February 2011 / Volume 63 / Issue 2|
The 10 percent dressing rule
By Dr. Jeffrey Badger
Dear Doc: I use a single-point diamond dresser and typically take a 0.001 " DOC when dressing 60-mesh wheels. But sometimes I use 240-mesh wheels. Can I take the same depth there?
The Doc Replies: Most grinding handbooks indicate to dress from 0.0002 " to 0.001 ": 0.0002 " for a fine dress, 0.001 " for a sharp dress. However, this doesn’t take into account grit size.
A rule of thumb for a sharp dress is a DOC of 10 percent of the grit diameter. Use a DOC of about 5 percent of the grit diameter for a medium-sharp dress and around 2.5 percent for a dull dress.
The equation for grit diameter is grit diameter (in.) equals 0.6 divided by mesh size, so a 60-mesh wheel has an average grit diameter of 0.010 " (0.6/60). Therefore, the DOC for a sharp dress (10 percent) of a 60-mesh wheel is 0.001 ", a medium dress (5 percent) is 0.0005 ", and a dull dress (2.5 percent) is 0.00025 ".
For a 240-mesh wheel, a sharp dress is 0.00025 ", medium is 0.000125 ", and dull is 0.00006 ".
Dear Doc: I grind tungsten-carbide shafts with a diamond wheel. The shaft has two diameters, and I sometimes get temperature-induced cracking during grinding where those two diameters meet. What can I do to stop the cracking?
The Doc Replies: Perhaps your problem is not in the grinding but in the part itself. What is the design radius at the circle of contact between the two diameters? If it is 1.0mm, then we’ll have to work on the grinding side. But if the answer is “sharp” or “0.050mm,” then there’s an issue with stress concentrations that are forming because of a small radius.
Stresses develop in carbide because of grinding temperatures. That’s normal. But if there is a very small corner radius, these stresses will be magnified several times, which can lead to cracking. This is called a “stress concentration.” You can work hard to reduce grinding temperatures—and perhaps increase cycle time—or decide on a reasonable corner radius that generates only a modest stress concentration.
I observed this exact situation at a company grinding WC. When I asked what the corner radius size was, they didn’t know. They just plunged the edge into the part. If the edge of their 400-mesh wheel was sharp, so was the part. The solution was to decide on a radius, in this case 0.50mm, and then dress the radius into the part. This reduced the stress concentration and solved the problem.
Dear Doc: I grind nickel-base alloys with CBN wheels. I dress the wheel with a diamond disc that traverses the wheel to put a form in it. The polycrystalline diamonds in the roll have radii of 0.25mm. Everything was fine when the disc was new, but burn problems appeared after about a week. So I stuck on a new disc and burn disappeared—for about a week. I can’t be purchasing a new disc every week. What can I do instead?
The Doc Replies: Those PCD inserts in the disc may start with a nice 0.25mm radius, but they quickly develop a flat area at the contact point. Now, instead of each point on the wheel hitting a round diamond two or three times, it’s hitting the flat region maybe eight times. Those repeated hits dull the CBN grits, producing your burn problem.
The solution is to increase the traverse speed. With the faster speed, the diamond will hit the flat part of the diamond two or three times, creating a sharp CBN wheel, and your burn problem should disappear.
If you want to get fancy, you can measure the length of the flat area. However, it’s nearly impossible to see with the naked eye. Try sticking the disc in a shadow-box projector and focusing at the edge of the diamond. You’ll see the 0.25mm radius interrupted by a flat region, maybe 0.1mm long, where the diamond is contacting the CBN grits. As that flat grows, keep increasing the traverse speed so the “overlap ratio,” or the number of times a point on the wheel hits the diamond, stays constant with the increasing length of the flat. CTEAbout the Author: Dr. Jeffrey Badger is a grinding consult-ant. Web site: www.TheGrindingDoc.com. His “High Inten-sity Grinding Course” takes place May 23-25 in Chicago.
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