How to grind the inner circle? What are the methods of internal grinding?
2021-11-15

What are the methods of internal grinding?

In grinding processing, internal grinding is also a very common type. Internal grinding can process through holes, non-through holes, step holes, etc., on machine parts, so it is widely used in machining. The dimensional accuracy of processing can generally reach the tolerance level IT6-IT7. If we use high-precision grinding technology, the dimensional accuracy must be controlled within 0.005mm. So, what are the methods of internal grinding?


1. The form of internal grinding

Internal grinding can be divided into the following three forms:


(1) Central type internal grinding

During grinding, the workpiece rotates around the centerline of the main shaft of the headstock. Ordinary internal grinders or universal cylindrical grinders are in this form and are suitable for grinding the inner holes of sleeves, gears, flanges, and other parts.


(2) Planetary internal grinding

During grinding, the workpiece is fixed and does not rotate. In addition to high-speed rotation around its axis, the grinding wheel also rotates around the center of the grinding hole at a relatively low speed to achieve circular feed. In addition, the grinding wheel also performs longitudinal feed motion and periodic transverse feed. The lateral feed of the grinding wheel is achieved by increasing the radius of gyration of the planetary motion. The internal planetary grinder is mainly used for grinding parts with large volumes and is not easy to rotate.


(3) Centerless internal grinding

On the centerless internal grinding machine, the workpiece is supported on the support wheel and the pressure wheel with its ruined outer circle and is driven by the guide wheel to rotate. This grinding method is suitable for grinding the inner hole of the thin-walled ring part.


2. Features of internal grinding

Compared with external grinding, internal grinding has the following characteristics: 

(1) In internal grinding, the diameter of the grinding wheel used is small, and the speed of the grinding wheel is restricted by the speed of the internal grinding tool, so the grinding speed is generally between 20m/s and 30m/s. Due to the low grinding speed, the roughness value of the workpiece surface is not easy to decrease.


(2) When internal grinding, because the grinding wheel is in contact with the workpiece in the inner tangent circle, the contact arc between the grinding wheel and the workpiece is larger than that of external grinding, so the grinding heat and grinding forces are larger, the grinding grain is easily dulled. The workpiece is easily heated up or burned.


(3) When internal grinding, cutting fluid is not easy to enter the grinding area. Wear debris is not easy to discharge. When wear debris accumulates in the inner hole of the workpiece, it is easy to cause the grinding wheel to block and affect the surface quality of the workpiece—especially when grinding brittle materials such as cast iron grinding chips and cutting. When the liquid is mixed into a paste, it is easier to block the grinding wheel and affect the grinding performance of the grinding wheel.


(4) The rigidity of the long shaft of the grinding wheel is relatively poor, which is prone to bending deformation and vibration, which greatly influences the processing accuracy and surface roughness and limits the increase in the amount of grinding.


3. The method of internal grinding

We must adjust the position of the grinding wheel before grinding. When grinding the inner hole on the universal cylindrical grinder, the grinding wheel is in contact with the front wall of the hole. At this time, the lateral feed direction of the grinding wheel is the same as when grinding the outer circle. On an internal grinding machine, the grinding wheel is in contact with the back wall of the hole, which is convenient for the operator to observe and longitudinally grind the work surface.


The longitudinal method and plunge method are commonly used in internal grinding.

(1) Longitudinal grinding method

This grinding method is the same as the cylindrical longitudinal grinding method. When grinding the through-hole, we first select the diameter of the grinding wheel and the long axis according to the diameter and length of the workpiece. The rigidity of the long shaft is better, and the length only needs to be slightly larger than the length of the hole.


(2) Cut-in grinding method

This grinding method is the same as the external plunge grinding method, which is suitable for grinding workpieces with a shorter inner hole length and has higher production efficiency.

When using the cutting-in method for grinding, the rigidity of the connecting shaft should be good. The grinding wheel is easy to be blocked and blunt during continuous feeding. We should repair the grinding wheel in time. And a lower cutting speed should be used for fine grinding.


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