What is surface finishing in manufacturing?
A surface is what we touch when holding an object, such as a manufactured part. Designers specify section sizes relative to different surfaces to each other. These nominal surfaces represent the contour around the part’s design surface, defined by the lines in the engineering drawings.
Surface Texture
Surface texture includes repeated and/or random deviations from the nominal surface of the object; it is defined by the following four features: roughness, undulation (corrugation), surface coverage (the textures, structures on the surface are shaped in different shapes, lying flat) and spread evenly across the surface) and cracks.
Roughness refers to small, evenly spaced deviations from the nominal surface, determined by the material properties and the process of forming the surface. The ripple is defined as the deviation of the larger distance.
They occur due to bending, vibration (oscillation, movement), heat treatment and similar factors. Roughness is the superposition of ripples. Surface coating form is the dominant direction (conspicuous, prominent, and dominant) or the geometrical shape of the texture, surface structure. It is determined by the manufacturing method used to create the surface, usually resulting from the action of the cutting tool.
Surface Roughness and Surface Finishing
Surface roughness is a property that can be measured based on roughness deviation as defined in the previous section. Surface finish is a more subjective term that denotes the smoothness and overall quality of a surface. In common application, surface finish is often used as a synonym for surface roughness.
The most commonly used property to measure surface texture is surface roughness.
In there:
Ra = arithmetic mean of roughness, m (in);
y = longitudinal deviation from the nominal surface (converted to absolute value), m (in);
Lm = specified distance over which surface deflection is measured.
Vertical deviations: Vertical deviations (the undulating height of the surface)
Actual surface: Actual surface
Nominal surface: Nominal surface
In there:
Ra has the same meaning as above (Ra = arithmetic mean of roughness, m(in));
yi = longitudinal deviation (converted to absolute value) and is determined by the index i, m (in);
n = number of offsets included in the distance range Lm.
The units in these equations are meters and inches.
In fact, the scale of the deviation is very small, so the more appropriate units are μm (μm = mx 10 -6 = mm x 10 -3 ) or μ-in (μ-in = inch x 10 ) or μ-in (μ-in = inch x 10 ) -6 ). These are commonly used units to express surface roughness.
AA roughness values
The AA method is the most widely used arithmetic averaging method for surface roughness at present. Another alternative, sometimes used in the United States, is the root mean square (RMS), which is the square root of the mean of the squared deviations over the measured length. RMS surface roughness values will always be larger than AA roughness values because larger deviations will be more prominent in the RMS value calculation.
How to measure surface roughness?
Surface roughness is also flawed by any single measure used to evaluate a complex physical property. For example, it does not take into account the effect of surface covering shapes; Therefore, the surface roughness can vary greatly, depending on the direction chosen, the location chosen, and the distance to be measured.
Another shortcoming is that ripple can be included in the Ra calculation. To get around this, a parameter called shear length is used as a screener that separates the ripple in a measured surface from the roughness offset.
In practice, the cut length is the sampling distance along the surface. Sampling distances shorter than the wave width will eliminate the ripple-related longitudinal deviations and include only the roughness-related factors. The most common cut length used in practice is 0.8 mm (0.030 in).
The limitations of surface roughness
The limitations of surface roughness have prompted the development of additional measures that more completely describe the topography of a given surface. These measures include three-dimensional rendering of the surface, as described in section.
The notation specifying the parameters of the surface texture is a check mark (looks like a square root sign), with entries as specified for mean roughness, undulation, shear length, and surface coating shape and maximum roughness distance.
Surface Roughness Symbol
What is the symbol for surface roughness? There are 3 symbols: the rough symbol does not specify the machining method, the rough symbol indicates the cutting method, and the roughness symbol indicates the chipless machining method, respectively, are shown as follows.
Basic symbols
On the basic symbol, there are 4 positions to record the following parameters with h being the height of the font size on the drawing.
Location 1
Write the value of Ra or Rz (if you write the parameter Ra, you do not need to write the parameter symbol)
Position 2
Record special processing method (scraping, grinding, polishing)
Symbol on position 2
Position 3
If it is necessary to specify the reference length, write the selected length in this position.
4 positions
Record undulation direction. On the surface of the work piece, there are usually the following undulating directions:
- Parallel undulating direction
- Perpendicular undulation direction
- The undulating direction intersects each other
- undulating direction any EMA
- Direction of circular undulation
- CHEAP radial undulation direction
- The roughness of unmachined surfaces is denoted by the sign.
