Metal Loading in The Wheel
During bore grinding you want your wheel to stay nice and clean, but sometimes it becomes caked with metal flakes. This condition is caused by metal loading and when it occurs the loaded areas produce metal on metal rubbing instead of clean cutting. This rubbing will result in various problems, including scratches, deflection, and potential cracking due to excessive heat.
Metal loading on the surface of a CBN wheel
The first line of defense against metal loading is your coolant. The purpose of coolant is to minimize heat while washing away the metal chips that might otherwise load into the wheel. Adjusting the coolant nozzles may be all you need to do. Make sure you have enough pressure and flow, and be sure the nozzles are pointed in the right direction. Also check that the filtration system is working properly and that chips aren’t being re-circulated into the machine. If there is a chiller to maintain a constant temperature, check that too.
The next thing to check is the diameter of your wheel. It may be just a little too big for the bore you are grinding and this can “starve” the flow of coolant to critical areas. In this case, reducing the wheel diameter could fix the problem. A properly sized bore wheel is typically 80%-90% of the unground bore size, however there are exceptions, and a small adjustment can sometimes make a big difference.
The third thing to check is whether the wheel is unknowingly being asked to remove more stock, more quickly than it was designed to. A common example of where this happens is when something changes in the manufacturing processes of pre-machined bores that you are trying to grind and they are coming in undersized. These often undetected changes in bore diameter can starve the coolant and accelerate metal loading as the wheel makes contact sooner than it should, often when it is still on rapid approach.
Always keep in mind that grinding is a finishing operation. So you want to start out with parts that are as close as possible to the final diameter you are looking for. Ideally, grinding will just remove a little material and leave a nice finish. Bad things happen to parts when grinding is used to correct dimensional issues that should have been addressed in earlier stages such as machining, heat-treating, etc.
Finally, it is possible the wheel may not be sharp enough to scoop out material efficiently and avoid metal loading. If this is true, try sharpening the wheel with more aggressive dress parameters. This is the ultimate fallback solution for a wide variety of grinding problems and is easy enough to do if you use Meister’s Dressing Calculation Tool, which was designed to assist you in setting up the optimal parameters for rotary dressing of vitrified CBN and Diamond grinding wheels. (Click on the link to download it.)
