Transcripts
1. Introduction and overview for the GD&T class: Welcome to the
geometric dimensioning and tolerancing course. Commonly refers to as GD and T. In this section
one of the course, we will explain what GD and T
is and why is it useful and beneficial for designers and manufacturers of
mechanical parts. By the end of this course, you will be able to
explain what GD and T is. Describe the purpose
of GD and T. And identify and
describe the purpose of the notations used for GDN. Then we will jump into
Section two of the course, which is called
position tolerances. Position tolerances and
dimensions define where features are located on the part with respect
to other features. Position tolerances are
typically used on holes, pins, tabs, and slots, and other features of size. As you will see, GD and
T position tolerances are particularly useful when dealing with pattern of holes. By the end of this section
two of the course, you'll be able to
describe how to apply position tolerances to
various features of size. And explain how LMC
and MMC designators apply to tolerance
values and to datums. Then describe how to specify
a projected tolerance zone and describe how to apply position tolerances
to coaxial features. In Section three of our course, we will talk about the
form and size tolerances. In this course, we will discuss size tolerances and
form tolerances, as well as cylindricity,
and cecularity. By the end of this course, you will be able to discuss the purpose of form
and size tolerances. Describe the envelope
principle as defined by ASME standard y 14.5. List and describe the
different types of form tolerances and
how they are applied to different features and define bonus tolerance
and virtual conditions. In Section four of our course, we'll be talking about profile
and run out tolerances. Profile torances are typically used on irregular surfaces where flatness and position tolerances are insufficient to describe
the part requirement. Runout tolerances are typically applied to rotating parts to maintain the form
and location of features with respect to
their bearing surfaces. By the end of this course, you will be able to
describe how to apply profile tolerances to
surface or linear elements. Explain how to correctly identify and prioritize
datum references and relate how to interpret and apply a runout or total
runout tolerances.
2. GD&T symbols: If you have seen or studied
mechanical drawings, you may have already encountered
many of the symbols and notations used by GD
and T. At first glance, they may seem complicated
or confusing. But they are actually
quite simple. Symbols are used to define the allowable variation of
each feature in a part. This is done by first defining the perfect theoretical form or dimension of that feature. Then describing the
allowable variation from that perfect form, usually within a defined zone. Each symbol describe
a different type of feature and the
allowable tolerance zone for that particular surface. We use three main
types of notations to define the perfect geometry
and allowable variation. These are first
features control frame. These symbols define the size and shape of the
tolerance zone and call out a frame
of reference for the feature by use
a data references. The datum features identifiers. These symbols identify which features on the part
are used as a datum. A datum can be any surface
axis or point that provide a frame of reference for the interpretation of the
feature control frame. Basic dimensions. The basic
dimensions shown as symbol, linear dimensions in
a rectangular box, representing the theoretical
exact definition from a feature to its
datum references.
3. Positional Tolerances Special conditions: In some cases, the function
of the feature may not be accurately reflected
by a tolerance on the feature within
the part itself. Consider a pin
pressed into a hole. Any angular error
in the hole will be magnified along the length
of the pin after assembly. To account for this issue, we can specify a
projected tolerance zone that extend beyond the
boundary of the part. The projected zone
is indicated by a P modifier behind
the tolerance value. The length of the
projected zone can be indicated in one of two ways, and numeric value behind
the pin modifier or a bold phantom line and
dimension on the drawing itself. The dimension refers to the minimal length by which
the zone must be extended. The effect is that
the feature axis is more tightly constrained
by the longer tolerance zone. For many parts with
whole patterns to ensure a good fit
within the made in part, it is important for
the holes to be positioned accurately
relative to each other. But it is okay for
the whole pattern to float relative to
other part features. For these situations, we introduce a composite
feature control frame. In the composite
feature control frame, the top half of the frame locate the whole pattern with
respect to datums, A, B, and C. It is called the pattern locating tolerance zone
framework or plots. It is interpreted exactly as the earlier examples except this time within a
larger tolerance zone. The bottom of half of
the frame is called the feature related tolerance
zone framework or frets. It controls the position of the holes relative
to each other, and the orientation
relative to datum A. The datums B and
C are not listed, so the basic dimensions
do not apply. The dimensions between
the holes do apply. So the effect is
that we have defined four tolerance zones of diameter 14,000 that can
move as a pattern. Each hole in the pattern has a virtual condition
of 226,000 diameter. But they can move
together as a pattern within the larger tolerance
zone defined in the plots. Looking closer at
the tolerance zones, we can get a better feel for why this works
the way it does. The upper half of
the control frame, the plots describes
four tolerance zones precisely located
from the datums. The frets defines four
smaller tolerance zones, precisely located with
respect to each other. That can float within the
larger plots tolerance zones. The axis of the actual holes must pass through both zones. Take note, the tolerance
