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Metrology > CMMs, Measurement, Encoders Probes, > Flashcards

CMMs, Measurement, Encoders Probes, Flashcards

1
Q

What is COORDINATE MEASUREMENT?

A
  • Coordinate measurement allows an engineer to compare one component to another using the dimensions of the two components.
  • Sample components off the production line can be assessed with respect to a master specimen or a master set of dimensions, assumed from the design drawings.
  • Coordinates or dimensions can be measured using Cartesian or Polar axes.
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2
Q

What are the 5 alternatives to CMMs?

A

OPTICAL CMMs
MULTI-AXIS MEASUREMENT ARMS
HAND HELD SCANNERS
TRACKING SYSTEMS
iGPS (GPS)

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3
Q

What are the KEY CMM ISSUES?

A
  • Coordinate measurement principles
  • Encoder design
  • Touch Trigger Probes
  • GD & T principles
  • Errors and uncertainties
  • Verification and calibration
  • CMM measurement application
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4
Q

What are the 8 General CMM Measurement Stages?

A
  1. Selection of features on the workpiece to be measured.
  2. Definition of the workpiece datum(s) used within the coordinate system.
  3. Selection of the workpiece orientation.
  4. Selection of the workpiece holding method.
  5. Qualification of the CMM probe.
  6. Definition of the probing strategy.
  7. Programming of the CMM.
  8. Analysis and recording of the results.
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5
Q

Why some WORKPIECE FEATURES may not be measurable?

A
  • Some features may not be measurable using a CMM !!
  • Some features may be impracticable to measure with a CMM !!
  • Some features may not be cost effective to measure with a CMM !!
  • Consideration should therefore be given to the minimum number of measurements required, in order to determine component accuracy and conformity to specification.
  • Ideally the strategy should only require one set-up to measure all features, and in reality a minimum number of set-ups should be used.
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6
Q

What is the COORDINATE SYSTEM of work piece and CMM and why is it needed?

A
  • Coordinate systems may be rectilinear with familiar notation (X,Y,Z).
  • However, polar coordinates may also be present, and in many cases more useful for measurement purposes (θ, r).
  • The CMM is “dumb” until the component coordinate system is identified.
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7
Q

What and how many COORDINATE SYSTEMS are NEEDED FOR:
Cylindrically polar coordinate system
Spherically polar coordinate system

A

Cylindrically polar coordinate system - One rotary and two linear axes
Spherically polar coordinate system - One linear and two rotary axes

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8
Q

How are Datum Points identifed on a CMM?

A
  • Drawing datum points, planes and surfaces need to be identified from the relevant component drawings.
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9
Q

What is PROBE TIP QUALIFICATION?

A
  • Before any measurement can begin, the CMM needs to know where the probe tip is in space.
  • This is achieved using a probe tip qualification routine.
  • The routine in its simplest form involves taking five points around a sphere.
  • The sphere is of a precise size and sphericity.
  • The ruby tipped stylus is of a precise size and sphericity.
  • The five data points are typically 0, 90, 180 and 270 degrees around the equator, and one at the pole.
  • The data points are fitted to a sphere in software.
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10
Q

What is the BS 7172: 1989 standard for?

A

This standard is typically used by the CMM software designers to write the code for processing data points.
The governing standard which specifies the “approved” mathematical fitting techniques and equations is BS 7172:1989.

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11
Q

PROBING STRATEGY - Number of contact points required?
Straight Line
Plane
Circle
Sphere
Cone
Ellipse
Cylinder
Cube

A

Straight Line Min 2 Rec 5
Plane Min 3 Rec 9
Circle Min 3 Rec 7
Sphere Min 4 Rec 9
Cone Min 6 Rec 12/15
Ellipse Min 4 Rec 12
Cylinder Min 5 Rec 12/15
Cube Min 6 Rec 18

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12
Q

What should be considered with CIRCULAR MEASUREMENTS?

A

If lobing is suspected, great care with measurements should be taken

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13
Q

Check if spherical, cylindrical, cronical measurement equations are needed

A
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14
Q

What are the Several key features that lead to issues of CMM probing?

A
  • Functional understanding of the component.
  • Understanding of the manufacturing processes.
  • Dimensional Inspection Plan.
  • CMM probing strategy.
  • CMM Manual programming.
  • Feature based programming.
  • CMM off-line programming and modelling.
  • 3D modelling software inspection path generation.
  • CMM Programme testing
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15
Q

What are the basic features of a probe?

A
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16
Q

What is the Lord Kelvin’s principle of kinematics?

A

A free body to move in space has six degrees of freedom
- These are three degrees of linear freedom and three degrees of rotary freedom.
- The principle also states that one point of contact will remove one degree of freedom.
- Hence for complete constraint, in space, only six contact points are required.

17
Q

How the touch probe work?

A
  1. The trigger signal is generated, whenever movement of the stylus causes any of the three sets of contacts to be broken.
  2. The trigger signal causes the CMM software to read the encoder positions, thereby giving a coordinate position for the point of contact with the work piece.
  3. The spring force causes the probe to re-seat itself after every operation, and allows sub-micron repeatability.
18
Q

How is probe switching done?

A

Electric current breaks

19
Q

What is Pre-travel?

A

Pre-travel is the difference between when the stylus touches the object surface, and when the probe system actually gives the trigger signal.
- Pre-travel will depend on the probe orientation and the direction of force on the probe tip.
- Pre-travel needs to be assessed and compensated for, in order to measure repeatedly and accurately.
- On a day-to day basis, this is achieved via “probe tip qualification”

20
Q

How to calculate pretravel?

A

FC x L = FS x R
L and FS are constant
∴FC is proportional to R

21
Q

What is PROBE HYSTERESIS?

