The difference in surface conditions (surface roughness) between milling and polishing.

What is milling?

Milling is a widely used process in metalworking.

Milling machine processing can be broadly categorized into two types: face milling and peripheral milling. In both cases, the milling cutter secures the workpiece in the chuck and removes material. This machining method necessitates the capability to firmly hold the material using a chuck.

The premise of a magnetic sucker is that it can be affixed to magnetic materials; non-magnetic materials or very small items cannot be processed. Similarly, a standard sucker requires a certain thickness to effectively clamp and secure the workpiece. As the size of the workpiece increases, a larger chuck is necessary, which means that large milling operations require a greater initial capital investment. Furthermore, in recent years, the use of CNC tools for machine automation and labor-saving has become increasingly common.

Milling Type

◆ Front Milling

Face milling employs face milling cutters to machine large surfaces. This technique is particularly common in metalworking for shaping materials. However, face milling often results in noticeable wear marks, which are characteristic of metal processing products.

◆ Flat Milling

When it comes to milling, the most common type is face milling, which you may be familiar with. However, there is also a process known as flat milling. In a horizontal milling machine, the axis of rotation is perpendicular to the surface during face milling, while it is parallel to the surface in flat milling. Although flat milling offers excellent processing speed, it is primarily used for roughing, as the resulting surface will inevitably be rough.

End Mill Machining

It is nearly identical to a face mill, but it utilizes a tool known as an end mill. Due to the limited processing area, the machining process requires more time.

What is polishing?

The polishing process encompasses various methods, ranging from hand tools to specialized polishing equipment. With hand tools, a wide array of processes can be performed with an investment of only tens of thousands of yen, allowing for versatility in the size, shape, and performance of the workpiece. However, hand tools are not ideal for shape processing, as the accuracy of shape control relies heavily on the operator's skill. Even when utilizing specialized polishing devices, they remain unsuitable for shape processing and are primarily employed for surface treatment and enhancement of surface conditions.

Types of Polishing Processes

◆ Sander Polishing

Double-action sanders are widely used. Typically, polishing is achieved by attaching sandpaper or sand film with felt backing to the sander (sand, not thunder).

There are three common types of sanders available on the market: double-action sanders, single-action sanders, and random orbit sanders.

The double-action grinder's performance varies based on the size of the built-in track diamond. A larger track enhances polishing performance, while a smaller track improves surface conditions. This is due to the fact that, even at the same rotational speed, the operating distance (amplitude) increases with a larger orbit.

◆ Abrasive Belt Polishing

Abrasive belt polishing is typically categorized into three types based on the size of the abrasive belt: wide abrasive belts, narrow abrasive belts, and small abrasive belts.

Wide sand belts are typically employed for the surface polishing of large areas. There are various types of narrow bands, each suitable for different applications depending on the flexibility of the substrate. For regrinding general steel, a robust X-weight substrate is utilized, whereas a flexible J-weight substrate is generally preferred for light metals and small objects. Notably, belt polishing offers the highest polishing performance, which translates to greater processing efficiency in the polishing process.

Surface Grinding Machine Processing / Double-Head Grinding Machine Processing

Surface grinding machines utilize cylindrical rolls for polishing. For instance, there are 100 impellers constructed from emery cloth, arranged in a wing-like configuration and secured with metal fittings (our company's NP wheel), as well as non-woven wheels made from molded non-woven fabric (NL, etc.).

Since the material is cut by rotating the rolled product, the process is similar to the surface milling described above. However, when using a surface mill, the mill behaves more like a rigid body, making it suitable for roughing. In contrast, these abrasive products are flexible, which makes them more appropriate for tasks such as pre-finishing compared to the surface mill.

◆ Disk Grinding Machine Processing

The grinding materials used in disk mills typically include metal brushes, grinding disks, and diamond cutters, which are specifically designed for grinding purposes. It is important to note that these tools are among the most abrasive of hand-held devices and operate at high rotational speeds, making safety a top priority.

About Surface Parameters

The number indicates surface roughness. Because it is intuitively understandable, it remains commonly used in charts. Additionally, its clarity makes it particularly easy to comprehend when the surface condition does not require high precision. This system is based on the 1952 JIS standard.

Ra (Arithmetic Mean Roughness)

The simple mean assumes that all bumps on a surface are not averaged to form convex surfaces. This is by far the most commonly used parameter. In my opinion, the aspects related to PCBs are presented in the following table.

Rz (formerly known as Ry)

Ra is currently the most widely used parameter; however, I recall that prior to the revision of JIS in 2002, many customers regarded Rz (Ry) as the most significant. This parameter, known as maximum height roughness, represents the average of the sum of the highest and lowest points within the evaluation cell.

Specifically, it remains a crucial parameter because deep lines (scratches) can result in product defects and are challenging to polish out or remove during post-processing.

Rpk/Rvk

This parameter is particularly interesting to me. Rpk represents the height of the convex peak, while Rvk denotes the depth of the convex valley. For sliding materials, such as engine cylinders, Rpk is considered effective for initial wear—essentially, how quickly the material breaks in—whereas Rvk is crucial for oil leakage resistance. Additionally, Rvk influences the total number of scratches mentioned earlier, making it a parameter that should be closely monitored based on the specific application.

RSm

After polishing, a series of peaks and troughs is always present. The average length of these peaks and valleys is referred to as RSm. The finer the grain, the more polished the surface appears.