in the lower frame, the frets will always be smaller than the tolerance in the
upper frame, the plots. Of course, position tolerances are not restricted
to holes and pins. They can also be
applied to tabs, slots, notches, and grooves. In these cases, the
tolerance zones are usually rectangularly ed. So we have a part with two
slots located from datum A, B, and C. Because the tolerance for the horizontal slot
position can be different from the tolerance for the vertical slot position. We need two feature
control frame. The notation boundary is
added beneath each frame to indicate that the tolerance applies to the entire
boundary of the feature. For the slot at the
top of the part, the tolerance zone is
at 20,000 of an inch wide and centered
1.5 " from datum B, the center plane of the slot feature must
applied within the zone. Because we specified that the
tolerance applies at MMC, we can define a
virtual condition for the slot at 970,000
of an inch wide. If the actual slot is above MMC, bonus tolerance would apply. For the slot in the
middle of the part, the tolerance zone
is rectangular. The center plane in each direction must be
within the tolerance zone. Like the first slot, we have specified that the
tolerance applies at MMC. Therefore, we could make a functional gauge with
an oblong pin that is one and 9,600 by 9,700 to test if the
part are within spec.
4. Coaxial Position Tolerance: So far, we have been discussing positional tolerances
of features that lies in the same plane. But positional
tolerances can also be used to control
coaxial features. In this example, the datum
feature is a feature of size. In we have applied an MMC modifier to both the position tolerance
and the datum reference. In practice, this means
that bonus tolerance is available if either the feature or the datum is below MMC. A functional gauge could be constructed for the part
and would look like this. There are conditions
where we will need a opposite positional tolerance to control coaxial features. In this example, we have four coaxial hole that must
be in a close alignment, but the functional
requirement allows them to float as a pattern relative
to the datum surface. The composite tolerance
plots controls the whole pattern
location while the fritz controls the coaxiality of
the whole to each other. The plots creates a 30,000 diameter tolerance zone that is parallel to Datos A and B. The center axis of each hole must pass
through these zones. The fret creates a second
10,000 diameter tolerance zone. This tolerance zone does not need to be parallel
to any datums, but the axis of all four holes must also
pass through the zone. A functional gauge to test
the upper frame would require a pin of diameter
460,000 of an inch. That was precisely located from Datum simulators for A and B. The functional gauge for
the lower frame would be a se pin of a diameter
480,000 of an inch. So far, we have looked at a
number of applications where the MMC modifiers is useful to specify the functional
requirement of a feature. We can also make use of the least material
condition requirement, where the function of
the feature requires it. In this example, we have
a tubular part with both a datum reference and a whole position
specified at LMC. LMC for the whole is when the whole is at its
maximum diameter. LLC for the datum is when the outside diameter is
at its minimum diameter. This means that additional
position tolerance is allowed as the feature of the part from the least
material condition, the virtual condition for
the whole describes a zone, which there will
always be material. The practical result
is that we have specified the minimum
allowed wall thickness for the tube while also allowing the maximum flexibility in
size and position tolerance.
5. Position Tolerances Conclusions: S position tolerances
define where features are located with
respect to other features. Position tolerances
are used on holes, pins, and slots, and
other features of size. First, you need to define a frame of reference for
the target position. Then you can define the theoretically exact
position for the features. Finally, you can define the allowable deviation in position from the exact
position we defined. MMC and LMC modifiers
are used to specify the functional requirement
of the feature to allow manufacturing flexibility while ensuring part
functionality.
6. Tolerances of size: One of the simpler concept in dimensioning is the
tolerance of size. The size dimension is used to identify the
diameter of a hole, the thickness of a bar, and the width of a
slab, and so on. Size tolerances can be applied to these basic
dimensions using a plus or minus tolerance
to let the manufacturer know the allowable limits
for the size of the feature. This seems simple, but the question arises when
there is a variance in form form refers to
error or distortion in the feature that are not described by the simple
plus or minus tolerances. If a shaft is at a maximum
allowable diameter, but it is slightly
bulged, is it acceptable? GD and T addresses
this question. Spoiler alert. The answer is no. Rule number one. In
the ASME standard, which defines the envelope
principle for the shaft, the envelope
principle means that the maximum allowable
diameter defines a cylindrical envelope in
which the entire shaft fit. A shaft at its maximum
diameter is said to be at MMC or maximum
material condition. Since any bend in a shaft that MMC would push it
outside the envelope, it would have to be perfectly
straight to be acceptable. However, the shaft at LMC or lease material
condition could be bulged by the entire
length of the size tolerance. Simply stated, rule number one requires perfect form
for features at MMC. There are some exceptions
to rule number one. First, stock dimensions such as the thickness of bar stock
or sheet are exempt. The designer must note on the drawing that the dimension
is a stock dimension. Second, rule number one does not apply to part that are
flexible in their free state. For instance, a piece
of rubber tubing, application of specific
geometric tolerance to size dimensions also can allow variation to exceed
the MMC envelo.