- On older drawings, surface textures are shown in grades with an inverted triangle symbol. It is the designation of the surface gloss according to the old Soviet standard.
Ghness conversion table between inches and meters. And between the old Soviet system and the present.
Roughness Symbol On Each Face
The roughness symbol of each surface on the drawing is recorded only once on the visible contour, or extension of the visible contour, with the sharp tip of the symbol directed at the surface to be recorded.
General roughness symbol for the whole drawing
If all surfaces of the part have the same roughness level, write the general roughness symbol in the upper right corner of the drawing.
General roughness symbol for the whole drawing
Roughness symbols for most faces throughout the drawing
If most surfaces of the part have the same roughness level, the common symbol is in the right corner of the drawing and enclosed in parentheses.
Roughness Symbol for a face with two different roughness
If on the same surface there are two different roughness levels, use a thin solid line to draw the dividing line, the dividing line must not be drawn on the material dashed line of the section.
Roughness symbol on the surface of coal teeth
The roughness of the tooth surface, the shank key is recorded on the dividing surface, when there is no frontal figure on the drawing.
Roughness symbol on threaded surface
The thread working surface roughness designation is indicated next to the thread tip diameter size or thread profile.
To understand what is surface roughness?
Surface roughness nature
Surface roughness is also known as surface gloss. The surface of the part after machining is not ideally flat but has undulations. These bumps are the result of plastic deformation of the surface layer of the part when cutting the metal layer. Is the effect of the cutting motion, is the blade mark left on the machined surface and many other causes…
However, not all surface bumps belong to surface roughness. It is a set of bumps with relatively small steps and is considered within the limit of the standard length (which is the length of the part of the surface chosen to measure the surface roughness).
- The bumps with the ratio between the step of the bump (p) and the height of the bump (h) 50 belong to the surface roughness, and the bump has a height of h 3
- The bump that 50 p/h ≤ 1000 belongs to the surface wave, the bump has a height h 2
- The bump that p/h > 1000 belongs to the shape deviation, the bump has height h 1
Surface Roughness Importance:
Then the roughness will be flattened, the larger the roughness, the greater the leveling, the more redundancy of the joint will be reduced, reducing the durability of the joint. The selection of Rz in accordance with the characteristics of the joints can be according to the following empirical formula where δ is the redundancy:
- When mounting diameter Φ > 50 mm: Rz = (0.1 – 0.15)δ ( μm)
- When mounting diameter Φ 18-50 mm: Rz = (0.15 – 0.2)δ ( μm)
- When mounting diameter Φ < 18 mm: Rz = (0.2 – 0.25)δ ( μm)
For parts with large redundancy when pressing two parts together
Surface roughness greatly affects the working quality of machine parts
For details in dynamic joints (sliding bearings, guide rails, sliders…). Therefore, the larger the roughness, the more difficult it is to ensure the formation of lubricating oil film on the sliding surface. Under the effect of load, the rough peaks come into contact with each other, causing semi-wet friction, even dry friction.
This leads to a decrease in working efficiency and an increase in the working temperature of the joint. On the other hand, at the contact peaks, the concentrated force is large, the stress is too great to exceed the allowable stress, causing melt deformation, which destroys the contact surface, and the working surface wears out quickly. The shorter the initial wear period, the shorter the service life of the part.
For parts working under cyclic loads and loads
Then roughness is a stress concentration factor that easily causes cracks to reduce fatigue strength of the part. At the bottom of the undulations is the place where the stress is concentrated with a very large value. There will appear micro cracks – that is the cause of detail damage. So if the standard surface roughness increases. The bottom radius of the large bump will improve the fatigue strength of the part.
The smaller the grit, the smoother the surface
Better resistance to corrosion. Another intuitive way can explain it by a phenomenon that we often see: the smoother the surface, the longer it takes to rust.
Evaluation criteria and surface roughness standards
Surface roughness is assessed by the protrusion of the profile. It forms by the intersection between the real surface and the plane perpendicular to the real surface. The standard for roughness assessment is the geometrical elements defined within the standard length l. Calculated against the surface profile average.
