A COMPREHENSIVE GUIDE TO SURFACE ROUGHNESS IN ENGINEERING DRAWINGS

Audio: A Comprehensive Guide to Surface Roughness in Engineering Drawings

So far, we have covered a range of topics on engineering drawings, from introduction to fits. In this article, we are going to talk about surface roughness in engineering drawings.

Introduction to surface roughness

At its core, surface roughness refers to the minute variations or deviations in a material’s surface profile compared to a perfectly smooth, ideal surface. These deviations can be in the form of peaks, valleys, and other irregularities that occur on a microscopic scale. But what causes these irregularities, and why are they crucial in engineering and manufacturing? Let’s explore.

Origins of surface roughness:

• Manufacturing processes: Almost every manufacturing process, whether it’s machining, casting, forging, or additive manufacturing, introduces some level of surface roughness. For instance, milling might leave behind tool marks, while casting could result in slight granular irregularities.
• Material properties: The inherent properties of materials can also influence roughness. For example, the grain structure in metals or the composite’s fibrous structure can lead to natural surface irregularities.
• Environmental factors: Over time, external factors like wear, corrosion, or even the deposition of contaminants can alter a surface’s roughness.

Scales of roughness:

Surface roughness can be categorized based on the scale of observation:

• Macro roughness: This is a roughness that can be felt by touch or even seen by the naked eye. Examples include the texture on a concrete wall or the grooves on a vinyl record.
• Micro roughness: This refers to irregularities that are typically observed under magnification. It’s this scale that engineers often refer to when discussing surface finish in manufacturing.
• Nano roughness: With technological advancements, we can now observe and measure roughness at the nanometer scale. This is especially relevant in industries like semiconductors or advanced coatings.

Why measure surface roughness?

The importance of measuring and specifying surface roughness lies in its impact on the functionality and performance of a component. For instance:

A hydraulic cylinder needs a specific roughness to retain a thin film of oil, ensuring smooth motion and reducing wear.
In optical applications, a mirror might require an extremely smooth finish to reflect light accurately.

Understanding surface roughness is not just about achieving a smooth finish. In many cases, a controlled level of roughness is desirable. For example, certain adhesive bonds might require a roughened surface to achieve optimal adhesion.

In essence, surface roughness is a nuanced and multifaceted parameter that plays a pivotal role in determining the performance, aesthetics, and longevity of a wide range of products and components.

Surface finish, surface roughness, and surface texture

Surface finish, surface roughness, and surface texture are terms that are often used interchangeably in various industries. However, they have distinct meanings, especially in the context of manufacturing and metrology. Let’s break down the differences:

Surface texture

Definition: Surface texture is the broadest term among the three. It refers to the intricate pattern or arrangement of the surface’s deviations from its nominal or ideal form. Surface texture encompasses all the irregularities you’d find on a surface, and it’s a combination of all the microscopic features present.
Components: Surface texture can be broken down into three main components: waviness (larger-scale, more spaced-out irregularities), roughness (finer, closely spaced irregularities), and flaws (defects or unintended deviations like scratches or pits).
Representation: The surface texture is often represented using profile traces, which are graphical representations of the surface’s deviations across a specified length.

Surface roughness

Definition: Surface roughness is a subset of surface texture. It specifically refers to the finer, closely spaced irregularities on the surface. These irregularities are usually introduced by the manufacturing process.
Measurement: Surface roughness is quantified using parameters like Ra (average roughness), Rz (mean peak-to-valley height), and several others. These parameters provide specific values in micrometers or microinches that describe the surface’s roughness.
Importance: Surface roughness is often a critical parameter in manufacturing because it can influence properties like friction, wear resistance, and the ability of surfaces to form seals.

Surface finish

Definition: Surface finish is a more general term that refers to the overall quality or appearance of a surface. While it often includes considerations of surface roughness, it can also encompass other attributes like coating quality, presence of defects, and aesthetic appeal.
Context: The term “surface finish” is often used in contexts where the visual or tactile quality of a surface is of importance. For instance, in consumer products, a “good surface finish” might refer to both the tactile feel and the visual appeal of the product.
Representation: Surface finish might be specified using roughness parameters, but it can also include specifications related to coatings, treatments, or even visual inspections.

