Your rotating parts wobble and fail during tests. Bad parts waste your time. Bad parts waste your money. You need to pick the right tolerance symbol to fix this problem. Let us look at concentricity and runout to see which one you should actually use.
Runout is the best tolerance for most rotating parts because it measures surface wobble directly. Concentricity is much harder to measure because it checks theoretical center points. You should use runout for shafts and bearings to save money. Only use concentricity when you need perfect mass balance in your design.
You might think these two terms mean the exact same thing. Many smart engineers make this mistake on their drawings. Mixing them up causes huge delays on the shop floor. Let us break down these concepts so you can design better parts today.
What is the tolerance for concentricity?
You put concentricity on a drawing. The machine shop calls you and complains. This tolerance is very hard to check. Let us see what this symbol actually means for your parts.
Concentricity tolerance ensures that the median points of all diameters on a cylindrical part line up with a common datum axis. It controls the theoretical centerline of the part. It does not control the actual outer surface. This makes it a very strict and difficult requirement to check.
Understanding the Theoretical Center
Concentricity is all about the middle. Imagine a long metal shaft. We cut this shaft into many thin slices. We find the exact center point of every single slice. Concentricity says all these center points must sit in a perfect straight line. This line must match the main axis of the part. This sounds easy in your CAD software. It is very hard to do in real life. You cannot touch the middle of a solid piece of metal. You must use complex math to find these points.
Why It Causes Trouble
I remember a project from five years ago. An engineer wanted a motor shaft with a tight concentricity callout. My team at QuickCNCs spent days trying to measure it. We had to map the entire part using a CMM machine. We had to calculate the median points. It cost the customer a lot of extra money. The part did not even need this strict control. The engineer just wanted the part to fit into a hole.
When To Actually Use It
You should only use concentricity for very special cases. These cases involve mass distribution. Think of parts that spin very fast in an airplane. They need perfect balance. The table below shows when to use this tolerance.
| Application Type | Do You Need Concentricity? | Reason Why |
|---|---|---|
| Standard bearing shaft | No | Surface control is enough |
| High-speed turbine | Yes | Mass balance is critical |
| Basic turned pin | No | Over-engineering wastes money |
| Precision aerospace gear | Yes | Exact symmetry is required |
Concentricity causes over-engineering. Most parts work perfectly fine without it. Always ask yourself if you really care about the theoretical center points. Do not use this tolerance if you only care about how the part fits.
What is the difference between concentricity and runout tolerance?
You need your parts to fit perfectly. You feel confused about which control to apply. Using the wrong one ruins your assembly. Here is how they differ in practice.
The main difference is that concentricity controls theoretical center points, while runout controls the actual physical surface. Runout measures the total surface variation as the part rotates. Concentricity only checks if the middle points align. Runout combines form and location. This combination makes it much easier to measure.
Surface Measurement vs Center Measurement
Runout checks the outside of the part. You put a dial indicator on the surface of your shaft. You spin the shaft. The needle on the dial moves. This movement tells you the runout. It is very simple. It measures the wobble directly. Concentricity checks the inside. You cannot touch the inside center of a solid metal shaft. You have to use computers to calculate it. Runout is so much easier for machinists.
Time and Cost Differences
I always tell my European clients like Alex to think about the cost. Time is money in CNC machining. A runout check takes two minutes on the shop floor. A concentricity check takes two hours in a clean room. You pay for that extra inspection time. You also pay for the expensive tools needed to check it.
Comparing the Two Controls
Let us look at a simple breakdown. This table helps you decide.
| Feature | Runout Tolerance | Concentricity Tolerance |
|---|---|---|
| What it controls | Actual surface | Theoretical median points |
| Inspection method | Dial indicator | CMM mapping and math |
| Shop floor friendly? | Yes | No |
| Cost impact | Low | High |
| Main goal | Stop surface wobble | Ensure mass balance |
Practical Manufacturing Advice
Runout is a composite measurement. It controls roundness and location at the same time. If a part is not round, the dial indicator shows it. If the part is not centered, the dial indicator shows it. Runout gives you everything you need for most rotating parts. You should always default to runout. Only switch to concentricity if your specific design demands it. This simple choice makes your supply chain much smoother. It saves everyone a lot of headaches.
