CMM.pdf

Dr Swadesh Kumar Singh
Ex-IES, Ph.D. IIT Delhi
Coordinate Measuring Machines

Need For Faster Inspection
— Use of conventional devices like micrometer, vernier callipers,
dial indicators, protractors etc used to measure only a single
feature of the work piece.
— Large inventory of metrological instruments with different
ranges and different accuracies is needed for measuring different
features of a work piece.
— Time taken for the inspection will also increase.
— There is a growing need for a device that can do faster first piece
inspection and many times 100% dimensional inspection.
— The Coordinate Measuring Machine (CMM) plays a vital role in
the mechanization of the inspection process.
Dr Swadesh Singh, Ex-IES, Ph.D. IIT Delhi

Coordinate Measuring Machines
— Coordinate Measuring Machine (CMM) is an electromechanical
device for measuring the physical geometrical characteristics of an
object.
— Also known as Measuring Machines.
— This machine may be manually controlled by an operator or it may
be computer controlled.
— Parts are fixture on a work table attached to the structure.
— Measurements are made using a probe attached to the third
moving (Z) axis of the machine.
— Data collection system typically includes a machine controller,
desktop controller and application software.

Coordinate Measuring Machine

Features of CMM
— All the moving members, the bridge structure, Z-axis carriage
and Z- column are made of hollow box construction to give
maximum rigidity with reduced weight.
— Errors built up in the machine are fed into the computer for
error compensation to be built up in the software.
— All the CMM machines are provided with their own computers
with interactive dialogue facility and friendly software.
— Thermocouples are incorporated to the machine and interfaced
with the computer for temperature gradients compensation.
— Versatile in their capability to record measurements of complex
profiles with high sensitivity (0.25µm) and speed.

Components of CMM
— Mechanical structure and a displacement transducer
— Probing system.
— Drive system and control units to move each of the three axes.
— Digital computer system with application software

Mechanical Structure
— CMM machine structure incorporates precise movements in X, Y
and Z directions.
— Displacement transducers fitted to each axis sense the direction of
linear movement, measure the coordinate values of each axes and
gives digital display.
— They are classified into six configurations:
Ø Cantilever type
Ø Moving Bridge type
Ø Fixed Bridge type
Ø Column type
Ø Gantry type
Ø Horizontal arm type

Cantilever Type
— A vertical probe moves in the Z-axis.
— This is carried by a cantilevered arm that moves in theY-axis.
— This arm also moves laterally through the X-axis.
Advantages:
Ø Small size, low cost and minimum floor space requirement.
Ø Convenient access to the work table.
Ø Capacity to measure large, thin work parts.
Ø High rate of mounting and measuring on CMM.
Disadvantages:
Ø Limited accuracy due to cantilever nature of the system.
Ø Lower rigidity than most of other CMM constructions.

Cantilever Type

Moving Bridge Type
— It is the most widely used structure.
— It has a stationary table to support work piece to be measured and a
moving bridge.
Advantage:
Ø Reduce bending effect.
Disadvantage:
Ø With this design, the phenomenon of yawing (sometimes called walking)
can occur- affect the accuracy.
https://www.youtube.com/watch?v=oGwfp1euyJI
https://www.youtube.com/watch?v=m5be1CWYRc0
*Yawing – This happens because two legs of the bridge move at slightly
different speeds and result in the twisting of the bridge.

Fixed Bridge Type
— In this type, the bridge is rigidly attached to the machine bed.
— This design eliminates the phenomenon of walking and provides
more accuracy and high rigidity.
— In this deign, throughput is somewhat affected due to involvement
of additional mass.
— https://www.youtube.com/watch?v=3B2diS2IMqw

Column Type
— These machines are also referred as Universal Measuring
Machines(UMMs)
— The column type CMM construction provides exceptional rigidity
and accuracy.
— These machines are usually reserved for gauge rooms rather than
inspection.

