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Internal cylindrical grinding questions-roundness and taper

2023.11.07More SuperHard Products Co., Ltd48

The most common problem with internal grinding is the taper produced by grinding.

When the ground hole is not straight enough, there will be a taper problem. The inner diameter gradually becomes larger or smaller towards one or both ends. In some cases, tapers occur at both ends, resulting in an "hourglass" or "barrel" longitudinal section shape. How to solve the taper problem?

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1. Change the oscillation stroke

If the hole becomes smaller at one or both ends, it is likely that the two ends are not being ground sufficiently, while the middle of the hole is being sufficiently ground with each grinding wheel pass (two grinds per stroke). The solution is simply to increase the stroke so that it extends further to or beyond one or both ends of the hole. This ensures that each area of the hole gets the same amount of grinding time. On the contrary, if the center of the hole is small but the two ends are large, do the opposite and reduce the stroke of one or both ends, which is equivalent to increasing the time the grinding wheel is in the middle.

2. The grinding wheel is dressed with the opposite taper

If the hole is relatively shallow, the grinding wheel is large enough to cover its entire length but still create a taper. What is the reason for this? You might think that the grinding surface of the hole is exposed on the surface of the grinding wheel. But this is where "deflection" comes into play. The pressure of the grinding wheel on the part causes the grinding wheel bar and wheel to deflect from the surface of the hole. Therefore, the surface of the grinding wheel is never completely parallel to the surface it is grinding. Slight parallel deviations from the curvature of the grinding wheel and grinding wheel bar may result in unacceptable taper. Sometimes, a simple solution to this problem is to dress the grinding wheel with a reverse compensating taper.

 3. Added spark-free grinding

Spark-free grinding refers to a period of time when the grinding wheel oscillates but does not feed, usually consuming 2-5 seconds in a 20-second hole grinding cycle. Therefore, spark-free grinding requires approximately 25% of the cycle time. Due to the cycle time requirements, a sparkless grind that is too short may create taper problems. Here's why: During sparkless grinding, all the forces accumulated during the grinding cycle are released. If the grinding wheel deforms during the grinding process, it will be released to the part where the grinding taper occurs during non-spark grinding. Therefore, adding one to two seconds of no spark is often a quick and easy solution to correcting these types of taper problems.

4.  Rollback and regrinding

If you are worried that sparkless grinding will take up more cycles, retracting and regrinding is an effective method. When retracting, the grinding wheel and workpiece are not in contact, and any deformation can be quickly restored. You can then go back to sharpening the taper area quickly without adding too much time to the beat.

5.  Sharpen the grinding wheel to make it sharper

Since the most common portion of taper is caused by a deflected grinding wheel bar, the sharper the wheel, the less deflection of the grinding wheel bar. So sometimes, a sharper grinding wheel means less deflection and less taper. Adjust parameters to create a sharper grinding wheel.

After addressing the taper issue, the next most common internal grinding issue is roundness. Maintaining roundness within specified limits is critical to part quality.

Roundness, commonly known as the degree of roundness, refers to the fact that the actual shape of the circle elements on the part is equidistant from its center. The roundness tolerance is the maximum allowable variation of the actual circle from the ideal circle on the same cross-section.

Even nominally round parts are not perfectly round. Some are oval, some are leaf-shaped, and some have other "non-circular" shapes.

During the grinding process, if the part deviates from the specified roundness. What's the most likely solution?

1. Added spark-free grinding

Roundness problems are usually caused by deformations or forces in the wheel bar or wheel that need to be relieved during the grinding process. The simple solution is to add spark-free grinding to ensure that all deformations and forces are released before the grinding wheel is retracted.

2. Increase workpiece speed

Sometimes increasing the workpiece speed can solve roundness problems. Especially for roundness problems caused by harmonics, the corresponding harmonics can be eliminated by changing the rotation speed of the workpiece. Increasing the speed or reducing the feed usually reduces grinding forces, deformation, and vibration.

3. Sharper grinding wheel

Sharp wheels grind more efficiently, require less force, and produce less vibration, both of which have an impact on roundness. Therefore, a sharper grinding wheel can produce rounder parts. Optimizing dressing parameters is the basic way to ensure a sharp grinding wheel.

4. Reduce coolant pressure

If you are grinding long parts, you need to increase the coolant, which is beneficial to cooling and removing excess. But sometimes in the small space of a long cylinder, high-pressure coolant can deflect the grinding wheel, causing the parts to be out of round after grinding. In these cases, reducing the coolant pressure can resolve the roundness problem.

----EDITOR: Doris Hu

---POST: Doris Hu


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