August 2009 / Volume 61 / Issue 8|
Grinding Doc: Using the Ra eyeball technique
By Dr. Jeffrey Badger
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.
In spite of all this, I think it provides the best Ra measurement. Here’s how it works.
Purchase a roughness gage that has Ra values ranging from about 2μin. to 80μin. Round up three machine operators, say, Joe, Rex and Butch. (Don’t get Barney the purchasing manager because he won’t have a clue. Machine operators have “the magic eye.”) Next, set the workpiece right next to the roughness gage so you can view both simultaneously under a low-magnification microscope or a high-power magnifying glass. Then bring Joe into the room and ask him which Ra sample on the roughness gage best matches the workpiece surface. Then ask Joe to leave the room and have Rex come in and give his opinion. Then ask Rex to leave and ask Butch to come in. Whatever you do, don’t tell anyone what the previous guy said. That’ll prejudice their opinions.
Finally, average the three values. That’s the Ra for the surface.
It may sound crude, but I use the technique all the time, and I trust it as much as any profilometer measurement—or more. This technique will cost you about $50 for the roughness gage.
Whenever you do the measurement, use the exact same three guys, as every eye is different. This technique is good for getting you in the ballpark of the true Ra value for a surface, but it’s also great for giving you relative values, i.e., whether the changes to a grinding process are making your surface finish better or worse. CTEAbout the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com.
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