Understanding GD&T

Summary

This script offers an in-depth exploration of Geometric Dimensioning and Tolerancing (GD&T), a critical aspect of mechanical design ensuring parts fit and function correctly. It explains the shift from traditional dimensional tolerancing to GD&T, which accounts for a part's intended function by controlling 14 geometric characteristics across five categories: Form, Orientation, Location, Profile, and Runout. The video delves into the application of GD&T, including feature control frames, datums, modifiers, and inspection techniques, providing a comprehensive foundation for mechanical designers.

Takeaways

• 🔍 Tolerances are critical in mechanical design to ensure parts fit and function as intended.
• 📏 Dimensional tolerancing has limitations as it doesn't account for how parts will be used in practice.
• 📐 Geometric Dimensioning and Tolerancing (GD&T) is a more functional approach, controlling 14 different geometric characteristics.
• 🔺 GD&T categorizes these characteristics into Form, Orientation, Location, Profile, and Runout.
• 📋 Features in GD&T can be surfaces, holes, or slots, and are controlled using feature control frames.
• 🔳 Surface features are individual surfaces, while features of size have defined dimensions and can be measured.
• 🔲 Geometric tolerances applied to surface features or features of size can have different implications.
• 📏 Datums are reference surfaces used during inspection and are crucial for establishing a part's position and orientation.
• 🔄 Modifiers in GD&T, like MMC and LMC, can adjust tolerance zones based on the feature's size relative to its limits.
• 🔍 The Envelope Principle in ASME ensures parts fit together by controlling both size and form, unlike ISO's Independency Principle.

Q & A

What is the primary challenge in designing and building mechanical systems?

The primary challenge in designing and building mechanical systems is ensuring that all parts, once manufactured, will fit together and function as intended.

Why is tolerancing important in the mechanical design process?

Tolerancing is important because it allows for variations in manufacturing while still ensuring that parts will fit together and function correctly.

How does geometric dimensioning and tolerancing (GD&T) differ from dimensional tolerancing?

GD&T differs from dimensional tolerancing by allowing control of tolerances in a way that reflects the intended function of the part, rather than just defining how much each dimension can deviate from its nominal value.

What are the five categories of geometric characteristics controlled by GD&T?

The five categories of geometric characteristics controlled by GD&T are Form, Orientation, Location, Profile, and Runout.

What is the difference between surface features and features of size in GD&T?

Surface features in GD&T are individual surfaces, while features of size are features that have a defined dimension and can be measured with tools like calipers.

How are geometric tolerances applied to features in GD&T?

Geometric tolerances in GD&T are applied to features using feature control frames, which contain all the necessary information to control a specific geometric characteristic.

What is the purpose of datums in GD&T?

Datums in GD&T are reference surfaces that need to be considered during inspection. They are used to locate features and define how a part should be immobilized when inspecting a geometric tolerance.

How does the Envelope Principle, also known as GD&T Rule Number 1, affect the form of a feature?

The Envelope Principle states that the surface of a regular feature of size shall not extend beyond an envelope that is a boundary of perfect form at MMC, meaning that the MMC limit of size controls both the size and form of the feature.

What is the difference between the Envelope Principle and the Independency Principle?

The Envelope Principle states that the size limits control the form of a feature, while the Independency Principle considers the geometric form and size of a feature separately, meaning that the size limits do not control form.

What are the two types of runout tolerances and how do they differ?

The two types of runout tolerances are circular runout and total runout. Circular runout controls the roundness of individual cross-sections relative to a datum axis, while total runout controls runout along the axial direction, using a tolerance zone defined by two concentric cylinders.

Outlines

00:00

🔍 Introduction to Mechanical Design and Tolerances

This paragraph introduces the complexity of mechanical system design, emphasizing the importance of tolerancing to ensure parts fit and function correctly. It contrasts dimensional tolerancing with geometric dimensioning and tolerancing (GD&T), highlighting GD&T's ability to control tolerances based on a part's intended function. GD&T focuses on 14 different geometric characteristics, categorized into Form, Orientation, Location, Profile, and Runout. The paragraph explains the difference between surface features and features of size, and how geometric tolerances are applied using feature control frames, which include symbols for geometric characteristics, tolerance values, datums, and modifiers.