7. Types of Form Tolerances: Tlrances of form describes the allowable variations in
the contours of features and surfaces on a part
that are sometimes more and sometimes less stringent than the size
tolerance envelope. Tolerances of form are flatness, straightness, cylindricity,
and roundness. Tolerances of form control
the shapes or the contour of the indicated features without reference to
any other features. Therefore, they are
no datum reference in the feature control frame. We'll start our
discussion with flatness. A flatness tolerance is
applied to a single surface and defined a flat tolerance
zone by two parallel planes. All elements of the surface must fall between
the two planes. Rule number one still applies, so a flatness
tolerance is used to put tighter restrictions on a feature than the
size tolerance alone. As you can see in the example, the flatness zone relates only
to the indicated surface. It does not need to be parallel to the size
tolerance zone. Straightness is
similar to flatness. But when applied to
a single surface, it applies only in one
direction at a time. In the example, we see that the individual line element are required to be
straight in one direction, but variation is allowed
in the other direction. Straightness can also be
applied to a feature of size. When applying this control
to a feature of size, the feature control
frame should attach to the dimension leader and not
to the surface of the part. Instead of applying
to the surface, the straightness
tolerance defines the cylindrical
tolerance zone in which the axis of
the part apply. Straightness tolerance
applies to a feature of size is considered to apply
regardless of feature size. That means that even apart
at MMC can be out of straightness by the amount of the tolerance thereby
exceeding the MMC velo. In many cases, it
is desirable to add a modifier to the
straightness tolerance to indicate that it
applies only at MMC. We indicate this with a circled letter M just
after the tolerance value. This means that a
part is smaller than MMC can be even further
out of straight. As we see in the example, a shaft at MMC can still be
out of straightness by 5,000, but a shaft at LMC can
use the extra 20,000 size tolerance as a bonus to apply toward the
straightness tolerance. That's why we call it
a bonus tolerance. Note that in no case, will the shafts ever
exceed the envelope defined by the size limit
plus the straight tolerance, in this case, a
diameter of 515,000. We call this envelope
the virtual condition. VC. Think of it as
a virtual shaft of perfect straightness or a zone that no material can cross. We can even use this concept
to make a functional gauge. In this case, a precision
hole of diameter, 515,000 if any shaft
fits through the hole, it's good provided it
is not undersized. Remember that the
concept applies to shafts can also
be applied to holes. In the case of a hole, the maximum material
condition is met when the hole is at
a smallest diameter. This example shows a hole which coincidentally also has a virtual condition
of 515th thousand. Since the pins and holes all have the same
virtual condition, they can never be
interference issue. Sometimes we want to specify straightness
on a localized level. If a shaft is four feet long, we don't want it to be perfectly straight with a big
hook at the end. We can specify that each
section of the shaft meet a section control while we allow the entire
shaft to have tolerance. As we see in the example, we can even apply controls to both the entire shaft and to shorter section
at the same time. Cylindricity it
is like flatness, but apply to round surfaces. The cylindricity
tolerance zone is two concentric
cylinders separated by the allowed tolerance. Cylindricity is used
when control is needed, that are tighter than the
feature of size that allows. Therefore, it will
always have a value that is less than one half
of the size tolerance. Otherwise, the size tolerance
would be in control anyway. Like cylindricity, circularity, sometimes
called roundness, is used for round parts. However, when
cylindricity applies to the entire surfaces, circularity applies only
to each circular element. Each cross sectional element must be round within
the tolerance, but size changes along the length may exceed
the tolerance. Circularity can
also be applied to cones or other non cylindrical
but round features.
8. Form and Size Tolerances Conclusions: Size tolerances define
the allowable variation in the size for the feature. Form tolerances describe
the allowable variations in the contours of features
and surfaces on a part. The four type of form
tolerances are flatness, straightness, cylindricity,
and circularity or roundness. The envelope principle
rule number one, states that a feature must fall within the size
tolerance limits. Therefore, the feature at maximum material condition
must have perfect form. Parts at LMC can apply the bonus tolerances
to the form tolerance.