Is concentricity half of runout?
Many people try to use simple math for GD&T. They guess the relationship between tolerances. This guessing leads to rejected parts. Let us clear up this common math myth.
No, concentricity is not simply half of runout. Runout often reads double the offset of a center axis. They still measure different things. Runout includes surface errors like out-of-roundness. Concentricity ignores surface errors. You cannot use a simple mathematical formula to convert one tolerance into the other.
The Origins of the Myth
Many engineers see a dial indicator move. A shaft is off-center by 0.1mm. The dial swings up 0.1mm and down 0.1mm. The total runout reads 0.2mm. People look at this math. They think concentricity is just half of the runout value. This is a very common mistake in the engineering world. It seems correct at first glance.
Why the Math Fails
This math only works if your part is a perfect circle. No part is a perfect circle in the real world. CNC machines leave tiny errors on the surface. A part might look like an egg shape under a microscope. Runout catches this egg shape. The dial needle jumps up and down over the bumps. Concentricity ignores the egg shape completely. Concentricity only cares about where the middle of the egg is located.
The Impact on Quality Control
I once saw a young engineer try to convert these numbers on a drawing. He changed a runout callout to a concentricity callout. He just divided the number by two. The parts failed in the final assembly. The surface was too wavy.
| Condition | Runout Result | Concentricity Result |
|---|---|---|
| Perfect circle, off-center | Shows total wobble | Shows center offset |
| Egg shape, perfectly centered | Shows high error | Shows zero error |
| D-shape, off-center | Shows high error | Shows center offset |
Focus on Function
You cannot swap these terms using simple math. Runout is a practical check. It ensures the part fits into a bearing without getting stuck. Concentricity is a pure geometric check. Keep them separate in your mind. Do not try to divide runout by two to get your answer. You must understand what your part actually does in the machine.
Is concentricity no longer used in GD&T?
You read a new engineering standard book. You notice something is missing. The rules have changed over time. This makes you wonder if you should stop using this symbol entirely.
Yes, the concentricity symbol was officially removed from the ASME Y14.5-2018 GD&T standard. The committee removed it because it was too confusing and costly to measure. You should use runout or position tolerance instead to achieve the same functional goals without the extra inspection headaches.
The Big Change in 2018
The ASME Y14.5 standard is the rule book for drawing parts. In 2018, the rule makers made a huge change. They threw out the concentricity symbol. They also threw out the symmetry symbol. This was a shock to many old-school engineers. It was great news for people on the shop floor. We hated checking concentricity.
Why They Removed It
The committee realized a big problem. People used concentricity wrong. Engineers just wanted the parts to not wobble 95 percent of the time. They did not actually care about the theoretical median points. The strict definition forced machine shops to buy expensive CMM machines. It wasted billions of dollars in the manufacturing industry over the years. The symbol caused too much arguing between designers and machinists.
What You Should Use Now
You need new tools if you design parts today. You can get the exact same results with better symbols.
Alternative Options
| Old Callout | What You Actually Want | New Better Callout |
|---|---|---|
| Concentricity | Stop surface wobble | Circular Runout or Total Runout |
| Concentricity | Line up two features | Position Tolerance |
| Concentricity | Control mass balance | Position Tolerance with modifiers |
I highly recommend updating your old drawings. I will call you if you send me a drawing with a concentricity symbol. I will ask if we can change it to runout. This change saves you money. It gets your parts delivered faster. It is better for everyone involved in the project. Follow the new standards and your work life will be much easier.
Conclusion
Always choose runout for your rotating parts to stop wobble and save money. Concentricity is outdated, hard to measure, and causes over-engineering. Stick to runout for easier manufacturing.