Flatness in GD&T: Ein detaillierter Leitfaden

Geometric dimensioning and tolerancing is a standard system. It helps in conveying the engineering tolerances and the intent of the design with the help of engineering drawings. The symbol of flatness is also a key consideration for controlling the tolerances of the flat surfaces.

The article below has a detailed discussion on the flatness in GD&T that will help you use it in your components. So, let's read it.

What is Flatness?

Flatness in GD&T is a type of engineering tolerancing tool that helps to control the size of the design. The flatness tolerance is opted for such surfaces that have a flatness that needs to be opted in certain bounds so that the component can function properly. It is a measure for controlling the surface form.

Flatness vs Other GD&T Features

Flatness can be compared with some other GD&T features as follows:

Flatness vs Straightness

Flatness is a three-dimensional characteristic that controls the surface, whereas straightness is a two-dimensional characteristic that controls one single line on the surface. Flatness has its tolerance zone, which is defined by two planes parallel to one another. Straightness has its tolerance zone defined by lines parallel to one another.

Flatness vs Parallelism

Flatness and parallelism are sometimes mixed. Parallelism needs a surface or axis with which it can be related; it cannot work without a datum. Whereas flatness does not require a datum, and it can be used on a surface that is unparalleled to another surface. Hence, the need for a reference point is not required.

Flatness vs Surface Finish

Flatness and surface finish both control the variations in the surface. However, the surface finish does this control in a precise way. Flatness, on the other hand, shows the difference between the path and maximum height.

Flatness vs Regular Tolerancing

Flatness is a tolerance type that manages the consistency of the surface, regular tolerancing determines deviation from design.

How to Show Flatness on a Drawing?

The GD&T flatness tolerance on a drawing is shown using a feature control frame. The first block contains the geometric characteristic symbol for the flatness and it is in the form of a parallelogram.

As the tolerance zone for flatness is a wider one, the symbol is not required in the second block due to it being the default zone for the tolerance type. The second block has material modifiers and tolerance values as needed.

The flatness doesn't need a datum for references and the leader arrow refers to the surface which is under the control. Sometimes, it may point towards the size dimension, indicating that the median plane is under the control of flatness.

What is Flatness at Maximum Material Condition?

Flatness at a maximum material condition is a valid callout that is applied to a feature of size, like width. Rather than the flatness control for the surface, it comes from the median plane instead. The call-out is used by designers to control the size, which is tighter than the entry form.

The tolerance zone restricts the countless features in the given measurements. Such an arrangement is not in effect when the flatness callout with MMC adds to the size of the tolerance using geometric tolerance.

Measuring the Tolerance

The final measurement of the tolerance can be checked using different methods, and these are as follows:

Single Planar Surfaces

Flatness measurement needs a surface plate, probe, or gauges for height. It cannot be measured by just placing the part on the slab.

Using a height gauge

The height gauge is used for measuring the flatness and for the reference features held parallel. Advanced CMM machines are used for inspecting the flatness. They can create virtual planes that look like the surface that is to be checked and measured.

The entire area is covered with the height of the gauge, and positive and negative elements are added to calculate the total variance. This variance needs to be less than the flatness tolerance value.

Using a surface plate

A surface plate inspects the flatness by holding the face on the surface plate using a height gauge. The height gauges and the components are moved in a way that the entire width and length of the surface are covered. The variance is calculated for the total surface.

Feature of Size (Flatness at MMC)

When measuring the flatness, the measurement of the derived median plane is basically conducted. There are two methods of measuring it.

Using a functional gauge

This method uses two height gauges, which are placed opposite to one another. The height gauge is used on the bottom and top surfaces and measures the local thickness. It is then moved along the surface to see if the entire surface is within the size tolerance or not.

The second method uses a gauge that has a cavity that fits the plate at the virtual condition boundary. It is the total available limit for tolerance. In order to be approved, the plate must fit in the gauge.

Using a CMM

CMM makes measurements and it requires covering the same plate, which is posted in a way that the probe reaches the two surfaces. The points are then marked, and the local thickness of these points is measured. If the thickness is within the size limits, the midpoint is calculated.

The flatness is calculated by subtracting the maximum local tolerance of the plate. If the median plane flatness variance is less than the allowed flatness tolerance, the path is approved.

Benefits of Using Flatness Tolerance

There are various benefits of using flatness tolerance, and some of these are as follows:

• Flatness tolerance ensures that parts can fit together, which reduces the chances of misalignment. Many sensitive applications like automobiles and aerospace require such a flatness tolerance.
• Flatness tolerance helps minimize stress, wear and tear, and friction, especially when chosen for the moving components.
• When the flatness is controlled in a component, the overall quality of the components increases and it contributes to the consistency in performance.
• The need for excessive post-processing is eliminated by having flatness control, which reduces the waste of material as well as energy.

Where Can Flatness Be Applied?

Flatness tolerance is opted for by various industries where the precision of components is crucial. The automobile industry, electronics and Luft- und Raumfahrt are such sectors where appropriate assembly and low friction are a concern in components. Some of the applications where flatness is applied are as follows:

• Dichtungen
• Flanges
• Optical parts
• Mating components

Practical Significance of Flatness in Manufacturing

The significance of flatness manufacturing cannot be disregarded; it plays a crucial role in determining if the component will function or not. The section below sheds more light on this subject.

Quality Control and Consistency

Flatness maintains consistency across the entire production run, ensuring that components meet the standards. It is one of the crucial requests in mass production. The use of measurement systems like CMM also helps in measuring flatness, ensuring that components of high quality are produced in each batch.

Cost Implications

Appropriate flatness control also eliminates the need for post-processing like Schleifen, which causes a lot of material waste. When the components meet the standards of flatness, the production efficiency is enhanced and also the need for extra spending is reduced.

Impact on Functionality

Flat surfaces offer functionality in various applications. Some mechanical components like gaskets when manufactured with appropriate flatness reduce friction between moving components and prevent stress.

Schlussfolgerung

The significance of flatness in GD&T plays a vital role not only in enhancing the aesthetics of the components but also in its functionality. The components can be assembled appropriately, the cost of post-processing is saved, and durability is achieved.

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