9. Profile of a surface: There are two types of
profile tolerances notations, profile of a line and
profile of a surface. A profile of a surface tolerance is designated with a
semicircle symbol. It is used to control the form or location
of a surface feature. Datum references and basic
dimensions describes the form and location of a
theoretically exact feature. The feature control
frame defines the allowable deviation
from the exact feature. A profile of a surface
tolerance can be used for any surface from a flat plane
to something very complex, but the principle is the same. Consider this example. The basic dimensions defines
the exact form of a surface, as well as the orientation and location with
respect to datums A, B and C. The control
frame defines a 2,500 of a millimeter
tolerance zone centered about the
exact feature. All points on the actual feature must lie within the zone. In some cases, it is more
convenient to define the inner or the outer boundary for surface than to identify
the center plane. In these situations,
you can designate that the tolerance
zone lies entirely on one side of the
exact surface using a dark phantom line and arrows on the
drawing as shown here. In some cases, the
part function may not require that a surface can be controlled
in two directions. In those cases, you can apply a profile tolerance to individual line movements
on the surface. The profile of a
line tolerance is designated with a phases arc. Profile of a line
tolerance applies only in the direction of the view in which the control
frame is applied. It means that the form of
each linear element of the surface must lie within
the designated zone. Although each
element can move in location within the limits
of the feature of size. Note that in this example, it was designated that the tolerance zone lies entirely below the
theoretical surface. Another use for application for profile tolerances is to control more than one
surface at a time. In this example, a profile of a surface tolerance is used to ensure that the two surfaces are co planar within the
designated tolerance zone. No datum reference is required because the surfaces are
only related to each other. Any number of surfaces may
be designated in this way. In some profile
tolerance applications, you want to control
multiple surfaces. In this example, the
profile tolerance applies to the entire
perimeter of the hole. This is designated by the small circle at the
start of the leader line. Each surface, all four sides and rounded corners must be within the eight tenth of a
millimeter tolerance zone, centered about the
theoretically exact feature. Profile tolerances can also
be used in composite form to provide greater control with respect to some
datum references. In this example, the upper part of the feature control frame controls the form and location of the feature with
respect to datums A, B, and C. The lower
control frame provide a tighter tolerance zone with respect to A and B only. The entire form of
the feature must fit in the narrower two tenth of
a millimeter tolerance zone, but the tolerance zone
can float within the larger a tenth of a
millimeter tolerance one.
10. Run-out: Runo tolerances are used to control the form of
circular surfaces, for rotating parts, relative
to their bearing surfaces. It is most often measured
with a dial indicator, which is why the symbols
resemble gauge pointers. There are two different
symbols for runout tolerances. A single arrow represent circular runout and applies to each circular
element of the surface. Double arrows indicate
total runout, which means that
the entire surface must be within the
designated tolerance zone. In this example, a
conical surface and the flat surface are both controlled by circular
runout controls. Each circular element of
each feature must have no more than 2000 of an inch
total indicator reading, TI R. Rotated around
the axis of data A. The cylindrical surface
must have no more than 10,000 of an inch TI R
for the entire surface. When moving the indicator
from left to right, the highest and lowest point must not exceed the
tolerance zone. It is important to mention
that runout is always used to control the form of a feature
regardless of feature size. Also, runout is always relative to the axis of
a different feature. So a datum reference
is always required. Some parts will rotate on more
than one bearing journal. This is the case
with motor shafts. Since the shaft is supported by two journals on the same axis. It is inappropriate to designate a primary
and secondary datum. In this case, you
designate them as equalizing datums in the
feature control frame. In practice for inspection, the part will be mounted
with two datum simulators, one for each datum.
11. Conclusions: Profile tolerances are typically used on irregular surfaces where flatness and position
tolerances are insufficient to describe
the part requirements. Run out tolerances are typically applied to rotating parts to maintain the form
and location of features with respect to
their bearing surfaces. You now should know how to
properly apply and interpret profile tolerances for both
surface and line elements, how to reference datums and apply basic dimensions
to describe features, and how to use composite profile tolerances to reflect specific
features requirements.
