Technical Note #2

Helmel Engineering Products, Inc. Customer Support -- Tech Note #2

Multi-Point Feature and CMM Inherent Errors:

Applies to: All Levels of Geomet, Geomet 501
Last updated: Tuesday December 08, 2009.

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Introduction

Technical Note #2 - Multi-Point Feature Inherent Errors
Published October, 14, 2003
Edward R. Yaris - Software Development Manager, Helmel Engineering Products, Inc.

Commonly found Errors

• CMM Calibration Errors
• Probing Errors
• Operator Influences
• Data Point Spread

System Errors

In all multi-point features, the ability to report accurate and repeatable results are subject to the quality of the gathered data points. Influencing your measured results will be the stacking of inaccuracies within your measuring system generated by the commonly found errors listed above.

Coordinate Measuring Machine

Scale Error

The CMM has inherent inaccuracies such as axial linear error and mechanical limitations on squareness, parallelism and perpendicularity. Every CMM has a scale attached to each axis of movement. This scale can be a mechanical / optical or laser configuration. In the case of a mechanical / optical there exists positioning error which CMM manufacturers will attempt to compensate for through hardware or software.

For example when a scale is installed, it will be compared to a master scale or laser. The CMM will be positioned along the scale at different points and the reported position will be compared to the master. If the position reported by the laser was 10.00000 and the CMM reported 9.99875 The difference will be the linear inaccuracies of the scale at that position. In a single linear compensation, a scale factor will be applied to the CMM’s position feedback to correct for the error. However, this method of compensation works with the overall average and does not correct for local inaccuracies along the scale.

The chart below demonstrates the Position Error measured compared to a Master Scale or Laser. The scale along the left side illustrates the deviation error.

figure 1, Original Positional Error

After single line compensation has been applied the deviation error has been minimized to create a more accurate position.

figure 2, Single Linear Compensation Applied

One limitation in single line compensation is that the variances along the scale can not be individually removed. Therefore the single line compensation method only averages out the entire scale without regard to multiple position errors. When the CMM is positioned along the scale, some error characteristic will still be built in.

Non-Linear Scale Compensation

Another method to minimize scale-positioning error is the Non-Linear method. This method builds a table of compensation values that can be measured and created along the various undulations along the scale. It requires that the CMM has a reproducible home position for all axes. The chart below illustrates a non-Linear method applied to the same scale shown above. The error has been reduced further.

figure 3, Non-Linear Compensation Applied

As with any linear compensation method, if the scale is not mapped out over every position, then an error will always be present influencing your results.

Mechanical Intrinsic Accuracy

The next error that a CMM can have is called intrinsic accuracy, the mechanical accuracy of the CMM. These include Squareness, Perpendicularity and Parallelism of the structure of your CMM. If the X-axis is not perpendicular to the Y-axis, then when the probe is moved along the X-axis a deviation in Y will occur. To test for this, place a certified square down on the CMM base. Place a .0001” or better indicator on the probe bar and lock the Z and X-axes from moving. Move the CMM in the Y-axis with the Indicator in contact with the square. Set the square edge parallel with the Y-axis travel until no movement is shown on the indicator. Unlock the X-axis and reposition the indicator so it contacts the square and lock the Y-axis. Move the indicator along the X-axis and you should not see movement on the indicator dial.

Repeat this procedure in the front and back location on your CMM to ensure the X to Y-axis relationship is perpendicular thoughout the measuring envelope. An additional test would be to raise the square to the highest point within your measuring cube and repeat the procedure.

Should you see a deviation, then your CMM is not square in the XY plane. Any measurements taken will have an inherent inaccuracy.

The Z-axis should be perpendicular to the CMM base. To test this, place a .0001” or better indicator on the probe bar. Place a certified square on the CMM base with the surface parallel to the Y-axis. Lock your X and Y-axis with the indicator in contact with the square. Move your Z-axis up and down, you should not see any movement on the indicator. Now repeat the test with the surface of the square parallel to the X-axis.

Should you see a deviation, then your CMM is not square in the ZX and / or YZ plane. Any measurements taken will have an inherent inaccuracy.

BallBar Test for CMMs

One method to accurately evaluate axis to axis relationships is found in the B89.4.1, Methods for Performance Evaluation of Coordinate Measuring Machines published by ASME International. This Standard establishes requirements and methods for specifying and testing the performance of coordinate measuring machines (CMMs) having three linear axes perpendicular to each other and up to one rotary axis positioned arbitrarily with respect to these linear axes. In addition to clarifying the performance evaluation of CMMs, this Standard seeks to facilitate performance comparisons among machines by unifying terminology, general machine classification, and the treatment of environmental effects.

The Ball Bar consists of a temperature stabilized bar with two high precision spheres mounted at each end, see figures 4 and 5. The length between the spheres is not necessary to be known. The Ball Bar is positioned within the measuring area of the CMM in 28 different positions as described in ASME B89.4.1.

figure 4, Ball Bar Set	figure 5, Evaluated Ball Bar Position

The process of gathering data involves the measuring of each sphere and calculating the direct line distance between them. One advantage of this method over other methods, such as a step gage, is the averaging effect of the probe lobe errors. In any given direction around the traditional touch probe, there exists a lobing error that can be measured, but not effectively removed from a feature calculation. By capturing data points evenly around the sphere, the lobe error variations are not removed, but averaged out to obtain a closer size and location reporting of the actual sphere.

