What Causes Deterioration in Surface Roughness During Machining? A Guide to Prevention

Factors Causing Deterioration of Surface Roughness

Surface roughness refers to the minute irregularities on the surface of machined products. The primary causes of increasingly pronounced surface irregularities, or in other words the surface becoming rougher, can be broadly divided into two categories: cutting conditions and cutting tool wear.

Factor 1: Cutting Conditions

During machining, the tool cuts away material as it rotates or moves linearly. In theory, the tool's cutting edge invariably leaves behind a series of minute peaks along its path (cusp height).
Cusp height represents the theoretical surface roughness. In particular, higher feed rates widen the spacing between peaks, resulting in greater surface irregularities.

For example, when using a ball end mill for plane machining, doubling the feed rate worsens the theoretical surface roughness (height of surface irregularities) by approximately four times. When surface quality is a priority in finishing, the basic approach is to reduce the feed rate.

Additionally, excessive cutting depth or long tool overhang can lead to self-excited vibration, also known as "chatter vibration." This vibration transfers to the machined surface, producing scaly patterns and periodic surface irregularities, and significantly degrading surface roughness.

Factor 2: Cutting Tool Wear and Built-Up Edge

Even new tools inevitably experience edge wear as machining progresses. Tools with edge wear lose their sharpness, causing various adverse effects on the machined surface.
Also "Built-up edge" is a particular problem when machining soft materials such as aluminum or mild steel. This phenomenon occurs when the high temperature and pressure generated during machining cause some of the removed chips to adhere and build up on the cutting edge of the tool.

The built-up edge grows and breaks off repeatedly. Portions may detach and adhere to or gouge the machined surface, resulting in nicks and scratches. Because it destabilizes the cutting ability of the tool itself, built-up edge is a particularly troublesome factor in surface roughness deterioration.

In addition to built-up edge, flank wear and rake face wear (crater wear) can also dull the tool's cutting edge. As a result, cutting resistance increases, making the surface finish more prone to gouging and burrs, and leading to deteriorated surface roughness.

Issues Caused by Deteriorating Surface Roughness

When surface roughness deteriorates and exceeds the values specified in design drawings, it not only compromises appearance, it can also lead to serious issues affecting product functionality and reliability.

Poor Air and Oil Tightness

Surface irregularities are a fatal flaw in parts requiring air or oil tightness, such as sliding surfaces of cylinders and valves, and flange mating faces. Large irregularities can create gaps between parts, resulting in oil or air leakage.

It also increases frictional resistance in sliding parts, potentially causing premature wear or seizure of parts.

Loss of Metallic Luster

A beautiful metallic luster is an important factor in determining the quality of exterior and decorative parts of products.

High surface roughness and pronounced irregularities cause diffuse reflection of light, reducing the surface's natural gloss and giving it a dull appearance. This significantly reduces the added value of the product.

Lack of Machining Allowance, Leading to Machining Defects

A rough surface can also be described as a condition in which the part exhibits unstable dimensions.

For parts with strict dimensional tolerances, even if the measured "peaks" of surface irregularities fall within specified tolerance, the "valleys" may already lie below the lower tolerance limit. In such cases, additional machining (re-machining) to improve surface finish may leave no remaining material allowance (machining allowance), resulting in undersized, defective products.

A Guide to Improving Surface Roughness

To improve surface roughness, it is effective to approach the issue from two perspectives: "tool selection" and "cutting conditions." By considering these approaches comprehensively and finding the ideal conditions for your company's machining environment, you can achieve both high quality and productivity.

Solutions Using Tool Selection

Choosing the appropriate tool can sometimes resolve surface roughness issues at their source.

Solutions Using Cutting Conditions

In addition to tool selection, solutions using cutting conditions are also essential.

For more information on cutting speed, see the following article.

SOLUTION NAVI "What Is Cutting Speed?"

Conclusion

Improving surface roughness in machining is a crucial issue for enhancing product value. The main factors that cause this deterioration are "inconsistencies in cutting conditions” and "tool wear.”

To address this, it is important to first identify the causes of surface roughness deterioration in your company's machining. Then, the fastest way to improve the situation is to select a tool with the appropriate coating and shape, and to optimize cutting conditions such as feed rate and rotational speed.

Using the solutions introduced here as a reference, aim to set optimal cutting conditions while maintaining a balanced approach.