12. Datum Systems: In GDN t, there are
three types of datums. Surfaces, axis and points. A datum is a theoretically
perfect feature. For example, it can be a theoretically flat surface or the central axis
of a cylinder. However, the actual
feature is never perfect. So you refer to the feature as a datum feature to differentiate it from
the actual datum. When you set up a
part for inspection, you will often use
precision devices, such as surface plates or V blocks to hold the
part in a place. These devices are used with datum features to simulate the theoretically perfect datum. Even these precision
surfaces are not perfect. So you call these datum
feature simulators. When we are selecting
Dayton features, we try to create a fixed frame
of reference from which to define the location and orientation of other
features on the part. Consider this simple example. You want to dimenion
a hole in the part, and be certain that the manufacturing and
inspection processes are always repeatable. In this case, you have selected three surfaces as datums
and labeled them K, G, and R. When naming datums, you are free to choose any
letters and any order. Most people choose A, B and C by convention, but it's not a rule. The feature control
frame for the whole make reference to the datums inside the feature
control frame. The order of the
datums does marror. So the first second and
third datum references are called the primary, secondary and tertiary
datums respectively. The next figure explains how
they are to be interpreted. The primary datum will set flat on the datum
features simulator. Because the datum
feature is imperfect, it will rest on the three highest points or more if there are
on the same plane. The secondary datum feature will contact the datum simulator
on at least two points, establishing the rotational
position of the part. The tertiary datum will contact a datum feature simulator
on at least one point, establishing the
position of the block. You have now completed
a three fixed frame of reference from which you can evaluate the whole position. You may be wondering
why you picked the K surface as
the primary datum. After all, the whole is
dimensioned from G and r. The reason we chose K is this datum defines the
orientation of the whole. Your goal is to define the theoretically exact
position for the whole. Then the tolerance in the feature control
frame tells you the allowable deviation
from perfect. Wherever the hole is, with respect to the
size of the part, you want it to be
perpendicular to the surface. That's why k is
the primary datum. Once you have
established that it is perpendicular to
the mounting surface, you can locate it
from the edges, and any imperfections
in the edges will not affect the perpendicular
orientation of the tolerance zone. You can see from this example
that the order of placement of the datum references in the feature control
frame is very important. If R was listed first, the control would have a
very different meaning. In many cases, you need to
use round surfaces as datums. Look at the next example. The flat surface of the part is shown as the primary datum, just like the last example, but there is only one
other datum listed. This is because a circular
datum can provide two reference planes that meet at the axis
of the cylinder. The theoretically
exact positions of the holes are fully defined. In some cases, you will need to choose a feature of
size as a datum. Instead of a singular surface. For instance, the datum
may be the width of a slot where neither side has a functional
presidence over the other. In other cases, the datum may be a hole or a pin with
a specified diameter. In the case of a slot, the datum is the geometrical
center plane of the slot. The datum feature
simulator would be a ground block that
fits into the slot. So when you use a feature
of size as a datum, the allow variation and the
size must be considered. By default, the datums called out in a feature
control frame are considered to apply regardless
of feature size or RS. You can see in this example
that the datum simulators for RFS datums must be adjusted to match the actual size of
the datums on each part. This is appropriate when the function of
the part requires such a tolerance as in the
case of a press fit shaft. In many cases, though, the designer will purposely leave clearance between
features for assembly purposes. The designer can specify
the tolerance that must apply for the closest fit part to make sure they
assemble correctly. Consider the next example. A modifier has been added
to the datum references. The circled M indicates that
the tolerance applies when the datum is at maximum
material condition or MMC. A tab or a pin is said to be at MMC when it is at the
largest allowed size. A slot or hole is at MMC when it is at the
smallest allowable size. When MMC is a specified
for a datum reference, the datum simulator is made to match the MMC of the feature. When a part is not at this MMC, there is extra clearance
between the part and the datum simulator that
will allow the part to meet. This is considered
a bonus tolerance that gives the manufacturer an extra manufacturing
flexibility while ensuring that the
part fits together. In the final use of the part, this movement represent
assembly clearance. In some cases, we do not want to wish to use an entire
features or surfaces as a datum, but instead, we want
to limit the datums to a smaller area in
these situations. We use datum target to designate the precise points in
which to reference. Datum targets can be points, can be lines or small designated areas that are located with
basic dimensions. The target indicates
the specific locations from which the other
features are dimensioned. Point datums are indicated by an x placed at
the desired spot, and they are located
by basic dimensions. S split balloon is used
to identify the datums. Note that the
standard datum label is still attached to
the datum surface. Line datums are indicated
by a heavy phantom lines, located by basic dimensions and labeled with
a split balloons. Area targets are indicated by
bordered cross hashed area, located by basic dimensions and labeled with split balloons. F round targets, the
diameter of the target may be shown in the balloon
or dimensioned directly. You will notice that
multiple targets can be used to designate
a single datum. They can be any
combination of points, lines or areas as long as they reflect the final
function of the part.
Omar Koryakin, Metrology Manager
