Motorcycle helmets are not unfamiliar to everyone. This is a must-have safety tool for every safety-conscious person on a bicycle, especially for male friends who like to use a trailer.

 

Today, I will introduce to you how to make a motorcycle helmet lens mold core.
Everyone knows that plastic lens on a good motorcycle helmet can protect against wind, sand and slow speed, which directly affects safety of rider and sports.
This article will let you understand five-axis CNC machining of motorcycle helmet lens mold core. Taking numerical control machining of helmet lens mold core as an example, numerical control machining method of mold part is introduced. In current mold manufacturing industry, three-axis CNC machining technology has been widely applied and relatively mature, but with development and advancement of five-axis CNC technology, advanced five-axis CNC machining technology has shown obvious superiority in the market.
Generally, plastic optical lenses on the market need to have precise molds to ensure that injection molded lenses can meet required optical specifications (zero diopter, zero luminosity), and it is difficult to process a helmet lens.
Lens mold belongs to high-precision plastic mold, and its mold surface has complex shape, difficult material cutting and high precision. Its manufacturing and processing technology has higher requirements than general precision plastic mold.

 

Figure 1 Lens on a motorcycle helmet

 

Figure 2 helmet lens mold core parts
Looking at above figure, part drawing can be seen: size of part is 330X200X213mm, clearance with cavity is less than 0.02mm, and raw material is STAVAXESR (S136) advanced stainless steel.
Parts are mainly curved, and most of them are steep faces. Height difference of surface is about 185mm. There is a water-cooled cavity on bottom side. Size of surrounding round is small at R0.8. Shape is generally curved, which is also difficulty.
Among them, yellow surface shown in Figure 2 is required to be higher: surface roughness value of surface is Ra=0.8μm and profile accuracy is higher; surface height difference is larger; there is a convex undercut, as shown in red part of Figure 2. Therefore, main processing difficulty of part is on molding surface.
This is general process data of helmet mold. I hope that you will not be scared by data of these crafts. I believe that data will speak and will help you build a specific product model.

Through above process analysis, machining precision and surface quality of part are relatively high, machining is carried out by a combination of mechanical and grinding methods. Machined surface can be machined as much as possible after Ra0.8. Then artificial polishing is used to achieve Ra0.01, so that surface of lens mold maintains a good flatness, achieving zero diopter and zero astigmatism.

Yellow surface shown in Figure 3 is main machining difficulty of part. If a conventional three-axis machining center is used to machine surface. Due to reversed position, small rounded corners and excessively high drop, multiple clamping (at least three times) or EDM must be performed.
Let's compare advantages and disadvantages of several processing methods~~~
If multiple clamping is used, it is difficult to ensure machining accuracy and surface quality of curved surface due to accuracy of fixture manufacturing and operation level of operator. There will be problems such as knife marks, uneven machining allowance, uneven surface quality, etc., increasing difficulty of subsequent polishing and grinding, increasing polishing time.
If electric machining is used, required copper size is large due to large size of parts, and special fixtures need to be manufactured. Therefore, above problems caused by multiple clamping are also caused; cost of manufacturing fixture and copper is higher, efficiency is low, and production cycle is long.
Figure 3 3D drawing of helmet lens mold core
If five-axis machining center is used to machine curved surface, clamping can be completed without a special fixture, which reduces cost of fixture and avoids problems caused by multiple clamping.
Moreover, in five-axis machine tool processing, since position angle of tool/workpiece can be adjusted at any time, not only can tool be overhanging very short, rigidity and workability can be ensured, and cutting point can be fully utilized at all times by using optimum cutting point of tool. Or use a line-to-form spiral end mill instead of a point-contact ball-end milling cutter, or even further optimize tool/workpiece's pose angle for higher cutting speeds and cutting line widths.
It is appropriate to compare analysis or use a five-axis machining center to machine surface, which can achieve higher cutting efficiency and better surface quality.
However, according to characteristics of mold parts processing and requirements of new machine tools, in production process of mold parts, it is impossible to complete all processing steps of parts by one or two CNC machine tools - roughing, semi-finishing, finishing and superfine machining. Reason is obvious – it is completely impossible to maintain good precision on a machine that is suitable for all types of processes and workloads.
For example, in roughing, guide rails, ball screws and spindle bearings of machine tool will be under heavy pressure and load, which will have a great impact on accuracy of machine.
Therefore, under premise of taking into account economy and high precision, machining of part should be carried out on a special three-axis CNC milling machine, finishing is performed on special three-axis machining center, and machining allowance is left in forming surface. Finishing of formed surface is completed on shaft machining center.

Processing scheme of proposed helmet lens mold core is as follows:
1. Cut mold blank by electric discharge wire cutting. Blank pretreatment hardness is HB200, bottom and top surface of fine grinding are used as reference plane.
2, With top surface as reference, installed on CNC milling machine table, divided into four sides, rough machining bottom groove, leaving a 0.3mm margin.
3. Finish bottom groove, leaving a margin of 0.
4, Processing bottom face, first hit center drill.
5. Holes of φ5 and φ12 are machined by boring.
6. Turn side workpiece, machine side hole, and drill center first.
7. Machining hole of φ12 with a boring method.
8. Use bottom surface as reference, divide four sides, and rough top surface, leaving a 0.5mm margin.
9. Roughly process uneven residue with a knife.
10. Remove workpiece, heat treat and temper to HRC 50 degrees. Heat-treated workpiece is mounted on finishing center on basis of bottom surface, top surface is finished with a margin of 0.015, wherein curved surface of molded portion is left with a margin of 0.1 mm.
11. Based on bottom surface, it is installed on five-axis machining center to finish curved surface and reverse position of forming part, leaving a margin of 0.01mm.
12. Polishing.

This part is CNC programmed using DELCAM's POWERMILL software. NC programming program table is as follows.

 

CNC programming program table
Finished machining results of helmet lens mold core are shown in Fig. 4. Surface quality of workpiece is required, and surfaces are uniform.

 

Figure 4 Processing results of helmet lens mold core

Molding surface and reverse position of helmet lens mold core are processed by five-axis machining center in one clamping process, surface quality has achieved expected effect, production efficiency is improved, technical parameters required by customer are achieved,  and reflecting superiority of advanced five-axis CNC machining technology in the market.This processing solution can also provide reference for processing of similar mold surfaces.