Everything you need to know about surface roughness

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Everything you need to know about surface roughness

Whether it’s CNC Machining, injection molding, metal casting, or 3D printing services.  You can see that each services has a different surface roughness, The surface roughness of products is a topic that cannot be removed.  If the products you plan to order also have surface roughness requirements, then please continue to read.  

In this article, we will discuss surface roughness in detail. We will explore what surface roughness is, how it is formed, and how it can be assessed. Additionally, we will discuss some of the most common representations of surface roughness. We hope that this article provides you with a better understanding of surface roughness and its effects on engineering surfaces!

What is surface roughness?

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Surface roughness refers to the roughness of the machining surface of parts with small spacing and small peaks and valleys. In engineering, surface roughness is defined as small, local deviations of the surface from the nominal form. The deviations may occur due to manufacturing processes (machining, casting, etc.), or they may be naturally occurring (oxidation, corrosion, etc).

These local surface irregularities can have a major impact on the function and performance of engineering surfaces. For example, surface roughness can cause increased friction and wear, reduced fatigue life, and decreased corrosion resistance.

How is surface roughness formed?

There are many factors that can contribute to the formation of rough surfaces. In machining processes.For example, the friction between the tool and the surface of the parts in the process, the plastic deformation of the surface metal when the chip is separated, and the high-frequency vibration in the process system, the discharge pit of the electric machining, etc.

Due to the different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.

Other common surface roughness formation factors include:

  • Material properties (hardness, ductility, etc.)
  • Manufacturing process parameters (cutting speed, feed rate, etc.)
  • Machine tool condition (alignment, rigidity, etc.)
  • Workpiece setup (clamping force, fixturing, etc.)

Surface Roughness VS Surface finish, What is the difference?

Surface roughness is a measure of the irregularities on the surface of a material. It is quantified by the surface waviness parameters, which are used to describe the surface profile. it always has an absolute value, this is independent of requirement or human percepiton.

Surface finish is not measurable, It can only be expressed qualitatively(using attribute). No such formula is available to directly estimate surface finish.

The main difference between surface roughness and surface smoothness is that surface roughness is a statistical parameter while surface smoothness is an optical will now talk about surface roughness measurement.

How to Measure Surface Roughness?

There are three main surface roughness assessment basis: visual inspection, stylus profilometry, and optical interferometry.

Visual inspection

It is the most common surface roughness assessment method. It is quick and easy to do, but it is also relatively subjective. As a result, visual inspection is only suitable for surface roughness assessment in general terms.

Stylus profilometry

It uses a mechanical stylus to trace the surface of the part. The resulting data can be used to generate a surface profile, which can then be analyzed to quantify the surface roughness. This method is more objective than visual inspection, but it is also more time-consuming and expensive.

Optical interferometry

It is the most accurate surface roughness assessment method. It uses light to measure surface deviations, and can achieve sub-nanometer accuracy. However, optical interferometry is also very expensive and requires specialized equipment.

Standard execution sheet for surface roughness measurements
Document
Ra / μm >0.008~0.02 >0.02~0.1 >0.1~2.0 >2.0~10.0 >10.0~80
Sampling length / mm 0.08 0.025 0.08 2.5 8.0
Evaluation length / mm 0.4 1.25 4.0 12.5 40
Sampling length:

The sampling length LR is the baseline length used to evaluate the surface roughness. The sampling length should be selected according to the actual surface formation and texture characteristics of the part, which can reflect the surface roughness characteristics, and the sampling length should be measured according to the overall direction of the actual surface contour.

Evaluation length:

Evaluation length LN is the length necessary to evaluate the profile, which may include one or more sampling lengths. Because the surface roughness of each part of the surface is not uniform, a sampling length can not reflect the characteristics of a certain surface roughness reasonably, so it is necessary to take several sampling lengths on the surface to evaluate the surface roughness.

Common surface roughness representations:

There are three main ways to represent surface roughness: graphical, numerical, and textural.

Graphical representations of surface roughness are usually in the form of a surface profile. This can be generated using stylus profilometry data, or it can be estimated from a drawing or photograph.

