Portable surface gages have come a long way in the past 25 years. Instruments that once encompassed three separate pieces, took a hand cart to move and cost tens of thousands of dollars can now be carried in the palm of your hand. Specially-created hardware modules are no longer needed for each surface finish parameter—today’s products provide all the most common parameters as standard features.
While the products are more powerful, smaller and less expensive, one thing hasn’t changed: like any other gage, they are no better than the way they are used. Here is a checklist to follow when using a portable surface finish gage.
Check the gage. Prior to a gage being used, it should be inspected to ensure that it is operating properly. With a bench gage, a zero and span master can verify that the gage is reading properly. Similarly, most surface gages are proved with a separate test patch. Some gages have a test specimen built into the unit and the gage can be verified simply by moving the probe to the internal test specimen and making a trace.
Check the environment. The second thing to do is to check out the environment in which the surface check will be made. Dirt is always the number-one enemy of any precision measurement, surface checking included. Make sure the part is free from coolants, oils and burrs. The surface probe is a delicate tool with a fine diamond contact, a fragile sensor, and very little space between the contacts protruding from the probe. Oil or sludge on the part will certainly affect the trace and increase the chances of misinterpreting the true reading.
Unlike bench gages with adjustable arms and posts, surface finish gages are self-contained with nothing to tighten up. The second biggest environmental source of surface measurement error is vibration. Vibration from a stamping or turning machine, a passing fork truck or even a loud radio can pass through a part and become a component of the measured results.
Check the print for the parameter to be measured. If surface finish is called out on a drawing but not otherwise specified, it is standard practice to assume Ra (average roughness). But no single parameter is best for all types of parts, and many applications are best served by using two or more parameters. For example, Ra in combination with Rmax (maximum roughness) may provide a good general idea of a part’s performance and alert QA to the presence of potentially damaging surface anomalies. Every application reacts differently to different combinations of roughness and waviness, and industry has responded by creating more than 100 different formulas with which to calculate surface finish parameters from the same measurement data. Thus, it’s critical to understand what the print is calling for and why.
Check the parameter setting. When the process or quality engineer designed the inspection process, they should have specified the test conditions under which the gage should be used. To correlate these, compare the settings of the gage to the callouts of the inspection process. Make sure all settings are the same. Newer gages may allow you to print out the settings, facilitating the comparison.
Check the results. One of the most important characteristics of an inspector or gage operator is to know when things are right and when something is not right. If the surface callout is for 60 Ra and parts suddenly start measuring 5’s and 10’s, something may be wrong. This is where having a reference patch at hand is important. Make a quick check on the reference: if it reads surprisingly low, there is apt to be a probe problem.
Check that results are saved for future review. It’s great that surface finish checking has moved from the lab to the shop floor. However, the main reason for doing this has been to get results faster in order to keep the process under control. It’s also important to document when the measurements were made and what the results were. Today’s gages often have internal storage capabilities or ports for USB memory cards. They can store hundreds of results, manage them easily, and with a little analysis work, improve the manufacturing process.
Following this list before beginning the measurement process will assure results that can be trusted and used for proper process decision making.