What is Accuracy?
The accuracy of a measuring instrument indicates the deviation of the reading from a known input
What is Precision?
The precision of a measuring instrument indicates its ability to reproduce a certain reading with a given accuracy
What Sensitivity is?
The sensitivity of a measuring instrument is the ratio of the instrument scale change to the change in the measured variable
What is Error?
The error of a measuring instrument is the quantified deviation between the instrument reading and the known input
What is Uncertainty?
The uncertainty of a measuring instrument is the deviation between readings when a known reference value does not exist
What are Systematic errors?
Systematic errors remain constant or change in a regular fashion in repeated measurements of one and the same quantity. Corrections can be introduced to remove the error
What are Random errors?
Random errors are differences between the results of separate measurements which cannot be predicted individually
What are potential sources of random errors? 5
- Human errors
- Environmental errors
- Equipment errors
- Data analysis errors
- Calibration errors
Sources of errors in CMM measurements can be classified as? 2 types
spatial errors or computational errors
What are Spatial errors?
are errors in the measured position of a point:
- Accuracy of components of the CMM
- Environment in which the CMM operates
- Probing strategy used
- Engineering characteristics of the workpiece
What are Computational errors?
are the errors in the estimated dimensions:
- CMM software used to estimate the workpiece geometry
- Precision of the host PC
- Number and relative position of the measured points
- Geometry departure from the ideal geometric form
Interim Checking
A full CMM verification (calibration) will typically only occur once a year, due to issues of time and cost
Machine interim verification is strongly suggested by
- Reference test piece
- Ball-ended bar
- Kinematically located reference arm
- Ball plates/hole plates/ring gauges
MACHINE CHECKING GAUGE
The MCG and other techniques of interim checking provide an ability to routinely and quickly (15 minutes) produce a relative health check of the CMM.
* A database of results should be kept.
* Any deviation away from an interim check performed very close to a full CMM verification, should be a cause for concern.
PROBING ERRORS
- Manufacturers will often state a “Probing Error” known as “R”.
- “R” is the error within which the range of radii of a material standard can be determined with a CMM, using a sphere as a reference artefact (ISO 10360).
- Stylus to ideally be at 45 degrees to the X, Y and Z-axes.
- The probe should have previously been qualified using the normal routines.
- 25 points should be taken randomly over the surface of one hemisphere of the reference sphere.
Equation for r
All 25 measurement points are used to calculate the Gaussian (least squares) solution to the sphere.
* For each measurement point, the radial distance “r” from the sphere centre to the probed point is calculated.
20°C
On April 15th 1931, the Comité International des Poids et Mesures (Paris) agreed that henceforth, all length measurements should be referenced to 20°C.
THERMAL EXPANSION
- All materials will contract and expand when subject to changing conditions and temperatures
- The use of metrology instrumentation must consider the thermal Environment, including the handling of the instruments
NOMINAL DIFFERENTIAL EXPANSION
- Corrections must be made for the Nominal Differential Expansion (NDE) if consistent, accurate measurements are to be made with respect to 20°C.
- NDE calculations cannot be exact, because information about coefficients of linear expansion for a material and a component will not be known in detail.
THERMAL MEMORY
- Components all have a thermal memory, the reactivity of which is dependant on material type, mass, component dimensions and the environment.
- Components moved from one environment to another which is at a different temperature, will start to change temperature, dimensions and consequently shape.
- Typically components require a “soaking” or “soak-out” time during which they stabilise to the new environment.
- Soak-out may take minutes or even days. Serious errors will occur if measurements are attempted when the component is dimensionally unstable.
THERMAL MANAGEMENT
- Accurate use of the CMM requires control over conduction, convection and radiation. This can be achieved in a number of ways:
- Direction of air flow
- Type of air flow
- Velocity/volume of air flow
- Temperature sensors (location/type)
- Air lock and/or soak room
- CMM temperature compensation
- Control of lighting (left on)
- Control of other heat generating appliances
- Reduction of human contact of components
Becomes a significant issue when the CMM is situated in the manufacturing area
What is ISO 10360 PART 2 and what does it state?
- Part 2 of ISO 10360 deals specifically with the procedures for performing a comprehensive verification of the CMM accuracy within a measuring volume
- For each of the seven configurations, take and record measurements of five test lengths (in each of these seven orientations).
- The total number of length measurements is thus 105.
- Measure one point only at each end of the test length for each length measurement.
The performance of the CMM is verified if?
The performance of the CMM is verified if none of the 105 values of error of length measurement (converted to microns) is greater than the manufacturers stated error value (EL,MPE).