05:07

📏 Understanding Geometric Tolerances and Datums

The second paragraph delves into the specifics of geometric tolerances within the Form category, discussing flatness, straightness, circularity, and cylindricity. It explains how these tolerances are applied and inspected, using tools like dial test indicators and coordinate measuring machines (CMMs). The paragraph also introduces the concept of datums, which are reference surfaces used during inspection to immobilize a part and ensure accurate measurement. Datums are identified with letters and symbols on drawings and can be actual surfaces or theoretical perfect surfaces, with datum simulators being the real, imperfect surfaces used for immobilization.

10:09

🛠️ Controlling Feature Geometry with Datums and Orientation Tolerances

This paragraph continues the discussion on datums, explaining the difference between a datum feature, a datum, and a datum simulator. It describes how restraining datum features reduces a part's degrees of freedom, allowing for accurate inspection. The paragraph then moves on to orientation tolerances, which control the angles between features and datums, including parallelism, perpendicularity, and angularity. It outlines how these tolerances are inspected, using methods like sine bars and by placing parts on datum simulators.

15:13

🔩 Position Tolerances and the Benefits of Geometric Tolerance Approach

The fourth paragraph focuses on position tolerances, which define the acceptable location of a feature of size relative to its true position. It contrasts the geometric tolerance approach with traditional dimensional tolerances, highlighting the advantages of using cylindrical tolerance zones and the ability to explicitly define relevant datums and their order. The paragraph also introduces modifiers in GD&T, such as MMC and LMC, which adjust tolerance zones based on the actual size of a feature, providing flexibility and ensuring parts fit together properly.

20:16

🔄 The Envelope Principle and Modifiers in GD&T

This paragraph discusses the Envelope Principle from the ASME standard, which states that a feature's surface must not extend beyond a boundary of perfect form at the maximum material condition (MMC). It contrasts this with the ISO standard's Independency Principle, which considers geometric form and size separately. The paragraph explains how modifiers like E and I can be used to apply the Envelope or Independency principles, respectively. It also touches on special cases where the Envelope Principle is overwritten, such as when explicit flatness or straightness tolerances are called out for a feature of size.

25:17

🌐 Profile and Runout Tolerances in GD&T

The final paragraph covers profile and runout tolerances, which are versatile and can control form, orientation, and location simultaneously. It explains how profile of a surface and profile of a line tolerances create tolerance zones that follow the shape of the feature. The paragraph also discusses runout tolerances, including circular runout and total runout, which control eccentricity relative to a datum axis. It mentions inspection methods for these tolerances, such as using dial gauges. The paragraph concludes by acknowledging the complexity of GD&T and encourages viewers to explore further resources, including exclusive content on Nebula and a special offer for CuriosityStream.

Keywords

💡Tolerancing

Tolerancing is a critical aspect of mechanical design that ensures parts fit together and function as intended despite manufacturing imperfections. It is the allowance made for variations in size, shape, or position of a part to ensure it performs its intended function. In the video, tolerancing is discussed as a key challenge in mechanical system design, highlighting the importance of parts fitting together correctly.

💡Dimensional Tolering

Dimensional tolerancing is a traditional approach where drawings define the maximum allowable deviation of each dimension of a part from its nominal value. It is a method to control manufacturing tolerances by specifying limits on dimensions. The video script points out its limitations, stating that it doesn't always reflect how a part will be used in practice.

💡Geometric Dimensioning and Tolerancing (GD&T)

GD&T is a more sophisticated approach to tolerancing that considers the intended function of a part. It allows for the control of 14 different geometric characteristics, such as flatness and perpendicularity, which are crucial for part functionality. The video emphasizes how GD&T complements dimensional tolerancing by focusing on features rather than dimensions.

💡Features

In GD&T, a 'feature' refers to a part of a component, such as a surface, a hole, or a slot, to which geometric tolerances are applied. The video explains the distinction between surface features and features of size, noting that geometric tolerances have different implications when applied to each.

💡Feature Control Frames

Feature control frames are graphical notations used in GD&T to specify which geometric characteristic is being controlled and the tolerance applied. The video script describes these frames as containing essential information to fully control a geometric characteristic, using leader lines or extension lines for application.

