FAQs

Accuracy refers to the closeness of a measured dimension to a standard or known value. It is identified as the difference between the measurement of a dimension and the accepted value for that dimension from a trusted external source or the percentage by which the two values differ.

Measuring the accuracy of a Profile Projector or a Video Measuring Machine

For example, suppose a measurement of a length of a gage block on a Profile Projector gave a length of 9.997 mm. If the manufacturer of the gage block certified the gage block has a value of 10.001mm then :

The accuracy of the measurement is 9.997-10.001 = + 0.004 mm.

Such a calculation gives the absolute deviation of the measurement. A measure of the accuracy can only be determined if some prior knowledge of the true value is available

The precision of an instrument refers to the dispersion of measurements.   The closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions. Measurement precision is usually expressed numerically by measures of imprecision, such as standard deviation, variance, or coefficient of variation under the specified conditions of measurement.

If determinate or systematic error was the only source of uncertainty in a measurement, the job of the experimenter would reduce to a sequence of operations to eliminate each source of determinate error, after which one would presumably measure the “accepted” value of some parameter. Measuring the parameter would always give the same number at each measurement. However, there are additional sources of variation that ultimately determine how “well” one may measure a quantity. These are indeterminate errors (also called random errors). They generally cannot be positively identified as their values from Uncertainty Analysis. Some are inherent to the way the experiment is set up; some are simply a result of the way nature acts. A good experiment reduces or eliminates systematic error and provides an estimate of the indeterminate error, expressed as uncertainty.

Precision or Repeatability of a Profile Projector or a Video measuring machine

Consider a simple act such as measuring a Gauge Block again. This may be carried out several times on a single sample, simply to be certain of the value. Suppose the object of an exercise is to measure a gage block that has a true value of 10.000mm. To be sure of this length, the experimenter measures the length in five repeated measurements. It is unlikely that the five lenghts will be exactly the same, as shown below:

Length as Measured in Five Different Experiments

a) 001 mm b) 10.000 mm c) 9.997 mm d) 9.998 mm e) 10.004 mm

The question is the following: is this sample 10.000 mm? It seems that one measurement indicates this is the case. However, the other four measurements deviate from this value. The variation across the set of measurements produces some uncertainty about the length. Any expression of the length must include some indication of this uncertainty.

The uncertainty is a function of the type of sample, the conditions under which it is being measured, the Profile Projector, and the person doing the measurement. Presuming there is no determinate error, one may state these measurements reflect something about the random error associated with the measurement of the length. The uncertainty of measurement is a calculation made to describe the bounds within which you have every reason to believe the true value lies.

The last digit contains some information. It shows that all of the five measurements fall between 9.997 mm and 10.004 mm. Thus, one could say that the actual value, based only on these measurements is 10.000±0.004 mm. This gives a statement of the uncertainty by including the range of all values in the set. The statements above are attempts to quantify a quality of the measurement. This quality is the precision. It is defined as the degree of agreement between replicate measurements of the same quantity.

In 1962, the US department of defense was the first to outline the guidelines of industrial traceability. The guideline clearly describes that the shop floor measuring and testing instruments shall be calibrated utilizing the reference standards which by themselves have the calibration being traceable to a national standard. Thus the traceability relates individual measurement results to national standards or nationally accepted measurement systems through an unbroken chain of comparisons. The emphasis on traceability is important because it enables measurement consistency from laboratory to laboratory in a logical and consistent manner.

Image processing technique for finding the boundaries of objects within images. It works by detecting the discontinuities in brightness. By using it features like corners, lines and curves can be extracted from an image for dimensional measurement uses. The four steps of edge detection are smoothing, enhancement, detection and localization. Firstly, the noise is suppressed in the image, without destroying the true edges, then a filter is applied to enhance the quality of edges. The next step is determining which pixels should be discarded as noise and which should be retained . The last step is determining the exact location of the edge.

