CNC Machining Light Guides with Fine Optical Details: Considerations and Solutions

Automotive light guides are essential components in modern car lighting. Their complex shapes and high-precision optical features demand advanced machining techniques. In this article, WayKen will share the CNC machining process of light guides and their splitting and post-finishing methods.

Machining Characteristics of Light Guides

The design of light guides must meet both aesthetic and precise optical performance requirements. Their complex shapes and high light transmission properties make the machining process challenging. Here are their machining characteristics:

Irregular Feature

The shape and structure of automotive light guides are irregular, typically arc-shaped or S-shaped cylinders with a diameter between 7-10mm. Commonly, these types of parts emphasize appearance and assembly feasibility.

Optical Details

Light guides usually have fine tooth-like structures that curve or form S-shapes along the cylinder’s contour. These details typically require 5-axis CNC machining to produce.

High Light Transmission

Light guides are usually made from transparent PMMA material. And they are polished to achieve high-light transmission.

Raw Material Usage for Light Guide

Due to the irregular structure of light guides, the following example shows their material usage: a 50g light guide part requires 9.1kg of raw material! The final part’s weight is about 5‰ of the blank’s weight, just excluding the materials of machining for positioning.

As you can see, the following dimensions marked for the light guide are the maximum part dimensions, whereas the actual blank size is 675*90*125mm.

Machine Travel Limitations When Machining Light Guide Part

Due to the advantage of multi-axis rotation on 5-axis machines, the details of optical parts can be processed very well. However, let’s also take the light guide above as an example, the blank of 675*90*125mm of the light guide exceeds the maximum travel of typical 5-axis machines, making the whole workpiece difficult to machine one time. A split machining approach is necessary.

In addition, even with split machining, the light transmission areas still require whole machining, making 5-axis CNC machining still not suitable for processing these light guide transmission areas. To overcome these limitations, a combination of split machining and 3-axis machining is used. 3-axis machines typically have much larger travel, which is suitable for producing longer parts.

Machining Considerations of Optical Details

The tooth-like structures on light guide the curve along the cylinder’s contour, making it a big challenge for 3-axis CNC machining. As a professional manufacturer, WayKen specializes in plastic CNC machining and has years of experience in automotive part manufacturing.

Here is the WayKen’s solution. Firstly, all optical details must be machined accurately. It requires finding a suitable machining direction to clamp and process all optical features at once. After identifying the correct direction, a beveled reference must be machined to fix the workpiece. Thus achieve to machining the optical details of the light guide with one clamping.

Appropriate Machining Parameters

Light guides are polished for high transparency. Due to the light transmitting along specified reflective paths, it requires optical details of the guide to strictly meet 3D design standards.

Typically, the minimum machining radius for optical details is R0.1-R0.15mm, with a surface tolerance of ±0.02mm and post-CNC machining surface roughness of Ra0.2μm. To achieve these machining requirements, the stability of the machining equipment, tool durability, lubrication and cooling of CNC coolant, and rationality of machining parameters should be considered. Therefore, based on WayKen’s machining experience, optical details are usually machined using high-speed, slight milling, and low feed rate.

Overall Machining for Optical Areas

From the following light guide part’s structure, the yellow area is mainly for assembly positioning, while the green area is for light transmission. Splitting the cross-section of the guide (green area) would cause the light transmission to lose the optical function, so this transmission area must be machined as a whole.

Obstructions on Workpieces

However, after adjusting the machining position, some areas at one end of the light guide (indicated by the red arrow) still exist in the obstructions, making it impossible to process. Upon careful evaluation by WayKen’s engineers, this area can be processed in splitting without affecting light transmission.

Fine Surface Treatment for High Optics Results

Achieving high-quality optical surfaces requires careful attention to detail in the polishing and bonding processes.

Polishing Process

As mentioned, light guides are usually arc-shaped or S-shaped cylinders with a diameter of 7-10mm and a length of about 600mm. Since transparent PMMA is easy to crack during machining, the polishing process must carefully control the force to avoid indentations or part breakage.

At WayKen, our skilled craftsmen strictly follow the polishing procedures, carefully removing machining marks with various grades of sandpaper and polishing wax. The surface roughness after our polishing process can reach Ra0.02μm.

Bonding Process

Transparent PMMA materials usually use straight-splitting methods along the part’s contour. However, it should avoid light transmission and guidance areas. This splitting method is aesthetic and facilitates bonding. It’s important to note that no-mark adhesive and UV curing are commonly used for bonding components, ensuring minimal visible bonding marks.