💡Datums

Datums are reference surfaces used in GD&T to establish a coordinate system for measuring and inspecting parts. They define how a part should be oriented during inspection. The video script explains the concept of datums and their importance in creating a datum reference frame for accurate part inspection.

💡Modifiers

Modifiers in GD&T are used to adjust the tolerance zones applied to features of size, providing additional control over tolerancing. The video script discusses how modifiers like MMC (Maximum Material Condition) and LMC (Least Material Condition) can alter the tolerance zone based on the actual size of the feature.

💡Flatness

Flatness is a geometric characteristic that ensures a surface is evenly parallel to a plane. The video script uses flatness as an example of a simpler geometric characteristic, explaining how it is measured and its importance in surfaces that need even contact with other parts.

💡Straightness

Straightness is a tolerance that controls the alignment of a line to ensure it is straight. It is applied to individual lines on a surface rather than the entire surface. The video script describes how straightness is inspected and its application to features of size, such as the axis of a pin.

💡Cylindricity

Cylindricity is a form tolerance that ensures a feature, like a shaft, maintains a uniform roundness along its entire length. The video script contrasts cylindrical tolerance with circularity, emphasizing that cylindricity applies the tolerance uniformly along the feature's length.

💡Position Tolerance

Position tolerance in GD&T defines the maximum allowable misalignment of a feature of size from its theoretically exact position. The video script explains how position tolerance is used to control the location of holes and the advantages it has over traditional dimensional tolerances.

Highlights

Tolerancing is crucial for ensuring parts fit and function as intended in mechanical design.

Dimensional tolerancing defines how much each dimension can deviate from its nominal value.

Geometric Dimensioning and Tolerancing (GD&T) complements dimensional tolerancing by controlling 14 different geometric characteristics.

GD&T allows for better communication of design aspects by applying tolerances to features like surfaces, holes, or slots.

Geometric tolerances are assigned using feature control frames that contain all necessary control information.

Flatness tolerance defines a tolerance zone between two parallel planes for surface features.

Straightness tolerance is applied to individual lines on a surface feature, defining a tolerance zone between two parallel lines.

Circularity tolerance controls the roundness of a surface with a tolerance zone defined by two concentric circles.

Cylindricity is similar to circularity but with a uniform tolerance zone along the full length of the feature.

Datums are reference surfaces used during inspection and are identified on drawings with a letter and symbol.

Position tolerance defines the maximum distance a feature of size can be located away from its theoretically exact position.

Modifiers in GD&T allow for additional bonus tolerance depending on how close the feature is to its size limits.

The Envelope Principle in ASME states that a feature's surface must not extend beyond a boundary of perfect form at MMC.

Profile tolerances control form, orientation, and location of features with a tolerance zone following the shape of the feature.

Runout tolerances describe the eccentricity of a surface relative to an axis and are used for rotating parts like shafts.

GD&T is complex, but understanding its fundamentals is essential for accurate mechanical design and manufacturing.

Transcripts

Designing and building any kind of mechanical  system is a complex process that needs to properly

account for many different parameters, like  cost, materials, and manufacturing techniques.

But one of the key challenges is  making sure that all of the parts,

once manufactured, will fit  together and function as intended.

And that's why tolerancing is such an important  part of the mechanical design process.

The easiest way to define tolerances  is using the dimensional approach,

where drawings define how much each dimension of a  part is allowed to deviate from its nominal value.

But dimensional tolerancing doesn't work  particularly well in a lot of scenarios because

it doesn't really reflect how the part will be  used. It doesn't let you specify that you need a

surface to be flat because it has to create a seal  with another part, for example, or let you control

how close to perpendicular the axis of a hole  needs to be to the surface it's drilled into. 

Geometric dimensioning and tolerancing,  usually called GD&T, is a different

approach to tolerancing that allows you to  control tolerances in a way that reflects the

intended function of the part. It complements  dimensional tolerancing by letting you control

14 different geometric characteristics,  helping you better communicate which aspects

of your design are important. These characteristics can be

split into 5 categories that control Form,  Orientation, Location, Profile and Runout.

