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The Role of Surface Finish in Machining Processes

surface finish

Table of Contents

In manufacturing processes, surface finish plays a crucial role in ensuring the durability and performance of a product. It involves modifying a surface’s texture, appearance, and properties through processes like machining, polishing, or coating. The benefits of doing this are numerous and include protection against corrosion, extending the product’s life span, and enhancing wear resistance. Surface finish also improves fatigue strength by minimizing stress concentrations that can lead to premature failure. Additionally, a well-finished surface enhances the product’s visual appeal and consumer perception. Overall, surface finish is a critical factor in manufacturing that can greatly impact the durability, reliability, functionality, and aesthetics of a product.

What to Expect with Surface Finishes

Surface finishes vary greatly depending on the type of machining process used. Whether the process involves cutting, polishing, or casting, each manufacturing method will interact with the part surface in a unique way. This unique interaction with the part surface can result in unique properties, roughness, and aesthetics. Therefore, it is important to remember that surface finish and manufacturing processes are deeply interrelated. Before selecting a manufacturing process for a project, forethought should be given to the desired surface finish. For example, the surface finishing resulting from cutting stock material by sawing will be different from the surface resulting from milling, grinding, or a final electro-polishing process. 

There are general characteristics related to surface finishes that help describe or characterize a surface finish. These include roughness, waviness, and lay. 

Roughness

Roughness is sometimes used interchangeably with the term surface finish, however it is actually just one of many surface finish characteristics. Roughness refers to the irregularities and deviations on the surface of a material. It quantifies the texture and unevenness of the surface at a microscopic level. 

Waviness

Waviness represents the larger-scale variations on a surface. It refers to the deviations that occur over a longer wavelength than roughness. Waviness can be caused by factors such as tool marks, machine vibrations, or inherent material variations. 

Lay

Lay describes the direction, alignment, and pattern of the surface texture. It is characterized by the predominant direction of the surface features, such as the tool marks left by machining or the alignment of the polishing strokes. The lay can significantly affect the performance and functionality of certain applications, such as sealing or friction.

Helpful insight can be found in our materials pages for what to expect for surface finishes when using our services. We encourage you to reference this information so that you can be prepared for color, texture, or finish variations. 

Surface Finish vs. Surface Roughness

Surface finish and surface roughness are related but distinct concepts. Surface finish refers to the overall characteristics of a surface, including its texture, appearance, and properties. It includes characteristics such as roughness, waviness, and lay. On the other hand, surface roughness specifically focuses on the microscopic irregularities and deviations on the surface. Roughness is a quantitative measure of the fine-scale variations in height between peaks and valleys on the surface. It provides information about the texture and smoothness at a small scale. While surface roughness is a component of surface finish, surface finish encompasses a broader range of factors.

How to Calculate and Measure Surface Roughness

In general, surface roughness is calculated by determining the arithmetic average of surface heights across the surface of an object. This is usually done along an “Evaluation Length.” Several measurement systems exist but mainly fall into the categories of direct measurement methods, non-contact methods, comparison methods, and in-process methods

  • Direct Measurement Methods
    • Direct Measurement Methods use a stylus that runs along the surface. A resulting registered profile determines the roughness. 
  • Non-Contact Methods
    • Non-contact methods use optical equipment, light, and optical sensors to measure the roughness.
  • Comparison Methods
    • Comparison methods take physical samples and compare them against surfaces of known roughness parameters. 
  • In-Process Methods
    • In-process methods use inductance and electromagnetic energy to make surface roughness measurements.

Surface Finishing Units and Symbols

Many parameters can be identified using the roughness measurement methods. These parameters represent numerical values that quantify different aspects of the surface roughness, waviness, or texture. They are typically expressed in units of length, such as micrometers (μm) or microinches (μin). They provide a standardized way of describing the characteristics of a surface’s finish. Some of the main surface roughness parameters are: 

  • Ra (Arithmetic Average Roughness): Ra represents the average of the absolute values of all measured surface roughness deviations from the mean line within a sampling length. It is the most commonly used parameter for surface roughness.
  • Rz (Average Maximum Height): Rz measures the average values of the five largest differences between peaks and valleys. The measurement is done using five sampling lengths. It helps to eliminate error since Ra is quite insensitive to some extremes. 
  • Rms (Root Mean Square Roughness): Rms is the root mean square of the measured surface height deviations from the mean line within a sampling length. It provides a measure of the dispersion of the surface roughness and is less sensitive to extreme peaks and valleys.
  • Rmax (Maximum Valley Depth): Rmax measures the depth of the deepest valley to the highest peaks within a given evaluation length. 

