WEBVTT

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>> This lecture is
for lab number 16

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on geometric dimensioning
and tolerancing.

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Now, we've talked about
tolerancing before.

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It's the allowance for
specific variation in the size

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and geometry of a part.

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So when you're manufacturing
or creating something,

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it's expected that it
would not be perfect

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and therefore would
be subject to errors.

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However, if we allow errors,
we need to be specific

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about how much error or what
is our tolerance for error

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that is acceptable for our part.

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The need for tolerancing
is because it is impossible

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to manufacture a part to
an exact size or geometry.

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And since variation from the
drawing cannot be avoided,

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we must specify the
acceptable degree of variation

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and for our part to
still function properly.

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Because if you allow
too much variation,

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the part may not be
functional anymore.

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However, if you allow
too small of a variation,

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it may affect the cost
of manufacturing the part

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because you would require
precise manufacturing.

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And also, it would result
in incorrect rejection

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or in quality control of parts

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that otherwise would
have been functional.

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So we need to be careful about
being too precise as well.

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We have to remember

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that tolerance should
follow the function.

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For assemblies, for instance,
parts will not fit together

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if their dimensions do not fall

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within a certain
range of values.

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And we don't want that,
because you won't be able

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to assemble the different
parts of the assembly.

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And also, for interchangeable
parts, if a replacement is used,

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it must be a duplicate
of the original part.

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But of course within
certain limits of deviation

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because you cannot expect it

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to be exactly the
same as the original.

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The relationship between
functionality and size or shape

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of an object varies
with the part.

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So for instance, for automobile
transmission, the transmission,

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it's very sensitive to the
size and shape of the gears.

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On the other hand, for
bicycles, it's not as sensitive.

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Therefore you can
allow bigger variation.

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There's two forms of
physical tolerance.

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One is size.

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Okay? Which gives limits
to the allowed variation

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in each dimension,
maybe the length, width,

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height, diameter, et cetera.

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And a second part, which
is a little more difficult

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to specify is tolerance
with respect to geometry,

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meaning a cylinder has to be --

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say if you have a cross
sectional area that's circular.

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Of course, you cannot
get a perfect circle.

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So how would you specify
the deviation from a circle

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that you would allow,
the deviation

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from being a perfect circle?

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And that's what we deal with
in geometric dimensioning,

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in tolerancing, or GDT.

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Allows for specification of
tolerance for the geometry

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of the part, not just the
size, but the actual geometry,

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its deviation from a
perfect form or geometry.

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Geometric dimensioning
tolerance uses special symbols

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to control the different
geometric features of a part.

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And it's an elaborate system.

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And it would take
more than just one lab

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to understand it, to master it.

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But I just wanted to have an
understanding of how it works.

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Now, there's generally
three levels of tolerances:

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the default tolerance, which is
placing the drawing title-block

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by the engineering firm.

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And it typically conforms
to routine tolerance levels.

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So it's what applies
to tolerances for parts

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within your drawing, okay,
unless otherwise specified.

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Okay, that's the default.

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The general one is
placing the drawing

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by the design engineer
as a note, okay?

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It applies to the
entire drawing.

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So there might be a note saying:
So-and-so is the tolerance

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for all parts, or
say, for all circles.

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And it supersedes, of
course, the default tolerance

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because there's an explicit
statement in the note

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through a note that is
a general tolerance.

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We also have specific
tolerances associated

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with a single dimension
or geometric feature.

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And of course, it supersedes
the two previous types

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of tolerances.

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And there's different ways
of specifying a tolerance.

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Limit dimensions.

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This is called limit
dimensioning, okay,

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where you give the minimum
and the maximum, the upper

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and the lower value
of the dimension.

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So that an acceptable part
may be at the upper limit,

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lower limit or any
value in between.

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The advantage of using this
is it's easy for fabricators

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to just measure the total
distance and make sure

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that it's really in this limit.

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Okay, here's another way

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of specifying deviations
by limit dimensions.

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A plus or minus, it's
called unilateral deviation

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if only in one direction.

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And bilateral if the error is
allowable in both directions.

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It will be slightly
bigger that the idea value,

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and it could also
be slightly smaller.

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Basic size dimensions
given with tolerances noted

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as a plus or minus range.

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So you have a target
value and a plus or minus.

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Okay? And in terms of bilateral,
you can have it equal,

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meaning the error in
the positive and error

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in the negative are
exactly the same.

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Here an example.

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If it's equal or bilateral
tolerance is easiest

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to fabricate because most
errors are random in nature.

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And most errors have more
or less the same probability

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of being too big or too small.

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All you have to do is
give the target values

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and the random error can be
added or subtracted depending

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on the random case
that you have.

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Yeah, so the fabricator
can simply set the machine

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to the target value.

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And the error could be plus or
minus and it would be acceptable

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as long as it's within
the limits.

