What is roughness profile?

Surface roughness is the most commonly used parameter to describe surface microtopography in machining. It reflects the microscopic geometric shape error of a machined part's surface. With the development of the machining industry, surface roughness measurement technology has also made significant progress. Especially since the mid-to-late 1970s, with the increasing popularity of microcomputers and the advancement of modern optical and laser technologies, roughness measurement technology has become increasingly important in precision machining industries such as machining, optical processing, and electronics.

Surface roughness measurement methods can be generally divided into two categories: contact and non-contact. Contact methods primarily include comparison, die impression, and stylus methods. Non-contact methods commonly used include light sectioning, speckle pattern analysis, astigmatism, optical heterodyning, AFM, and flying optical sensor methods.

Contact Profilometers

Contact surface roughness profilometers use the stylus method. The stylus method, also known as the stylus tracing method, is a method in which a very sharp stylus (diamond stylus tip with a radius of micrometers) is placed vertically on the surface to be measured and moved horizontally. The stylus will move vertically along the contour of the surface to be measured.

This tiny displacement is converted into an electrical signal through a circuit and amplified and processed to obtain the roughness parameter value of the working surface. It is mainly divided into several types, such as inductive, piezoelectric, and inductive. This instrument has the advantages of good stability, objective and reliable readings, and is easy to use. Its vertical resolution can reach several nanometers.

Advantages:

Large measurement range, high resolution, stable and reliable measurement results, and good repeatability.

Disadvantages:

(1) The hardness of the diamond measuring head is generally very high, which can easily scratch the workpiece and is not suitable for measuring high-quality and soft material surfaces.

(2) In order to meet the wear resistance and rigidity requirements of the measuring head, the measuring head cannot be made too small and sharp, which can easily affect the measurement accuracy.

(3) When measuring the microscopic surface contour, in order to ensure the accuracy and lateral resolution in the scanning path direction, the feed step is very small, so the measurement speed is not high.

Working Principle

A stylus-type surface roughness profilometer consists of a sensor, a drive box, an indicator, a recorder, and a worktable. The inductive sensor is one of the primary components of the profilometer. A stylus (mostly made of diamond due to its wear resistance and high hardness) is mounted at one end of the sensor's measuring rod. The tip of the stylus requires a very small radius of curvature to fully capture the surface conditions. During measurement, the stylus tip rests on the workpiece's surface, maintaining perpendicular contact with the surface. The sensor is then moved at a slow, even speed by a drive mechanism.

Because the surface being measured has a contour with peaks and valleys, the stylus moves up and down along the surface as it slides across it. This motion is then transmitted to the magnetic core through the fulcrum using the principle of leverage, causing it to move up and down synchronously in opposite directions within the inductor coil. This motion is amplified, causing the inductance of the two differential inductor coils surrounding the core to change, converting the stylus's minute vertical displacement into a synchronous, proportional electrical signal.

The sensor's coil and measurement circuitry are directly connected to a balanced bridge fabricated later. Changes in the coil's inductance cause the bridge to lose balance, generating an electrical output proportional to the stylus's vertical displacement. This electrical output is relatively weak and difficult to detect, requiring electronic amplification. After phase-sensitive detection, a signal representing the magnitude and direction of the stylus's displacement is generated.

This signal is then divided into three paths: one path is applied to an indicator to indicate the stylus's position; one path is fed to a DC power amplifier for amplification and recording; and one path, after filtering and amplification by an averaging amplifier, enters an integrator for integration, resulting in a direct readout of the surface roughness parameter from the indicator.

The measurement range of this instrument typically ranges from Ra 0.02 to 10μm, though a few models can measure even smaller values. The instrument is equipped with a variety of accessories to accommodate surface measurements on workpieces with various shapes, including flat surfaces, internal and external cylindrical surfaces, conical surfaces, spheres, curved surfaces, small holes, and grooves. Measurements are quick, convenient, and highly accurate.

Non-contact

Non-contact surface roughness profilometers measure surface roughness by indirectly obtaining information about the surface without affecting the surface topography. The greatest advantage of this method is that the probe of the measuring device does not come into direct contact with the surface, protecting the device and avoiding measurement errors that would otherwise be introduced.

Non-contact surface roughness profilometers measure surface roughness by indirectly obtaining information about the surface without affecting the surface topography. The greatest advantage of this method is that the probe of the measuring device does not come into direct contact with the surface, protecting the device and avoiding measurement errors that would otherwise be introduced.

Roughness profiler

FAQ

Q1: What is roughness?

Surface roughness is a measure that describes the micron-level peaks and valleys on a part's surface. Whether it's high-precision CNC-machined parts or blanks from injection molding or casting, they all contain microscopic irregularities invisible to the naked eye. These tiny fluctuations can affect part performance in terms of friction and wear, seal leakage, coating adhesion, and fatigue crack initiation. Therefore, precise control of surface roughness is crucial to ensuring product quality and extending product life.

Q2: What are some common roughness parameters?

In practical engineering applications, selecting the appropriate roughness parameter is crucial. Ra (arithmetic mean deviation) is the most common evaluation metric, measuring the average of the absolute values ​​of all deviations on either side of the profile centerline to provide an average roughness reading for the entire surface. Rz (ten-point height) focuses more on extreme peak-to-valley differences, averaging the distance between the highest peak and the lowest valley. It is suitable for applications where deep valleys or sharp peaks are a concern and could cause scratches or fatigue.

In addition, Rq (root mean square deviation) uses the square root of the sum of squared deviations and is more sensitive to large deviations. Rmax (maximum peak-to-valley height) precisely quantifies the maximum single peak-to-valley difference within the measurement length and is often used to detect single-point defects. To comprehensively evaluate part performance, both Ra and Rz are often referenced, taking into account both overall smoothness and extreme characteristics.

Q3: How to Use Roughness Charts?

Charts compare different manufacturing processes with typical Ra values, making it easy for designers and process engineers to quickly compare and select parts. During the preliminary design phase, the roughness level can be determined based on functional requirements. When marking drawings, refer to ISO 1302 standard notation to accurately indicate the Ra value, measurement length, and surface orientation to ensure consistent processing and inspection.

Q4: What is the difference between surface texture and roughness?

Surface "texture" encompasses all undulating characteristics of a part's surface, including roughness, waviness (long-wave undulations caused by machine tool vibration or workpiece thermal deformation), and orientation (the dominant direction of the machining path on the surface). Surface "roughness" specifically refers to the short-wavelength, microscopic peaks and valleys, which are the core indicator affecting friction, sealing, and fatigue performance. To comprehensively evaluate part surface performance, roughness, waviness, and orientation must be considered and fully expressed in ISO notation on the drawing to ensure high consistency between design intent and machining results.

Roughness: Short-wavelength fluctuations determine friction and adhesion performance;

Waviness: Long-wavelength fluctuations can affect smooth motion;

Lay: The main direction of the machining path, which can affect lubrication and coating adhesion.

Surface roughness has evolved from an "inspection item" to a "design parameter." Driven by intelligent manufacturing, online adaptive control, and digital twin technologies, future factories will achieve real-time roughness monitoring and closed-loop optimization, thereby improving product performance while reducing manufacturing costs and shortening delivery cycles.