Unlike the traditional tolerancing approach  that applies tolerances to dimensions, GD&T 

applies them to features instead. A feature could  be a surface, a hole, or a slot, for example. 

It's important to differentiate between surface  features, which are just individual surfaces,

and features of size, which are any  features that have a defined dimension,

meaning that they can be measured with callipers,  whether that's a hole, or a feature defined by two

opposed parallel surfaces. This is because  in the GD&T world geometric tolerances can

mean very different things if they're applied  to surface features or to features of size.

Geometric tolerances are assigned to features  using feature control frames. These little grids

contain all of the information needed to fully  control a particular geometric characteristic. 

They can be applied to  features using leader lines,

extension lines, or for features of size they  can be attached directly to dimensions.

Let's look at how they're structured. The  first box in the frame contains a symbol

that defines which of the 14 geometric  characteristics is being controlled.

The next box specifies the tolerance  to apply. This value defines the size

of a tolerance zone within which the  entirety of the feature must be located.

The shape of the zone depends on the geometric  characteristic being controlled - a diameter

symbol can be added to indicate that the  tolerance zone is circular or cylindrical. 

Next is a series of letters that defines  datums, the reference surfaces that need

to be considered during inspection. And finally modifiers can be added to

either the tolerance or to the datums, to  get even more control over tolerancing.

We'll talk about datums and modifiers in more  detail later on, but first let's look at an

example, starting with one of the simpler  geometric characteristics, flatness.

The feature control frame for a  flatness call-out looks like this.

When the call-out is applied to a surface it  defines a tolerance zone between two parallel

planes that are separated by the distance  shown in the feature control frame.

All manufactured parts are imperfect  – for a part to meet this tolerance

all points on the surface must be  located within the tolerance zone.

The two planes defining the  tolerance zone are parallel

to each other, but they don't have to  be parallel to any other surfaces.

Flatness tolerances are often  specified on surfaces that mate

with other parts and need to have even  contact, like the face of a flange.

Flatness tolerances can also be applied to  features of size, in which case the tolerance

zone applies to the derived median plane of the  feature. The derived median plane is constructed

by taking the midpoints of opposite points on  the two surfaces that define the feature of size.

Since the surfaces are imperfect, the  derived median plane will be too.

When learning about GD&T it's  often useful to think about how

a part will be inspected to see if  it meets the geometric tolerance.

Flatness of a surface can easily be measured using  a dial test indicator. First the part is mounted

on three jacks, and the jacks are adjusted so that  the indicator reads zero at three defined points.

This creates a reference plane.

The indicator is then swept across the  surface to identify high and low points.

If the distance between the highest  and lowest points is less than the

width of the tolerance zone, the  flatness requirement is met. 

Another very common inspection approach is  to use a CMM, a coordinate measuring machine.

A computerised probe is used to take  measurements that are fed into software

that then uses algorithms and curve fitting to  determine deviations from the perfect size.

CMMs are expensive pieces of equipment but can be used  to inspect any kind of geometric tolerance. 

Next in the Form category of geometric tolerances  is Straightness. It's similar to Flatness,

but is applied to individual lines  instead of to an entire surface. 

When applied to a surface feature, any line  on the surface in the same direction as the

line the call-out is pointing at must be within  a tolerance zone defined by two parallel lines.

For inspection the probe is swept along multiple  straight lines instead of being swept across the

entire surface.

When straightness is applied to a feature of  size instead of a surface,

the tolerance zone is cylindrical and it  applies to the axis of the feature. 

You might apply a straightness  tolerance to the axis of a pin,

for example, to make sure it will  engage properly with a hole. 

The circularity tolerance is used  to control how round a surface is.

The tolerance zone is defined  by two concentric circles,

the radial distance between the two circles  being equal to the specified tolerance.

Circularity controls the roundness of individual  cross-sections completely independently.

This means the tolerance zones  don't need to be on the same axis,

and the diameter of the concentric circles  can vary along the length of the feature.

Circularity can be inspected in a few different  ways but ideally the part should be rotated and

a probe used to measure displacements  at several different cross-sections. 

The measurements are plotted on a polar graph  to determine if the tolerance is met.

Cylindricity is the last of the four form  tolerances. It's similar to circularity

except the tolerance zone is uniform  along the full length of the feature. 