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Geometric dimension

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and tolerancing stresses
tolerance levels not just

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for sizes, how big or small it
is, but in terms of tolerances ,

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in terms of deviations
from perfect geometry.

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That's why it's called
geometric dimension.

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It uses standard symbols
to indicate tolerances

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that are based on the
feature's geometry.

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It's also called, sometimes
called feature-based

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dimensioning and tolerancing

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or true position
dimension and tolerancing.

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But everyone knows
what GDT means.

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It's based on feature control
frames to indicate tolerances.

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In fact, for your labs most

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of it will just be creating
these feature control frames

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in order to specify the
tolerance that is being required

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for a particular dimension.

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And it's the state of the art in
terms of indicating tolerances.

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Just a brief history
of tolerancing.

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In the 1800s many factories
used the cut and try,

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file and fit approach.

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So basically you cut
it and try if it fits.

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If it doesn't fit you
file it until it fits.

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But it's sort of
a trial and error.

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And then a slight improvement of
that would be the plus and minus

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or coordinate system
of tolerancing.

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So basically you are just giving
tolerances in terms of size.

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If it is slightly bigger or
slightly smaller.

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But in the 1900s, they developed
the first GDT geometric

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dimension and tolerance
standards in order

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to improve quality and utility
of engineering drawings.

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And it evolved quite
a bit since then.

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In 1966 the united GDT
standards were published.

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The geometric dimensioning
tolerancing is based

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on what's called
envelope principle,

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which establishes an envelope
of and acceptable part

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that satisfies the functionality
of that particular part.

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Any deviation in form
is acceptable as long

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as it's within this envelope.

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So, here's an illustration of
the envelope principle, okay?

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So the red dashed lines
represent the lower limit.

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This is the smaller one.

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The upper limits.

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So anything in between
them will be an envelope.

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So those are the dashed red
lines indicate the envelope.

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So as long as the actual
part is inside that envelope.

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As you can see, the geometry,
it's not perfect anymore, right?

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The cross section is probably
not perfectly circular.

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And the surface of the
cylinder is also crooked.

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But as long as it's
inside the envelope,

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it is acceptable according
to the envelope principle.

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So, note that the deviation
is not just in size.

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It deviates from the perfect
geometric properties of,

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say, a perfect cylinder.

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Limits of size indicates
a variation in form

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that is allowable between
the least material condition

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and the maximum material
condition.

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Okay? So the conditions wherein
you are using the least amount

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of material to make the part.

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And the other extreme is
using the maximum material

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for the part and still be
acceptable to do its function.

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So here's another illustration
of the envelope principle.

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Giving the minimum
and the maximum.

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And anything that's inside the
envelope will be acceptable.

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So the envelope principle
defines the size.

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It's not just the size,
but also the form,

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geometric relationships
for the parts.

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And as I said earlier,
GDT is based

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on the feature control frame.

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Okay? And here's the
simplest illustration of that.

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So, they are the basic
dimension symbols, for instance.

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And we've talked
about it before.

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Any dimension that doesn't
have a tolerance is identified

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as basic dimension by putting
it inside a rectangular box.

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Letter B. All these
examples here are samples

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of GDT feature control frames.

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So for instance, this
definition here is a definition

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of a datum reference letter.

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Okay, a datum is either a
surface or an axis or an edge

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that can be used to refer
geometric properties.

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Okay, here is geometric
characteristic symbol.

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Okay? That is used for, this an
example, that's called flatness.

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Let's talk about that.

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Here is the actual
tolerance level.

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And we'll understand
how it's interpreted.

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Here's another example.

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We have this position a
geometric characteristic symbol.

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Here, this symbol is for
cylindrical tolerance.

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This is the tolerance value.

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And this modifier here.

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M is for maximum
material condition.

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Here is a specific example
of a feature control saying

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that that has a datum reference.

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So the interpretation of this
particular one here means

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that the particular feature

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to which you attach this
feature control frame should be

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perpendicular within a tolerance
of .05 with respect to datum,

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therefore it's the same.

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Okay? So this is other example
of feature control frames.

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So these feature control
frames are based on symbols

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that determine form
and orientation,

00:13:40.086 --> 00:13:42.096 A:middle
terminal control
form and orientation.

00:13:43.226 --> 00:13:48.076 A:middle
And these 14 characteristics
may be controlled.

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There's three basic
types of controls.

00:13:56.026 --> 00:14:00.236 A:middle
Those are based on controlling
forms, orientation and location.

00:14:00.236 --> 00:14:02.976 A:middle
So for instance, with
respect to controlling form,

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we have flatness
shown by this symbol.

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Straightness, okay.

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Circularity, this
is self-explanatory.

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And cylindricity or
the cylindrical center.

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So these are measures of
form, geometric forms.

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On the other hand, and
there's no individual --

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there's no datum
reference needed.

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So, if something is flat,
it's flat relative to itself

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or straight or circular
or cylindrical.