In summary:

Surface texture is the all-encompassing term that describes all the deviations and irregularities on a surface.
Surface roughness specifically looks at the finer, closely spaced irregularities and is quantitatively measured.
Surface finish is a broader term that considers the overall quality or appearance of a surface, which can be influenced by roughness but also includes other factors.

The importance of surface roughness

Understanding and controlling surface roughness is vital for several reasons:

Friction control:
- Bearings: In bearings, the right surface roughness ensures that there’s adequate lubrication retention while minimizing direct metal-to-metal contact, reducing wear and heat generation.
- Clutches & brakes: The roughness of clutch and brake surfaces can influence the coefficient of friction, affecting the performance and lifespan of these components.
Wear and tear:
- Gear teeth: The roughness of gear teeth can influence how gears mesh and wear over time. A smoother finish can reduce pitting and other wear mechanisms.
- Engine cylinders: The walls of engine cylinders are intentionally textured to a specific roughness to aid in oil retention, reducing wear on the piston rings.

Aesthetic appeal:
- Consumer electronics: The feel of a smartphone or a laptop’s surface can influence a consumer’s perception of its quality and luxury.
- Automotive finish: The paint finish on cars, especially luxury models, demands a very smooth surface to achieve a mirror-like reflection.
Sealing efficiency:
- Hydraulic pistons: The surface roughness of hydraulic pistons and their cylinders can influence how effectively they seal, impacting the efficiency and reliability of hydraulic systems.
- O-rings: The mating surfaces for O-rings need to have a specific roughness to ensure a proper seal without excessive wear on the O-rings.
Thermal conductivity:
- Heat sinks: The surface roughness of heat sinks can influence their efficiency. A smoother surface can improve contact with components, enhancing heat transfer.
- Engine components: In components like pistons, a specific roughness can aid in heat transfer, ensuring the component doesn’t overheat.
Adhesion:
- Paints & coatings: The roughness of a surface can influence how well paints and coatings adhere to it. A slightly rough surface can improve adhesion, ensuring the coating doesn’t peel off easily.

- Medical implants: Implants like hip replacements often have a specific roughness to promote bone adhesion and growth around the implant.
Fluid dynamics:
- Pipelines: The roughness of the interior of pipes can influence flow rates and efficiencies, especially in viscous fluids or high-speed flows.
- Aircraft & automobile bodies: The surface roughness of these vehicles can influence drag and overall aerodynamic/hydrodynamic efficiency.
Stress concentrations: Rough surfaces have peaks and valleys. These microscopic irregularities can act as stress concentrators. When a material is subjected to stress, these points of irregularity can experience higher localized stresses than smoother areas, making them prime locations for crack initiation.
Crack initiation sites: The inherent irregularities in a rough surface provide numerous sites for crack initiation. As a result, rougher surfaces might experience multiple crack initiations, which can merge and form larger, more detrimental cracks.
Crack path deviation: Interestingly, a certain level of surface roughness can actually impede the straight-line propagation of a crack. When a crack encounters a rough or irregular surface, it might be forced to find the path of least resistance, causing the crack to deviate or branch. This can increase the energy required for the crack to propagate, potentially slowing down its growth.
Fatigue life: In cyclic loading conditions, the initiation and growth of fatigue cracks can be influenced by surface roughness. Rougher surfaces can reduce fatigue due to the early initiation of cracks from stress concentrators.
Corrosion and environmental effects: Rough surfaces can trap contaminants or moisture, leading to localized corrosion. Corrosive environments can exacerbate crack propagation, especially if the roughness promotes the retention of corrosive agents.
Fracture mechanics implications: The energy required to propagate a crack can vary based on surface roughness. Rougher surfaces might require more energy to push a crack forward, especially if the crack path is deviated by the surface’s irregularities.

ISO standards for surface roughness

ISO 21920-1:2020 is the current international standard that specifies the terms, definitions, and surface texture parameters. This standard is a part of a series that focuses on the methodology and instruments for determining the surface texture.

