To modify the external diameter and obtain a smooth surface texture of CNC machined parts, we have two categories of cylindrical surface grinding operations i.e. centerless grinding and centered grinding. Grinding the outside diameter of the parts has many advantages like smoother surface finishes, tighter tolerances, and workpiece distortion eradication.
Centered grinding keeps parts fixed then the grinding wheel goes all over the part to eliminate excessive material pieces. To hold the part stationary, small indentations on the ends of the part are used. Centerless grinding keeps the grinding wheel fixed and to remove the unwanted chips, parts travel through the machine and pass the grinding wheel.
What is Centerless Grinding?
A machining process that uses an abrasive wheel to remove unwanted materials from the external diameter of a part is referred to as centerless grinding.
In centerless grinding, the part is not held stationary. But centered grinding has a fixture to hold the part still. To perform grinding action, the part is supported in between two rotating wheels. One wheel moves the part and the other removes the abrasive material. Centerless grinding is preferred over other grinding methods when requiring a high volume production in a short time.
Centerless Grinding Machine Setup
Centerless grinding uses purpose-built centerless grinding machines. But every machine must consist of a robust base, grinding wheel, regulating wheel, and an angular-topped work rest blade. Since automation is the need of the hour. Therefore, computer numerical control has been introduced in these machines for optimization and precision purposes.
These modern machines are capable of interchange the grinding wheels to allow them for different grits and shapes. Moreover, the angle of the regulating wheel can be adjusted. It will assist to accommodate parts of varied sizes.
Working Mechanism of Centerless Grinding
The working mechanism of centerless grinders is not very complicated. But if not understood and applied properly then it can cause difficulties. First of all, the part is held between two wheels, i.e. regulating wheel and grinding wheel, with the help of an angular-topped work rest blade. The work rest blade holds the workpiece when it passes through the grinder.
The grinding wheel, (mentioned as the stationary wheel in the diagram above), is a fixed wheel that has more tangential speed than the part at the point of contact. And it is responsible to perform grinding action. The regulating wheel, (mentioned as a moving wheel in the diagram), can move easily. It controls the rotational speed and is responsible to exert pressure on the part.
Both the wheels rotate in the same direction at different speeds and their speeds result in grinding. And also, the rate of disposal of excess material is determined by their relative speed.
When the proper operation starts, then the part moves with the regulating wheel at the same speed and the grinding wheel moves at relatively higher speeds. When part slips and past the grinding wheel, it removes small chips from its surface. After some time, the part will have a better surface texture.
Many different abrasive types are utilized in grinding wheels. Which type is the best for a particular part depends on its material. To execute grinding on ferrous materials, aluminum oxide, ceramic aluminum oxide, and cubic boron nitride (CBN) grains are usually used. But for non-ferrous materials, silicon carbide and diamond grains are the best. These grains are used in vitrified or resin bonds to make a grinding wheel.
Normally, to grind delicate parts vitrified bonds are used. Otherwise, they may deform when high grinding pressure is applied. For stronger parts, resin bond grind wheels are used. Regulating wheels are made in rubber, plastic, or vitrified bonds.
Types of Centerless Grinding
Based on the technique utilized to feed the workpiece by the machine, centerless grinding has been categorized into the following three types
In in-feed grinding, the part is fixed manually into the grinding machine. After that, using some lever arrangement, the regulating wheel is moved towards the grinding wheel, and then grinding starts. In this type, there exists no axial movement of the workpiece as the length of grinding has to be controlled. The only movement that occurs during the grinding is the rotating movement. In-feed centerless grinding is used for relatively complex shapes, such as headed, stepped, or taper shapes.
In this type of grinding, the workpiece is fixed axially into the machine on one side and comes to rest against an end stop. After that, the grinding operation is executed, and then the workpiece is fed in the opposite direction to exit from the machine. It is used for headed components (means those parts where their length is greater than the width of the grinding wheel) and for tapered shapes also.
In through-feed centerless grinding, the gap between the regulating wheel and the grinding wheel is set according to the diameter of the part. Then the part is passed through the grinding wheels completely. In the end, we have our part with a modified external diameter. This method is very simple and efficient. As it does not require any separate feed arrangement. But it is limited to only simple cylindrical shapes like roller pins and straight long bars.
Benefits of Centerless Grinding
- Centerless grinding requires less grinding stock.
- It can modify the external diameters of fragile parts too.
- It is capable of grinding high volume production in a short time.
- The centerless grinding operation is easy to control.
- It does not require any specified skilled laborers to operate.
- Loading/unloading of the workpiece is easy and simple.
- Higher grinding wheel rotation velocities can be achieved if required.
Industrial Applications of Centerless Grinding
Centerless grinding is preferred in the industry where a large number of parts need to be processed in a short period. It is used to manufacture many high-volume automotive parts like control rods, camshafts, crankshafts, pistons, sleeves, etc. Centerless grinding is ideal for industries where roundness and extreme accuracy of cylindrical surfaces is demanded.
Below is the list of industries that majorly use centerless grinding
- Power generation
- Oil and gas etc.
Parts that can be ground by centerless grinding machines are injector plungers and bodies, copy rollers, cylinders, motor shafts, valve pistons and valve stems, drills, spools, piston pins, pivots, rotors, tubes, valve lifters, axles, balls, gear shafts and hubs, injector needles, isolators, small crankshafts, bearings, bolts, bushings, camshafts, etc.
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