What Is Runout in GD&T?
In the world of small metal parts manufacturing, the Geometric Dimensioning and Tolerancing (GD&T) system uses a combination of symbols and GD&T tolerances to provide insights into how to produce the features that are essential to a part’s form and function.
In this blog, we take a look at two related GD&T features that are sometimes called out on engineering drawings. For those of us who cut and machine small metal parts, the concept of circular runout versus total runout is not as simple as it may at first seem.
And for the manufacturers who ultimately use the parts we make as components in their own products, understanding the difference between circular runout and total runout can be important to creating specifications that will yield the best results.
Circular Runout Definition
As the name implies, circular runout is typically used to control variation in circular features of a part, such as shoulders, tapers, and fillets. In the GD&T system, the circular runout symbol is an arrow.
Often simply called runout, circular runout specifies how much the feature can vary in relationship to a reference point as the part is rotated around an axis. In basic terms, that means it looks at how much “wobble” there is in the feature of the part in relation to the reference point.
Along with the runout symbol, a diagram will typically show the circular runout tolerance zone — a two-dimensional area into which all points on the feature’s surface must fall.
Total Runout Definition
Total runout, on the other hand, is used to control variation in all the points on the surface of a rotated part with regard to the part’s rotation on its axis.
It is a characteristic that is generally called out when you have a part that must rotate around a central axis and have all points on the entire surface of the part be in spec. In GD&T, the total runout symbol is a double arrow.
The total runout tolerance zone is a three-dimensional, cylindrical area surrounding the reference point, into which all points on the rotating part must fall.
The Difference Between Circular Runout and Total Runout
What, then, is the easiest way to think of runout vs. total runout? Circular runout controls only a particular circular cross section of a part, while total runout controls the entire surface of the part.
That means total runout seeks to limit the cumulative variation across an array of part surface characteristics, such as:
- Straightness
- Circularity
- Concentricity
- Cylindricity
- Coaxiality
- Taper
- Angularity
- Perpendicularity
- Parallelism
- Profile
When to Specify Runout and Total Runout
Both circular runout and total runout are frequently used where you have components that must rotate quickly, such as drills, gears, shafts, and axles. That being said, a total runout tolerance is not called out as often, because it puts tight constraints on the entire surface of a part.
However, total runout tolerance is still very important for preventing vibration, oscillation, and surface taper on rotating parts that have large areas of surface contact, such as large pump shafts, transmission shafts, and complex gears.
Measuring Runout and TIR in GD&T
You can measure runout by spinning the part while an indicator gage takes readings across the pertinent surface — either a cross section to measure circular runout or the entire part surface when measuring total runout.
This method allows you to determine the total indicated runout (TIR), or the total amount the gage moves. If, for example, the gage moves 0.001” (0.0254 mm) across the part and that value is within the allowed tolerance, then your part meets that specification.
Cumulative TIR in Machining
Whether you are using a vertical mill or a lathe, the goal is to determine how balanced, accurate, and correct the spinning device is. The challenge is that everything in the machining business has a TIR.
For instance, when you make something using a spinning device, there are typically TIR values for three different sources of wobble — the spindle, the collet (or chuck), and the cutting tool or drill itself — any or all of which can create errors in the part.
Therefore, the fundamental concept is the TIR value of a part cannot exceed the cumulative TIR of all the tools used to make that part. What’s more, the TIR values of the tools are additive; one won’t cancel out another, and all can add to or worsen the wobble.
For example, having a perfect spindle won’t erase the effects of a wobbly collet, and a bad drill can ruin the perfection of the most perfect machine tool. So inherently, you are likely to have a TIR that exceeds what is called out due to the cumulative effects of the tools’ TIR plus the runout TIR.
Don’t Forget the Proper GD&T Runout Symbol
Another issue is that a machinist will often receive an engineering drawing that specifies a TIR value — but the drawing does not make it clear what exactly that means. For example:
- Is it the TIR for the entire part surface (total runout)?
- Or is it for just a particular circular feature of the part (circular runout)?
- Or is it the maximum total value — taking into account the TIR of both the part and the tools that will be used to make it?
Providing all the details in an engineering drawing, including runout or total runout symbols and tolerances as appropriate, can help to alleviate confusion.
Expertise in Compensating for TIR
Here at Metal Cutting, where every day we produce thousands of small metal parts, one of the first things we do when we see a drawing with TIR values for circular runout and total runout is evaluate whether our equipment is capable of producing the customer-specified values.
If the TIR in a drawing is lower than our machine specs, as well as the TIR of any tools or accessories we would use, we know to alert the customer and decide the best course of action for moving forward.
In the end, remember that total runout controls the total 3D surface of an entire part while circular runout only controls specific circular, 2D cross sections. And whichever feature is most appropriate for your application, choosing the best production method is critical to meeting your specified tolerance and ensuring your product achieves the desired functionality.
It helps to have a partner like Metal Cutting, where our goal is to work with you to deliver high-quality precision parts that meet your specifications, while also keeping your production costs within budget.
For tips on how to improve the accuracy of your specs and project quotes, download our free guide, How to Fine-Tune Your Quote Request to Your Maximum Advantage: Frequently Asked Questions in Small Parts Sourcing.
