Chapter 13 Coordinate Measuring Machines

1
Chapter 13 Coordinate Measuring Machines

• Overview
• The role of CMMs
• Types of CMMs
• Types of probes
• Probe calibration
• Machine programming
• Modes of operations
• Metrological features

2
The Role of CMMs

• Reduce inspection time (80-90)
• Widely use in manufacturing
• Short runs
• Apply for Multiple features
• Flexibility
• Avoid Production interruption

3
Structure of a CMM

• 3 Axes XYZ
• Carriage on each axis the 1st carriage travels
  ?guide the 2nd one ?3rd travels along its axis
• Each carriage has a fitted positioning transducer
  ?record the displacement of them ? computer or
  display
• The 3rd carriage ? probe ? contact ? all
  displacements
• Envelope limits of travel in all three axes (not
  limit size of work pieces)

4
Types of CMMs

• 10 standard types
• 5 common types
• Cantilever type large envelop
• Bridge type rigidity
• Column type rigidity, accuracy
• Horizontal arm type large work pieces
• Gantry type extreme large work pieces
• 5 variation types
• Special type rotating work table or probe
  spindles (column type) speed up inspection

5
Five Basic Types of CMMs
6
Five Variations of Basic CMM Types
7
Types of Probes

• Hard contact make contact manually
• Soft contact (electronic probes) probe head,
  probe, stylus
• Can record movement directions 4-way, 5-way,
  6-way probes
• Use soft probes compare as zero point
  standard null process, standard built-in scale
• Non-contact type probes Avoid deforming objects
  by stylus
• Light beam stylus
• Microscope
• Optical probes

8
(No Transcript)
9
The Ball Styli, Multiple Styli Probe, Joints and
Extensions
10
Probes Calibration

• Probe calibration remove the deflection problem
  of probe

11
Machine Programming

• Benefitsaccuracy, speed, precision
• Functions
• Resolution selection
• Measurement units conversion
• Conversion of rectangular coordinates to polar
  coordinates
• Axis scaling
• Datum selection and reset
• Circle center and diameter solution
• Bolt-circle center and diameter
• Save and recall previous datum (datum reference
  ? data)
• Nominal and tolerance entry
• Out-of-tolerance computation
• Statistical process analysis and control (contour
  program)
• Use of subroutines for repetitive inspection
  work ? reduce operator errors caused by fatigue

12
Typical Useful Subroutines
13
Modes of Operations

• Manual mode CMMs obtain measurement data and the
  program provides the measurements in the required
  form of user
• Automatic mode the computer provides the
  measurements in the proper sequence, then uses
  the data to compute the results
• Programming mode self-teach ? record sequence
  or add off-line instructions
• Statistical analysis mode use the batch mode
  for large number of similar parts ? trends that
  may lead to out-of-control conditions, CMMs use
  for complex parts with many features

14
Metrological Features

• Repeatability 2.54?m (?when size of CMM?)
• Accuracy 10.16?m (?when size of CMM?)
• CMMs can reduce computational errors, systematic
  errors (check before measure)
• CMMs do not diminish thermal error,
• User must be careful about normalization of the
  machine, vibration
• CMMs have the alignment built-in function (line
  of measurement//the axis of standard)
• Avoid the misinterpretation the source of error
  in angle measurement
• Easy control the distortion of the workpiece with
  CMMs
• High reliability check itself.