Measure flatness

Measuring the flatness of surfaces is of great importance in various applications. Some examples are the testing of sealing surfaces between two bodies, such as the cylinder head gasket in automobiles. Measuring flatness is also an important factor in the production of preliminary products in the metal industry, such as the production of axis guides for measuring systems, or in the production of wafers in the semiconductor industry.

Definition flatness

Flatness is the difference between the highest and lowest points of a surface located between two ideal flat (planar) surfaces. The distance between the two planar surfaces defines the flatness tolerance.

In the real world, flatnesses of a few nanometers can be achieved, e.g. for highly polished flat glass surfaces. For metallic measuring objects, which are manufactured in large quantities using the usual production methods, flatness tolerances of a few µm to a few tenths of a mm are common.

Principle of flatness measurement of a surface

Basics of flatness measurement

A surface is flat if it has no structures in its surface over its entire length and width. These structures can look as follows:

Mountains and valleys as interruptions of a centerline of length or width.
Cracks are non-continuous interruptions of varying depth in the plane of the base material.
Wrinkles are elongated elevations.
Scores and scratches are mechanical damage to the surface caused by sharp objects.
Waves are alternation of mountains and valleys.
Roughness is the regularly recurring and very small deviation in shape along a wavy line.
Bumps are convex interruptions along the plane.
Pores usually have a concave shape.

There are optical (non-contact) or tactile methods for measuring a plane for planarity.

Methods of flatness measurement

Optical methods include:
- Chromatic Confocal Metrology
- White light interferometry
- Laser scanning
The following methods are available for tactile measurement
1. 3-D coordinate measuring method
2. Tactile step method

The differences of the methods

Optical chromatic confocal metrology uses the effect of different refraction of different wavelengths in glass.
Measurements with white light and laser interferometry uses the scattering behavior of reflected light to determine a surface structure.

In laser scanning, the optical measurement method used by QuellTech, a surface is continuously scanned with a laser line. The diffuse reflections of the laser line are recorded by a sensor. From the profile data recorded in this way, a computer can record a 3D point cloud of a scanned surface. Laser scanning as an optical measuring method is therefore particularly well suited for checking the surface flatness of starting material, profiles, rails and strip material.

The tactile scanning step method, on the other hand, performs the flatness measurement with the aid of a contacting measuring head. This measurement is very well suited for very small measuring distances where maximum precision is required. Since the probe head usually has a spherical shape with a certain diameter, it cannot plunge into any small valley of a few µm.

When measuring a flatness by the tactile 3-D coordinate measuring method, it is necessary that a tactile contact of the surface with a measuring head is required.

Flatness measurement with QuellTech

QuellTech uses the laser scanning measurement method. Laser profiles with up to 4096 points per profile are recorded. Accuracies of up to 1-2 µm can be achieved.

In contrast to tactile measuring methods, it is possible to record several million measuring points in a few seconds on an area of e.g. 50 x 100 mm: Flatness measurement in highest precision.
This high number of measuring points enables a precise and complete recording of the flatness in a short time. The advantage over tactile measurement is that a laser light can penetrate the smallest surface structures.

Furthermore, the laser scanning method can also be used to measure coplanarity. For example, one laser scanner scans the top surface of a target and a second laser scanner simultaneously scans the same bottom surface of the target. In this way, the parallelism of these two surfaces to each other can be calculated.

3D point cloud of the flatness measurement of the surface at a sealing surface

What are the advantages of laser scanning flatness measurement from QuellTech?

High resolution, accuracies down to 1µm
High speed: acquisition of a 3D point cloud with millions of measurement points in a few seconds
Laser light penetrates the finest structures
Robust against external disturbance variables due to differential method
Skew of the surface in the room can be easily corrected

Application areas for flatness measurement

Production of preliminary products in the metal industry
Installation in metal plate straightening machines for inline flatness measurement
Final inspection of linear axis guides
Testing of sealing surfaces for exhaust systems and hydraulic flanges
Production of wafers in the semiconductor industry.
Testing the coplanarity of metal bodies
The measurement of measuring plates

Examples of flatness measurement with QuellTech

In the past, we have used Quelltech solutions for flatness measurement in a wide variety of projects. Here you can find a small excerpt of these projects:

Precisely measure the flatness of surfaces with these QuellTech high-performance laser scanners:

Q6 Laser scanner

The QuellTech Q6 Laser Scanner product series has a proven track record in demanding industrial applications that require high levels of precision, process stability and high travel speeds.

Further information: Q6 Laser scanner

Q5 Laser scanner

The QuellTech Q5 laser scanner series combines advantages of small form factors with high resolution and fast scan rate. The Q5 is suitable for measurement applications with limited space and rugged environmental requirements.

Further information: Q5 Laser scanner

Q6 Laser Scanner

Q5 Laser Scanner

Can QuellTech laser scanners solve your measurement task?

If you have any questions about flatness measurement or would like advice from QuellTech on this topic, we would be happy to help.

We would like to help you evaluate your specific measurement task. By performing an initial free test measurement of your application, we can make an early assessment of feasibility.

There is always potential for improvement, we will help you with that. Contact us for more information or to make an appointment for a consultation.

Your personal contact:

Stefan Ringwald
Tel: +49 89 124 723 75

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