Factors affecting the surface roughness include cutting edge geometry, accuracy, built-up edge, tool wear, machine tool accuracy, chip breaking properties and cutting conditions. These also differ according to the type of machining (external, internal, etc).
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Arithmetical mean roughness(Ra) is one way of calculating roughness. Ra means the value obtained by the following formula and expressed in micrometer when sampling only the reference length from the roughness curve in the direction of the mean line, taking X-axis in the direction of the mean line and Y-axis in the direction of longitudinal magnification of this sampled part. The roughness curve is expressed by y = f(x):
Tool grade selection
The most important factor in avoiding a poor surface finish is to avoid built-up edge from occurring. To achieve this, the choice of tool grade is important.
A built-up edge is a piece of work hardened material that adheres to the top of the cutting edge. The built up edge itself protrudes from the cutting edge and actually machines the workpiece. This generally results in causing a greater depth of cut and therefore creating component inaccuracy. Additionally, a built-up edge can cause the insert surface to peel and repeat the built up edge process, greatly worsening the surface finish. Therefore, a built-up edge is the biggest factor adversely affecting surface finish roughness.
To avoid a built-up edge, a tool grade with low affinity to the workpiece material should be chosen. For machining steel, cermet, mainly containing TiC (titanium carbide) and TiN (titanium carbide) is effective. TiC and TiN have a lower solubility to iron than WC (tungsten carbide), and can help prevent chip welding.
Tool geometry selection
The tool geometry can also affect the surface roughness. The insert corner radius has the most effect of all features of an inserts’ geometry. The surface finish condition that the corner radius geometry generates, without any imperfections during machining is called the theoretical surface roughness.
The theoretical surface roughness depends on the feed and the size of the corner radius. Higher feeds create a rougher surface finish. A larger corner radii create a smoother surface finish.
A large tool rake angle also helps improve surface finish. This is because tool sharpness is increased and welding is less likely to occur, therefore preventing peeling of the insert.
Honing also changes tool sharpness and generally a smaller hone produces a better surface finish. Meanwhile, the honed edge roughness can be copied to the machined surface. To avoid this, a good surface roughness of the honed edge is important. However, these copying properties change greatly according to workpiece material and are influenced by the cutting conditions.
The above image showing the relationship between corner radius and surface finish roughness.
Change in cutting conditions
The theoretical surface roughness is influenced by feed rates. A low feed rate improves surface roughness. However, feed rates that are too low, lower the overall machining efficiency, so it is important not to set the feed rate to worsen the surface finish roughness.
Meanwhile, in real examples of machining, avoiding welding and a built-up edge is an important point. To prevent a built-up edge, it is important to keep the cutting temperature over the re-crystallization temperature of the workpiece material. This softens any built-up edge, preventing it from adhering to the cutting edge. In other words, increasing cutting speed prevents a built-up edge from occurring and thereby helps to improve the surface roughness. Increases in feed and depth of cut also raise cutting temperatures, and improves surface roughness.
Use a wiper insert
Even if the feed rate is doubled, the surface roughness does not become any worse.
To improve the surface finish, inserts with a wiper edge geometry are available.
Improvement in surface finish is not only due to the advantage of using wiper inserts. The feed can be increased while maintaining surface roughness and this leads to a dramatic improvement in machining efficiency and a reduction in machining costs. Moreover, increased feed improves chip control and reduces the number of inserts required per production batch making wiper inserts ideal for high efficiency machining.
However, wiper inserts also have disadvantages. This is due to increases in cutting resistance and vibration when machining workpieces with low machine tool and clamping rigidity. Additionally the wiper effect only works in the horizontal and vertical directions, making them unsuitable for copying, but if used effectively they can greatly improve productivity.