The Essential Guide to Runout in Precision Machining

Runout is a critical concept in precision machining and plays a significant role in ensuring the quality of machined parts. It refers to the variation in motion or position of a surface as a rotating part turns about an axis. The concept is often integrated with Geometric Dimensioning and Tolerancing (GD&T), a symbolic language used on engineering drawings and CAD models to define the allowable geometric variations in part geometry. Runout is one of the geometric tolerances specified in GD&T, and it is crucial to understand its implications to produce parts with high precision and accuracy.

In this guide, we will explore different aspects of runout, including GD&T runout, runout symbols, applications, and common issues related to runout in GD&T.

 

The Fundamentals: What is Runout?

Runout is a fundamental concept in the field of precision engineering and manufacturing. It is a term used to describe the deviation or inaccuracy that occurs when a rotating object, such as a spindle, shaft, or wheel, does not rotate perfectly around its central axis. In simpler terms, runout is the amount by which a rotating surface deviates from its ideal path during rotation. It is a critical parameter that influences the quality, performance, and functionality of machined parts and assembled products.

Example of radial and axial runout

Runout can occur in any rotating object, and it is often considered a composite of two types of errors: radial runout and axial runout. These two components of runout affect different aspects of the machined part and can lead to various problems if not adequately controlled.

1. Radial Runout

Radial runout refers to the variation in the radial distance between the surface of the rotating part and its central axis. It is usually measured at a specific distance from the axis of rotation, and it affects the roundness of the machined part. Essentially, radial runout is the deviation from a true circular form experienced by a rotating part during its rotation. This deviation can cause the rotating surface to move closer to or farther away from the central axis as it rotates, leading to an out-of-round condition.

For example, consider a rotating wheel. If the wheel has radial runout, it means that as the wheel rotates, some points on the wheel’s surface will be closer to the axis of rotation, and some points will be farther away. This inconsistency can lead to uneven wear on the wheel and the surface it contacts, resulting in vibrations, noise, and reduced performance.

Radial runout is commonly measured using a dial indicator placed against the rotating surface at a specified distance from the axis of rotation. The part is rotated one full revolution, and the maximum and minimum readings on the dial indicator are recorded. The difference between the maximum and minimum readings indicates the amount of radial runout.

2. Axial Runout

Axial runout, on the other hand, refers to the variation in the axial distance between the rotating surface and a reference plane perpendicular to the axis of rotation. It affects the flatness and parallelism of the machined surface. Essentially, axial runout is the deviation from a true planar form experienced by a rotating part during its rotation. This deviation can cause the rotating surface to move up and down as it rotates, leading to an out-of-flat condition.

For example, consider a rotating disc. If the disc has axial runout, it means that as the disc rotates, some points on the disc’s surface will be higher, and some points will be lower relative to the reference plane. This inconsistency can lead to uneven contact with mating surfaces, resulting in vibrations, noise, and reduced performance.

Axial runout is commonly measured using a dial indicator placed against the rotating surface with the spindle in a horizontal position. The part is rotated one full revolution, and the maximum and minimum readings on the dial indicator are recorded. The difference between the maximum and minimum readings indicates the amount of axial runout.

 

What is GD&T in Runout ?

Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used on engineering drawings and computer-aided design (CAD) models to define the allowable geometric variations in part geometry. It is a standardized system that allows engineers and manufacturers to communicate and specify the geometric characteristics of parts and assemblies accurately. One of the geometric tolerances specified in GD&T is runout.

Runout in GD&T and interpretation

GD&T runout is a geometric tolerance that controls the amount of runout (both radial and axial) that a feature or surface can have when referenced to a datum axis or plane.

There are two types of runout specified in GD&T: circular runout and total runout.

1. Circular Runout

Circular runout controls the amount of variation of a surface as it is rotated one full revolution about a datum axis. It is a 2D tolerance that restricts the amount of runout at a particular cross-section of the part. Circular runout is a composite tolerance that controls both the surface elements of the part and its relation to a datum axis. Essentially, it combines the effects of variations in roundness, flatness, straightness, and co-axiality into a single tolerance.

For example, consider a rotating shaft with a circular flange at one end. If the circular runout of the flange is controlled, it means that as the shaft rotates, the surface of the flange must not deviate from a perfect circle by more than the specified tolerance.

2. Total Runout

Total runout, on the other hand, is a 3D tolerance that controls the entire surface of the part as it is rotated 360 degrees about the datum axis. It takes into account both the radial and axial variations along the entire length of the surface. Total runout is a more stringent tolerance than circular runout because it controls the entire surface of the part, not just a particular cross-section.

For example, consider a rotating shaft with a cylindrical surface along its length. If the total runout of the cylindrical surface is controlled, it means that as the shaft rotates, the entire surface of the cylinder must not deviate from a perfect cylinder by more than the specified tolerance.

 

Decoding the Runout Symbol in GD&T

The language of Geometric Dimensioning and Tolerancing (GD&T) is riddled with symbols, each carrying its own significance and implications. One such crucial symbol in the GD&T lexicon is the runout symbol.

