Ra, Rz, Rt Surface Roughness Measuring & Finish

Contents Overview of Lapping Techniques The Lapping Process Insights on Lapping Hand Lapping Explained Single-Plate Lapping Machine Conditioning Mechanisms Functionality of Pressure Plates Workholding Methods Speed Dynamics Machine Variants Centerless Cylindrical Lapping and Its Characteristics The Lapping Oil and Powder Quality of Surface Finish Production Accuracy Factors Analyzing Lapping Idling Times Polishing Process Cost Calculation Measurement Techniques for Laser Flatness Instrumentation for Working Plate Assessment Workpiece Types Suitable for Lapping Diamond-Based Lapping

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Quality of Surface Finish

When employing a specific grit size and fluid viscosity, adjusting lapping pressure can lead to different material removal rates, while also affecting film thickness and overall surface finish. Initially, the pressure is typically kept low and gradually increased throughout the lapping operation, tapering off towards the conclusion of the process. This method optimizes material removal, ensures superior surface finish, and achieves exceptional flatness, ultimately perfecting overall surface finish quality.

Waviness, characterized by peaks and valleys, denotes larger surface variations occurring beyond the scope of surface roughness. Such irregularities generally emerge from factors like warping, vibrations, or deflections during machining.

Connecting Grit Size and Surface Finish

To illustrate, when machining a steel component hardened to 60 HRc, applying silicon carbide grit of 500 and a pressure of 250 g/cm² results in a surface finish of approximately Ra= 0.2 my (N4) or Rz 0.6-0.8. Conversely, reducing the pressure to 50 g/cm² can yield a surface finish nearing Ra= 0.05 my (N2) or Rz 0.2-0.3. This correlation highlights the interdependence between grit size and surface finish quality.

It’s crucial to understand that surface roughness refers to minute height variations on any given material. The average of these variations defines the Ra value, while Rz quantifies the maximum deviation from peak to valley. Surface roughness is typically measured in microns. For instance, a surface exhibiting Ra of 8 will average peaks and valleys that do not exceed 8 μm over a specified distance. Additionally, roughness can be evaluated by comparing the workpiece surface with a known standard.

International standards such as DIN and ISO clearly define surface finish quality in terms of Ra or, more accurately, Rt metrics. In practical applications, Rz is often noted as it results from averaging five independently measured Rt values. Numerous commercial measuring devices exist to obtain these readings (refer to Figure 54).

Understanding Flat Surfaces and Their Readings

Ra denotes the arithmetic mean of all absolute roughness deviations from the centerline across the measurement length, while Rz represents the average peak to valley height calculated over five successive sampling lengths. It’s essential to note that Ra accounts for all dimensions without differentiating between rejected and acceptable surfaces.

The mean roughness value, Ra (DIN), calculates the average deviation of the roughness profile R from the mean line across the measuring distance lm.

Maximum peak to valley height Rt (DIN) identifies the vertical distance between the highest and lowest peaks in the roughness profile R over the total measuring distance lm, effectively highlighting the elevation difference within the specified range.

Mean roughness depth Rz (DIN) is the calculation of average individual roughness depths across five sequential measuring distances, focusing on peak and valley deviations from the mean line.

Systems for Designating Surface Roughness

Comparative tables for machine manufacturers and surface profiles illustrate various roughness metrics (Figure 51 showcases turned steel workpiece Ra 7.51, Rt 31.1). Figures 52 and 53 display the same workpiece post-lapping with applied silicon carbide and diamond, respectively, showcasing significant improvements in surface metrics Ra and Rt.

Measuring Instruments

Understanding the distinction between surface roughness and flatness is paramount in measurement. Current electronic instruments for surface finish quality typically rely on microprocessor systems and integrated printers (as shown in Figure 54). However, the accuracy of results can fluctuate, contingent upon the specific device utilized. It's critical to consider factors such as probe type, needle pressure, measuring distance, and filtering methods (refer to DIN Standard).

Factors such as workpiece material, microstructure, hardness, and machining type must also be actively assessed, especially concerning measuring distance orientation against machining traces. Remarkably, even a minimal pressure of 1 mN applied via a diamond probe with a 5-micron radius can compress a non-ferrous material's surface by up to 50% of its roughness depth.

For materials such as oxide ceramics or sintered metals, it's vital to consider microstructural porosity. Bearing ratios are often measured at varied roughness levels and expressed as percentages. Visual inspections through comparisons of polished versus unpolished surfaces provide practical insights into texture differences.

Figure 57 captures a matte-lapped aluminum component under magnification with the corresponding measuring diagram.

Figure 58 showcases the same workpiece post-polishing using fine paper, illustrating material removal and corresponding measurements.

Related Topics

Introduction to Surface Roughness Measurement

Surface roughness parameters are defined rigorously by international standards. However, obtaining accurate results heavily relies on the metrologist's selection of various parameters.

Key Definitions

To grasp this topic, several key definitions are necessary:

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• Roughness: This quantifies the process marks resulting from the creation of the surface, and other factors like material structure, distinct from Waviness or Form, which possess longer wavelengths.
• Filter: This is a process utilized to eliminate frequencies below or above specific thresholds. Measurement systems may include mechanical filters, while software can implement mathematical filtering.
• Cut-off: This denotes the wavelength at which a filter becomes effective, typically analyzed within upper and lower cut-off ranges, designated Ls or λs (shortest) and Lc or λc (longest). The term cut-off is often interchangeable with Sample Length.

Identifying Potential Issues

As represented in Figure 1, the profile demonstrating a roughness wavelength of 0.25mm with an Ra around 20 μm can yield contrasting results based on cut-off settings (Lc). For instance, an Lc setting of 0.08mm may suggest near-zero Ra, while Lc at 0.25mm reports 10 μm, and at 0.8mm or 2.5mm provides the true value of Ra=20 μm. Setting Lc at 8mm or more will lead to inflated Ra values due to the incorporation of larger waviness into the analysis.

Fundamental Principles

Understanding filtering concepts in surface roughness profiling is crucial. Selecting the appropriate stylus tip and filters based on the specifics of the surface in question is vital. Typically, a singular measurement is conducted to evaluate surface texture.

Choosing Lc/ λc

The selection of λc hinges on the evaluated process, supported by guidelines like ISO - (refer to the accompanying table). Ultimately, the selection of cut-offs should be informed by the spacing of profile features (peaks and valleys) attributed to the machining process, designated as RSm. A general rule is to set λc to approximately five times this spacing.

Choosing Ls/ λs/Bandwidth

The ISO introduced the bandwidth concept in dated formats. In this structure, shorter wavelengths for surface roughness analysis are governed by a specific short wave filter (termed λs). Bandwidth is thereby managed in relation to surface features rather than the measuring system limitations. An optimal bandwidth of 300:1 is recommended, enhancing measurement reliability.

Filter Selection

The Gaussian filter is typically preferred for its phase accuracy and rapid roll-off without undesirable ripple effects. Taylor Hobson Ultra software offers both 2CR and 2CR-PC filters, available for backwards compatibility with older instruments.

The filter choice’s effect may seem subtle, resulting in minor percentage changes in roughness parameters. However, these variations can become significant when working towards consistency across diverse sites or instruments.

Feel free to reach out for a copy of our booklet, 'A Guide to Surface Texture Parameters', for additional technical insights.

Content reproduced with full acknowledgement from Taylor Hobson Ltd.

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