Have you ever looked at a complex part and wondered how on earth anyone makes sure it’s perfectly balanced? It’s not magic, I promise. It’s a clever system called GD&T, and one of its most interesting tools is Symmetry. When you hear that word, you probably think of the wings of a butterfly, right? A perfect mirror.
And you’re right! That’s the whole point of GD&T Symmetry. It’s a special instruction on an engineering drawing that tells the manufacturer, “This feature needs to be perfectly centered and balanced.” It’s not very common, and as we’ll see, it’s thought to be a bit old-fashioned now, but it’s very powerful.
So, let’s dive in and get the full story on Symmetry GD&T in a way that actually makes sense. We’ll decode the symbols and see how it all works.
What is Symmetry?
Symmetry is a 3D geometric tolerance that makes sure the median points of a feature are perfectly centred around a datum centre plane or datum axis.
Wow, that’s a lot to say, isn’t it? Let’s break it down. Picture a block with a slot cut into it. Symmetry makes sure that the slot doesn’t move to one side of the datum. It has to be perfectly even.
We call it a derived feature. This means that we are not measuring the surfaces directly. We don’t do that; instead, we find the halfway point between two points on opposite sides of the feature.
That middle point is the median point. The Symmetry control says that all of these middle points must be very close to each other in a very small area right in the middle. It’s one of the main groups of location profile orientation controls we use to describe the shape of a part.
Symmetry Symbol
There is a small icon for each GD&T control, and the Symmetry symbol is easy to find. It looks like three lines that are all the same length and are stacked on top of each other. But you won’t ever see this GDT Symmetry symbol by itself. The feature control frame (FCF) is a special box on the drawing where it always lives.
The tolerance’s command center is this Symmetry feature control frame. It’s a rectangle with a few smaller boxes inside it. The first box in this block of Symmetry geometric characteristics has the Symmetry symbol in it.
The Symmetry tolerance value is in the next box of this geometric tolerance block. The last box, the datum block, tells you which datum the feature is connected to. This whole feature control frame (FCF) for Symmetry has all the information you need.
Symmetry Tolerance Zone
This is where the real magic happens. The area where all the median points can be is called the Symmetry tolerance zone. For most features, the default tolerance zone type is a total wide tolerance zone with two parallel planes.
Think of two walls that are perfectly flat and very close to each other. The space between these two parallel planes is the symmetry tolerance zone that the drawing shows.
If the feature tolerance block says that the tolerance should be 0.03 mm, this means that two planes are 0.015 mm away from the datum center plane on each side. This thin zone is perfectly centered on the main plane.
All of the median points must be within the tolerance zone for the part to be good. The callout ensures the feature is perfectly balanced, thereby maintaining symmetry control. If any median point moves outside of these two parallel planes, the part is not good.
How to Measure Symmetry
So, how do we really check this? How do we check the symmetry of a part to make sure it meets the requirement? It’s not as easy as just grabbing a ruler, though, because we’re talking about those imaginary median points.
It’s actually one of the hardest GD&T callouts to check. We need special tools and methods to check, control, and measure this quality. This process allows us to verify the part and confirm the form distribution is accurate. Let’s look at a few ways to do it.
Using a Caliper or a Micrometer
It’s almost impossible to get an accurate measurement of symmetry with just a calliper or a micrometre. You can definitely use these tools to find out how wide a slot is or how far apart two flat surfaces are that are parallel to each other.
But they can’t tell you where the centre plane is. They can’t find those very important middle points. So, for all intents and purposes, fewer points aren’t enough. It’s possible in some very simple cases, but because of the operator’s skill and instrument error, this method isn’t usually recommended. These hand tools just aren’t good enough for a reliable check.
Using a Coordinate Measuring Machine
This is the real thing. A coordinate measuring machine (CMM) is the most modern and accurate way to check for symmetry. A coordinate measuring machine has a probe that is very sensitive and can touch the part in hundreds or even thousands of places with amazing accuracy.
First, the coordinate measuring machine (CMM) will touch the datum plane to find out where it is. After that, it will touch a lot of points on both sides of the feature being controlled to get measurements of its different parts.
The CMM’s powerful software then figures out where all the midpoints are. It creates a virtual tolerance zone around the datum and checks if all median points must fall within it. It’s the best way to be sure.
