A Guide to Surface Finishes
Machining processes, such as milling and turning, impose characteristic irregularities on the surface of a part being produced. Factors ranging from cutting tools to machine tool conditions, all influence these characteristic irregularities on the part’s surface.
Simply put, surface finish can be viewed from two different angles. From the part designer’s view, the surface finish of a part is a condition which affects part functionality. The machinist view is that surface finish is a result of the processes used and that the finish can be changed by changing or altering the process.
It is now the manufacturing engineer’s responsibility to determine how the machinist will produce the part’s surface finish defined by the design engineer.
Surface finish, or surface texture, can be broken down into three components:
Note that, as the cutting tool is used, it leaves a groove with a width and depth, and in cases where grinding is used, each abrasive granule on the wheel can be seen as a small cutting tool, which also leaves a mark on the surface of the part being produced.
Waviness is caused by small fluctuations in the distance between the workpiece and the cutting tool during a machining processes. These small fluctuations are caused by vibrations and cutting tool instability.
Some of the sources of the vibrations can be internal, such as the power motor of the machine and worn spindle bearing. Other sources are external, such as other machines operating on the shop floor.
Assuming that the part being produced is normally flat and/or straight, errors of form is caused by the lack of flatness or straightness in the machine’s tool ways. Because the machine will always follow the same ‘out-of-straight’ path, this irregularity is highly repeatable.
All three of these surface finish components exist simultaneously and each condition is examined separately.
Generally, surfaces finishes tend to be a stable condition. This means that surface finish should not change from part to part (unless a condition in the process is changed).
Manufacturing engineers can, in general, predict the surface finish a certain process will deliver. Because of this, surface finish measurements are used to monitor the stability of the manufacturing process.
This allows a machinist to take a measurement and establish whether the entire process is running correctly.
In cases where the machinist’s measurements show a change, it is a sign that a factor in the process has changed. This change can range from anything between cutting tool limits or an insufficient amount of coolant.
By measuring the three components described above, the search for the cause of the error can be narrowed. With cause in hand, its effects can be reduced or eliminated.
Using parameters, manufacturers are able to compare surface finishes in a quantitative manner.
Parameters are defined using algorithms to turn the raw measurements into a useable numerical value. There are over 100 parameters, but machinists traditionally rely on just one or two of these parameters.
Average Roughness (Ra) is the most widely specified and measured parameter. The algorithm used to for Ra calculates the average height of the entire surface within the sampling length from the mean line. This calculation serves as an effective to way to monitor the stability of the process.
Rmax measures the vertical distance from the highest peak to the lowest valley within five sampling lengths. It then selects the largest of the five values.
This means that Rmax is extremely sensitive to irregularities such as burrs and scratches on the part’s surface, making it a useful parameter for inspecting these types of irregularities. Note however, that this parameter is not useful in measuring the stability of a manufacturing process as burrs or scratches are not always caused in the manufacturing process.
Also note that Ra is an averaging function known to be fairly insensitive to occasional irregularities, making it an insufficient parameter for detecting features like Rmax is able to.
Rz is used in favor of Ra. Rz is also based on the evaluation of five lengths. Unlike Rmax, it does not select the largest peak-to-valley distance of five, but rather averages these five values. The use of Rz can be seen in Germany as well as other European countries.
Design and Production
Surface finishes affect the performance of the part. Where some parts require a smooth surface finish, others require a rougher finish, with some even requiring more complex surface finishes.
Design engineers will specify a surface finish parameter and value, fully understanding how these parameters and values will affect the performance of the part being produced.
After a surface has been defined and specified, the manufacturing engineering will determine how to effectively produce this part in the most cost-effective way possible without influencing the part’s reliability.
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