With the implementation of touchscreen operation, canned measurement routines and automatic ranging, form measuring machines have become nearly foolproof and have successfully migrated from the lab to the shop floor. Still, the form machine is only as good as its setup, and without due attention to the basics, there are many pitfalls that can make even the most basic measurement useless. In this column, we’ll highlight some of the most common problems.
Erroneous Staging—As with any dimensional check, unless a part is staged correctly, unwanted errors are apt to appear in the measurement results. A three-jaw chuck is often used to stage a workpiece, but using one on a thin-walled part may introduce a three-lobed condition in the measurement. Likewise, special clamping may be needed to stabilize a tall, narrow part.
Using the wrong stylus tip—Often, the operator of a form measuring system will use only the probe initially supplied with the machine, regardless of the size of the part being measured. However the stylus tip really acts as a mechanical filter and needs to be selected according to part diameter and the maximum number of undulations per revolution relative to the application. The manufacturer of the form machine probably has a whole host of measuring probes available for the transducer. In addition to the need for different ball sizes and contact shapes, other materials may be required. Sometimes conflicts between the part and the tip causes probe chattering or even "picking up" material on the probe, thus changing its characteristics over time.
Choosing the wrong type of signal processing filter—Originally, the filters in form measuring systems were analog and used resistors and capacitors. These were referred to as 2RC filters. When computers started taking over the form tasks, digital filters were created to simulate these 2RC filters. The newest systems use digital Gaussian filters, which are more accurate than the 2RC filters and tend to introduce less distortion. Because of these characteristics, they are becoming the filter of choice in the newer standards. Please be aware that older systems will not correlate with new form systems, which may make them obsolete to the standards.
Confusion over the way the results look—The results of a form measurement rarely look "round" on the computer screen or printout. It is quite common for the trace to have sharp high and low points that can fool the user into thinking the part has terrible form errors.
Computer-based form systems normalize the deviation around a fixed circle. The operator then chooses the best magnification to balance the form error to the specific surface deviations. While the computer trace does not accurately represent the part’s actual profile, the relationship between the highs and lows remains consistent with the system’s magnification and represents accurate angular relationships around the circumference of the trace.
Measuring the wrong parameter—Geometry parameters are typically a source of confusion. For example, in the case of a simple indicator gage, the term "runout" and "out-of-roundness" are used interchangeably. However, they are not the same when measuring form and should be measured differently. Similarly, when gaging perpendicularity, parallelism and other parameters involving relationships between two surfaces, the user must understand which surface needs to be measured first.
For example, the statement often seen on a print, "B is parallel to A," is not the same as "A is parallel to B." By definition, the second surface in the statement is considered the reference or datum surface. All of these parameters and more are defined in the ANSI Y14.5M Standard.
Inattention to centering and leveling—As part of staging, proper centering and leveling are critical elements for correct form measurement. A round part that is not level will produce an oval trace, while an off-center part will produce a trace that almost resembles a heart. If the form gage has a fixture to quickly align the part, it is wise to make sure that the part is in the fixture properly and is centered and aligned. Even if the form machine has centering and tilt correction as part of its software package, the closer you get to true center and level, the better your results will be.
The above-mentioned examples are only a few of the most common form measurement problems.