Column Type

Horizontal Arm Type
— These come in a variety of configurations such as moving ram,
moving table and fixed table.
— These are mainly used to measure the dimensional and geometric
accuracy of the machined or fabricated workpieces.
Advantage:
Ø Provides a large area, unobstructed work area.
Ø Ideal configuration for measurement of automobile parts.
Disadvantage:
Ø Due to cantilever design of the horizontal arm, it becomes less
rigid and hence less accurate.
Ø https://www.youtube.com/watch?v=Zt7lmsDRFBM

Horizontal Arm Type

Gantry Type
— The support of the work piece is independent of the X and Y axes,
both are overhead, supported by four vertical columns rising from
floor.
— This set up allows us to walk along the work piece with the probe,
which is helpful for extremely large pieces.
— https://www.youtube.com/watch?v=Bl2mnAPcTkA
— https://www.youtube.com/watch?v=BdzO1Sigw6U

Gantry Type

Probing System
— Part of a CMM that senses the different parameters required for
the calculation.
— The probe is fastened to mechanical structure that allows
movement of probe relative to the part.
— The tip of the probe is made of ruby ball ( Ruby is a form of
Corundum (Aluminum oxide)).
— High hardness for wear resistance and low density for minimum
inertia are the required characteristics of the ruby in the probe.

Probe Assembly

Probing System
Types of Probes:
— Contact type probes
1. Touch trigger probe
2. Analog scanning probe
— Non- contact type probe
ü Used when the object being measured
would be deformed by the contact of
stylus. For inspection of printed circuit
board, measuring a clay of wax model
etc.
1. Laser probes
2. Video probes
Outline of Probe Head

Contact Probes
1. TouchTrigger probes
Ø These are the most widely used probes.
Ø The probe actuates when the contact is
made with part surface. It works on
Triggering mechanism.
Ø The computer records this contact
point coordinates with the help of a
transducer.
Ø An LED light and an audible sound
usually indicate contact.
Ø Touch probe assemblies usually consist
of three components: probe head,
probe and stylus.
TouchTrigger
Part section of
probe head

Contact Probes
Ø VariousTriggering mechanisms which are commercially used are:
ü The trigger is based on the principle that, when the tip of the probe is
deflected from neutral position then the highly sensitive electrical
contact switch starts emitting signal.
ü The trigger actuates when there is an electrical contact between probe
and metallic part surface.
ü The trigger uses a piezoelectric sensor that generates a signal based on
tension or compression loading of the probe.
Ø Probe occupies its neutral position when it has been separate out from
the contact surface.

Probing system

Contact Probes
2. Analog scanning probe
Ø Used to measure contour
surfaces, complex and
irregular surfaces.
Ø Remains in contact with the
surface as it moves.
Ø Improve the speed and
accuracy

Non-Contact Probe
1. Laser scanning probe
Ø Laser probes project a light beam on the work surface.
Ø When the light beam is triggered, the position of beam is read by
triangulation through a lens in the probe receptor.
Ø Laser tool has a high degree of speed and accuracy.
2. Video probe
Ø The features are measured by computer count of the pixels of the
electronic image.
Ø The camera is capable of generating multitude of measurement
points within a single video frame.

Styli

Calibration of Three Axes Coordinate
Measuring Machine

CMM Operation And Programming
— Positioning of CMM probes relative to work surface can be
accomplished by manual operation or direct computer control
(DCC).
— Computer controlled CMM’s operate much like CNC machine tools
and must be programmed.
CMM Controls
— The methods of operating and controlling a CMM can be classified
into four main categeories:
1. Manual drive.
2. Manual drive with computer assisted data processing.
3. Motor drive with computer assisted data processing.
4. Direct computer control with computer assisted data processing.

CMM Controls
1. Manual drive CMM
• The operator physically moves the probe along the machines axes
to make contact with the work surface and records the
measurements.
• A digital readout provides the measurements that the operator
records either manually or with paper printout.
• Any calculations on the data must be made by the operator.

CMM Controls
2. Manual drive with computer assisted data processing
— It provides some data processing and computational capability for
performing the calculations required to evaluate a given work part
feature.
— The types of data processing and computations range from simple
conversions between units to more complicated geometry
calculations such as determining the angle between two planes.
— The probe is free floating and permits the operator to bring it into
contact with the desired part surfaces.