The reference curve has the shape of the nominal profile of the surface. Divide the real profile within the standard length l such that the sum of squares of the distances from the points of the real profile to this line is minimized. In other words, the standard curve is the line that divides the real profile into two parts. Have the same total area of convex vertices and concave bases F 1 +F 3 +F 5 =F 2 +F 4 +F 6
The reference length is the length of the part surface selected for roughness measurement. No other undulations with steps greater than standard length l are involved. The standard specifies standard lengths with the following values 0.01; 0.03; 0.08; 0.25; 0.8; 2.5; 8; 25 mm
- Average arithmetic error of amplitude (profile) Ra: is the arithmetic mean deviation of the absolute values of the profile error in the standard length range. The wrong profile is the distance between the points to the moving average.
- Average height according to 10 points Rz: Is the average height of 5 distances from 5 highest peaks to 5 lowest bottoms of roughness. Calculated within standard length.
Surface roughness is expressed by surface roughness
According to TCVN 2511: 1995, 14 roughness levels and values of roughness parameters Ra and Rz are specified. The smaller the roughness value, the smoother the surface will be. The selection of the Ra and Rz criteria depends on the required quality of the surface and the texture properties of the surface.
In production, Ra is often used to evaluate surfaces with average roughness from (level 6-12). For surfaces with roughness (grade 13-14) or very fine (grade 1-5), the Rz criterion is used. because it evaluates more accurately.
How to determine roughness level?
The Rz criterion is also used for surfaces where the Ra parameter cannot be directly checked, for example. Small sized surfaces or complex profiles (tool cutting edges, clock details, etc.).
Depending on the working conditions and the use properties of the detailed surfaces, the roughness level is determined. The contact surfaces require roughness parameters with small values. Non-contact surfaces require high roughness parameters. The higher the dimensional accuracy, the smaller the roughness parameter required.
Part surfaces have different roughness, requiring different machining methods. The smaller the surface roughness value, the more sophisticated machining is required.
Tolerances and dimensions
In addition, surface roughness is related to dimensional tolerances and shape tolerances.
Methods for checking surface roughness
There are two methods of checking the surface roughness: by sample comparison or by machines due to roughness.
Sample comparison method
Types of surface comparison pattern inspection methods are used to compare surfaces by probe or by sight. The condition for comparison is that the comparison specimen and the work piece are of the same material and have the same manufacturing method, e.g. longitudinal turning. Probe comparisons are made with fingernails or small pieces of copper (the size of a coin). Visual comparison is more convenient when looking at the right angle of light and using a magnifying glass.
Devices due to roughness:
Working surface measuring devices with profile measurement method (probe cutting method). Record surface deviations with a diamond probe tip.
The ideal shape of the probe tip is a cone (60° or 90°) with a rounded top. at roughness Rz > 3 um choose the tip with peak radius r = 5 µm, at Rz > 50 µm choose r = 10 µm. With roughness depth Rz < 3 m, the recommended value for the peak radius is 2 µm. Because of the smaller peak radius, it is possible to better detect the small depressions of the profile.
In this detection system, the probe picks up the roughness profile relative to the path of the slider. Most of the wave is filtered out mechanically through a 25 mm radius of the slider.
Portable measuring device with slider detection system.
Probe system with reference surface
Also known as a freewheeling system, a slider (guide bar) is very precise. In the feeder device form the reference surface. Through tilt adjustment, the reference face is adjusted as parallel as possible to the surface of the work piece.
When the D profile (primary profile) is not filtered, it shows an overly inclined position. Then you have to adjust the standard face better. Through the relative motion of the transducer relative to the reference plane, all parameters can be measured.
Features of the surface roughness measurement system
Also known as a freewheeling system, a slider (guide bar) is very precise. In the feeder device form the reference surface. Through tilt adjustment, the reference face is adjusted as parallel as possible to the surface of the work piece.
When the D profile (primary profile) is not filtered, it shows an overly inclined position. Then you have to adjust the standard face better. Through the relative motion of the transducer relative to the reference plane, all parameters can be measured.
- The radius of the probe limits the ability to detect very small grooves.
- Probe with movable slider only captures roughness.
- Detector systems with reference faces can measure roughness, waveforms and components of shape deviation.
Measurements of different measuring instruments can only be compared if information about the measurement method can be known. Examples are the detection system, the radius of the probe and the profile filter.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.