Internal grinding is mainly used for grinding the surface and end surface of the inner hole of the workpiece, which is an important grinding method. There are 5 kinds of common internal grinding problems

1. The surface of the inner hole is rough (internal grinding problems)

It is often seen that the surface to be ground is rough, and the grinding marks are deep, and sometimes the whole wall is roughened in the internal grinding. This is because

the long shaft of the grinding wheel has deviated, and the gap between the grinding head bearing is too large, causing the grinding wheel to sway during the grinding process. It makes the grinding wheel not dressed to be round and flat, resulting in the rough surface of the workpiece when dressing grinding wheels.

The method that prevents such defects is to disassemble the grinding rod, insert the lever dial indicator into the spindle cone hole, and then move the belt by hand to check the rotary accuracy of the spindle according to the dial indicator stability. There is a bias, indicating that there is a problem with the grinding head bearing.

If there is no deviation, the grinding rod is loaded, and then the grinding rod is close to the grinding wheel by dial indicator to check whether the grinding rod is bent ( usually the grinding rod allows the deviation to be within 0.05mm).In addition to the above two points to ensure the improvement of the surface roughness of the grinding, and also pay attention to choose the right grinding wheel and rotary dresser.

The machining quality is affected by small internal grinding wheels and slender rods in the internal grinding process. In order to facilitate the smooth surface of the inner hole, the following two points should be noted when dressing the grinding wheel.

• The top angle of the diamond blade should be sharp and can dress the flat and multi-blade abrasive grains, making the cutting light and reducing the surface roughness.
• If internal grinding with universal cylindrical grinding machine, the height of the center of grinding wheel dressing tools should be the standard of the center of the cylindrical grinding machine, and it is 1-2mm below the center. When it is used in the inner grinding head, since the direction of rotation is opposite to that of outer grinding head, so the height of the diamond blade should be 1-2mm above the center. So that it can be kept stable when dressing to avoid smashing. It is good for smoothing of the wheel edge, improving the accuracy of the workpiece and reducing the surface roughness.

2. The inner hole is an ellipse(internal grinding problems)

The deformation in the internal grinding can be divided into two types. One is that the workpiece is deformed before removing from the grinding machine after grinding. The other is that the workpiece is deformed after being removed from the grinding machine.

The former is caused by the large bearing clearance of the grinding machine spindle, the bearing, and the spindle surface is not well bonded, and the two-speed bearing rotation center is not on the same axis. In addition, it should consider whether the workpiece clamping is reasonable. Such deformations are usually symmetrical deformations.

The second deformation is mainly caused by elastic deformation when the workpiece is clamped (This deformation has actually been introduced before, see Fig.5-38). The workpiece is clamped by the three-claw self-centering chuck, and the workpiece has changed due to the clamping force. After the machining is finished, the three claws of the chuck are relaxed, and the workpiece is reset to cause deformation.

In order to avoid such deformation, the method of strengthening the wall thickness of the workpiece to increase the rigidity may be applied, that is, fix an elastic open gasket(Fig. 5-51) on the cylindrical clamping position of the workpiece, making the three claws of the chuck clamp the gasket, and finally fasten the workpiece by the gasket. So that the three-point clamping force can be dispersed on the circumference of the workpiece without causing uneven deformation of the workpiece and causing elastic deformation.

Fig.5-51 Enhancing the clamping rigidity with the opening gasket

The other is that the workpiece is clamped on the chuck, and the platen is pressed against the plane and then do grinding, and the platen is deformed after being loosened. This deformation is mainly due to unevenness of the plane of the workpiece or the plane of the chuck, and the workpiece is deformed after being pressed. In order to avoid the deformation, thick cardboard can be placed between the workpiece and the chuck to cushion the unevenness of the surface of the workpiece or the chuck.

3. The inner hole is cone(internal grinding problems)

This defect is shown in Fig.5-52. It is mainly caused by the centerline of the grinding head frame and the longitudinal feed direction of the table are not parallel or the axis of the grinding wheel is not parallel with the feed direction of the table in the vertical plane. At this time, the grinding machine needs to be adjusted or dressed.

In the batch grinding, if the hole is found to be a cone, do not blindly adjust the grinding machine. Because the grinding wheel is blunt or is blocked both cause the wall thickness difference of the workpiece hole may be too large, and the thermal expansion may be inconsistent, resulting in the inner hole being cone. Therefore, should take the measures after the cause is identified.

4. The inner hole is flared(internal grinding problems)

The ground hole has a defect that has two large and small in the middle. Generally speaking, in addition to adjusting the distance accuracy of the work surface returning direction, most of the grinding rods ( long shaft) are thin and the rigid is poor.

Under the action of the grinding force, the horn is formed by the grinding rod to make the tool elastic, and the defect occurs when grinding the small and deep hole. In order to eliminate this defect, high-speed steel or carbide grinding rods can be used instead of the conventional steel grinding rods to enhance rigidity and improve processing quality.

If there is a horn mouth at both ends of the inner hole, it is usually caused by the grinding wheel staying at both ends of the hole for a long time or the length of the grinding wheel over that of the hole.

Fig.5-52 The inner hole is cone

a) Hole is left big and right small

b Hole is left small and right big

5. The grinding head of the internal grinding machine is prone to heat and inflexible rotation or even stuck(internal grinding problems)

This problem is mainly because impurities and small sand particles enter the oil injection hole or the bearing. Because the lubrication system of the grinding head itself requires the addition of secondary lubricating oil per shift, such as the dirt of the oiling device or the sand around the filling port on the grinding head directly enters the bearing with the oil, which is easy to produce the bearing heat and inflexible rotation of the grinding head or even stuck.