We will explore the basics of flatness within the GD&T framework in this article, examining how it functions as a critical tool for ensuring the precision of a surface.
What is Flatness?
Flatness is the deviation of the macro-convex and concave heights of a substrate relative to an ideal plane. It is an indicator that limits the variation of an actual plane relative to its ideal plane, used to control the shape error of the measured surface. Flatness belongs to the category of form errors in geometric dimensioning and tolerancing (GD&T), its tolerance zone is the region between two parallel planes separated by the tolerance value t.
How to Annotate Flatness?
As shown in the figure (omitted), this indicates that any 100*100 area of the surface must lie within two parallel planes separated by a tolerance value of 0.1mm.
As shown in the figure, this indicates that the workpiece surface must lie within two parallel planes separated by a tolerance value of 0.08mm.
How to Measure Flatness?
In the measurement of mechanical geometric errors, the measurement of flatness is crucial. It involves comparing the actual measured surface with an ideal plane; the linear distance between the two is the flatness error value. Alternatively, it can be determined by measuring the relative height differences of several points on the actual surface and converting them into a flatness error value expressed as a linear dimension. Common methods for measuring flatness include:
Optical Flat Interference Measurement
This method uses the working surface of an optical flat to represent the ideal plane, directly determining the flatness error of the measured surface based on the degree of curvature of interference fringes. This method is primarily used for measuring small surfaces, such as the working surfaces of gauges and the measuring faces of micrometer anvils. While traditionally suited for small surfaces, the development of optical flat interferometers has enabled this method to be used for larger planes as well.
Operating steps include：
- Place the optical flat: Place the optical flat on the surface to be measured, ensuring a very small wedge angle is formed between them.
- Light source illumination: Use a monochromatic light source to produce interference fringes. The position of these fringes depends on the angle at which the light is incident.
- Observe interference fringes: If the fringes are straight, parallel, and evenly spaced, the surface flatness is excellent. Curved fringes indicate poor flatness.
- Error calculation method: The error value is calculated by multiplying the ratio of the fringe curvature to the distance between adjacent fringes by half the wavelength of the light wave. White light is often used as the source, with a typical wavelength of 0.6 μm.
Dial Indicator Measurement
The workpiece and a micrometer/dial indicator are placed on a standard surface plate, using the plate as the measurement datum. The indicator is moved along the actual surface point-by-point or along several linear paths. This method is widely used in machining and automotive manufacturing for detecting geometric errors like flatness and straightness. It is simple to operate and low-cost, making it suitable for rapid on-site inspections. However, results are heavily influenced by operator skill, and precision is relatively low, making it suitable for general inspections.
Operating steps include：
- Preparation: Place the workpiece on the surface plate and adjust the pointed supports and dial indicator to ensure the measurement surface is leveled.
- Measurement process: Level the plane (using methods such as the three-point, four-point, or horizontal method), then measure 9 equally spaced points on the surface. The difference between the maximum and minimum readings on the indicator is recorded as the approximate flatness error.
- Data processing: The error value is the difference between the maximum and minimum readings, which serves as the basis for evaluating flatness.
Liquid Level Measurement
This method uses a liquid surface as the measurement datum, formed by liquid in “communicating vessels,” and is measured using sensors. It is primarily used for measuring the flatness error of large surfaces, such as large motor test platforms and flexible welding platforms. It provides relatively accurate results for large planes and is simple and cost-effective. However, measurement time is long, it is sensitive to temperature, and it is only suitable for planes with lower precision requirements.
CMM Measurement
Using coordinate measuring machines (CMM) allows for fast and accurate flatness measurement, suitable for applications with high-precision requirements. Its advantages include high precision, automation, and efficiency.
Operating steps include：
- Installation and calibration: Mount the workpiece on the CMM and perform zero-point calibration and datum setting to ensure accuracy.
- Select measurement points: Select multiple points on the surface. Generally, more points provide a more realistic reflection of the surface’s flatness. Regardless of the method used, at least 4 points must be measured for flatness.
- Data measurement and import: Use the CMM or an optical measuring machine (OMM) to measure the object directly, record the coordinate values, and import the data into computer software.
- Data analysis and error correction: Software improves accuracy through data analysis and error correction. Specifically, feature points can be calculated and constructed according to the “minimum condition” to derive the flatness error.
- Result evaluation: Select the plane for evaluation in the software and input the tolerance to obtain the result. If needed, the software can output the results and display graphs of single-point deviations and form errors.
How Getzshape Can Help
Getzshape delivers high-quality custom CNC machining, sheet metal fabrication, electrical discharge machining and more. Leveraging advanced equipment and strict quality control, we ensure accuracy and on-time delivery for prototypes to large production runs. As your end-to-end manufacturing partner, we streamline sourcing, machining, post-processing, and logistics.
The table below shows the flatness tolerances by demonstrating typical ranges for CNC machined parts.
| Nominal length, mm | Tolerance Class H | Tolerance Class K | Tolerance Class L |
| ~10 | 0.02 | 0.05 | 0.1 |
| 10~30 | 0.05 | 0.1 | 0.2 |
| 30~100 | 0.1 | 0.2 | 0.4 |
| 100~300 | 0.2 | 0.4 | 0.8 |
| 300~1000 | 0.3 | 0.6 | 1.2 |
| 1000~3000 | 0.4 | 0.8 | 1.6 |
Getzshape will ensure the tight flatness for your valued, customized CNC machined projects by CMM inspection.
