E- “What exactly dictates the kind of coating that is applied to each abrasive? Are there specific coatings for certain materials, or is the main focus more on grinding performance?”
J- “There are certainly plenty of different coatings that can be used, and a lot of the decision depends on the bond that is used. More often than not if we are using coated abrasive, the coating will be nickel, copper, or titanium-based. Nickel coating adheres to the bond very well, and depending on the thickness of the coating the longer those individual grits will hold onto the bond. So, abrasive coatings will extend the life of your wheel as long as there isn’t any excess heat generation to worry about. The coating is removed or greatly reduced when designing wheels for a dry grinding environment due to the fact the high heat grinding operations can wear the wheel surface quite quickly, neglecting to expose new abrasive particles quick enough.”
E- “There seem to be a lot of different ways grit size can affect your grinding result. In what ways does grit size impact the overall performance of the wheel?”
J- “There are two main ways grit size impacts the performance of the grind. First, the bigger the grit size the more aggressive the cut will be. Large grit sizes will remove more material, whereas finer grits will result in a better surface finish while not having the ability to cut as aggressively. Second, abrasive wear down over time reducing sharp cutting edges to rounded surfaces. When this happens, the rounded grit needs to be released from the bond and reveal a new, sharp grit underneath. Larger grit sizes will have more cutting points to dull before needing to be removed, meaning that they can be used more before they need to reveal a new grit with fresh cutting points. The drawback here is that with larger grit sizes, comes a lesser amount of grit particles present in the bond. This could mean more of the bond surface will need to be worn away before a new grit can be exposed.”
John continued his explanation with an example involving grinding in round tool production, which was the perfect summation of a custom wheel with a very specific grit size. Check out Eagle’s grit size chart here!
J- “In round tool manufacturing, often involving a gashing or a cup wheel, the corner radius on the wheel has to be a very specific size in order to achieve the desired geometry. Over time with wear, if that radius changes by two-thousandths of an inch, it no longer does the job right. At a minimum, you need two grit particles on a surface to form a radius. If each grit is five-thousandths of an inch in diameter, with the minimum two-grit requirement, it will create a grinding radius that is ten-thousandths of an inch. If the customer wants to achieve a smaller radius, they will have to size down the grit.”
I had previously imagined grit size would affect the surface finish and cutting speed of the wheel, as well as the lifespan. However, the fact that grit size has a specified impact on high-tolerance geometry grinding was a surprise. As John mentioned in our interview, there are times when the manufacturer may want a more aggressive cut, meaning a coarser grit, but the geometry they need to achieve won’t allow for such a large grit size. In this case, a finer grit must be chosen. End mill production is a good example of this. The fluting process has to be precise, so it is more beneficial to choose a finer grit that has a better chance of maintaining corner geometry and conducting more passes with the grinding wheel than an aggressively cutting, large grit size. This is why 240 grit is commonly used for fluting.