Runout is a critical parameter in ensuring the functionality and efficiency of rotating parts. The runout symbol in GD&T is represented by a circle with two parallel lines running through it. This symbol is used in engineering drawings and CAD models to specify the allowable runout of a surface or feature relative to a datum axis or plane.

The runout symbol is often encountered in two primary contexts: circular runout and total runout.

Circular Runout Symbol

Circular runout symbol in GD&T

The circular runout symbol is the same as the general runout symbol (⌔), but it is used specifically to control the runout of a surface at a particular cross-section of a part as it is rotated 360 degrees about a datum axis.

• Circular runout is a 2D tolerance that limits the runout at a particular point along a feature’s length.
• It is essential when a rotating shaft has multiple diameters, and you need to control one specific diameter’s runout concerning the shaft’s axis.
• The circular runout symbol (⌔) is used to denote the maximum allowable runout of that diameter.
• This symbol is positioned above the dimension line of the feature it regulates, and the datum reference is inserted in a rectangular box at the end of the dimension line.
• For instance, if the circular runout of a diameter is controlled concerning datum axis A, the notation on the drawing would appear as: ⌔0.030|A.

Total Runout Symbol

Total runout symbol in GD&T

The total runout symbol is also represented by the same runout symbol (⌔), but it is used to control the runout of an entire surface along the length of a feature as it is rotated 360 degrees about a datum axis. It is a 3D tolerance that restricts the amount of runout across the entire surface of the part.

• When dealing with a rotating shaft with an extensive cylindrical surface, you may need to control the runout of the entire surface concerning the shaft’s axis. In this situation, the total runout symbol (⌔) is used to specify the maximum allowable runout of the entire surface.
• This symbol is located above the leader line that points to the controlled surface, and the datum reference is included in a rectangular box at the end of the leader line.
• For instance, if the total runout of a cylindrical surface is controlled concerning datum axis A, the drawing notation would appear as: ⌔0.030|A.

 

The Crucial Role of Runout Control in Precision Engineering

In the realm of manufacturing and engineering, precision is of the utmost importance. A critical aspect of ensuring precision in rotating components is the control of runout. Runout is the deviation of a surface from its ideal form as it is rotated about an axis. Even the slightest deviation can result in vibrations, noise, and wear, leading to decreased performance, reduced lifespan, and potential failure of equipment. Hence, controlling runout is paramount in the design, manufacturing, and assembly of rotating components.

Table: Importance’s of runout control

Importance	Description	Impact on Machinery and Operations	Impact on Costs and Productivity
Ensuring Proper Functioning of Machines	Controlling runout is essential for the smooth and efficient movement of rotating parts, reducing the risk of vibrations, noise, and increased wear and tear.	Decreased performance, increased maintenance requirements, and potential premature failure of components.	Increased lifespan of machinery, reduced maintenance, and operational costs.
Maintaining Product Quality	Runout can lead to surface irregularities during machining processes, affecting the fit, form, and function of parts.	Rejection of components, increased manufacturing costs, and potential product recalls.	Higher quality products, reduced manufacturing costs, and avoidance of costly recalls.
Reducing Vibrations and Noise	Runout can cause imbalances in rotating components, leading to vibrations and noise transmitted to other parts of the machinery and the operating environment.	Additional wear and tear, uncomfortable or hazardous operating environment.	Safer and more comfortable operating environment, reduced wear and tear on machinery, and reduced maintenance costs.
Increasing Equipment Lifespan	Runout-induced wear and tear on rotating components and other parts of the machinery affected by vibrations can lead to premature failure and increased maintenance requirements.	Premature failure of components, increased maintenance requirements.	Increased equipment lifespan, reduced operating, and maintenance costs.
Enhancing Performance	Runout in rotating components can lead to inefficiencies in machinery, such as uneven firing of cylinders in an internal combustion engine, resulting in reduced power and efficiency.	Reduced power and efficiency, decreased productivity.	Increased productivity, reduced operating costs, and optimal performance of machinery.

 

How to Measure the Runout?

As discussed in earlier sections, controlling runout is imperative for various reasons. However, the question remains – how is runout accurately measured? There are two primary ways of measuring runout, dial indicator and laser measurement. Let’s discuss them in detail.

Dial Indicator

The dial indicator is one of the most commonly used instruments for measuring runout. It is a precision tool that can measure small linear distances with high accuracy. A typical dial indicator consists of a graduated dial and a spindle that moves in and out as it contacts the surface being measured. The needle on the dial indicates the deviation of the surface from its true position as the part rotates.

Setting Up the Dial Indicator

• Secure the Workpiece: The first step in the process is to securely fix the workpiece on a fixture or a surface plate to prevent any movement during the measurement process.
• Position the Dial Indicator: Next, position the dial indicator so that the spindle contacts the surface to be measured. Ensure the dial indicator is perpendicular to the surface being measured for radial runout and parallel to the surface for axial runout.
• Zero the Dial Indicator: Rotate the workpiece and adjust the dial indicator until the needle points to zero on the graduated dial.