Form tolerances control the  shape of a single surface,

axis or plane. But most other tolerance types  control the geometry of a feature relative to

one or more references, that are called datums.

Datums are identified on drawings using  a letter and this triangle symbol,

and can be attached to features  in a few different ways.

Datums are usually defined using surfaces,  but if the symbol is attached to a feature

of size the datum is the corresponding  centreline or centre plane.

Datums are used to locate features by defining  how a part should be immobilised when inspecting

a geometric tolerance. To be accurate with the  terminology we need to differentiate between

a datum feature, which is the feature on the  object that's restrained, a datum, which is the

theoretical perfect surface corresponding to that  feature, and a datum simulator, which is a real

imperfect surface that will be used to immobilise  the part to approximate the perfect datum.

A part just floating in space is said to have  six degrees of freedom - it can translate left

and right, up and down, and forward and back,  and it can rotate around those three axes.

If we hold the datum feature against a datum  simulator, three of the six degrees of freedom are

immobilised. The part can now only translate left  and right, and up and down, and it can only rotate

around a single axis. Restraining one more datum  feature constrains another two degrees of freedom. 

And by restraining a third the part is  fully immobilised and can be inspected.

These datums establish a datum reference frame,

the coordinate system used  to inspect the feature.

The order in which the datums are applied  is important because all real surfaces are

imperfect - using datums in the same order ensures  that measurements are repeatable. To see why let's

look at an example where we want to measure where  the centre of the hole is located.

Since we held the part against datum simulator B

first it will have a minimum of three  contact points with datum feature B.

Datum simulator C will have a minimum of  two contact points with datum feature C,

and datum simulator F will have a minimum  of one contact point with datum feature F. 

The datums are listed in order  in the feature control frame.

If we change the order in which the datums  are applied, or if we use different datums,

the part will be set up slightly  differently for measurement.

All of the remaining geometric tolerances  use datums. Let's look at the orientation

group of tolerances next. They're used to  control the angles between features.

Parallelism controls how close a  feature is to being parallel to a datum.

The tolerance zone is defined by two planes  that are parallel to the specified datum.

Perpendicularity works in the same way, but the  tolerance zone is at 90 degrees to the datum.

And angularity is a more general orientation

tolerance that controls the angle  between a feature and a datum.

When applied to features of size, the  orientation tolerances apply to the centre 

plane or axis of the feature. The diameter  symbol is used in this feature control frame

to specify that the tolerance zone for  the axis of the feature is cylindrical.

To meet the tolerance the axis must be  contained within the tolerance zone.

Parallelism is inspected in a similar way  to Flatness, but instead of mounting the

part on jacks the datum feature on the part  is placed directly on the datum simulator.

Perpendicularity can be  checked in the same way.

And angularity can be checked using a sine bar,  that allows angles to be measured accurately.

There are three location tolerances -  position, concentricity and symmetry.

Although they all appear in the ISO standards,  the concentricity and symmetry tolerances were

removed from the 2018 edition of ASME Y14.5,  so I won't cover them in this video.

Position is one of the most commonly  used geometric tolerances. It defines

the maximum distance the axis or median plane  of a feature of size can be located away from

its theoretically exact position. It's often  applied to control the location of holes.

The theoretically exact position of the feature, 

called the true position, is  defined using basic dimensions, 

which are enclosed in a box to show that normal  dimensional tolerances don't apply to them. 

The position tolerance establishes a cylindrical  tolerance zone around the true position.

To be acceptable the axis of the hole must  be contained within the tolerance zone. 

The position of a hole can obviously be defined  using dimensional "plus and minus" tolerances,

so you might be wondering why the  geometric tolerance approach is

any better. There are a few reasons. One big advantage is that dimensional

tolerances define a rectangular tolerance zone,  but in almost all cases it makes more sense to use 

a cylindrical zone, which is evenly distributed  around the true position of the hole.

Another advantage of the geometric tolerance  approach is that it allows you to explicitly

define the relevant datums and the order  in which they should be considered. 

For holes the primary datum is usually  chosen to be the datum perpendicular to

the axis of the hole, because the hole  axis being perpendicular to the mating 

surface is usually more important  than its position on the surface.

