Milling of curved surfaces belongs to the category of contour/contour CNC milling. It involves machining irregularly shaped contours or continuous curves with different angles (inclined, concave or convex). This is the key process to complete most of the customized parts with unique shapes and requires CNC machinists to have advanced expertise in basic machining procedures and principles.
This article will give a detailed introduction to the surface CNC milling technology. What aspects should be paid attention to when milling curved surfaces? What should the complete curved surface machining flow look like?
The Machining Flow Of Curved Parts
The processing of curved surface parts also follows the processing rules of CNC machining centers. Before reaching the final processed parts, they are first subdivided into different categories: rough machining/semi-rough machining, semi-finishing, finishing and super finishing. Because free-form curves and surfaces cannot be clearly expressed by geometric drawing and mechanical drawing, it has become the primary problem to be solved in the roughing stage of the machining center.
In order to improve the efficiency of milling when the rough parts of the curved surface parts are preliminary processed, according to the margin given by the surface to be processed, an end mill can be used to mill them layer by layer according to the contour surface. After rough milling, the shape of the curved surface will appear hierarchical distribution, and the height of the steps depends on the rough milling accuracy. During rough machining, due to the large machining allowance and cutting amount, a large amount of cutting heat is generated during the cutting process, which is easy to make the tool wear quickly. At this time, the temperature of the cutting area should be lowered, and the cutting fluid should be applied at this stage. Lubrication and cooling effect.
Semi-roughing is indispensable. It is an important stage from roughing to finishing. Its purpose is to mill out the extra part left in the roughing step. Semi-finish milling should be carried out with a ball-end milling cutter, and its line and step distance should be larger than that of fine milling. It should be noted that semi-finishing milling should leave a machining allowance of about 0.5 mm for the subsequent finishing milling process. After this process, the shape of the processed surface is close to the theoretical curved surface.
Finishing is the process of finally processing the theoretical surface. Ball-end milling cutters are the first choice for finishing tools, and line cutting is usually used. Attention should be paid to the selection of the turning point and the determination of the feed speed during programming. For parts with better openness, the turning point should be selected outside the curve table, that is to say, the curved surface should be appropriately extended outwards during programming.
If the part requires mirror polishing or ultra-smooth contours, the operation will proceed to ultra-finishing. To this end, high-speed machining technology and high-precision tools are used.
Know The Tools Used For CNC Milling Curved Surfaces
We use various round-end milling cutters to machine smooth curved surfaces on the manufactured parts. These include ball head indexable end mills, round inserts and ball head solid carbide. We prefer to use circular contour tools in contour processing because it will not leave any obvious marks on the tool path.
Insert Type End Mills With Round Blades
These types of end mills have larger diameters. The fillet-type end mill has impressive productivity and high stability, and is an ideal tool for roughing.
Solid Ball End Mill
These types of end mills usually leave a very good surface finish on the machined parts. Due to their structure, they may have lower stability. Therefore, it can be used for finishing instead of roughing.
Indexable Head Ball End Mill
These types of end mills are similar to solid carbide end mills, but have removable cutter heads. This is a special tool with a detachable end that can be easily replaced as needed.
What Are the General Tips for CNC Milling Technology For Curved Surfaces?
What are the general techniques of surface CNC milling technology?
1. Prepare In Advance
Before choosing to manufacture custom parts through contour milling, it is important that you first be aware of the various factors that may affect the entire milling process. In addition, in order to choose the ideal processing method, we must also determine some specific matters. This is a further understanding of the main preparatory work we carried out.
Pay attention to the cavity depth of your part design.
This plays a key role in helping us choose the right cutting tool. We also determined how long the meter length will take.
Determine how much material must be subtracted from the original block.
This helps us plan the operations needed to manufacture your parts. After that, we evaluate whether additional procedures are needed after semi-finishing and semi-roughing.
Consider how to clamp the workpiece in place.
This will help prepare the necessary fixtures that we may need.
2. Get The Correct Feeds And Speeds
The feed and speed determine the speed at which we subtract material from the part. This is why it has a major impact on obtaining an excellent surface finish. In order to achieve this goal, we need to follow some calculation methods. The feed rate may also depend on the tool used, the depth of cut, the material cut, the required accuracy, and the contour of the machined part.
3. Tool Utilization
In today’s market, there are many tools available. We prioritize the rigidity of the tool for roughing operations to suit the feed requirements and the aggressive cutting depth for roughing. At the same time, the end mill must be very sharp in finishing, so as not to leave any obvious traces of the tool path. Another difference is the tool diameter. Generally, the diameter of the roughing tool is larger than the diameter of the finishing tool.
4. Minimize Vibration During CNC Milling
Vibration is one of the main causes of poor surface finish, chattering, and damage to cutting tools. During milling operations, vibration can come from a variety of sources, including clamping stability, tool stiffness, material hardness, and errors in the machine spindle. Here are some tips to reduce this processing barrier:
- Use tools with high beating accuracy
- In the rough machining process, avoid the overhanging time of the end mill for too long, use extension rods or extension tool holders
- Make sure to maintain proper grip and balance during the cutting operation to secure your work fixture.
- When handling hard metals, avoid cutting drastically.