Measuring the Runout

Slowly rotate the workpiece by hand or using a slow-speed motor. Ensure to rotate the workpiece a full 360 degrees to measure the entire surface. Then, observe the movement of the dial indicator’s needle as the workpiece rotates. Record the maximum deviation of the needle from zero on the graduated dial. This value represents the runout of the surface being measured.

Laser Measurement

Laser measurement is another accurate method used for measuring runout. This technique uses a laser beam directed at a rotating component, and a sensor detects the reflected light. The sensor then converts the light into an electrical signal, which is processed by a computer to determine the runout.

Setting Up the Laser Measurement System

• Secure the Workpiece: Similar to the dial indicator method, start by securely fixing the workpiece on a fixture or a surface plate.
• Position the Laser and Sensor: Position the laser and sensor so that the laser beam is directed at the surface to be measured, and the sensor can accurately detect the reflected light.

Measuring the Runout

Rotate the workpiece, ensuring a complete 360-degree rotation. The sensor will detect the reflected light as the workpiece rotates and convert it into an electrical signal. The computer will process this signal and display the runout of the surface being measured.

Both the dial indicator and laser measurement methods are accurate and widely used in various applications. However, the laser measurement method is usually preferred for high-precision applications or when measuring large components that may be challenging to measure using a dial indicator.

 

GD&T Runout with an Example

Let’s consider the example of a rotating shaft. A shaft is a critical component used in many machines, from electric motors to car engines. The proper functioning of the shaft is crucial for the smooth operation of the entire machine. One important characteristic that needs to be controlled is the shaft’s runout.

• Example Scenario

Suppose we have a shaft with a diameter of 50 mm that needs to be manufactured for a high-speed motor. The motor’s design specifies that the shaft’s maximum allowable runout is 0.05 mm relative to datum axis A, which is the central axis of the shaft. This means that as the shaft rotates, the surface of the shaft should not deviate more than 0.05 mm from a perfect circle centered around datum axis A.

• GD&T Specification

The GD&T specification for this scenario would include the runout symbol (⌔), the tolerance value (0.05 mm), and the datum reference (A). This specification would be placed on the engineering drawing of the shaft as follows: ⌔0.05|A.

• Interpretation

This notation specifies that the circular runout of the shaft, relative to datum axis A, should not exceed 0.05 mm. The manufacturing and quality control teams would then use this specification to produce and inspect the shaft.

• Manufacturing Process

During the manufacturing process, the machinist would use a lathe or a CNC machine to produce the shaft. The machinist would carefully control the machining parameters, such as the cutting speed, feed rate, and depth of cut, to ensure that the shaft is produced with the specified runout tolerance.

• Inspection Process

After the shaft has been manufactured, it needs to be inspected to ensure that it meets the specified runout tolerance. The inspector would use a dial indicator or a laser measurement system, as described in the previous section ( Laser & dial Indicator) , to measure the runout of the shaft.

If the maximum deviation of the surface from the true circular form is less than or equal to 0.05 mm, the shaft would pass the inspection and be deemed suitable for use in the motor. If the runout exceeds 0.05 mm, the shaft would be rejected, and the manufacturing process would need to be adjusted to produce a shaft that meets the tolerance.

 

Summing Up

Runout is a critical concept in precision machining that affects the quality, functionality, and lifespan of machined parts. Understanding the different aspects of runout, including GD&T runout, runout symbols, and common causes, is crucial for producing high-quality machined parts. Implementing proper procedures and using high-quality equipment can help reduce runout and improve the overall quality of your machined parts.

Prolean’s CNC Machining Services provide high-quality, precision machined parts with tight tolerances and minimal runout. Our state-of-the-art CNC machines and experienced machinists ensure that every part is machined to the highest standards of quality and accuracy.

 

FAQs

What is runout in machining?

Runout in machining refers to the inaccuracy or deviation that occurs when a rotating object, such as a spindle or shaft, does not rotate perfectly around its central axis. This deviation can cause uneven wear and tear on tools and workpieces, reducing their lifespan and overall performance.

What is GD&T runout?

GD&T runout is a geometric tolerance that controls the amount of runout (both radial and axial) that a feature or surface can have when referenced to a datum axis or plane. There are two types of runout specified in GD&T: circular runout and total runout.

What is the difference between radial and axial runout?

Radial runout refers to the variation in the radial distance between the surface of the rotating part and its central axis, whereas axial runout refers to the variation in the axial distance between the rotating surface and a reference plane perpendicular to the axis of rotation.

How is runout measured?

Runout can be measured using a dial indicator or a laser measurement system. The part is rotated, and the measurement device is placed against the surface being measured. The device will indicate the amount of runout as the part rotates.

What are the common causes of runout?

Common causes of runout include spindle error, toolholder error, and workpiece clamping error. Any inaccuracies or misalignments in these components can lead to runout in the machined part.

How can runout be reduced?

Runout can be reduced by ensuring proper clamping of the workpiece, using high-quality toolholders with precise concentricity and balance, and regularly checking and maintaining the spindle to ensure it is accurately aligned and free of any inaccuracies or damage.