Surface Grinding is a typical grinding process performed in many industries, especially in general metalworking, industrial and job shops. While larger facilities are set up with optimized grinding machines, coolant and dressing capabilities for high-volume manufacturing, some smaller facilities don’t have the same luxury.
Many job shops producing a lower volume of parts, and even some larger manufacturers, are challenged with older machinery, limited coolant available and inferior wheel-dressing capabilities, all of which can lead to inefficient grinding processes and poor part quality. This article offers some recommendations for wheel selection and tips for proper grinding parameters to help improve part quality and increase surface grinding productivity. Resin Bonded Abrasive Wheel
One of the most common mistakes in surface grinding is using a grinding wheel with an abrasive grain that is too durable. If a durable abrasive grain is used, a significant amount of force is required to fracture the grain in order to expose new sharp points for continued cutting. When under low pressure, durable grains will not fracture. They become dull and rub against the metal instead of cutting it. This causes friction, resulting in chatter and burn or producing a poor surface finish.
Often times in surface grinding, the depths of cut are smaller, sometimes under 0.001" [0.025 mm], which may not create enough force to efficiently fracture a duller grain. Where forces are too low to effectively fracture the grain, dressing is needed to sharpen the wheel and sufficient coolant is required to cool down the grind zone. However, if dressing and/or coolant are not available, it is critical to use a grinding wheel with a more “friable” grain—to break down and expose sharp edges.
A more friable grain fractures with lower forces and lower depths of cut, which are typically the case in surface grinding. If heat generation is a problem in a surface grinding operation, it’s important to know whether the abrasive is a friable or a durable grain. An abrasive wheel with more friability is recommended because it will create a cooler cut and allow the grains to fracture, providing more sharp points that cut instead of rub the metal.
While ceramic grinding wheels are considered superior technology and provide excellent material removal rates, they aren’t always ideal for surface grinding applications for several reasons.
Ceramic grains can be slightly more durable in general compared with conventional aluminum-oxide grains. Even a highly friable ceramic will be stronger and harder to fracture than a highly friable aluminum-oxide grain.
However, ceramic wheels can actually be too durable for surface grinding applications. This doesn’t necessarily apply to all types of surface grinding/materials/parts, but if a grinding process using a ceramic wheel incurs heat, a more friable ceramic grain may be required. Or switching to a friable conventional grain altogether may solve the issue.
It is also important to use coolant and/or dressing when using ceramic wheels to keep the cut cool and to sharpen any durable ceramic grains that are dulling. If a process has either or both of these elements, then ceramic may be an option. Otherwise, conventional aluminum-oxide abrasives will be a better choice.
Many parts that are surface ground have a large surface area that is wider than the grinding wheel. For example, in order to grind the entire surface of a 3' x 3' [0.9 x 0.9 m] slab with a 2"- [50.8-mm] wide grinding wheel, the wheel will need to do multiple traversing passes or cuts. These cuts can all be in the same direction, or the wheel may alternate between upcuts and downcuts.
During traversing passes, it is necessary to have a slight stepover from one grind pass to the next, meaning that the second cut slightly overlaps the first one to ensure a continuous grind. Without a stepover, some material between passes may be left unground if the grind passes don’t align perfectly.
For traverse surface grinding that alternates between upcuts and downcuts, a large stepover may induce heat. A 50% stepover means that half of the first will be ground again in the second pass. For example, if the wheel is 2" [50.8 mm] wide, the second pass will regrind 1" [25.4 mm] of the previous pass and 1" of the new pass.
If the section being ground doesn’t have time to cool down before the stepover, then it’s likely that this section will still be hot when the next grind pass occurs. This is especially true if there is substantial heat generated during the initial grind pass from using a durable grain or a lack of coolant.
Heat can generate internal damage to a material and poor surface finish in the form of chatter and burn. In this scenario, it’s better to have a smaller stepover amount, such as 20% or less. In addition, decreasing the stepover amount will require less passes to grind the entire part, decreasing overall cycle time, which is an added bonus.
In a surface grinding process where dressing is used, there are multiple ways to manipulate the dresser to ensure that it’s most effectively sharpening the wheel. These tips aren’t just for surface grinding but can also be used for many types of grinding wheels. For a quick reference, the following parameters can be used to improve sharpness for wheels being dressed by a traverse dresser, which is the most common dresser type for surface grinding:
Increasing the dress traverse rate will improve the aspect ratio of the dresser to allow for a more open wheel face.
Increasing the dress depth ensures that all grains are being conditioned enough to induce fracturing for a sharp wheel face. Shallow dress compensation may knock off the tips of sharp grains and dull them instead of sharpening them.
Similar to a slow traverse rate, a multipoint tool can close off the wheel face. A single point tool simulates an improved aspect ratio and can assist with improving the sharpness of a grinding wheel during dress.
Fine Squaring Wheel While there may be other ways to alter grinding machine, wheel or dress parameters to improve the efficiency and quality of surface grinding, the recommendations provided here can help alleviate common issues. Applying them can be a simple fix to help create the most productive surface grinding process.