GD&T Profile of a Surface
By Lucas Lo | Updated: May 13, 2025
Table of Contents
As we all know, in GD&T Symbols, the Profile Tolerances include GD&T Profile of a Line and GD&T Profile of a Surface. In the last article, we have produced GD&T profile of a line. So, we will illustrate knowledge of GD&T profile of a surface in this article.
1. What is Profile of a Surface in GD&T?
Nowadays, many engineering features such as airplane wings, automotive BiWs and turbine blades etc. have extremely complex surfaces with tight accuracy requirements for key functions.
GD&T profile of a surface plays huge role in such cases. As a powerful GD&T callout, profile of a surface can be applied on mostly all kinds of complex surfaces where other tolerances may not work well.
Therefore, it is important to comprehensively know about GD&T profile of a surface. In this part, we will explain what is profile of a surface in GD&T from its definition and tolerance zone.
GD&T Profile of a Surface Definition
GD&T profile of a surface is one of the GD&T Symbols that is used to control the actual profile of the measured surface, including its form, location and orientation.
GD&T profile of a surface confines the permissible deviation zone of a profile surface from the ideal profile to ensure the consistence of actual profile of surface with the designed one.
It establishes a 3D tolerance zone around a surface(usually an advanced curve or shape) to ensure the surface profile manufactured as required.
GD&T Profile of a Surface Tolerance Zone
As GD&T profile of a surface is used to control the surface profile, the tolerance zone is comprised of two parallel surfaces(usually curved), forming a 3D zone where all points of the measured surface must lie.
The two parallel surfaces must follow the contour of the theoretical surface profile across the entire length of the surface and locate at sides of the ideal surface with equal distance.
In short, the GD&T profile of a surface tolerance zone is the region between two parallel surfaces that are equidistant form the theoretical profile surface.
And the tolerance value indicated on engineering drawings is the total distance of the two parallel surfaces while the acceptable variation is only half of the value.
2. How to Show GD&T Profile of a Surface?
In this part, we will talk about the callout of GD&T profile of a surface on engineering drawings, mainly including the feature control frame and indication of measured surface.
The feature control frame of GD&T profile of a surface consists of three parts, including geometrical tolerance block, feature tolerance block and datum block.
The first block, geometrical tolerance, contains a closed semicircle that stands for GD&T profile of a surface.
The second block contains only the tolerance value for GD&T profile of a surface.
Because its tolerance zone is a total wide area that has no need to be particularly indicated and modifiers like MMC or LMC are not applicable with it.
The third block houses the datum symbols. Since GD&T profile of a surface may refer to datum planes or not, this block then may appear or not.
And you should know that if GD&T profile of a surface does not refer to any datum surface, it is only used to control the form of the surface profile.
However, if any datum symbol presents in the feature control frame of GD&T profile of a surface, it means that except form of surface profile, the orientation and location are also controlled.
And the feature control frame should be connected with a leader arrow that points to the measured surface directly or leading to the extension lines of measured surface.
3. How to Measure GD&T Profile of a Surface?
After learning about what is GD&T profile of a surface and how to show it in engineering drawings, you are supposed to know how to measure it then. In this part, we will produce various common means for GD&T profile of a surface measurement.
What’s more, the pros and cons comparison among them would also be provided for you to choose the most suitable one in manufacturing and inspection.
Using Coordinate Measuring Machines
Coordinate Measuring Machines(CMMs) are one of the common machines that can be used to measure GD&T profile of a surface.
CMMs are suitable for measuring complex surfaces with high precision, and they can provide direct analysis with CAD models.
However, they are sensitive to environmental factors like temperature and vibration, and the equipment cost is very high. The concrete steps for GD&T profile of a surface measurement by CMMs are as follows:
Firstly, plan the measurement paths and density for measured points based on CAD models or theoretical profile on drawings.
Secondly, select a suitable probe(contact or optical one) and calibrate the probe. Then measure surface points along the paths and collect their cloud data.
Thirdly, compare the measured data with the theoretical model to calculate the deviation of surface profile.
