{"id":9757,"date":"2021-03-30T17:21:08","date_gmt":"2021-03-30T14:21:08","guid":{"rendered":"https:\/\/fractory.com\/?p=9757"},"modified":"2024-01-26T14:58:15","modified_gmt":"2024-01-26T12:58:15","slug":"flatness-gdt-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/flatness-gdt-explained\/","title":{"rendered":"Flatness (GD&T) Explained"},"content":{"rendered":"

The fourteen types of geometric tolerances in GD&T are divided into 5 main types of control. These are form, location, orientation, profile, and runout.<\/p>\n

Form controls determine the shape of individual features in a part. They consist of the following four types of geometric tolerances – straightness, flatness, circularity<\/a> and cylindricity<\/a>.<\/p>\n

In this post, we shall learn about the flatness callout and how to use it in the right place for maximum efficiency.<\/p>\n

What is Flatness?<\/h2>\n

Many applications need parts with a flat surface. No surface is perfectly flat but using GD&T<\/a>, we can develop parts with a surface that is flat enough<\/em> for our application.<\/p>\n

The flatness callout controls the uniformity of a surface or a median plane as needed. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. All the points on the specified surface must lie between these two planes for part approval.<\/p>\n

As flatness refines a surface, we can also use it in a tolerance stack<\/a> without interfering with other requirements.<\/p>\n

Flatness vs Other Characteristics<\/h2>\n

It may seem like flatness is very similar to other geometric as well as regular tolerances in terms of the final result. So let’s do some 1:1 comparisons in order to make sure the difference is clear to everybody reading this article.<\/p>\n

Flatness vs straightness<\/h3>\n

\"Tolerance<\/p>\n

Flatness is the 3D equivalent of the surface straightness control<\/a>. While straightness has parallel lines<\/strong> representing its tolerance zone, the flatness tolerance zone is formed by two parallel planes<\/strong>.<\/p>\n

Thus, while straightness only makes sure that a single line on a surface has to be within the limits, GD&T flatness does the same for a collection of lines – a surface.<\/p>\n

Flatness vs parallelism<\/h3>\n

These two are often confused. Parallelism<\/a> is not a standalone callout. It needs another feature such as an axis or a surface to relate to. It cannot function without a datum.<\/p>\n

On the other hand, flatness does not need a datum. We can use flatness on a surface that is not parallel to any other surface, so there is no reference point to compare the result with other than the closed system itself.<\/p>\n

Flatness vs surface finish<\/h3>\n

This probably causes the most confusion out of these comparisons.<\/p>\n

While both control surface variations, the surface finish does it on a much finer scale. The measurement for the surface finish is shown as an average while for flatness, the difference between the maximum height and depth is shown as the worst case.<\/p>\n

Flatness vs regular tolerancing<\/h3>\n

\"Thickness<\/p>\n

The image above has a tolerance of +\/- 0.1 mm for thickness. All in all, this gives exactly the same result in terms of flatness – it guarantees it as does the one below.<\/p>\n

\"Flatness<\/p>\n

But this one has both the flatness callout AND a +\/- tolerance for part thickness. As you can see, the flatness is still inside the same limits – 0.2 mm in total. But now the thickness of the part can vary up to 0.4 mm in both ways or 0.8 mm in total.<\/p>\n

Thus, flatness can be achieved without restricting any other dimensions<\/strong>, making it easier to obtain and lowering the total cost.<\/p>\n

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\n Scale Your Manufacturing from Prototyping to Series<\/span>\n\n