Five-axis CNC programming technology for aspheric lenses based on UG/NX

Based on the characteristics of the aspheric lens surface, the lens processing technology was formulated, and the processing trajectory of the base mold and lens was planned based on UGNX3.0. Among them, cavity milling is used to plan the rough machining trajectory of the wooden bottom mold, fixed contour milling is used to plan the finishing trajectory of the wooden bottom mold, and the swarf method is used to plan the rough and finishing trajectories of the lens.

1. Lens processing five-coordinate CNC machining center

Generally speaking, a five-coordinate CNC machining center consists of three translation axes and two rotation axes. According to different motion axis configurations, five-coordinate CNC machining centers can be divided into three basic types:

1) Double swing of the tool means that both rotary axes act on the tool.

2) The worktable has double rotation, that is, both rotation axes act on the workpiece.

3) The tool and the workbench rotate respectively, that is, the two rotary axes act on the tool and the workbench respectively.

In order to ensure the rigidity of machining, this article uses a five-coordinate machining center with a double-rotation workbench, which has three translational axes X, Y, and Z and two rotational axes A and C that act on the workbench.

The production batch of lenses is generally relatively large, so the processing efficiency is particularly important, and the automatic tool change function is essential. Depending on the number of tools used in processing, it is recommended that there be no less than 5 tool changing positions.

2. Lens processing and clamping

There are currently two main clamping methods for lens processing: 1) machine clamp type; 2) air suction type. The machine clamp type clamping is relatively firm, but it is prone to the defect of clamping deformation. Air suction clamping is relatively smooth. As long as the air suction force is sufficient, the clamping task can be completed well. In order to make the clamping more stable, two kinds of clamping can also be used at the same time.

3. Bottom mold CNC machining trajectory planning

The bottom mold includes two parts: wood mold and rubber mold.

1. Wood mold processing trajectory planning

It can be seen from the shape of the wooden mold that its processing trajectory can be planned in a three-axis manner. The wooden mold blank is made of wood pulp and has a soft texture, so the processing feed speed and spindle speed can be higher, which can be set to 1000mm/min and 20000r/min respectively.

Generally speaking, the roughing machining strategy is cavity milling (CAVITY_MILL), which uses reciprocating machining to improve machining efficiency. The progressive height of the machining layer by layer is 1mm, and the generated machining trajectory is generated.

The finishing strategy is fixed axis contour milling (FIXED_CONTOUR), the upper surface is selected as the drive geometry (DriveGeometry), and the residual height is generally set to 0.02mm.

After the trajectory is converted into NC code, the wooden mold is actually processed.

2. Rubber mold processing trajectory planning

Since the rubber mold is a material in direct contact with the lens blank, the upper surface of the rubber mold must fit well with the lens blank ground, so as to ensure sufficient clamping force and make the lens processing process stable. The processing technology of the rubber mold is: first bond the rubber mold to the surface of the processed wooden mold, process the required inner and outer contours of the rubber mold, and then smooth the upper surface of the rubber mold according to the shape of the bottom surface of the lens blank (the residual height is generally is 0.01mm), ensuring that the lens blank and the rubber mold fit tightly.

After the rubber mold is bonded to the wooden mold, it deforms into an aspherical surface. Therefore, the processing trajectory of the inner and outer rings cannot be planned using three-axis processing. Five-axis processing must be used to plan the processing trajectory. The sides of the inner and outer rings of the rubber mold are straight-grained surfaces, which can theoretically be processed and formed in one go.

Considering the characteristics of the rubber material, one-time molding can easily cause the material of the inner and outer rings to tear, which will lead to insufficient clamping when processing the lens. This article uses three processes to complete the processing of the outer ring. Therefore, it is still divided into several passes for processing and forming. UGNX3.0 provides a large number of multi-axis machining trajectory planning methods, among which the Swarf method is mainly used for trajectory planning of ruled surfaces.

After processing the inner and outer rings of the rubber mold, it is necessary to use fixed-axis contour milling to smooth the surface, so that the gap between the lens and the rubber mold can be small and the clamping stable enough during lens processing.

After the trajectory is converted into NC code, the rubber mold is processed.

4. Lens processing trajectory planning

The five-axis CNC programming function of UGNX3.0 is very powerful. Commonly used driving methods are curve/point driving method (Curves/Point), boundary driving method (Boundary), spiral driving method (Spiral), and area milling driving method (Fixed_Contour). , surface area driving method (SurfaceArea), tool path driving method (ToolPath), etc. Commonly used tool axis control methods include NormaltoPart (perpendicular to the part surface), RelativetoPart (relative to the driving surface), Interpolate, NormaltoDrive, SwarfDrive, RelativetoDrive, etc. Among them, the driving method that is more suitable for lens processing is the surface area driving method (SurfaceArea), and the tool axis control method is SwarfDrive.

According to the processing technology of the lens, the planning of its processing trajectory is mainly divided into two steps:

1. Lens roughing trajectory planning

The lens roughing process should remove excess lens blank material to prepare for the finishing process, and the tool used is a flat-bottomed end mill. The lens blank material is made of glass or other polymer materials, which are relatively hard and brittle in nature. In order to facilitate the blank waste to fall off as soon as possible, we try to make it in one piece.

The rough processing trajectory planning method is Swarf, and the processing drive geometry selects the lens side. Taking into account the nature of the lens blank material, it is necessary to set the processing direction and advance and retreat knives reasonably. Roughing requires a certain machining allowance, which is completed by the rounding process.

2. Rounding processing trajectory planning

The rounding process rounds the edge of the lens to make the edge of the lens smooth, making it easier to install the finished product into the target frame. The generally used tool is the R rounding knife. Since the lens material easily forms cutting burrs when heated, high-pressure gas cooling is required during the rounding process to reduce deformation after processing, and cutting waste is continuously removed.

The method of planning the rounding machining trajectory is similar to roughing. It should be noted that the processing trajectory should be planned according to the center line of the lens side, so that the rounded lens side can be smooth and consistent and meet the process requirements.

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