Grit Size & Impact on Performance: Engineer Interview
The further I probe into the world of custom grinding wheels, the more I realize just how many considerations must be taken in the selection process. One of the many components of creating a custom wheel is abrasive selection, and more specifically choosing a grit size. There are several abrasive options available and each come in different sizes, ranging from 60 grit (US size) to 6000 grit (or 1 micron in diameter). But what are the criteria for selecting an opportune grit size? Let’s investigate the ways grit size can affect grinding wheel performance with an Eagle expert, John Coleman.
Grit Size Q&A
John utilizes over twelve years of experience to provide technical customer service as an Application Engineer at Eagle Superabrasives Inc.
E- “So John, can you briefly explain how one might go about choosing an abrasive for a grinding wheel?”
J- “The first aspect of abrasive selection involves considering what kind of material you are grinding. For example, diamond abrasive tends to be used on non-ferrous materials like carbide, glass and ceramics whereas CBN is used more on ferrous materials such as high speed, hardened or stainless steels, and cast iron. The second thing you need to consider is the type of abrasive coating needed, and the friability of the abrasive particles. Typically when choosing a grit size we will consider not only the application, but the bond structure we select as well. Blockier diamond and CBN are often used with metal bonds, whereas more friable, sharp powders are typically paired with a resin or polyimide bond.”
Using bond structure as a guide for choosing grit size makes perfect sense. Different bonds will wear in varying fashions, exposing more or less grit over time depending on the application. Smaller grit sizes will dull quicker, but the wheel will resurface new grit particles faster than if a large grit size is used.
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 wears down over time reducing sharp cutting edges to rounded surfaces. When this happens, the rounded grit needs to release 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 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. 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 conduct more passes with the grinding wheel than an aggressively cutting, large grit size. This is why 240 grit is commonly used for fluting.
John’s final message to manufacturers regarding grit size and cutting speed says it all:
“One of the biggest misconceptions out there is, if I want to cut faster I need a coarser grit. I can stick with the same wheel formula I have and go up in grit size. That is not always the case. The rest of the bond formulation almost always has a bigger impact on cutting ability. If the grit size is too large in a hard bond that isn’t conducive to breakdown and aggressive cutting, it will take too long for the bond to wear and reveal new grit particles. This means more time spent grinding on dulled abrasive, not a faster cut.”
Summarizing the interview
In summation, there are three ways grit size impacts grinding wheel performance. First and most obviously, the larger the grit size the more aggressive the cut you can obtain. In contrast, smaller grit sizes produce better surface finishes but have a lower material removal rate. Second, larger grit sizes have more cutting points to expose throughout use than smaller sizes. However, we cannot forget the bond! Larger grit sizes also take more time to release from the bond and reveal fresh abrasive. Smaller grit sizes may dull faster, but new particles are able to surface more quickly. Finally, grit size matters when manufacturers require specific geometry on their tools. It is an essential criterion to consider in tight tolerance grinding operations in order to maintain accuracy throughout use.
Although there seem to be a lot of specifications involved in choosing an abrasive, I can say after speaking to John the process feels a great bit less daunting. His wealth of knowledge and years of experience allow him to provide excellent technical customer service. If you have questions regarding your grinding operation, I would highly recommend reaching out to Eagle’s team of application engineers. They really know how to get down to the nitty gritty.
Whether you are looking for general guidance or are ready to get a quote, we are dedicated to helping you find the right solution – and if we can’t provide the exact wheel you need, we will let you know. Eagle values communication and we will never lead you on. We hope you’ll explore the ways our team can help your business realize its potential.