CMM Controls
3. Motor drive with computer assisted data processing
— In this electric motors are used to drive the probe along the
machine axes under the operator control.
— A joystick or a similar device is used to control the motions.
— These are generally equipped with data processing to accomplish
the geometric computations required in feature assesment.

CMM Controls
4. Direct computer control with computer assisted data processing
— This operates just like a CNC machine tool.
— It is motorized and the movements of the coordinate axes are
controlled by a dedicated computer under program control.
— Various data processing and calculation functions are also performed
by the computer.
— The DCC CMM requires part programming as with a CNC
machine tool.

DCC CMM Programming
— Two principle methods of programming a DCC measuring machine:
1. Manual leadthrough method
2. Off-line programming
— In the manual leadthrough method the operator leads the CMM
probe through the various motions required in the inspection
sequence, indicating the points and surfaces that are to be
measured and recording these into the control memory.
— During regular operation, the CMM controller plays back the
program to execute the inspection procedure.

DCC CMM Programming
— Off-line programming is accomplished in the manner of computer-
assisted NC part programming. The program is prepared off-line
based on the part drawing and then downloaded to the CMM
controller for execution.

Applications
— CMM’s find applications in automobile, machine tool, electronics,
space and many other large companies.
— These machines are best suited for the test and inspection of test
equipment, gauges and tools.
— For aircraft and space vehicles, a100% inspection is carried out by
using CMM’s.
— These can be used for determining dimensional accuracy of the
components.
— CMM’s are ideal for determination of shape and position, maximum
metal condition, linkage of results etc which cannot do in
conventional machines.

Applications
— CMM can also be used for sorting tasks to achieve optimum pairing
of components within tolerance limits.
— CMM’s are also best for ensuring economic validity of NC
machines by reducing their downtime for inspection results. They
also help in reducing cost, rework cost at the appropriate time with
a suitable CMM.

Advantages
— Precision and accuracy obtained is very high.
— Flexibility in operation
— Reduced set up time
— Single setup
— Improved accuracy
— Reduced operator influence
— Improved productivity

Limitations
— The table and probe may not be in perfect alignment.
— The probe may have run out.
— The probe moving in Z-axis may have perpendicular errors.
— Probe while moving in X and Y direction may not be square to each
other.
— There may be errors in digital system.

Causes of Errors in CMM
— The table and probes are in imperfect alignment. The probes may have a
degree of run out and move up and down in the Z-axis may occur
perpendicularity errors. So, calibration of CMM with master plates is
necessary before using the machine.
— Dimensional errors of CMM are influenced by
Ø Straightness and perpendicularity of the guide ways.
Ø Scale division and adjustment.
Ø Probe length.
Ø Probe system calibration, repeatability, zero point setting and reversal
error

Causes of Errors in CMM
Ø Error due to digitization.
Ø Environment.
— The length of the probe should be minimum to reduce deflection.
— The workpiece must not exceed maximum limit since it can change
the geometry of the guide ways.
— Variation in temperature of the specimen, measuring lab influence
the uncertainty in measurements.
— Translation errors occur from error in the scale division and error in
straightness perpendicular to the corresponding axis direction.
— Perpendicularity errors may occur if three axes are not orthogonal.

Features of CMM Software
— Measurement of diameter, center distances, lengths, geometrical
and form errors in prismatic components, etc.
— Online statistics for statistical information in a batch.
— Parameter programming to minimize CNC programming time of
similar parts.
— Measurement of plane and spatial curves.
— Data communications.
— Digital input and output commands for process integration.
— Program for the measurement of spur, helical, bevel and hypoid
gears.
— Interface to CAD software.

CMM Software Capabilities
— Resolution selection.
— Conversion between SI and English (mm and inch).
— Conversion of rectangular coordinates to polar coordinates.
— Axis scaling.
— Datum selection and reset.
— Circle centre and diameter solution.
— Bolt-circle centre and diameter.
— Save and recall previous datum.
— Nominal and tolerance entry.
— Out of tolerance computation.

CMM.pdf

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