines associated with most CMMs. |
Figure 9. Form
Deviation subroutines measure the amount of deviation from tru-form.
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Figure 10. Feature measurement subroutines
measure the size or location of the features of a workpiece.
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Figure 11. Orientation subroutines measure
workpiece surface relationships.
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Conclusion
The CMM does not introduce any new principle of measurement. However,
it does extend our ability to measure parts efficiently and effectively.
The CMM cannot and should not be used to measure all workpieces. The
application of the CMM should only be used on those parts which truly have
the characteristics necessary for this type of measuring tool. For those
workpieces which need the application of the CMM, this machine will reduce
inspection time drastically and will reduce machine down time when first
part inspection is required. It must be remembered that the CMM is a
precision measuring tool and should always be treated as such.
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