Surface Metrology: Understanding Texture and Measurement Parameters

Functional Areas and Surface Types

Functional Areas: Those that directly affect the operation of the machine. They may be in contact with other surfaces or moving about. Require careful finishing.

Support Surfaces: The underlying or supporting surfaces. Require finish grinding.

Free Surfaces: Those that have no special function and are not mechanized.

Types of Defects

• Macrogeometric Defects: Defects related to position (e.g., planarity) and ripple (periodic, $\ge 1 \text{mm}$).
• Microgeometric Defects: Roughness, including creases and stretch marks (random, $< 1 \text{mm}$).

Surface Area Definitions

• Theoretic Area: The area designed on the plans.
• Real Surface: The surface before the machining process.
• Effective Surface: The surface after machining.

Profile Definitions

• Geometric Profile: Design profile in the background.
• Real Profile: The profile of the billet of the piece.
• Machined Profile: The profile after machining.

Ridge Profile Parameters

$Z_p$ (Crest Height): Height of the crest between two consecutive intersections of the profile with the X-axis.

$Z_v$ (Valley Depth): Depth of the valley.

$Z_T$ (Total Profile Element Height): $Z_T = Z_P + Z_V$.

Length Parameters

Basic Length ($l$): The length of the geometric profile used to assess roughness.

Length of Assessment ($l_m$): The length measured for surface quality assessment.

Roughness Parameters Based on Basic Length

$R_p$: Crest height within a basic length.

$R_m$: Valley depth within a basic length.

$R_y$: Total of $R_p$ and $R_m$ within a basic length.

Average Roughness Parameters

$R_a$ (Arithmetic Mean Roughness): The arithmetic mean of the absolute values of the distance "y" of the roughness profile from the midline.

$R_z$ (Mean Peak-to-Valley Height): Mean distance between the 5 highest peaks and 5 deepest valleys: $\frac{z_1 + z_2 + z_3 + z_4 + z_5}{5}$.

Maximum and Total Roughness

$R_{max}$ (Maximum Roughness): The maximum value of roughness found in the measurement path, considering the amplitude of each segment.

$R_t$ (Total Height of Profile): The height difference between the highest peak and the deepest valley across the entire measurement path.

$R_p$ (Mean Peak Height): The sum of the highest peaks in each core length divided by the number of basic lengths.

$P_i$ (Maximum Peak Height): The highest peak across the entire measurement path.

Bearing Surface

Bearing Area: The area remaining after removing irregularities following the finish process.

Bearing Surface Ratio: The relationship between the bearing area and a smooth surface (100%). This represents the parts that are smooth to the touch and bear load.

Roughness Measurement Instrumentation

Probe Movement and Sensing

Progress Unit: Its role is to move the probe at a constant speed and in a straight line on the surface to be measured.

Inductive Sensor: Generates more tension as the vertical displacement increases.

Piezoelectric Sensor: Similar to the inductive sensor but uses a different mechanism.

Optoelectronic Sensor: Varies the captured light intensity based on vertical displacement, transmitting energy accordingly.

Probe Tip: Formed by a cone with a defined angle.

Signal Processing Components

Transducer: Translates the vertical oscillation signal.

Amplifier: Amplifies the signal.

Analog-Digital Converter: Converts the analog signal to a digital signal.

Profile Recorder: Maintains the flange height reference for the probe's vertical movement.