Why Should You Use Symmetry?
You might be wondering why someone would choose such a complicated tolerance when there are easier ones. The answer is in situations where balance and even stress distribution are very important.
Symmetry controls are very helpful for high-speed applications like turbines, engines, and rotating shafts. If any of these parts are out of balance, they can cause dangerous vibrations, too much noise, wear and tear too quickly, or even a catastrophic failure. Symmetry makes sure that static and dynamic balance are correct.
When there is a lot of weight on your part, symmetrical features spread the weight evenly. This lowers the risk of localised stress concentrations that could lead to early failure. Bearings, gear teeth, and support brackets are all examples of things that benefit from symmetrical load distribution.
Another problem that symmetry helps with is loads that change or happen in cycles. Asymmetrical parts can take on more stress than they should when machinery or structural parts move back and forth or vibrate. Over time, this leads to failure due to fatigue.
Symmetry controls help keep cracks from starting and spreading by making sure the form distribution is even. When features are balanced around a central plane, stresses are spread out more evenly, which greatly increases the life of the components.
Even though it is important, symmetry tolerance is not often spelt out unless it is absolutely necessary. It costs a lot to put this control into place because it is hard to inspect and maintain. There is no substitute for proper symmetry control when balance is very important.
Symmetry VS Other GD&T
In GD&T, the symmetry callout is part of the location control family. It makes sure that features are in the right places in relation to the datum plane. But how does it stack up against other location controls that look like it?
Other location controls can sometimes get the same results by using different methods and types of tolerance zones. The symmetry tolerance can do the same things as concentricity and true position. But each one has its own strengths and uses.
When you look at symmetry and other callouts, you can see some interesting differences. Position tolerances use coordinate systems to control where features are located, but symmetry is only concerned with balance around a central plane. This makes symmetry perfect for situations where balance is the most important thing.
These controls use the tolerance zone approach in very different ways. Symmetry always uses the full wide tolerance zone with two parallel planes. You can use circular zones for position tolerances and cylindrical zones around a datum axis for concentricity.
The difficulty of taking measurements varies greatly between these controls. It is fairly easy to check position tolerances, but symmetry needs advanced tools and skilled workers. Designers often choose simpler options when they can because of how complicated this is.
Symmetry vs Concentricity
People often get Symmetry and Concentricity mixed up, and it’s easy to see why. They are very similar ideas. In fact, the standard, ASME Y14.5M-1994, even states that Symmetry and Concentricity controls are the same basic concept, just applied to different shapes. The big difference is the shape of the feature they control.
Concentricity is used exclusively for cylindrical surfaces whereas Symmetry controls are for features with two parallel surfaces, like a slot. Concentricity controls create a cylindrical tolerance zone around a datum axis.
Symmetry creates a tolerance zone of two parallel planes around a datum center plane. So, Concentricity is for round parts, and Symmetry is for flat, opposing features. Concentricity derives an actual central axis instead of a median plane.
Symmetry vs True Position
This is another big comparison. Symmetry and True Position both control the location of a feature. However, True Position is much more common and flexible. It can do everything Symmetry can, but the reverse isn’t true.
A huge difference is that True Position can use modifiers like Maximum Material Condition (LMC/MMC). This can create bonus tolerances whereas Symmetry cannot; it is always applied RFS (Regardless of Feature Size).
True Position is also more versatile because it can allow datum feature shift and use projected tolerance zones, which Symmetry can’t. True Position locates a feature relative to datums, while Symmetry ensures a feature is balanced around a central plane and creates a different kind of control.
Conclusion
So, what’s the big takeaway here? GD&T Symmetry controls are a very specific and powerful tool for ensuring a feature is perfectly balanced. It uses a Symmetry symbol in a feature control frame FCF to define a tolerance zone made of two parallel planes.
The rule is simple: all the calculated median points must lie inside that zone. While it’s tricky to measure, requiring a coordinate measuring machine CMM for real accuracy, it’s the best way to control balance for critical parts.
It’s one of the most important points to remember that this control was actually removed from the 2018 edition of ASME section Y14.5 because it’s so difficult to verify. However, you will still see it on thousands of older drawings, so understanding how it works is still essential for any engineer or machinist.