7 steps to ensure measurement results meet specifications

Are calibrated measurements consistent?

Throughout the industry, it is a matter of course that customers, material suppliers and component manufacturers use calibrated equipment for measurement. Calibrate equipment to:

Ensure that readings are consistent with other measurements, i.e. measurements of the equipment are compared and traceable to known and accepted standards

Determine the accuracy of equipment readings

Determine the reliability of equipment

But just because we all use calibrated tools, there's no guarantee that everyone measuring the same part will get the same result.

How is that possible? For starters, even if you use NIST traceable calibration measurement equipment, there is a tolerance in each device that indicates accuracy within plus or minus X quantities. More importantly, the tools used to calibrate the equipment (for example, pins) also have their own tolerances. In addition, the tolerances of the tools used to calibrate the pins... You get the idea.

The good news is that any manufacturer worth its salt will consider tool tolerances when determining how to achieve dimensions that meet your specifications. Tolerances aside, there are other factors (easy to control) that may have an impact on calibration measurements and whether the end result meets your specifications.

Success is in the details.

There are some simple "rules" that can help ensure calibrated measurements produce accurate and consistent results - especially for very strict tolerances, where slight differences in measurements can mean differences between specification and off-specification.

It may seem basic, but it's worth mentioning:

1. Make sure you and your supplier or manufacturer use the same type of equipment for measurement. For example, if the manufacturer uses optical measurements to verify the specified ID, but you use pin gauges to check the finished part, there may be discrepancies in the results. If you are measuring length, will you use calipers, micrometers or rulers?

2. Ensure that the measuring equipment used by you and your supplier/manufacturer is properly cross-calibrated. Otherwise, the two devices might measure parts in different ways. Z pin gauges do not produce the same results as ZZ pin gauges. If you are using a digital micrometer, be sure to use the same equipment calibrated in the same units, calibrate to the same standard, and round to the same decimal number.

3. If possible, provide your manufacturer with the measuring instrument or other equipment you plan to use to verify the dimensions of the part. For example, for functional tests that require use/nonconforming gauges, you can send a copy of the calibrated pin or screw gauges to the supplier. If a large amount of work requires more complex testing, your supplier may be willing to purchase the same calibration measurement equipment as in the field for use in production runs.

4. The method of measurement, not just the equipment to be used, should also be specified. Have a pre-production discussion with your supplier and provide detailed instructions on how to make measurements. Such as:

For ID, at what point in diameter (the highest point) will you measure? Low? The average?

If you are using something like a handheld digital micrometer, how many decimal places do you both measure? How many of these decimal places are valid (that is, all 0-9 values)? Finally, is the last decimal place rounded (that is, looped between 0 and 5)?

5. Ensure that all your measurement requirements are included in your drawings and that the details of the drawings do not conflict with your measurement methods. For example, if you provide a qualified/unqualified thickness gauge, but your drawings requirements pitch of screws parts manufacturer of you may have to be coordinated by pass/reject test parts, but if carried out in accordance with the drawings specifications, cannot pass the test of pitch - or if from a point and not another point measurement, could pass the test.

6. Think carefully about what you really need to measure. Avoid the risks of over-design, such as more waste, more waste and higher costs, by distinguishing between critical and non-critical dimensions. Ask yourself, do you really need to have a very strict tolerance on the X dimension? Or is any measurement suitable for the highest and lowest point (± tolerance) sufficient?

Equally important, during the quote and specification phase, be sure to identify your really key dimensions for your vendors and how to measure them so that you will all be on the same page from day one.

7. When practical, please use functional testing to meet your testing needs. A simple pass/fail test using a pin or screw thickness gauge may be a perfectly acceptable measure of quality and consistency and more cost effective than specifying specific tolerances.

What should we do if our measurements are still inconsistent?

When there is a discrepancy between the manufacturer's measurements and what you get when you examine the finished part, it is usually only necessary to remeasure the problem. For example, on re-measuring and finding that these same parts are now within the specification range, measurement errors may be detected, such as parts rotating during the initial measurement, resulting in non-circular diameters.

If the source of the measured differences cannot be determined, formal correlation studies can be conducted using best practices of the Gage R&R method. This usually involves taking samples of 30 numbered pieces, and having three people on the manufacturer side measure and record the dimensions of each part, and then repeating the process on the client side with the same samples.

If the correlation study does not find measuring equipment problems or method errors to be the cause of the difference, the manufacturer may need to cut the standard rod to a closer tolerance than specified on the drawing in order to obtain the desired results according to the customer's calibrated measuring equipment and method.

Fortunately, in metal cutting companies, we find very little demand for correlation studies. By closely handling tolerances throughout the production and testing process and manufacturing each part to its nominal size as much as possible, we are able to reliably achieve measurements within the specification range.

On your part, as a customer, you can also help ensure success by following the rules we have listed above. By working up front with your supplier or part manufacturer to define how parts should be measured and to agree on the calibration equipment and measurement procedures to be used, you can greatly improve the chances that parts will meet specifications.

Please contact us at zhang@pride-cnc.com