The resolution of an instrument is a quantitative expression of the ability of an indicating

device to distinguish meaningfully between closely adjacent values of the quantity indicated. In It is the smallest difference between displayed indications that can be meaningfully distinguished Profile Projector or Video Measuring Systems from Sipcon come with a least count or resolution of 0.005mm, 0.001mm, 0.0005mm and 0.0001mm.

The tolerance is the difference between the upper and lower tolerance limits. A designer will specify these limits to indicate how well a component needs to be made to meet its specification. As a general rule the accuracy of the measuring Profile Projector or a Video Measuring system should be at least 1/10^th  of the minimum tolerance in the part to be measured in an Optical Comparator or Video Measuring Machine. Tolerance can be  classified into size tolerances and geometric tolerances. Size tolerances are for the basic dimensions of the part where are geometric tolerances are for the geometry of the features. The geometrical tolerances are further classified into 14 types as follows and Sipcon Video Measuring Software are fully capable of measuring all the below  tolerances with a great ease. Camera system together with touch probe gives 3D capability to measure the 3D form tolerances as well.  

• Form tolerances
  1. Straightness : Straightness is a condition where an element of a surface or an axis is a straight line. A straightness tolerance specifies a tolerance zone within which the considered element or axis must lie. The straightness tolerance is shown in the view where the elements to be controlled appear as a straight line.

  2. Roundness: Roundness (Circularity) tolerance defines the permitted deviation of any circular element of a feature from a theoretically true circle. Any circular element must lie between two concentric circles whose radius difference is equal to the specified tolerance.   3. Flatness: A flatness tolerance defines the permitted deviation of a surface from a theoretically flat plane.   4. Cylindricity: A cylindricity tolerance defines the permitted deviation of any circular element of a cylindrical feature from a theoretically perfect cylinder. Any circular element of the feature must lie between two concentric cylinders whose radius difference is equal to the specified tolerance.

• Orientation tolerances
• 1. Parallelism: A parallelism tolerance defines the permitted deviation from a theoretically exact parallel condition.
  2. Perpendicularity: A perpendicularity tolerance defines the permitted deviation of a surface, axis, or centerplane from a theoretically exact 90º datum plane or axis.
  3. Angularity: Angularity is the condition of a surface or axis at a specified angle (other than 90º) from a datum plane or axis. The angularity tolerance is the distance between two parallel planes, inclined at the specified angle to a datum plane or axis, within which the tolerance surface or axis must lie.
• Position Tolerances
  1. True position A position tolerance defines a zone within which the center, axis, or center plane of a feature of size is permitted to vary from its theoretically exact position. Position tolerancing provides a method of location to ensure assemble-ability and interchangeability at maximum tolerance
  2. Symmetry : Symmetry Toleranceis a three-dimensional geometric tolerance that controls how much the median points between two features may deviate from a specified center plane or axis
  3. Concentricity: Concentricityis a complex tolerance used to establish a tolerance zone for the median points of a cylindrical or spherical part feature
• Runout Tolerances
  1. Circular Runout: Circular runout tolerance specifies the maximum allowable deviation from perfect form of a line element of a surface as it rotates 360º about a datum axis.
  2. Total Indicator Runout: A total runout tolerance specifies the maximum allowable deviation from perfect form of an entire surface as it rotates 360º about a datum axis.
• Profile Tolerances
  1. Profile of Line: A profile tolerance defines a tolerance zone controlling the form of line elements or surfaces of a part outline or portion of a part outline as related to its own perfect counterpart. This control can be applied to a related datum if applicable.
  2. Profile of surface : surface profile tolerance directed by a leader to the part outline controls the total surface of the part outline

The error in an instrument is the difference between the indicated value and the known value of some material standard of size (for instance a gauge block). The discrepancy between an accepted value of a parameter and an experimentally measured value results from deviations in the manner in which the measurement is carried out. No two measurements are exactly the same. Some deviations can be controlled and some cannot. Those that can, in principle, be controlled by careful adjustment of the experimental procedure are systematic errors. They definite values that can, in principle, be measured and corrected. Systematic errors are sometimes called determinate errors. The most common types error are instrumental error, operator error, and method error.   Such errors are often unidirectional, so they slant the result of the measurement. If that is the case, the experiment is said to have a bias. Systematic errors can be corrected only after the nature of the bias is identified. A common determinate error is an incorrectly calibrated instrument that systematically gives results that are either too high or too low.

