Coordinate Measuring Machine , UPSC ESE & GATE , Production , (New Topic Added in GATE)

 Coordinate measuring machine (CMM)

• A CMM is a device for measuring the physical geometrical characteristics of an object.
• This machine may be manually controlled or it may be computer controlled.
• Measurements are defined by a probe attached to the third moving axis of this machine.
• Probes may be mechanical , optical , laser or white light , amongst others.
• A machine which takes reading in six degrees of freedom and displays these readings in mathematical form is known as CMM.

Main components

• A CMM consist of four main elements:

1. The main structure which include three axis of motion .
2. Probing system
3. Machine control and computer hardware
4. Software for three dimensional geometry axis

Main structure

1. Cantilever

• A vertical probe moves in the z axis
• Carried by a cantilever arm that moves in the y axis
• This arm also moves laterally through the x axis
• Advantage - a fixed table allows good accessibility to the work piece
• Disadvantage - the bending caused by the cantilever design
• The cantilever design offers a long table with relatively small measuring ranges in the other two axis.
• Suitable for measuring long , thin parts

2. Bridge type

Advantage:

• Most widely used 
• High rigidity owing to compact bridge design and thus small measuring deviations.

Disadvantage:

• Limited accessibility caused by the bridge.

Application:

• For medium to large measuring range

3. Column type 

• Often reffered to as universal measuring machine instead of cmm
• This column type cmm construction provides exceptional rigidity and accuracy 
• These machines are usually reserves for gauge room rather than inspection

4. Gantry

• The support of the workpiece is independent of the x and y axis , both are overhead , supported by four vertical column rising from the floor.
• This setup allows you to walk along the workpiece with the probe , which is helpful for extremely large pieces.

5. Horizontal

• Also referred to as layout machine .
• Has a moving arm and the probe is carried along the y axis
• Advantage - provides large area
• Ideal configuration of measurement of automobile parts.

Probing system

• It is the sensory part of the cmm responsible for sensing different parameters required for the measurements.

Stylus:

A pointed instrument used as  an input device in the probe of a CMM

Types of probe:

• Transmission trigger probes
• Optical transmission probes
• Motorized probes
• Multiple or cluster probes
• Proximity or non contact probes

Inductive transmission probes

• They have been developed for automatic tool changing.
• Power is transmitted using inductive linking between  modules fitted to the machine structure and attached to the probe.

Optical transmission probe

• These probes also have been developed for automatic tool changing  .
• It allows probe rotation between gaging moves , making it particularly useful for datuming the probe.
• The wide angle system allows greater axial movement of the probe and is suitable for the majority of installation.

Motorized probe

• With the motorized probe , 48 positions in the horizontal axis , 15 in the vertical axis can be programmed for a total of 720 distinct probe orientation .
• A range of light weight extension , the head can reach into deep holes and recesses.
• The head of the probe is sufficiently compact to be regarded as an extension of the machine quill.
• This enables the inspection of complex components that would otherwise be impossible or involve complex setups.

Multiple stylus probe heads

• Wide range of styli have been developed to suit many different gaging applictions.
• The selection of stylus is done based on the application for which the probe is to be used .

Proximity or non contact probes

• Uses laser , capacitive or video measurement technology.

Laser probe

• A cmm equipped with a laser probe can convert a part of physical model into a digitized file.
• Such a file can be compared with other files and can be manipulated by designers to improve quality.
• Manufacturers can verify that each finished part measures exactly as designed.
• Laser triangulation probes are used to scan the surface and after scanning it transmits a continuous flow of data .
• Line lasers are the fastest way to inspect non linear surface .
• It is widely used in reverse engineering.

Automatic stylus change system

Machine control and computer hardware

• The control unit allows manual measurement and self teach programming in addition to cnc operation .
• The control unit is microprocessor controlled .
• Usually a joystick is provided to activate the drive for manual measurement.

Software for three dimensional geometry analysis

• In a cmm , the computer and the software are an inseparable part.
• They together represent one system
• The efficiency and cost effectiveness of a cmm depend to a large extent on the software.

The features of CMM software

• Measurement of diameter , center distance , length , geometrical and form errors in prismatic components, etc.
• Online stastics for statistical information in a batch.
• Parameter programming to minimize cnc programming time of similar parts.
• Measurements of plane and spatial curves.
• Data communication
• Digital input and output commands for process integration
• Program for the measurement of spur , helical , bevel and hypoid gears.
• Interface to cad software

Working principle of CMM

• CMM is also a device used in manufacturing and assembly processes to test a part or assembly against the design intent .
• By precisely recording the x , y , z coordinates of the target point are generated which can then be analyzed via regression algorithms for the construction of features.
• These points are collected by using a probe that is positioned manually by an operator or automatically via direct computer control.

Modes of operation

 1. Manual mode:

• CMM has a free floating probe .
• The operator moves the x,y,z axes to establish contact with the part feature to be measured .
• The differences in scale reading among the contact points are the measurements.

2. Manual computer assisted:

• Electronic digital displays are added to cmm for making zero setting, to select inch/mm , to print data in the standard format.
• These features save time , minimize calculations and reduced errors.
• A joystick is used to drive the machine axes.
• The operators manipulates the joystick to bring the probe into contact with the job.

3. Direct computer controlled:

• This is fully programmable
• The cmm uses 'taught' locations of cad data , to decide where the probe contact the job, and then collects measurement data.
• The fully automated cmm allows the operator to place the job in a fixture or on a table . run a stored program , collect the data points, and generate an output report/SQC record.

Advantages of CMM

• Flexibility 
• Reduce setup time 
• Single setup
• Accuracy 
• Reduce operator influence 
• Improve productivity
• Universatility and ease of operation

Application of CMM

• Aerospace engineering 
• Automobile engineering 
• Reverse engineering 
• Medical engineering

CMM in computer aided manufacturing

DMIS

• Dimensional measurement interface specification is a new standard in communication being used in CAM.
• It provides a bi-directional communication of inspection data between CAD system and inspection equipment so as to see what has to be made.