Understanding Filter and Outlier for CMM Application - Part 2
By Mark Boucher, CMM Quarterly
Part 2 of a 3 part series. We will cover diametrical features.
Diametrical Features
Reference Circle Selection
The programmer must select a fitting algorithm to be applied to the measured diameter. This will determine the reference circle applied to the measured data.
Roundness compares measured data with a computer-generated reference circle. Most measuring systems have four methods for comparing the measured data to a reference circle for evaluation. It is important to note the reference circle used should be related to the function of the finished component. The final numeric roundness result will be different with the different reference circles applied, so make sure the blueprint accurately reflects the desired result.
Least squares reference circle (LSCI) is the most commonly used reference circle. A line or figure is fitted to any data such that the sum of the squares of the departure of the data from that line or figure is a minimum. This also is the line that divides the profile into equal minimum areas. Out-of-roundness is then expressed in terms of the maximum departure of the profile from the LSCI, the highest peak to the lowest valley.
Minimum circumscribed circle (MCCI) is defined as the circle of minimum radius that will enclose the profile data. The out-of-roundness is then given as the maximum departure of the profile from this circle. MCCI is sometimes referred to as the ring gage reference circle.
This is a functional check as it is not as stable as LSQ.
Minimum zone reference circles (MZCI) or Chebyshev is defined as two concentric circles positioned to enclose the measured profile such that their radial departure is a minimum. The out-of-roundness value is then given as the radial separation of the two circles.
Less repeatable than LSQ as it fits around the feature's extreme points.
Maximum inscribed circle (MICI) is defined as the circle of maximum radius that will be enclosed by the profile data. The out-of-roundness is then given as the maximum departure of the profile from this circle. MICI is sometimes referred to as the plug gage reference circle.
This is a functional check as it is not as stable as LSQ.
Tip Selection and Filter Selection
There is a direct correlation between the bore diameter and the value of the UPR that should be applied.
Filter selection
For a more precise UPR selection the below formula may be used
Where d is bore diameter.
Tip Diameter Selection
The recommended stylus tip is 0.5mm radius or 1mm diameter for bores larger than 4mm in diameter, however when the bore diameter becomes quite small then the stylus tip diameter will need to be reduced.
The formula shows a calculation of the stylus tip when the diameter of the component becomes less than 4mm.
Where R is the stylus tip radius and d is the bore diameter
Applying Filter/ Elimination/ Filter Type
Fitting Algorithm - See Reference Circle Selection
Elimination - Use the Elimination box to set up the standard deviation algorithm (±3 sigma is the standard setting) used to apply a filter to the data. This means that you can reduce noise and achieve a good approximation of the surface. Standard Deviation is the number of standard deviations above which points are considered to be noise. Above this, points are filtered out.
Filter Type – We are scanning a bore so the selection would be circular. The UPR selection would be based on the diameter of the bore. See Tip Selection and Filter Selection
Apply Filtering
CMM Manager Advanced Options
In Part 3 we will cover Linear Features
