How to ensure the surface finish of CNC parts?

Why is a poor surface finish unacceptable?

In addition to being unsightly, a poor surface finish can adversely affect corrosion resistance and can make certain manufacturing processes, such as painting, more difficult and time-consuming. If the surface finish is poor, it will be difficult to adequately treat the part through appropriate post-processing, finishing processes such as anodizing or powder coating. That is, the coating may have a harder time adhering—causing pits in the uncoated portion and corrosion that may spread in or across the entire uncoated portion. Therefore, parts become more susceptible to corrosion due to improper (or untreated) surface preparation and exposed metal. A poor surface finish can also lead to more frictional effects between the part and other objects, as well as reduced strength and wear resistance.

What is a "processed" surface?

When a finished part is removed from a CNC machine, the surface of the part (before any post-processing surface finishing) will be dull and have visible tool marks. This is called a machined surface. Quantify the quality of the finished surface by measuring the average surface roughness (Ra) of the part. The standard commercial machine surface roughness of the parts is 3.2 μm. This is usually the cheapest surface treatment option because no additional processing is required. Some customers may be able to accept machined surfaces, others may require parts with lower surface roughness.

Four tips for improving machined surface quality

Improving machined surface quality requires revisiting machining basics. By modifying tool settings and optimizing tool paths and cutting parameters, manufacturers can ensure their parts have excellent surface quality. Here are 4 tips on how to avoid a poor surface finish on machined parts:

1. Increase speed and reduce feed

In order to obtain the desired machined surface, the CNC machine tool needs to be set up to work within the specified feed and speed range of the material being cut. By increasing the spindle speed of the tool, the contact time between the tool and the workpiece can be reduced. This reduces built-up edge (BUE) and extends tool life. Removing material quickly is the goal of roughing passes, while removing material slowly to achieve precise dimensions and excellent surface quality is the goal of finishing passes. By reducing the feed rate, lighter, shallower cuts can be made, resulting in a more uniform surface finish. By combining higher spindle speeds with lower feed rates, built-up edge on the part can be reduced, thereby improving surface finish.

2. Minimize tool chatter and deflection

Chatter is vibration that occurs on a tool due to forces during CNC operations. Deflection, on the other hand, is the distance the tool tip travels from the toolholder axis. Both are important causes of poor surface quality. While unavoidable, here are two tips to minimize tool chatter and deflection:

Make sure to use the correct tool and tool holder for the job and material to ensure rigidity. Make sure cutting parameters and feed rates are within recommended values. With the correct tool holder, tool overhang can be minimized, thus minimizing deflection.

Make sure the tool is securely seated in the chuck and the workpiece is securely clamped. The combination of both minimizes the possibility of tool or workpiece movement or vibration during machining operations.

3. Control chip removal

Improper chip breaking and removal can adversely affect surface finish, as chips can wear the workpiece and even cause damage to the tool. To solve this problem, consider using a chip breaker. Chipbreakers are built into or clamped onto the cutting surface of the tool. They promote chip breakage by deliberately bending the chip, making it shorter, less linear, and easier to break. Reducing the chip load reduces the forces on the tool. By controlling chip evacuation, manufacturers can get closer to achieving high-quality machined surfaces, avoiding built-up edges and extending tool life.

4. Increase the front angle

The rake angle is the angle between the rake face (the surface of the tool where chips flow after chip evacuation) and a plane perpendicular to the direction of the tool velocity vector. The increased rake angle helps facilitate chip evacuation and helps minimize built-up edge. By actively increasing the rake angle, cutting forces are reduced and chips are continuously formed and discharged, which also helps reduce the occurrence of poor surface finishes.

Poor surface finish of machined parts is a headache. Although machined to precise dimensions, parts with poor finishes are rarely accepted and can drain a company's profits. However, by applying the above tips, it is possible to achieve a surface finish that meets customer requirements in the machined state.