Errors in Circular Form Measurement

As with any precision measurement, the goal of circular form metrology is to provide a value that is as accurate as possible. Thus, it’s good to understand what adverse influences can affect these results. If the controllable influences are known and corrected before the measurement is made, the results will be more reliable.

The sources of circular form error can be divided into the following main categories:

The Environment

In the past we have been pretty consistent in saying that the environment is probably the biggest source of errors in precision measurement—particularly temperature variations. Here are some things to keep in mind when errors are evident:

• Thermal drift.

• Thermal deformation.

• Vibration.

• Cleanliness.

A sign that a thermal issue is affecting the results of a very high-resolution measurement is when the starting point of the trace does not match up to the ending point. This often indicates that there was enough thermal drift in the part, fixture or gage that it was picked up by the sensitive probing system during the trace. In this case, look for changes caused by part handling, air blowing over the form system, or the part not stabilizing to room temperature after manufacturing.

Setup

The measurement setup normally consists of the measuring stand, the precision rotary table, the sensing probe and the part to be measured. Measurement uncertainty can be caused by any number of systematic disturbances in the measuring loop. The most common causes are:

• Workpiece orientation (centrality, squareness).

• Workpiece and fixturing not demagnetized.

• Workpiece deformed by fixturing.

• Workpiece improperly staged or supported.

• Probe contact positioning (cosine error).

• Trace polarity.

• Position of the probes. When measuring with very high-resolution systems, errors can be introduced when turning the part into the probe as opposed to having it rotate away from the probe. The probe will actually “bend” very slightly from the friction of the part surface turning into it, which can cause non-repeatability errors.

• Vibration. In assessing fine form irregularities, the measuring accuracy can be highly affected by vibration. In these cases, the setup should be checked prior to making a measurement to assess the extent of environmental influence. Also, to reduce these influences, keep the measurement loop as small as possible.

• Dirt on the surface of the part. Dirt is one of the biggest enemies of precision measurement. An insufficiently cleaned surface will result in both inaccurate and unrepeatable measurements.

Equipment

The accuracy of the measurement system is affected by a number of factors inherent in the form measuring equipment, including:

• Quality grade of the instrument. The heart of any roundness system is the reference spindle. There are many different grades of spindles, and you should ensure that you have the right match for your application. Also, spindles can become worn or dirty over time, which can cause non-repeatability errors.

• Looseness in the system between the probes, probe arm and workpiece.

• Calibration accuracy of the measuring system (probe and amplifier/computer).

• Speed of response of the measuring system.

• Deformation of the workpiece either by the probing arm or the workpiece itself due to unbalanced mounting or being out of balance on the precision spindle.

• Sampling rate. With digital form-measuring instruments, the resolution of the traced form profile depends on the sample rate. Having too few measuring points may cause important features to be missed.

• And finally, the inherent accuracy of the mechanical components of the gage, such as slide positioning, pitch and yaw of the X and Y axes of the system. Incorrect positioning or alignment will show as part problems when really they are being introduced by the gaging system itself.

This last point is why it’s important to regularly check the total system for performance. Don’t certify only the probing accuracy. Use a master ball or cylinder to establish the machine baseline when first installed, and then check it regularly to ensure there are no changes over time.

To provide a circular form measured value that is as accurate as possible, it’s important to remember that the result depends on more than just the measuring system itself. It’s also critical to consider the influences of the workpiece and the environment in which the measurement is being made.