Surface Roughness Conversion Charts

A surface roughness chart is a visual reference tool used to compare the surface texture or roughness of a material with standardized samples that represent different levels of surface roughness. This provides a visual representation of how different surfaces appear and feel at various roughness levels. It helps individuals determine the desired or acceptable level of surface finish for a given application. The chart usually displays a range of roughness values or parameters, such as Ra, Rz, or specific surface texture symbols, along with corresponding visual representations of the surface. These visual representations may include textures like scratches, peaks, valleys, waviness patterns, or other common surface irregularities. 

A surface roughness conversion chart is a reference tool that allows for the conversion of surface roughness values between different measurement systems or units. It provides a convenient way to correlate and convert surface roughness parameters from one scale to another. Surface roughness measurement systems may use different units or parameters to express surface roughness, depending on the country or standard. For example, the Ra (Arithmetic Average Roughness) parameter is commonly used in the ISO system, while the Rz (Average Maximum Height) parameter is more prevalent in the ASME system. Additionally, different countries may have their own measurement units, such as micrometers (µm), microinches (µin), or roughness average (RMS).A surface roughness conversion chart typically includes a table that lists the different surface roughness parameters or units, along with the corresponding conversion factors or equations. By referring to the chart, users can convert surface roughness values from one unit or parameter to another, ensuring consistent interpretation of surface roughness data.

Surface roughness charts and conversion charts are commonly used in industries such as manufacturing, engineering, metalworking, automotive, and aerospace, where surface quality and performance are critical. They serve as a practical tool for assessing and specifying the desired surface finish in a clear and concise manner.

Below is an example of a simple surface roughness conversion chart comparing parameters between two common units, as well as demonstrating surface roughness ranges for commonly used production methods.

Production MethodRa (µm)Ra (µin)Rz (µm)Rz (µin)
Casting3.2 – 25125 – 100025 – 2001000 – 8000
Machining0.4 – 12.516 – 5003.2 – 50125 – 2000
Grinding0.1 – 3.24 – 1251 – 1640 – 650
Honing0.05 – 0.82 – 320.4 – 6.316 – 250
Polishing0.01 – 0.40.4 – 160.08 – 3.23 – 125

Please note that the values in microinches are approximate conversions from micrometers, and the ranges given are what you can generally expect. There may be slight variations based on specific measurement techniques and standards. It is always recommended to refer to the appropriate industry standards or consult with measurement equipment manufacturers for accurate and up-to-date surface roughness values.

Elevate Your Surface Finishing Partnering with SendCutSend

Surface finishing is of great significance in machining processes as it directly affects the quality and performance of machined parts. It helps to achieve desired dimensional accuracy, smoothness, and surface integrity. Proper surface finishing reduces friction, enhances wear resistance, and improves the functionality of the machined components. It also allows for better compatibility with other parts, facilitates assembly, and contributes to overall product aesthetics and end-user satisfaction.

We offer a variety of finishing services including anodizing, plating, and powder coating. In terms of surface finishing, we offer a free vibratory ceramic tumbling service to soften the edges of laser cut parts and remove burrs on small parts. We use specialized equipment that is harmonically tuned to the specific material we are tumbling which allows us to achieve a more consistent finish across the parts. We also offer free deburring to remove the worst of the dross, burr, and scratches.
Ceramic tumbling and deburring are not complete finishing processes and do not guarantee a uniform finish on the individual parts. However, they will prepare your parts for later surface finishes or chemical brightening. Check out our services page to learn more.

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Flush Standoff, 4-40, .250" Zinc plus Clear Chromate

Aluminum: 5052, 6061, 7075 Steel: Mild, G30

SKUSO-440-8
Thread Size4-40 x .250″
Hole size in sheet (+0.003/-.0.000).168″
Minimum sheet thickness0.040″
Maximum sheet thickness.125″
Fastener materialSteel
Minimum distance hole C/L to edge0.230″
When determining the distance between two or more fasteners, you can calculate the distance by the formula, C/L to edge + 1/2 the diameter of the second mounting hole..345″
Recommended panel materialSteel/Aluminum
Coating typeZinc
Length.250″
Aluminum material ranges (5052, 6061, 7075)0.040″-0.125″
Steel material ranges (CRS, HRPO, HR)0.048″-0.119″

We proudly use hardware by PEM

Flush Standoff, 4-40, .250" Passivated

Stainless Steel: 304, 316

SKUSO4-440-8
Thread Size440
Hole size in sheet (+0.003/-.0.000).166″
Minimum sheet thickness0.04″
Maximum sheet thickness.125″
Fastener material400 Stainless Steel
Minimum distance hole C/L to edge0.230″
When determining the distance between two or more fasteners, you can calculate the distance by the formula, C/L to edge + 1/2 the diameter of the second mounting hole. Example shown with x2 of the same hardware..345″
Recommended panel materialStainless Steel
Coating typePassivated
Length.250″
304 Stainless Steel material ranges0.048″-0.125″
316 Stainless Steel material ranges0.060″-0.125″