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Flashcards in Metrology Deck (24)
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1

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

2

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

3

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

4

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

5

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

6

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

7

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)

8

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

9

Modal analysis steps

1. approximation of machine tool structure, representation of structure by finite number of discrete points
2. recording compliance frequency response functions using a shaker (between spindle and workpiece) and an ACCELEROMETER
3. - 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
4. - 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

10

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

11

Parameters determined from compliance matrix

- static compliance, excitation frequency of 0 Hz
- resonance frequencies
- dynamic/resonance compliance at machines resonance frequencies
- damping

12

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

13

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

14

Moving bridge CMM +/-

+ relatively low cost, lightweight
- design allows for yawing, columns move at different speeds

15

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

16

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

17

Standard gantry +/-

cast iron frame with multi-point support
+ very large measuring volume, operator access
+ medium accuracy
- expensive, large bed & frame required

18

CMM parametric errors

- 1 scale + 2 straightness
- 3 rotations per axis
- 3 out-of-squareness between XY, XZ, YZ

19

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

20

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

21

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

22

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

23

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

24

Quasi-static load-deformation analysis steps

1. break down the machine tool into different components and identify relevant joint zones / interfaces
2. determine planes that describe both the deformation of components and possible dislocations in joint zones
3. measure the deflections at planes' corners
4. calculate the rigid body translations and rotations as well as component deformations
5. calculate pro-rata displacement of each component and joint zone in relation to the total displacement at the tool-workpiece interface