Pine Zhong

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Introduction

Thin workpieces such as washers, friction plates, templates, and thin metal sheets are widely used in mechanical manufacturing. However, grinding these parts on a CNC surface grinder can be challenging because they usually have low rigidity and are easily affected by clamping force, grinding force, and heat generated during the grinding process.

When a thin workpiece is directly clamped to a magnetic chuck, the magnetic attraction can cause the workpiece to deform. During grinding, the workpiece may also warp or bend due to insufficient rigidity and uneven stress. After grinding is completed and the magnetic force is released, the workpiece may return partially to its original shape. As a result, the final flatness may fail to meet the required accuracy.

Therefore, when machining thin workpieces, it is important to choose a suitable clamping method that minimizes deformation and allows the workpiece to remain as close as possible to its free state during positioning and grinding.

The following are several commonly used methods for grinding thin workpieces on CNC surface grinders.

1. Clamping with an Elastic Pad

One practical method is to use an elastic clamping arrangement. This allows the thin workpiece to be positioned and clamped while reducing the deformation caused by the magnetic chuck.

A rubber pad approximately 0.5 mm thick can be placed between the workpiece and the magnetic table. When the magnetic chuck is activated, the magnetic attraction compresses the rubber pad, allowing it to undergo a small amount of elastic deformation.

The elastic pad can absorb part of the uneven pressure and reduce the tendency of the thin workpiece to deform directly against the magnetic table.

During grinding, a relatively small grinding depth and feed rate should be selected. The workpiece can be ground several times, gradually approaching the required dimensions and flatness.

This method is relatively simple and does not require complicated auxiliary tooling. It is suitable for thin workpieces with relatively small deformation and moderate precision requirements.

The thickness and elasticity of the pad should be selected according to the workpiece size, thickness, material, and required grinding accuracy. An excessively thick or soft pad may reduce the stability of the workpiece during grinding.

2. Temporarily Increasing the Rigidity of the Thin Workpiece

For very thin workpieces with poor rigidity, a temporary support method can be used to increase their rigidity during grinding.

One commonly used method is to use epoxy resin to bond the thin workpiece to a flat supporting plate while keeping the workpiece in its natural, free state.

The workpiece is placed on the supporting plate, and epoxy resin is used to fill the gaps between the workpiece and the plate. Before curing, the epoxy resin has good flowability and can fill irregular gaps between the two surfaces.

After the epoxy resin has cured, the thin workpiece and the supporting plate form a relatively rigid assembly. The supporting plate therefore provides additional support and greatly reduces deformation of the thin workpiece during grinding.

The supporting plate and the workpiece are then placed together on the magnetic chuck of the CNC surface grinder.

First, one surface of the thin workpiece is ground. After the first surface has been finished, the workpiece can be removed from the supporting plate and placed on the magnetic table with the newly ground surface facing downward. The opposite surface can then be ground.

Because the workpiece receives continuous support during the grinding process, deformation caused by magnetic attraction and grinding forces can be significantly reduced.

In some applications, thick grease can also be used instead of epoxy resin to fill the gap between the thin workpiece and the magnetic chuck. This can provide additional support and improve grinding results.

This method is particularly useful for thin and easily deformable workpieces. However, after machining, the workpiece should be carefully separated from the supporting plate and any remaining adhesive or grease should be removed.

3. Mechanical Clamping

Another method is to use mechanical clamping equipment, such as a machine vise, to hold the thin workpiece.

The vise can be positioned and secured on the magnetic table, while the workpiece is held by the vise jaws.

Because the vise has a certain height above the magnetic table, the magnetic attraction acting directly on the jaws and workpiece is reduced. Therefore, the clamping force and grinding conditions should be carefully controlled to prevent deformation.

During grinding, a gradually decreasing feed rate can be used.

First, one surface of the workpiece is ground. As the final size and flatness are approached, the grinding amount and feed rate should be gradually reduced to minimize the grinding force acting on the workpiece.

