Flatness and surface profile lie in two different fundamentals of GD&T (out of a total of four) namely, flatness is categorized as “form”, while the surface profile is categorized as a feature of “profile”.
All “form” features are used for individual features; which is why they don’t require a datum for their complete definition. The case for tolerances of profile features is a little complicated as they may or may not require a datum for their control depending upon their applications.
Flatness
Flatness is a very simple and basic type of tolerance as compared to every other tolerance. Flatness determines how flat a geometrical feature or a surface (that is supposed to be exactly flat) really is without the reference of any other feature or datum.
In real-life applications, flatness is used in the manufacturing of many important products, for instance, in the production of wheel hubs and brake drums. In both of these products, the right and left side of the outer diameter needs to be perfectly flat for assembly. Otherwise, it would cause problems during operation. A brake drum having sides not having flatness up to a certain value may cause judder during operation.
Surface Profile
On the contrary, the surface profile controls the variation of a geometrical curve-like feature or shape as compared to the desired feature. This feature is used where an advanced curve-like feature needs to be controlled.
For the tolerance of surface profile, a datum may or may not be taken depending on its application.
One of the very important uses of surface profile tolerance is its use for defining the Coplanarity of two surfaces or features. Coplanarity is a condition where every element of any two or more surfaces or for a single interrupted surface exists on the same plane. To control Coplanarity, a tolerance zone consisting of two parallel planes is created and it is ensured that all the elements of the surfaces that need to be coplanar remain in this zone. The tolerance type used in the geometrical control feature for controlling coplanarity is the surface profile.
In real-life applications, tolerance of surface profile is critical in manufacturing those products that need to sit on planar interrupted surfaces or in manufacturing products with coplanar surfaces. A common example of such products is a clamper used for clamping various components in different assemblies.
In the manufacturing of these products, surfaces that need to sit adjacent to the flat surface products and surfaces that need to rest on equipment clamping ground need to be perfectly in line with each other for smooth and rigid clamping.
Correlations in Flatness and Surface Profile
A great correlation exists between flatness and surface profile as there are applications where both of these tolerance types could be used, and the definition for manufacturing specification of the desired product is complete.
Both of these tolerances are also similar in the way that both of these tolerances are used to avoid irregular surfaces. Such irregular surfaces can have very pointy peaks, which could damage the surface of the part assembled with them. If these pointy peaks are too high, there is a chance that the layer of corrosion-resistant paint cannot cover the irregular surface completely, leaving parts of that surface open, which can lead to corrosion of the whole product.
Differences Between Flatness and Surface Profile
The difference between these two types of tolerances can also be stated in terms of their significance, amount of usage, datum requirements, shapes of tolerance zones, procedure and tools for measurement and symbols used to denote them.
The flatness is a “form” controlling tolerance, while the surface profile controls the “profile” type of tolerances.
The usage of flatness is more common as compared to surface profile as flatness is applied to almost every product with flat surfaces, while the surface profile is only used for complex curves/shapes and for coplanar surfaces.
The flatness never requires a datum for its complete definition, while the surface profile may require a datum depending upon its application.
These tolerance types can be distinguishable in terms of their tolerance zones. The tolerance zone of flatness is always two-dimensional, which is established using two perfectly parallel lines or planes. While the tolerance zone of the surface profile is a three-dimensional space established using lines symmetric to the surface whose profile is to be controlled.
Another difference between these tolerances can be defined in terms of the application of MMC and LMC modifiers. Both of these modifiers can be applied to flatness while none of these modifiers can be applied to the surface profile type of tolerance.
The differences in flatness and surface profile are summarized in the table below;
| Category | Flatness | Surface Profile |
| Tolerance Type | Form | Profile |
| Feature Type | Individual Feature | Individual or Related Feature |
| Datum Requirements | No | Dependent on Application |
| Tolerance Zone | 2 – Dimensional | 3 – Dimensional |
| MMC/LMC Applicable | Yes | No |
Measurement of Flatness & Surface Profile
Both of these tolerances could be measured using modern technologies including CMM, lasers or conventional tools including dial gauge (dial indicator), height gauge, etc.
Measuring Flatness
Flatness, as already stated, is one of the very basic and common types of tolerance and can be very easily measured. For measuring flatness, various points (as many as possible) are taken on the feature or surface that needs to get measured and join these points into an imaginary line. Then an imaginary tolerance zone is established and every point on the imaginary line obtained earlier should remain in this zone. If by any chance, some points lie outside of this tolerance zone, then it would mean that the geometrical feature being considered is not flat to a required extent.
Conventionally, the easiest way to measure flatness is by using a height gauge with a dial test indicator. These are common tools to find in the workshop. The base of the height gauge is placed on a granite surface plate along with the object whose flatness is to be measured. Now, the dial test indicator is moved on the geometrical feature whose flatness needs to be measured and the variation of the flatness of that surface is determined.
Read also: Parallelism Vs. Flatness
CMM is employed where high accuracy is desired. CMM can create virtual tolerance zones that could be used to check the variations in the flatness of the surface. Using CMM, several points are taken on the surface whose flatness needs to be measured and imaginary line along with tolerance zone are drawn using CMM and it is verified that flatness lies in the given range or within the desired tolerance zone.
Measuring Surface Profile
Most of the time, the surface profile is applied to a complex curve and is not possible to accurately measure it using conventional methods. In places where high accuracy is not needed, dial indicator or thickness indicator can be used to measure the surface profile. For measuring the surface profile with a dial indicator, a customized mandrel will have to be manufactured first which can ensure that the dial gauge can stay at a specific distance as it is moved along that curve that needs to be measured. Then with the help of this mandrel, the dial indicator is moved over the surface, and variations in the surface profile are then measured.
Another conventional method for measuring surface profile is to measure it using a sticky replica tape, for instance, Elcometer Testex Replica Tape along with a thickness gauge. The replica tape is first pressed onto a surface and then the thickness gauge is used to measure changes in the thickness of the replica tape to predict the variations in surface profile.
Most of the time, the surface profile is measured using CMM due to the complexity of the curves on which this control is applied. The probe of CMM is moved over the profile of the targeted surface and a three-dimensional scan of the profile is made. This scan is then compared with the desired profile dimensions and variations in the surface profile are then measured.