A

Probe hysteresis is a problem which can occur as a direct consequence of a previous trigger followed by the re-seating of the probe, and may be considered as being analogous to mechanical backlash.
- Increasing stylus length or probe force, tends to increase hysteresis.
- This aspect of measurement uncertainty can be quickly identified using a setof gauge blocks, by measuring their inner and outer dimensions.

22
Q

2 Rules of Stylus Selection?

A

There are some basic rules for stylus selection, bearing in mind that most probe heads will take a range of different styli:
- KEEP THE STYLI SHORT AND STIFF
- KEEP THE STYLUS BALL AS LARGE AS POSSIBLE

23
Q

5 Reasons for Non-Contract Probes not being successful?

A

Some reasons for this are:
- Limited to line of sight
- Limited resolution and accuracy
- Engineering mistrust
- Reliability
- Compatibility

24
Q

What is a Displacement Transducers and what are the type and most common?

A
  • When a touch trigger probes touches a surface, its position is actually determined from the displacement transducers fixed along the three linear axes. A variety of displacement transducers can be found in CMMs:
    – Optical scales
    – Rotary encoders
    – Inductosyns
    – Magnetic scales
    – Laser interferometers
  • Many of the CMMs manufactured today will use optical scales
25
Q

What is used for Laser Based Measurement?

A
  • The laser must be incorporated into a system of optical components known as an INTERFEROMETER.
  • There are many different designs of interferometer, but one of the best known is the MICHELSON interferometer
26
Q

The 3 ideal criteria for measurement are?

A
  • A fixed reference reflector
  • A moving measurement reflector
  • A measurement of the environment
27
Q

What is the THE MOIRÉ EFFECT?

A
  • A more common approach for optical encoders, uses a phenomenon known as the Moiré Effect.
  • This phenomenon is observed when two regular patterns (usually linear gratings) are placed in contact, and almost in alignment.
  • In most applications, there is usually a fixed grating, variously described as either the reference, master or index grating.
  • This is used to form moiré fringes with a distorted or displaced specimen grating.
28
Q

Moiré fringes can be formed in three ways:

A
  • Two gratings in near contact.
  • Projection of one grating onto another by an optical system.
  • Projection of one grating onto another via an optical medium (mirror, liquid)
29
Q

How is the separation of the Moiré fringes D calculated?

A

D = d/θ
“D” is the separation of the Moiré fringes.
“d” is the grating interval.
“θ” is the angle between the rulings of the two gratings, and is assumed to be small.

30
Q

What is Fringe Measurement?

A

A single detector will indicate the extent of movement, but not the direction of movement.
* Ideally a minimum of two detectors are used, spaced preferably one quarter fringe pitch apart, giving rise to output forms in quadrature.
* This leads to a lead or lag in electrical phase of the sensor signal, which may be used to determine direction of movement.
* Typically four photocells or photodiodes will be used for fringe counting.

31
Q

What is the Abbé Error?

A
  • Describes the magnification of angular error over distance.
  • The largest error component is quite often the Abbé Error.
  • This error could typically introduce 1 micrometre per 200 mm offset, for each arc-second of angular change.
32
Q

What is the Golden rule of metrology?

A

Only measure using one degree of freedom at a time

33
Q

Why is Traceability important in a CMM?

A

CMMs can be prone to a large range of systematic and random uncertainties. The purpose of calibration and the traceability chain is to provide confidence in the CMM output.

34
Q

WHY ANALYSE MEASUREMENT DATA ? 3
One data point does not tell us everything we need:

A

One data point does not tell us everything we need:
- Measurement value
- Reliability of the measurement
- Measurement trend

35
Q

What are the Methods of Numerical analysis?

A
  • Mean, Median, Mode…
  • Statistical methods
  • Probability based methods
  • Linear/non-linear regression
  • Least squares methods
36
Q

What is REPEATABILITY and REPRODUCIBILITY?

A

Repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions.
* Reproducibility is the variability induced by the operators. It is the variation induced when different operators (or different laboratories) measure the same part.
* Reproducibility also takes into account longer timescales and the possibility of external influences such as the environment.
* Testing of R&R is an influential tool to help assess the quality of a gauging operation, across a range of different metrology tools and types.
* A common approach is to implement a Gauge R&R study, which specifically tests the repeatability and reproducibility of a process.

37
Q

Why is a GAUGE R&R Study performed?

A

The Gauge R&R is performed by measuring parts using the established and calibrated measurement system.
* The aim is to capture as many sources of measurement variation as possible, so they can all be assessed and addressed.
* To assess repeatability errors, the same part is usually measured several times by one operator.
* To assess reproducibility errors, multiple operators are needed. The specification for the number of operators will be dependent on the company/laboratory, and may vary between 2 or 3, and 10.
* To many companies, a %GRR under 10% is acceptable, 10% - 30% is marginal, and over 30% is unacceptable.

38
Q

What is SIX SIGMA?

A
  • This philosophy has changed the concept of “acceptable” quality from parts per hundred, to parts per million, or even parts per billion.
  • In statistical terms, typical levels of performance range between two and four sigma.
  • Three sigma identifies a defect rate of 66,810 per million components produced, or a defect rate of 6.68%.
  • Six sigma identifies a defect rate of 3.4 per million components produced, or a defect rate of 0.00034%.
39
Q

What is the QUANTIFICATION OF ERROR?

A

If a measuring instrument is calibrated with respect to a traceable calibration standard (known value) then the experimental error can be deduced. This value may be stated or more likely used as an adjustment for the measurement data:
- A micrometer is found to have an error of +20 microns
Readings could be: 20.480mm + 0.020mm or: 20.500mm
* If the micrometer suffers from backlash in the thread form which is random in its presence, then this may be estimated as an uncertainty:
- Readings would be: 12.740mm +/-0.050mm

Metrology (7 decks)

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