And finally, using a position  tolerance allows a bonus tolerance 

to be gained by applying modifiers. Modifiers are an important part of GD&T 

that allow the tolerance zones applied  to features of size to be increased by 

an additional bonus tolerance, depending on how  close the feature is to its size limits.

This hole is a feature of size, and its limits  of size are defined by dimensional tolerances. 

The hole has a maximum allowable  diameter of 9.8 millimetres, and a

minimum allowable diameter of 9.2 millimetres.

GD&T identifies three different conditions  for any feature of size - a maximum material

condition, a least material condition and  a regardless of feature size condition.

The maximum material condition occurs when the  feature is at the size limit where it has the

most amount of material. For a hole this is the  smallest allowable hole size. And for a pin it's

the largest allowable diameter. The least material condition is the

opposite case where the feature has the  smallest allowable amount of material. 

By default geometric tolerances apply at  the Regardless of Feature Size condition,

meaning that the size of the tolerance  zone is defined by the tolerance value

in the feature control frame,  and it doesn't change.

But this behaviour can be adjusted by  including a modifier, either the letter  

M for MMC or the letter L for LMC, next to  the tolerance in the feature control frame,  

which adds a bonus tolerance to the tolerance zone  depending on the actual size of the feature.  

Let's look at an example for MMC, which  is the most commonly used modifier.

Here it's applied to the position of a hole.

The modifier means that the tolerance zone  in the feature control frame applies at the

maximum material condition, which  is the smallest allowable hole.

If the hole is larger than MMC, a bonus tolerance  is added to the position tolerance that's equal  

to the difference between the actual size of the  feature and the maximum material condition.

One common use of the MMC modifier is where it's  applied to benefit from the fact that a hole

is oversized, and so its position can be less  accurate and still fit with a mating part.

The LMC modifier is less commonly  used but works in a similar way.

It adds bonus tolerance when the actual  size of the feature has more material.

This can be used if you have a hole close to an  edge for example, and you need to apply a tight

tolerance to the position of the hole to make  sure there's sufficient material between the

hole and the edge, but you want to relax  the tolerance if the hole is smaller.

The MMC and LMC material modifiers can  be applied to other tolerance types like

flatness or perpendicularity, and can even be  applied to datums if they're features of size,

although that's outside of  the scope of this video.

Material modifiers aren't the only way that  the form of a feature can be affected by its

size. A key concept that appears in the ASME  standard is the Envelope Principle, which is

also referred to as GD&T Rule Number 1. It states  that "the surface or surfaces of a regular feature

of size shall not extend beyond an envelope  that is a boundary of perfect form at MMC".

Let's look at an example to see what this  means. This pin has a nominal size of 12  

millimetres with a dimensional tolerance  of plus or minus half a millimetre.

Rule Number 1 establishes an envelope around  the pin based on the maximum material condition,

which is the largest possible diameter of  12.5 millimetres. According to Rule Number 1,

no part of the pin surface is allowed  to extend beyond this envelope.

This means that the MMC limit of size controls  not only the size but also the form of the pin.

If the actual size of a manufactured pin is  12.5 millimetres, meaning that it's at MMC,

then it will need to have perfect  form to fit into the envelope.

If the actual size of the pin is smaller than  MMC, the pin doesn't have to have perfect

form - it can be slightly bent, or barreled,  for example. To be acceptable the pin has to

be within the limits of size, be contained  within the MMC envelope, and meet any other  

geometric tolerances that have been defined. Rule number 1 is the default behaviour in the  

ASME standard, and its purpose is to ensure  that parts will fit together properly. This  

pin will always fit in a hole that's  larger than the MMC envelope. 

The ISO standard works differently - it  uses the Independency Principle by default,  

where the geometric form and the size  of a feature are considered separately.  

When using this approach the limits of size  do not control form at all. The pin could be  

completely bent and still acceptable so long as  it is within the limits of size. When using the  

Independency Principle there's no guarantee that  the pin will fit in a hole larger than the MMC  

envelope. Additional geometric tolerances need to  be applied to control how straight the pin is.

Modifiers can be used to change the default  behaviour. If the E modifier is used on a  

drawing to the ISO standard, it means  that the Envelope Principle applies.