Numerical representations of surface roughness are typically in the form of statistical parameters. The most common surface roughness parameters are:

Arithmetic mean surface roughness (Ra)

Set a length L on a surface contour curve and take the center of the length as the X-axis. Divide the sum of the areas of all oblique lines within the length by the measured length L. Is the Ra.

Root mean square surface roughness (Ry)

Set the length L on the surface contour curve. The vertical distance from the highest peak to the lowest trough of the curve in this length is the maximum rough value Rmax/Ry.

Maximum surface roughness (Rz)

Set the length L on the surface contour curve, and measure the distance between the top of the fifth peak and the bottom of the fifth trough at the center of the curve in this length, namely Rz.

Textural representations of surface roughness are usually in the form of a surface texture map. This is generated by measuring the surface roughness at a large number of points, and then mapping the data onto a grid.

Surface Roughness Chart

Surface roughness chart

The surface roughness chart is a graphical representation of surface roughness. It is generated by stylus profilometry, and it shows the surface profile of the part.

The surface roughness chart has two axes: the X-axis represents the distance along the surface, and the Y-axis represents the surface height. The surface roughness is quantified by the surface roughness parameters, which are shown on the surface roughness chart.

The surface roughness chart is a valuable tool for surface roughness assessment, because it can be used to compare different parts, or to compare the same part before and after processing.

SURFACE ROUGHNESS CONVERSION CHART

Document
Ra
(micrometers)
Ra (microinches) RMS (microinches) CLA (N) Rt (microns) N Cut-off Length (mm)
0.025 1 1.1 1 0.3 1 0.08
0.05 2 2.2 2 0.5 2 0.25
0.1 4 4.4 4 0.8 3 0.25
0.2 8 8.8 8 1.2 4 0.25
0.4 16 17.6 16 2.0 5 0.25
0.8 32 32.5 32 4.0 6 0.8
1.6 63 64.3 63 8.0 7 0.8
3.2 125 137.5 125 13 8 2.5
6.3 275 250 125 25 9 2.5
12.5 500 550 500 50 10 2.5
25.0 1000 1100 1000 100 11 8.0
50.0 2000 2200 2000 200 12 8.0

Selection of surface roughness

You need to consider the purpose of the surface finish. The surface finish must be appropriate for the intended purpose of the part. For example, a high surface roughness is usually required for parts that need to resist wear or friction.

Indicative surface roughness comparisons

Document
Manufacturing Process Ra μm >Ra μin(CLA)
TUBE FINISHING
Cold Drawn 1.6 ~ 3.2 63 ~ 125
Hot Extruded 25 ~ 37.5 1000 ~ 1500
Smooth Bore 0.4 ~ 0.8 16 ~ 32
Electropolished 0.1 ~ 0.4 4 ~ 16
METAL CUTTING
Sawing 1.6~ 25 63 ~ 1000
Planing, Shaping 1.6~ 12.5 63 ~ 500
Drilling 1.6~ 6.3 63 ~ 250
Milling 0.8~ 6.3 32 ~ 250
Boring, Turning 0.4~ 6.3 16 ~ 250
Broaching 0.8~ 3.2 32 ~ 125
Reaming 0.8~ 3.2 32 ~ 125
ABRASIVE
Grinding 0.1~ 1.6 4 ~ 63
Barrel Finishing 0.2~ 0.8 8 ~ 32
Honing 0.1~ 0.8 4 ~ 32
Elector-Polishing 0.1~ 0.8 4 ~ 32
Electrolytic Grinding 0.2~ 0.6 8 ~ 24
Polishing 0.1~ 0.4 4 ~ 16
Lapping 0.05~ 0.4 2 ~ 16
Superfinishing 0.025~ 0.2 1 ~ 8
FORMING
Hot Rolling 12.5~ 25 500 ~ 100
Forging 3.2~ 12.5 125 ~ 500
Extruding 0.8~ 3.2 32 ~ 128
Cold Rollming, Drawing 0.8~ 3.2 32 ~ 128
Roller Burnishing 0.2~ 0.4 8 ~ 16
OHTER
Flame Cutting 12.5~ 25 500 ~ 1000
Chemical Milling 1.6~ 6.3 16 ~ 250
Electron Beam Cutting 0.8~ 6.3 32 ~ 250
Laser Cutting 0.8~ 6.3 32 ~ 250
EDM 1.6~ 5.0 16 ~ 200

Will now talk about how to select surface roughness.