Once all 28 positions have been calculated, they can be entered in a Excel spreadsheet, download. The average of all 28 positions is first obtained and a companion column of actual - average is calculated. These values are then plotted as shown in figure 6.

figure 6, Plotted Ball Bar Test Results

The Maximum and Minimum values are found from the deviation column. The spread between these values is considered the working tolerance. This value is often reported as the volumetric accuracy of a CMM. It should be noted that the length of the ball bar is often a small fraction of the actual measuring area. Therefore, the reported working tolerance only reflects the area the cubic size of one ball bar length in the X, Y, and Z and should not be used to define the entire CMM.

figure 7, X / Y squareness

In figure 7, we can visualize how to interpret the Ball Bar tests. We can see the direct effect of a squareness deviation of the X versus Y axis. The distances of the measured lengths, d1 and d2, should be equal when the CMM has been properly setup. As the illustration shows, d1 being greater than d2 demonstrates that the relationship between the X and Y axis are not perpendicular. When properly interpreted by a trained calibration engineer, all 28 positions will effectively provide a picture of the intrinsic characteristics of your CMM.

Many CMM manufacturers have abandoned building intrinsic accuracy in favor of 3D software compensation. This allows building of a CMM with little attention to the accuracy of the assembly. The accuracy is acomplished by mapping the CMM with a known standard, such as a laser, then providing 3D compensation algorithms to correct the inherent errors. However using only a laser does not remove all the inherent inaccuracies within the entire measuring cube.

There is no correct answer as to which method delivers a more accurate CMM. In both cases, the ability to perform field calibrations often becomes an economic cost the user must absorb. To perform a field calibration by mapping the CMM with a laser system is a higher cost than adjusting for intrinsic accuracy.

Probing System Errors

Every probe system introduces inaccuracies to your data point gathering. These include capture speed, lobe errors, stylus lengths and probe contact pressure. Understanding the errors within the probe system can be very difficult. Most electronic touch probes have no index position when installed on a CMM and can rotate to a new location moving the lobe error. Additionally, electronic touch probes are a simple switch and can not determine the approach vector at the point of contact. This simple fact makes it very difficult to map out probe errors.

 

figure 8, Lobing Error

Lobe errors are mechanical limitations to the probe head. Lobes can have an error in excess of .0001” deviation when capturing a point moving at 0° compared to capturing a point at 60°. As shown in figure 8, the lobe error pattern is triangulated based on the mechanical layout of the kinematics probe alignment, see figure 9. The touch probe can be described as having low and high force directions when capturing a trigger, see figures 10 and 11.

figure 9, Force Direction	figure 10, High Force Dir.	figure 11, Low Force Dir.

Stylus lengths will add additional error to your system. A short length of about 10mm will not influence your data points, however a 100mm length will increase the error 10 fold to the lobe deviation. If you need to extend your stylus, we recommend the Renishaw PEL family of extensions. These are extensions that mount between the probe head and probe body.

When the Trigger Force pressure is set too high, the probe error will increase exponentially! Most mechanical probes allow you to adjust the tension applied to the trigger mechanism, see manufacturers technical manual. Some touch probe models, such as the Renishaw TP20, offer interchangeable modules with a preset trigger force. By increasing the pressure, you force the probe to trigger at a higher pressure that introduces stress and deflections in the stylus tip and extension.

Strain-gauge or piezo designed probes, such as the Renishaw TP200, minimizes the lobe error by eliminating the mechanical kinematics design. However, these gauges are susceptible to speed changes and the actual contact speed should be stable and consistent. Probes that use this technology should not be considered for manual CMMs.

Operator Influences

Finally the operator will provide the greatest error. If the operator captures data points with an inconsistent speed, repeatability errors are introduced. To test for speed difference, lock the Z and X-axes on your CMM. Place a part on your CMM and measure a +Y point. Increase the speed at which your capture points. At a certain speed you will begin to see variances in the reported results.

Other operator related issues include where you hold the CMM. If you have a large CMM, probing while holding the probe bar just above the probe will act differently that when you are holding as far up the probe bar as possible. The probe bar is a lever, and by moving that lever you are placing strain on the superstructure of the CMM. If you gather data points before the CMM has ‘settled’ you will get unreliable data points.

Data Point Spread

When measuring multi-point features, how you spread the data points will be the single greatest influence on the accuracy and repeatability of the feature. For example, if you capture data points on a circle using only a 15° sweep, the results are unreliable and not repeatable. If those same points were captured over a 300° sweep, the results would be accurate, see figure 12. The same holds true for Planes spheres, cylinders and cones. Cylinders and cones require a proper spread over the length otherwise the axis of the feature will not be reliable.

Imagine capturing two points on a line only .25” apart with a probing error of .0001”. The resulting line would have a repeatability error of .040” over 5.000” or 0.4584 degree! If that line were used as part of the Part Coordinate System, then all subsequent features would have questionable accuracy.