However, the ISO 1302 standard is still widely used for specifying surface roughness. Moreover, most CAD software still uses ISO 1302 standard symbols to indicate the surface roughness.

For this reason, we will use ISO 1302 standard symbols, but we will draw a parallel with the ISO 21920-1:2020 standard symbols.

Key terminology

Section length (lr)

Definition: The length over which the surface profile is evaluated. It’s a critical parameter when measuring and interpreting roughness values.
Significance: Different section lengths can yield different roughness values for the same surface, so specifying and standardizing the sampling length is essential.

Note: ISO standard defines units of roughness measurements as µm (micrometer)

Ra (Average roughness)

Definition: The arithmetic average of the absolute values of the profile height deviations from the mean line, measured within the section length.
Significance: Ra is one of the most commonly used parameters to describe surface roughness. It provides a general sense of the surface’s overall roughness but doesn’t capture peak-to-valley extremes.

Rq (Root Mean Square (RMS) roughness)

Definition: The square root of the arithmetic mean of the squares of the profile height deviations from the mean line, measured within the section length.
Significance: Rq is similar to Ra but gives more weight to extreme values. It’s particularly relevant in applications where wear or fatigue is a concern.

Rz (Maximum height of the profile)

Definition: The average difference between the five highest peaks and the five lowest valleys in the profile.
Significance: Rz provides a measure of the extremes of the surface texture, making it useful for applications where these extremes might be critical, such as sealing surfaces.

Rt (Total height of the profile)

Definition: The distance between the profile’s highest peak (Rp) and lowest valley (Rv) within the section length.
Significance: Rt provides a single value that captures the full vertical extent of the surface irregularities.

Rsm (Mean spacing of profile irregularities)

Definition: The average distance between consecutive local peaks within the section length.
Significance: Rsm gives insights into the spacing of the surface features, which can be crucial for applications like lubrication retention.

Rmr (Material ratio curve)

Definition: Represents the ratio of the profile material above a certain depth. It can be used to determine the bearing ratio or the proportion of the surface that carries the load.
Significance: Rmr is particularly relevant in applications like sealing, where the proportion of the surface in contact is critical.

Waviness

Definition: The larger-scale, more spaced-out irregularities on a surface, typically resulting from factors like machine vibrations, heat deformations, or material structure.
Significance: Waviness doesn’t capture the fine details of the surface texture but is crucial in applications where these larger-scale deviations matter, such as optical components.

Lay

Definition: The predominant direction of the surface pattern, usually corresponding to the direction of the last machining process.
Significance: Lay can influence properties like friction and can be critical in applications like sliding bearings.

Representing surface roughness on drawings

The requirements for the surface quality are defined on the engineering drawings with the use of graphical symbols. In the table, we will show both the graphical symbols according to ISO 1302 and ISO 21920-1:2020.

Graphical symbols can be expended and populated with additional information depending on the other needed requirements:

a – individual requirement for the surface quality
a1 – tolerance types: upper limit (U) or lover limit (L)
a2 – as appropriate, the type of filter and the transfer characteristics
a3 – length of measurement distance
a4 – surface parameter (Rz, Ra, Rt, etc) with the required limit value in µm
b – if needed, the second requirement for surface quality
c – an indication of the production process, surface treatment, coating
d – a symbol for the direction of surface grooves
e – machining allowance in mm
f – an additional circle representing all around the application.

Additionally, let us now look at the symbols for the direction of the surface grooves:

Common roughness height values

We created a two-page document with common roughness height values achievable with different manufacturing techniques. Download it for free!

