Grinding Doc: Using the Ra eyeball technique

Dear Doc: I grind small parts with intricate geometries. I’d like to get a handle on what kind of Ra values I’m generating and if the grinding process changes I’m making are improving or worsening the surface finish. I was told to purchase a profilometer. Is this the best route to go?

The Doc Replies: The Ra value is a measure of the average depth of the grinding scratches. A high Ra means a rougher workpiece surface finish; a low Ra means a smoother surface.

The typical way to measure surface roughness is with a profilometer. The instrument has a stylus that moves across the workpiece perpendicularly to the grinding scratches and traces the profile. The absolute values of the heights above and below the mean are then averaged to provide an Ra value. You can pick one of these up for a few thousand dollars.

However, profilometers can be maddeningly fickle. For example, one measurement might show a 16μin. Ra, and a second measurement taken in the exact same spot might read 24μin. Ra. If the part has a strange geometry or radius, measurements for the same surface can be even more sporadic. If you have a small area to measure, the problem is compounded.

Some of the more sophisticated profilometers map the geometry and filter out curvatures, but cost more.

If you’ve got an extra $100,000 to throw around, you can purchase a 3-D optical imaging system. Such a system provides a 3-D map of the surface from which you can obtain an accurate Ra value and lots of other cool stuff.

Or, you can use my patented “Ra Eyeball Technique.” It’s crude, doesn’t conform to ASTM standards and will get you laughed out of the room if you present your measurements at an academic conference.