Justifying calibration equipment

The definitive bench micrometer, the Universal Supermicrometer (USM), offers the advantage of being two instruments in one by providing the ability to measure both internal and external parts, gages, or standards.In order to help meet these goals, organizations should replace older calibration equipment with newer, more efficient models. While stated accuracy or uncertainty are always major considerations in any such purchase, many other factors must be considered.

Begin by considering instrument uncertainty, repeatability and resolution. It is important to remember that instrument uncertainty and accuracy are not the same. Simply put, accuracy is the difference between a true value and a measured one. For example, a measuring device with 0.0005" (0.0127mm) accuracy will provide readings that may be 0.0005" (0.0127mm) in error.

An instrument’s uncertainty statement incorporates the use of statistical analysis to convey the probable error in a measurement. It includes the sensor’s absolute accuracy, repeatability, and resolution, as well as any variability introduced by the equipment’s mechanics. This uncertainty is then used in conjunction with outside uncertainties to establish an overall measuring uncertainty. An instrument’s uncertainty statement contains more extensive mathematical calculations that better qualifies the instrument’s measuring capability.

In the case of a measuring instrument, repeatability is a measuring device’s inherent ability to consistently repeat its readings. A manufacturer’s repeatability study is usually conducted when all other factors that would affect the result, such as the master and operator, are not changed. Although this is an important specification to consider, it should not be assumed that a highly repeatable system would also be accurate.

Resolution refers to the number of trailing digits available on the display. It is also referred to as the smallest positional increment that can be seen. Once again, beware of dramatic product claims that only provide a resolution or repeatability statement. If the manufacturer’s literature does not provide accuracy or instrument uncertainty information, have the company send it to you.

Next, consider the sensor technology used. For instance, high-accuracy gaging units use either a precision linear encoder or a laser-based interferometer. Linear encoders come in different grades, and generally provide instrument uncertainties of 10μ (0.25μm). Laser-based units, on the other hand, can routinely achieve results in the 2μ to 3μ range (0.050μm to 0.075μm). Again, be sure that the accuracy specification you receive is for the complete unit, not just the sensor.

When considering accuracy, remember a phenomenon known as Abbe off-set errors. In simple terms, the idea is to minimize overall measurement error and to do so, the sensor and measurement axis must be in line. For example, dial calipers, where the measurement scale is off-set from the measurement axis, are subject to Abbe off-set errors. 

Also evaluate the unit’s style or configuration. Choices include direct reading units, which can make many different measurements within a specific range without remastering, and one-to-one comparators, which must be remastered for each size to be checked. For example, Pratt & Whitney’s External SuperMicrometer PC has a direct reading range of 1" (25.4mm) and a total measuring range of up to 40" (1,016mm). This allows the operator to make many measurements quickly and accurately over a 1" direct reading range before repositioning the anvil. For larger direct reading ranges, laser-based instruments provide direct readings well up to 120" (3,048mm) in length.

Most calibration labs historically have relied on one-to-one comparators for precision measurements. These units, while relatively inexpensive to purchase, require substantially more labor, compared to direct reading units, to operate properly. For example, it is possible to calibrate an 81-piece gage block set in less than one-and-a-half hours using a direct reading unit. This same task can be done in less than eight hours using comparators. The labor cost savings achieved in gage block calibration using this method can fully justify an equipment purchase. In addition, direct reading units reduce the number of masters required. Comparators require a separate master for each value measured; direct reading units only require an upper and a lower limit master to set a range in which to measure.

Additionally, users can choose between dedicated internal (ID) or external (OD) machines or those capable of doing both types of measurements. Besides the obvious cost savings associated with eliminating duplicated sensors, technicians must only become proficient with one machine. Combination units also require less bench space and allow one technician to quickly change from making OD to ID measurements.

Many other points should be evaluated to ensure that you get the right unit for your needs. Check both the instruments total range and the measuring table’s size. Make sure it is big enough to accommodate large parts, and that the table will handle the weight. The table should have integrated locating posts and T slots, which make part alignments and fixturing quicker. For ID measurements, make sure there are easy-to-use swivel, centering, tilt, and elevation adjustments. When considering combination units, note whether the unit has bi-directional probes or uses a separate setup for each. Many units with separate ID and OD stations typically only observe the Abbe off-set rule on one.


Ask for a demo
Before making an important investment in calibration equipment, take a test drive. If possible, include the instrument operators. They can help quantify differences in setup times and throughput.

When testing the equipment, perform a gage repeatability and reproducibility (GR&R) study, an excellent indicator of whether an instrument will actually perform to your expectations. GR&R studies also are referred to as machine capability studies. A typical GR&R study might consist of 10 parts of similar characteristics being measured three times by two or three operators. Once the data has been collected, a measurement error analysis and percent tolerance analysis can be made. Many gage management packages include a GR&R module to simplify these calculations.

Due to the versatility of combination ID/OD calibration systems, changeover from one setup to another sometimes becomes inevitable. When assessing throughput of a particular measurement, also address the time needed to switch over from one setup to another. For instance, when changing from outside dimensions to inside dimensions, do you need to change the contact probes, force dials, or some other levers? This will definitely increase calibration time. Some multifunctional gages have bi-directional probes that allow both ID and OD measurement, as well as an automatic force system that requires no adjustments at all.

The method of calibrating the unit – also know as mastering – is something else to consider. Questions include: How long does the calibration take? How long can measurements be taken without going back to the set point? Does the system allow for a few different methods of calibrations?

Other points to consider: How long is the warranty? Does the manufacturer have a return policy if the instrument does not perform to stated specifications? Do the specifications state, as a minimum, instrument uncertainty, repeatability, and resolution? Does the manufacturer have a reputation for quality products and good customer service?


Compare accessories
Manufacturers differ in the accessories they include with the unit. Because ID/OD machines are so versatile, nearly all of them include optional accessories including long reach probes, aligning fixtures, and special kits.

Multifunctional instruments have come a long way with the features they incorporate and benefits they can deliver. During the past several years, calibration lab computers are becoming networked which allows for data exchange between facilities. If PC-equipped, make sure the calibration equipment will work seamlessly. Equally important is the ability of some systems to send the measurement value to any user-specified software program or gage tracking system.


The bottom line
With nearly any product you buy, you get what you pay for. An instrument costing $5,000 or $10,000 less might be lacking the speed and throughput that would have paid back this amount in a relatively short period of time.

Writing a long, complicated justification is almost a necessity in order to get equipment that is long overdue. When writing justifications, remember the magic word: payback. Showing how the proper new equipment will improve overall quality, increase efficiency, and lower costs should enable you to get the equipment that will maximize your in-house gage calibration capabilities.
 

Pratt & Whitney Measurement Systems
Bloomfield, CT
prattandwhitney.com