After one surface has been finished, the workpiece is removed and the finished surface is placed against the magnetic table. The opposite surface can then be ground.

By repeatedly grinding the two surfaces and using smaller grinding amounts during the finishing stage, the flatness of both surfaces can gradually be brought within the required tolerance.

Mechanical clamping is relatively straightforward and can be useful for thin workpieces that are difficult to clamp directly with a standard magnetic chuck.

4. Vacuum Clamping

Vacuum clamping is another effective method for grinding thin workpieces. It uses atmospheric pressure to generate the required clamping force.

A vacuum fixture is equipped with a rubber sealing ring. The thin workpiece is placed on the sealing ring so that a sealed chamber is formed between the workpiece and the fixture.

A vacuum pump is then used to remove air from the chamber through the vacuum port. As the pressure inside the chamber decreases, atmospheric pressure acting on the upper surface of the workpiece presses it against the fixture.

Compared with conventional mechanical clamping, vacuum clamping distributes the holding force over a larger area and avoids highly concentrated clamping forces. This can effectively reduce local deformation of thin workpieces.

During grinding, an appropriate grinding method and grinding parameters can be selected according to the workpiece geometry and material. For example, circumferential grinding can be used in suitable applications.

After one surface has been ground, the workpiece can be repositioned and the opposite surface can be machined using the same principle.

Vacuum clamping is especially suitable for relatively large, thin, and easily deformable workpieces. However, good sealing between the workpiece, sealing ring, and fixture is essential. Insufficient sealing may result in inadequate vacuum pressure and unstable clamping.

Important Considerations When Grinding Thin Workpieces

The main challenges in grinding thin workpieces are generally related to low rigidity, clamping deformation, grinding heat, and residual stress.

If the workpiece has already been significantly deformed during clamping, it can be difficult to achieve the required flatness even when using a high-precision CNC surface grinder.

Therefore, in addition to selecting a suitable clamping method, the following factors should also be considered.

1. Minimize Clamping Deformation

The purpose of clamping is to keep the workpiece stable during machining, not simply to apply a large clamping force.

For thin workpieces, the clamping force should be distributed as evenly as possible. Excessive local pressure should be avoided.

2. Select Appropriate Grinding Parameters

Thin workpieces have relatively low rigidity and should not normally be subjected to excessive grinding depth or feed.

A smaller grinding amount combined with multiple grinding passes can help reduce deformation and improve dimensional accuracy.

3. Control Grinding Heat

Thin workpieces generally have limited heat dissipation capability. Excessive grinding heat may cause thermal deformation and affect the final flatness.

An appropriate amount of coolant should therefore be used, and grinding parameters should be controlled to prevent excessive temperature rise.

4. Use Multiple Finishing Passes

For workpieces with high flatness requirements, rough grinding, semi-finish grinding, and finish grinding can be used as separate stages.

During the final finishing passes, the grinding amount should be minimized so that the workpiece can gradually reach the required dimensions and flatness under low grinding forces.

Conclusion

Thin components such as washers, friction plates, templates, and thin sheets are prone to deformation because of their low rigidity, poor heat dissipation, and residual deformation after heat treatment.

If these workpieces are directly clamped with a conventional magnetic chuck, deformation may occur during clamping and grinding. More importantly, after the magnetic force is released, the workpiece may partially return to its original shape, making it difficult to maintain the required flatness.

For this reason, when grinding thin workpieces on a CNC surface grinder, different clamping methods can be selected according to the workpiece dimensions, thickness, material, and required machining accuracy.

The main methods include:

Elastic pad clamping

Temporary rigidity enhancement using epoxy resin or other supporting materials

Mechanical clamping

Vacuum clamping

The common objective of these methods is to minimize the influence of clamping on the workpiece and allow the workpiece to remain as close as possible to its natural state during grinding.

By combining a suitable clamping method with appropriate grinding parameters, cooling conditions, and finishing procedures, CNC surface grinders can achieve better

dimensional accuracy and flatness when machining thin workpieces.