When bores and holes – straight or tapered need to be ground to specific tolerances and finishes, internal cylindrical grinding is the application of choice. One of the major challenges faced with this grinding application, is the restriction of coolant, as the grinding wheel is encapsulated by the workpiece.
This encapsulation effect increases the area where the grinding wheel is in contact with the workpiece to be significantly bigger compared to that of external cylindrical grinding. The specification of these wheels is usually a coarser grain size, softer hardness and a more open structure compared to wheels used for external cylindrical grinding.
Dependent on your application, there are different machines available for this specialised grinding application. With Universal Cylindrical, or Plain Internal grinders, the grinding wheel spindle rotates in a fixed position and the workpiece rotates and reciprocates along the grinding wheel.
When parts that are too large to rotate, need to be ground, the Internal Planetary grinder is the grinding machine of choice. During this process the grinding wheel spindle rotates and moves in a circular / planetary movement, with the workpiece not rotating, but reciprocating to attain traverse.
For internal centerless grinding, a Chuck-type Internal grinder is used which allows for the workpiece to rotate in a fixed position and the grinding wheel to rotate and reciprocate through the length of the hole or bore being ground.
Apart from the grinding wheel setup, selection, and, workpiece characteristics, other key points to consider include, the amount of stock to be removed, condition of the grinding machine, speed of both the grinding wheel, and workpiece as well as the use of coolant during the grinding application.
When selecting a suitable grinding wheel, one should be careful not to select a wheel too large in diameter for the workpiece to be ground. As this could not only result in the wheel blocking the bore, but also prohibit the grinding swarf from escaping, causing the grinding wheel to rub against the workpiece with no actual grinding application, ending in the workpiece heating and distorting. To counter this problem the general rule of thumb is to select a grinding wheel 66% to 75% of the size in ratio to the internal diameter being ground. This will allow free flow of coolant between the grinding wheel and workpiece.
During the grinding application, it is critical to ensure that there is sufficient volume of coolant with the correct direction and pressure into the contact zone, to prevent the workpiece from heating up, causing it to crack and warp, or clog as grinding swarf cannot be washed away.
Considering the type of material to be ground, and its hardness is equally important. With hardened materials, a grinding wheel with a finer grit size is more suitable, as it is easier for a finer grit to cut through the material than a coarser grain. Our Andor grinding wheels have a superior bonding system that l keeps the grain in place long enough to facilitate the grinding application and release the dull grain at the optimum point to expose new sharper grains. Due to this, we are able to manufacture wheels with specialty grains such as Sintered Aluminium Oxide which is known for having a higher stock removal rate and cooler grinding characteristics compared to other Aluminium Oxide grains. Using this type of grain significantly decreases the amount of time it takes to grind difficult materials, saving on both labor and electricity costs.
Further to this, the amount of stock removal, and the required finish should also be taken into account. A coarser grain wheel will remove material quicker, but not necessarily produce a very good finish, whereas a finer grain wheel will take longer to remove stock but will render a much finer finish.
With Grinding Techniques, it is easy to find the correct solution to your precision grinding application. Our Business Development team is able to assist with a premium basket offering of specialised abrasive solution to the precision grinding needs of your business.