Using Optical Scanners
Optical scanners are another kind of tools for GD&T profile of a surface measurement. Compared with CMMs, optical scanners perform non-contact measurement, which is good for soft or delicate surfaces.
In addition, with fast speed, they are suitable for large-size surface profiles.
But optical scanners are easily affected by reflective or transparent surfaces. The concrete steps for GD&T profile of a surface measurement by optical scanners are as follows:
Firstly, fix the measured feature on the plate and align the scanner relative to the measured feature.
Note that if the surface is highly reflective, it should be sprayed with matte developer.
Secondly, regulate the scanning dot pitch according to the accuracy requirement as well the light power to get clear imaging.
Thirdly, scan the measured surface with optical scanner to collect data of it. And then use software to calculate the deviation.
Using Custom Templates
This means cost low and is suitable for simple profiles in mass. And the operation is easy and fast.
However, wear on template may reduce accuracy. The concrete steps for GD&T profile of a surface are as follows:
Firstly, machine transparent or metal templated, matching theoretical profiles.
Secondly, fit the template to the measured feature and then check gaps via light transmission or feeler gauges.
Thirdly, determine if the deviation falls within the tolerance.
4. Profile of a Surface VS Other GD&T Symbols
As GD&T profile of a surface is used to control the entire surface profile and can either be used with datums or not, it can effectively replace many GD&T Tolerances.
Here we will make comparison with GD&T profile of a surface and other GD&T Symbols.
Profile of a Surface VS Profile of a Line
GD&T profile of a surface can be served as the 3D equivalent of the GD&T profile of a line.
The former controls entire surface profile, which means that it controls every cross-section across the entire length of the surface, whereas the later controls only a specific edge line of cross-section on the feature.
In short, GD&T profile of a surface focuses on the whole surface profile while GD&T profile of a line focuses on the detail profile on a surface.
It decides that when these two tolerances are used together on one feature, the GD&T profile of a surface is commonly looser than the GD&T profile of a line.
Profile of a Surface VS Flatness and Cylindricity
The relationship between GD&T profile of a surface and GD&T flatness/GD&T cylindricity is similar to that between GD&T profile of a line and GD&T straightness/GD&T circularity.
GD&T flatness and GD&T cylindricity can be seen as the specific use of GD&T profile of a surface.
GD&T profile of a surface can be used to control any surface profile, including the commonly simple flat surfaces or cylindrical surfaces, and more complex curved surfaces.
More importantly, both GD&T flatness and GD&T cylindricity are simply form tolerances that do not take datums as reference; but GD&T profile of a surface can either refer to datums or not.
When it is used with any datum, it does not only control form, but also control orientation and location.
In short, compared with GD&T flatness and GD&T cylindricity, GD&T profile of a surface is more complex and synthetic, thus costing higher to measure.
You are supposed to select the suitable one based on concrete requirements.
Profile of a Surface VS Orientation Symbols
Actually, when used with datums, GD&T profile of a surface can replace all of the GD&T orientation symbols including GD&T parallelism, GD&T perpendicularity and GD&T angularity.
The key difference is that GD&T profile of a surface can control form and orientation at the same time while GD&T orientation tolerances can only control orientation.
Therefore, to reduce cost, if only orientation needs to be controlled or the form has been controlled with GD&T form tolerances, the GD&T orientation symbols are prior to be chosen.
But if the form and orientation of features with complex curved surface need to be controlled at the same time or the numbers of tolerance need to be reduced, the GD&T profile of a surface is prior to be chosen.
5. Conclusion
GD&T profile of a surface is an extremely useful and versatile GD&T symbol in engineering and manufacturing that can be used for complex surface profiles for all kinds of features.
Therefore, it is supposed to grasp its knowledge and application well. If you still have confusion about GD&T profile of a surface, you can contact us directly. And below we provide a list of other GD&T symbols that have been produced before.
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Lucas is a technical writer at ECOREPRAP. He has eight years of CNC programming and operating experience, including five-axis programming. He also spent three years in CNC engineering, quoting, design, and project management. Lucas holds an associate degree in mold design and has self-taught knowledge in materials science. He’s a lifelong learner who loves sharing his expertise.