  1. The selection of surface roughness parameters should not only meet the functional requirements of the parts surface, but also consider the economic rationality.

  2. Specific selection, can refer to the existing similar parts drawing, with analogy method to determine. Under the premise of meeting the functional requirements of the parts, the larger surface roughness parameter value should be selected as far as possible to reduce the machining cost.

  3. Generally speaking, the working surface of the parts, the matching surface, the sealing surface, the friction surface with high movement speed and high unit pressure, etc., requires high level and smoothness of the surface, and the parameter value should be smaller.

  4. For non-working surface, non-mating surface and surface with low dimensional accuracy, the parameter value should be the relation between parameter Ra value and machining method and its application examples, which can be used for reference.

SURFACE ROUGHNESS CONVERSION CHART

Document
Micrometers Rating Microinches Rating Applications
25 100 A rough, low surface resulting from sawing or rough forging. Therefore, the surface is suitable for some unmachined clearance areas.
1.25 500 These are rough, low-grade surfaces that cause roughage and heavy cuts. While cutting comes from turning, milling, disc grinding, and more.
6.3 250 This type of surface finish results from surface grinding, disc grinding, milling, drilling, and more. Therefore, they are suitable for interstitial surfaces with stress requirements and design permits
3.2 125 The coarsest surfaces are usually recommended for parts. It is also used for parts subjected to vibration, load and high stress.
1.6 63 Good machine roughness/finish, produced under controlled conditions. It also involves fine feed and relatively high speed.
0.8 32 Advanced machine polishing requires close control. It is relatively easy to produce with cylindrical, centerless or flat grinders. It is also preferred for products that do not require continuous motion or heavy loads.
0.4 16 High quality surfaces are usually polished, ground, or coarsely honed using emery. These finishes are therefore a good choice and smoothness is very important.
0.2 8 Fine, high-quality surface finish produced by grinding, polishing, or honing mechanics use this method where rings and fillers must slide over surface particles.
0.1 4 Polishing: A refined surface obtained by grinding, polishing or honing is used by the manufacturer only when mandatory design requirements are required. Therefore, it is the best finish in the measuring and instrumentation industry.
0.05 2 The finest surface finish produced by the finest polishing, honing, or superfinishing. Therefore, they are most suitable for fine and sensitive precision measuring blocks.
0.025 1 The finest surface finish produced by the finest polishing, honing, or superfinishing. Therefore, they are most suitable for fine and sensitive precision measuring blocks.
What is the difference between Rz andface Roughness (Ra)?

Rz is the maximum surface roughness, while Ra is the arithmetic mean surface roughness. Rz is a single point measurement, while Ra is an average of several measurements. Rz gives you a good idea of the highest and lowest points on the surface, while Ra gives you a good idea of whatitable surface roughness values depend on the application. For example, a high surface roughness is usually required for parts that need to resist wear or friction. A low surface roughness is usually required for parts that need to resist corrosion or have a good appearance.

Effect of surface roughness on component performance

Friction & Wear 

The rougher the actual surface of the part is, the larger the friction coefficient is, and the easier the part is to be woar.

Assembly Capability

When the interference fit of the parts, the peak of the parts will be squeezed flat in the process of assembly pressing, which reduces the actual effective interference amount and reduces the fit connection strength.

Brasive Resistance

Rough surface, it is easy to accumulate corrosive substances in the concave valley, and then gradually penetrate into the inner layer of the metal material, causing surface rust. The greater the depth of the valley, the more severe the corrosion.

Fatigue Strength

The rougher the part surface, the more serious the stress concentration caused by surface indentation. Especially when the parts are subjected to alternating loads, the possibility of fatigue fracture due to stress concentration is greater.

Conclusion

Surface roughness is an important surface finish parameter. It is a measure of the surface texture of a material. The surface roughness chart is a valuable tool for surface roughness assessment, because it can be used to compare different parts, or to compare the same part before and after processing.

You need to consider the purpose of the surface finish when selecting surface roughness parameters. If you have any questions about surface finish or surface roughness, please contact us and we will be happy to help.Thank you for reading!

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