Tips on incorporating surface roughness in design

Here are some tips for incorporating surface roughness in design:

Understand the application: Always start by understanding the specific requirements of the application. Does the component need to slide, seal, transmit heat, or reflect light? The function will dictate the surface finish requirements.
Specify clearly: When detailing surface finish requirements on drawings, be clear and specific. Use standardized symbols and ensure that the required parameters (like Ra or Rz) are explicitly mentioned.
Material matters: Different materials respond differently to machining processes. For instance, soft aluminum might achieve a smoother finish than hard stainless steel under the same machining conditions. Always consider the material’s properties when specifying surface finish.
Consider post-processing: Sometimes, achieving the desired surface finish directly from the primary manufacturing process can be challenging or expensive. In such cases, consider post-processing methods like polishing, grinding, or honing to achieve the desired finish.
Balance cost and quality: Ultra-smooth finishes often come at a higher manufacturing cost. Always balance the need for a high-quality finish with budget constraints. Sometimes, a moderately smooth finish might suffice for the application, saving costs.
Think about assembly: If the component is part of an assembly, consider how the surface finish will interact with mating parts. For instance, two parts with very rough finishes might not mate well, leading to assembly issues.
Account for wear and tear: Components subjected to wear, like gears or bearings, will see their surface finish degrade over time. Design with a margin to account for this wear, ensuring the component still performs adequately as it ages.
Stay updated with standards: Surface finish standards, like ISO or ASME, are continually evolving. Stay updated with the latest revisions to ensure your designs are compliant with industry best practices.
Collaborate with manufacturers: Often, the people machining or manufacturing the parts have valuable insights into achieving the best surface finishes. Collaborate closely with them, seeking their feedback during the design phase.
Test and validate: Whenever possible, prototype and test your designs, especially if the surface finish is critical. Real-world testing can reveal issues that might not be apparent in theoretical analyses or simulations.
Consider environmental factors: In corrosive environments or where there’s a risk of contamination, the surface finish can play a role in the component’s resistance to these factors. For instance, smoother surfaces might be less prone to corrosion or bacterial growth.
Stay curious and keep learning: The world of manufacturing and materials science is continually evolving. New techniques, tools, and processes emerge that can offer better surface finishes or more cost-effective solutions. Stay curious and invest time in learning about these advancements.

By keeping these tips in mind, you can make informed decisions, ensuring their designs are robust, functional, and cost-effective.

Tips on understanding the real-life application of surface roughness

Visit manufacturing facilities/departments: One of the most effective ways to understand the real-life implications of surface roughness is to see it in action. Arrange visits to manufacturing facilities/departments that produce parts requiring specific surface finishes. Witnessing the machining processes firsthand, observing the tools used, and understanding machinists’ challenges can provide invaluable insights.
Engage with experienced professionals: Collaborate and communicate with seasoned professionals in the field, such as machinists, quality control inspectors, and materials scientists. These experts often have years of hands-on experience dealing with surface roughness issues and can provide practical insights that aren’t found in textbooks.
Analyze failed Components: If possible, study components that have failed in service due to issues related to surface roughness. Analyzing wear patterns, understanding the reasons for seal failures, or examining how surface corrosion initiates can provide a deeper understanding of the criticality of surface roughness in real-world applications.
Stay updated with industry case studies: Regularly read industry journals, attend webinars, and participate in workshops or conferences. Case studies presented on these platforms often delve into real-world challenges and solutions related to surface roughness.
Experiment and prototype: If you’re in a position to do so, create prototypes with varying surface finishes and test them under real or simulated conditions. Observing how different surface roughness values impact performance can be enlightening.
Join professional organizations: Organizations like ASME, SME, or ASM International often have resources, courses, and seminars dedicated to topics like surface roughness. Being a part of such organizations can provide access to a wealth of knowledge and networking opportunities.

Closing words

Surface roughness, texture, and finish are more than just metrics or parameters on an engineering drawing; they are the embodiment of the intricate interplay between design intent, manufacturing processes, material properties, and real-world functionality. As we’ve explored, the details of a component’s surface can profoundly influence its performance, longevity, and interaction with its environment and other components.

For mechanical design engineers, understanding and effectively specifying these parameters is not just about adhering to standards or achieving a desired aesthetic appeal. It’s about ensuring that the component or assembly functions as intended, remains reliable throughout its lifecycle, and delivers value, whether it’s in the form of efficient operation, reduced maintenance, or enhanced user experience.

Now you have an excellent overview of surface roughness in engineering drawings. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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