Metrology Flashcards
Reducing pre travel
Length of stem - longer stems are softer and easier to bend, therefore the stem will bend more before contact point is recognised
Material of stem - stainless steel, tungsten carbide, ceramics. Steel exhibits the lowest rigidity, will deform the most
Trigger threshold - the higher the threshold the more the seating arrangement can be dislodged before a point is recognised. If this is too low vibrations in the machine could create a point in mid air
Approach velocity - with high velocity the touch probe is allowed to travel further due to reaction time of the software
Internal damping - this will allow the trigger threshold to be lower, resulting in a lower necessary contact force
Linear vs Pendulum approach
Linear: - points are approached successively, long time delay between first and last measured point
- reduced overall position error, ‘cold’ and ‘warmed’ measurement points taken on one position what cancel out
Pendulum: - position points recorded in quick succession, small variation and narrow scatter band
- short measuring time does not allow the machine to thermally grow or distort
- time between separate points is very long, thermal distortion will manifest as a large overall position error
Test component features
Perpendicularity of X&Y
two straight vertical faces, one along X in XZ plane, one along Y in YZ plane, angle between these features will give indication of angle between 2 axes involved with machining the feature
Test component features
Circular interpolation in XY
cylindrical feature that involves movement in both X&Y, must be extruded, measuring roundness will give info about circular interpolation of the 2 axes involved
Test component features
Position accuracy in Z
minimum of two surfaces in XY plane, at different Z heights, parallel to each other, distance between these surfaces provides info about accuracy of Z positioning
Test component features
Resolution of Z
very sharp ramp in Z, low resolution will show steps instead of smooth surface, machine failed to achieve a very low feed in Z
Direct vs indirect stiffness
Direct - stiffness measured in the direction in line with the applied force
Cross stiffness - defines stiffness perpendicular to direction of applied force (2 axes)
Under/over critical design
Under-critical: - natural frequencies are higher than the maximum excitation frequency expected to arise form any fast-spinning component
- usually favourable
Over-critical: - machine tool exhibits a natural frequency lower than most frequencies generated by its operation
- maximum cutting speed is a constraint that helps determine the correct design
- high speed machines use over-critical, frequency will only match during acceleration and deceleration
Modal analysis steps
- approximation of machine tool structure, representation of structure by finite number of discrete points
- recording compliance frequency response functions using a shaker (between spindle and workpiece) and an ACCELEROMETER
- Identification of natural/resonance frequencies and curve fitting
- curve fitting by determining natural frequency, damping and nominal stiffness
- multi-mass system can be represented as a finite number of single-mass systems, then these are super-positioned to give true behaviour of machine tool
- Identification of natural/resonance frequencies and curve fitting
- generation of vibration deformation diagrams in order to visualise results animations of dynamic behaviour of machine tool are generated
- animations exhibit dynamic deformation of machine tool structure at certain frequencies, can show which components exhibit most movement when machine is excited with certain frequency
- generation of vibration deformation diagrams in order to visualise results animations of dynamic behaviour of machine tool are generated
Compliance matrix measurements
9 directions total
- 3 directions in which force can be applied to main spindle (x, y, z)
- for each direction of force there are 3 directions in which the relative compliance has to be measured (x, y, z)
- accelerometer used instead of LVDT to determine dynamic performance of machine tool structure
Parameters determined from compliance matrix
- static compliance, excitation frequency of 0 Hz
- resonance frequencies
- dynamic/resonance compliance at machines resonance frequencies
- damping
Strain gauge probe
- 4 strain gauges mounted on webs inside probe body for X, Y, Z & thermal drift
- gauges trigger once a threshold force is breached
Piezo shock sensing probe
- piezo crystal detect mechanical shock signals generated when the stylus ball impacts
- can respond to frequencies higher than those detected by most other sensors
Moving bridge CMM +/-
+ relatively low cost, lightweight
- design allows for yawing, columns move at different speeds
Fixed bridge CMM +/-
moving granite table, ceramic Z ram \+ reduced yaw and increased stiffness \+ strongly reduced Abbe error - reduced operation - table plus workpiece must move, limited part weight
Horizontal arm CMM +/-
cast iron table with x-axis travel, single column providing vertical travel
+ ideal for large parts, excellent sideaccessibility
+ high throughput
+ two arms can be running opposite and parallel
- low dynamic stiffness, relatively large Abbe errors
Standard gantry +/-
cast iron frame with multi-point support
+ very large measuring volume, operator access
+ medium accuracy
- expensive, large bed & frame required
CMM parametric errors
- 1 scale + 2 straightness
- 3 rotations per axis
- 3 out-of-squareness between XY, XZ, YZ
Laser triangulation
- determine distances by calculating angles
- laser projects beam onto object, part of beam is reflected back onto detector
- depending on position of reflected beam, object distance is calculated
- a resonating mirror can be used to generate a line scan
Touch probing micro parts
- temperature controlled environment, mapping and compensating for thermal errors
- new materials, Invar, very low coefficient of thermal expansion
- low uncertainty calibration artefacts
- pagoda-shape CMM, higher rigidity than normal shapes, structure made of granite
Laser trackers
- use a laser beam to track position of retro reflector in space
- take point by point measurements
- head rotation angle Alpha, head tilt angle Beta and distance from the coordinate point
- spherical design of retro reflector allows its centre to always be at fixed offset distance wrt any surface being measured
- laser beam is reflected parallel but opposite in direction of incoming laser beam
Direct tool measurement
+ direct allocation of errors and their sources
+ separation of different error sources possible
- each property requires different setup
- very expensive and time consuming
Indirect tool measurement
+ fast & cheap
+ immediate result whether a machine is capable of performing a certain task
- cumulative error budget in test piece
- uncertainty of allocating the correct error source
Quasi-static load-deformation analysis steps
- break down the machine tool into different components and identify relevant joint zones / interfaces
- determine planes that describe both the deformation of components and possible dislocations in joint zones
- measure the deflections at planes’ corners
- calculate the rigid body translations and rotations as well as component deformations
- calculate pro-rata displacement of each component and joint zone in relation to the total displacement at the tool-workpiece interface