It is an operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.  Calibration of the apparatus is a process to correct this kind of error. Every Sipcon Profile Projector and Video Measuring machine is calibrated according to the relevant international standards for all the parameters.

There is a distinction between precision and accuracy one should always make. Even if the measurement’s precision is excellent, it may be inaccurate if a determinate error is present. Accuracy is a term relating the mean of the measurements to the true value, while precision is representative of the spread of these measurements. We can never achieve perfection in measurement so even a precise and accurate measurement will have some remaining uncertainty. It is best to use the term accuracy only as a qualitative term, or for broad comparisons between methods, and use the term uncertainty as the quantitative assessment. When the uncertainty of a measurement is evaluated and stated, then the fitness of purpose for a particular application can be properly understood. Accuracy or uncertainity in Sipcon Profile Projector or Video Measuring machine is 3+L/200 µm. Precision or Repeatability in Sipcon Profile Projectors and Video Measuring Systems is +/- 2µm with a standard ring gauge.

Statistical process control (SPC) is the term used to cover the application of statistics to the control of industrial processes. In its simplest form this may involve measuring the size of every tenth item off the production line and measuring and recording the dimension on a graph that has the upper and lower tolerances marked on it. By taking note of the trends displayed on the graph it is then possible to predict when the process is going to produce components with dimensions that exceed the permitted tolerances, and take corrective actions, such as adjusting the tool setting. The basic aim of SPC is to minimise variation. Sipcon Profile Projector and Video Measuring software come equipped with all the basic tools to carry out SPC so you do not need a special SPC software. All kinds of data charts, histograms, Scatter plots etc are possible in all measuring softwares like M2, Sipmeas etc.

Statistical quality control (SQC) is a broader topic dealing with the wider use of statistics to control the product from the design stage to the final shipped product. SQC includes such activities as process capability studies, SPC of that process, statistical based sample inspection and the statistical design of experiments.

The fractional increase in length of a material for a unit increase in temperature.

An estimate of the combined variation of repeatability and reproducibility for a measurement system. The GRR variance is equal to the sum of within-system and between system variances.

Inspection is the official process used to verify that a component or product complies with the design or legal requirements. This can range from assessing measurement results against a specification, detection of defects or simply ensuring all manufacturing processes/operations have been completed. The result of inspection is either pass or fail.

Measurement The process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity. Measurement presupposes a description of the quantity commensurate with the intended use of a measurement result, a measurement procedure, and a calibrated measuring system operating according to the specified measurement procedure, including the measurement conditions. Measurement System Any equipment, fixtures, people, environment, methods etc that are used to determine a quantitative or qualitative determination of a characteristic.

A non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurement, based on the information used. An estimated range of values about the measured value in which the true value is believed to be contained.

The science of measurement and its application. Metrology includes all theoretical and practical aspects of measurement, whatever the measurement uncertainty and field of application.

The apparent displacement of an object as seen from two different points that are not on a line with the object. If a measurement scale is not in direct contact with the indicator (e.g. a pointer slightly raise above the scale on a dial indicator, the increments on a vernier scale are slightly above the main scale) viewing the gauge from different angles will yield different results. This is a parallax error.

A quantity value consistent with the definition of a quantity. The value that would be obtained by a perfect measurement. (In practice this can’t be achieved, a measurement result will always be an approximation of the true value)