Coordinate Measuring Machines and Systems Flashcards
What is the primary function of a CMM
To measure the actual shape of a workpiece, compare it against the derired shape, and evaluate metrological information
Main components of a CMM
- mechanical frame with three axes and displacement transducers
- probe head
- control unit
- computer
Steps of a measurement with a CMM
- calibration of the stylus wrt the reference point of the head (center of the tip)
- determination of the Mathematical alignment: position and orientation of the workpiece reference frame wrt the machine reference frame
- measurement of the surface points on the workpiece
- evaluation of the geometric parameters of the workpiece
- representation or reporting of the results
Basic configurations of a CMM
- bridge (fixed or moving)
- cantilever
- horizontal arm
- gantry
- articulated arm
Moving bridge configuration: characteristics
- stationary table and moving bridge
- widely used
- reduced bending of the horizontal axis
- problem of yawing should be considered
Fixed bridge configuration: characteristics
- moving table and fixed bridge
- very rigid
- reduced speed with heavy loads
- no yawing problem
Cantilever configuration: characteristics
- fixed table, cantilever arm
- good accessibility
- bending of the cantilever reduces the accuracy
- suitable for long and thin parts
Horizontal arm configuration: characteristics
- ideal for measurements on car bodies
- moving long table
- possible dual arm configuration to reduce time
- excellent accessibility
Gantry configuration: characteristics
- suitable for very large parts
- fixed columns
- easy accessibility
Articulated arm configuration: characteristics
- robotic arm (non-cartesian CMM)
- probe in the end effector
- moved by an operator or by actuated joints
Hardware elements of a CMM
- structural elements
- bearing supports
- drive system
- displacement transducers
- probe head
- control system
ideal properties of the structural elements
- dimensional stability
- stiffness
- low weight
- high damping capacity
- low coefficient of Thermal expansion
- high thermal conductivity
Possible materials used
- Granite: very often used
- Aluminum: large thermal expansion compared to granite, but it has a very much higher thermal conductivity, so it is often used, since its deformation can be easily predicted and compensated
- composite: good properties, easy to manufacture, can be designed to obtain the requested properties; expensive
Bearing systems
- noncontact air bearings
- aerostatic air bearings use a thin film of air under pressure to provide load support
- accurate
- durable - mechanical contact bearings
- higher loads are supported
drive systems
- rack and pinion
- simple
- not very accurate for the backlash - belt drive
- simple
- not very accurate for its elasticity - friction drive
- simple, cheap
- low drive force - ball screw
- classical solution
- high stiffness - linear motor drive
- very high stiffness
- water cooling is required
- expensive
displacement transudcers
they have the role of determining the probe position when it touches the surface
- transmission scale
- made of glass
- scale with 50-100 lines per mm
- moving light source
- scanning reticle
- photocells measure fluctuation of the light, generating two sinusoidal signals phase-shifted by 90° - reflection scale
- made of steel
- alternate reflecting lines and diffusing gaps
- period signal produced by the photocell after reflection - interferential scale
- photoelectric head reads the interference fringes created by the light reflected from the scale - laser interferometer scale
- based on the light interference principle, discovered by Michelson and Morley
- one laser ray is directed to a fixed mirror as a reference
- one other laser ray is directed to a moving mirror
- the interference figures revealed in the two mirrors are different, and can be used to determine the displacement of the moving mirror
- much more accurate that optical systems, with resolution smaller that 1 nm
types of control strategies
- . Point-to-Point control
- continuous-path control
point to point control
- target position given as point coordinates
continuous path control
- precalculated scanning
- probe moves to the target position along a defined path
- used for known shape features
- force is not controlled
- force proportional to bending of the tip - adaptive scanning control
- adaptive control ensures that the tip Always maintains contact with the surface
- useful for unknown features
- actuators for each axis
- modulated contact force
fixturing
- it should be accurate and keep the part in position
- forces are very small so there are not many dynamics issues
- aluminum is usually used
types of probing systems
- contact probes
- non-contact probes
types of contact probes
in general, the information comes from the contact with the part
- hard probe: the operator detects the contact
- touch trigger probe: the machine detects the contact, then moves to another point
- measuring probe: the machine measures the displacement of the tip; the movement is with constant contact with the surface, while the measurements are taken with a certain frequency
types of touch-trigger probes
- kinematic resistive probe
- piezo-electric probe
- fiber probe (optical)
kinematic resistive probe: description
components:
- probing element to establish a mechanical interaction with the workpiece
- transmitting device (stylus stem)
- force generating element (spring) for producing a probing force
- sensor to evaluate contact information
- interface to CMM for transmitting the information
- there is a pretravel space from the contact to when the signal is triggered
- the pretravel, in a three point sensor probe, depends on the direction of approach like a triangle