And if the I modifier is used on a drawing to the  

ASME standard it means that the  Independency Principle applies.  

ASME defines a few other special cases where the  Envelope Principle is overwritten - for example  

if a flatness or straightness tolerance has been  called out explicitly for a feature of size. 

But anyway that's enough about modifiers  and the Envelope Principle - let's go  

through the remaining tolerance types. Profile tolerances are very versatile and  

can be used to control the form, orientation and  location of features all at the same time. 

The Profile of a Surface tolerance creates  a tolerance zone that follows the shape of  

the toleranced feature, with a width  equal to the specified tolerance.

Profile of a Line is similar to Profile  of a Surface, but it controls individual  

line elements of a surface, instead  of the entire surface at once.

The inspection of complex profile tolerances  can be difficult without a CMM, although  

it really depends on the application  and the complexity of the surface. 

In some cases profile tolerances can be  used instead of other tolerance types.  

Applying a Profile of a Surface tolerance to a  single nominally flat surface without datums,  

for example, is the same as  applying a flatness tolerance. 

Finally we have the runout category of tolerances.

Runout is a term used to describe the eccentricity  of a surface relative to a particular axis. 

There are two runout tolerances,  circular runout, and total runout. 

Circular runout controls the roundness  of individual cross-sections of a feature

relative to a datum axis. In this case the  datum axis is defined by datum feature A.

The tolerance zone is defined  by two concentric circles.

This is similar to the circularity tolerance zone,

except that circular run-out uses datums so the  tolerance zones must be centred on the datum axis.

Like circularity, circular run-out only  controls individual cross-sections,

so the radius of the tolerance zone can  vary along the axis of the feature. 

Circular runout can be inspected by rotating the

part around the datum axis and using  a dial gauge to measure deviations.

Total runout, which has two arrows in the  symbol, is used to control runout along

the axial direction as well, so the tolerance  zone is defined by two concentric cylinders.

During inspection the dial gauge is  moved along the part to see if there

are any deviations outside  of the tolerance zone. 

Runout tolerances are often applied to  rotating parts like shafts, because any  

significant eccentricity relative to the axis  of rotation can cause unwanted vibration.  

GD&T is a pretty complex topic and it's impossible  to cover everything in a single video, but  

hopefully this has given you a solid understanding  of the fundamentals. If you enjoyed the video,  

you might be interested in hearing about  Nebula, the creator-built streaming site  

where you can watch exclusive videos from The  Efficient Engineer that aren't on YouTube.

There's the video on The Future of  Engineering Drawings, for example,

that explores recent trends related to engineering  drawings, like the move towards model-based

design and the use of statistical tolerance  analysis. Or the video on Hydraulic Systems.

Or the one on Dimensional Analysis...

There's loads of amazing content from other  creators on there too, including exclusive  

videos from Mustard, RealLifeLore and Neo. And all of the videos on Nebula are completely  

ad-free and there are no sponsor messages,  so you can watch without interruption. 

To make Nebula even better, we've teamed up  with CuriosityStream, who have kindly sponsored

this video, to create an unbelievable bundle  deal that includes both streaming sites.

CuriosityStream is home to thousands of  high quality documentaries covering all

kinds of different topics, including space,  technology, history and the natural world.

I've recently been enjoying  Oddly Satisfying Science,

a really fun series that uses  loads of fascinating experiments

to investigate interesting science topics  like combustion, resonance and magnetism.

And here's why the bundle is such a good deal  - if you sign up to any of the CuriosityStream

annual plans using this link and enter the code  "efficientengineer", you'll not only get a 26%

discount, but you'll also get full access to  Nebula, for free. The landing page is a little

confusing, but if you use the "efficientengineer"  code when signing up for any annual plan you'll

receive an email from Nebula with instructions  on how to set up your free account.  

That's less than $15 to get access to both sites  for a full year. It really is a great deal.

So to get the bundle deal head over to  curiositystream.com/efficientengineer,

or click the on-screen link, and  use the code "efficientengineer".

Every single person who signs up using  this link will be directly supporting  

this channel, helping me continue  to create engineering videos.

And that's it for this introduction  to GD&T, thanks for watching!