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On the other hand, we also
have orientation controls

00:14:36.156 --> 00:14:40.866 A:middle
such as length profile,
surface profile, as shown here.

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Perpendicularity,
angularity and parallelism.

00:14:45.696 --> 00:14:48.886 A:middle
And of course, these orientation
controls would be relative

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to a datum.

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So for instance when you say
this particular feature is

00:14:54.456 --> 00:14:57.006 A:middle
perpendicular to within
a certain tolerance.

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Perpendicular with
respect to what?

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It's got to be with respect
to some reference or datum.

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Cylindrical tolerance zone.

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It indicates that the
position can vary along a

00:15:13.596 --> 00:15:15.056 A:middle
cylindrical region.

00:15:16.626 --> 00:15:19.496 A:middle
So here's the tolerance
cylinder, right here.

00:15:19.606 --> 00:15:26.386 A:middle
And if we say that, say, the
position of the center of, say,

00:15:26.386 --> 00:15:32.776 A:middle
this cylindrical hole has a
cylindrical tolerance region.

00:15:33.016 --> 00:15:38.786 A:middle
What that means is if you look
at the axis of the cylinder,

00:15:39.536 --> 00:15:40.936 A:middle
okay the cylindrical hole,

00:15:41.336 --> 00:15:46.066 A:middle
as long as it's inside the
cylindrical tolerance zone,

00:15:46.616 --> 00:15:48.286 A:middle
whether it's off-center

00:15:48.286 --> 00:15:52.656 A:middle
or whether the axis is
slightly tilted, okay?

00:15:52.996 --> 00:15:58.426 A:middle
It's okay as long as it's inside
this cylindrical tolerance zone.

00:15:58.786 --> 00:16:00.326 A:middle
So cylindrical means
that instead

00:16:00.326 --> 00:16:06.346 A:middle
of just varying the position
or location in one direction,

00:16:07.496 --> 00:16:09.606 A:middle
or in two directions,
it's actually

00:16:09.706 --> 00:16:13.296 A:middle
in a cylindrical zone
that you can vary.

00:16:13.736 --> 00:16:15.016 A:middle
Material conditions.

00:16:16.346 --> 00:16:21.006 A:middle
We have something called maximum
material condition, okay?

00:16:21.046 --> 00:16:24.916 A:middle
It's the largest acceptable
size for external features.

00:16:24.916 --> 00:16:29.306 A:middle
So if it's an external
feature, so it's a solid,

00:16:30.086 --> 00:16:34.036 A:middle
the bigger the size is, the
more material you're going

00:16:34.036 --> 00:16:34.556 A:middle
to be using.

00:16:34.556 --> 00:16:38.306 A:middle
That's why it corresponds to
the largest acceptable size.

00:16:38.906 --> 00:16:42.156 A:middle
It also corresponds to the
smallest acceptable size

00:16:42.496 --> 00:16:45.126 A:middle
for internal features,
meaning a hole.

00:16:45.456 --> 00:16:50.446 A:middle
So for instance maximum
materials condition is achieved

00:16:50.476 --> 00:16:56.006 A:middle
when this outer feature here,
the diameter is maximum.

00:16:56.416 --> 00:17:00.266 A:middle
On the other hand, it's
achieved by making the hole,

00:17:00.376 --> 00:17:02.766 A:middle
the internal feature, as
small as possible.

00:17:03.476 --> 00:17:08.296 A:middle
As a result of using
the maximum material,

00:17:08.866 --> 00:17:11.686 A:middle
the object weighs the
most under this condition.

00:17:12.076 --> 00:17:15.726 A:middle
On the other hand, least
material condition corresponds

00:17:15.816 --> 00:17:20.866 A:middle
to either the smallest
acceptable size

00:17:20.866 --> 00:17:24.366 A:middle
for external feature because
you'll be using materials the

00:17:24.366 --> 00:17:26.196 A:middle
smaller the object is.

00:17:26.736 --> 00:17:30.286 A:middle
Or the largest acceptable
size for internal features.

00:17:30.336 --> 00:17:32.166 A:middle
So the larger this hole is,

00:17:32.546 --> 00:17:34.066 A:middle
the less material
you're going to use.

00:17:34.576 --> 00:17:38.556 A:middle
The smaller the outer
external dimension is,

00:17:39.536 --> 00:17:41.196 A:middle
the less material
you're going to use.

00:17:42.406 --> 00:17:48.026 A:middle
Of course, it corresponds to
the least weight of the object.

00:17:49.286 --> 00:17:53.576 A:middle
If the size of the
feature doesn't matter,

00:17:53.576 --> 00:17:57.606 A:middle
we have this condition called
regardless of feature size.

00:17:58.386 --> 00:18:00.346 A:middle
Indicates that the
material condition is not

00:18:00.426 --> 00:18:01.936 A:middle
to be considered.

00:18:03.406 --> 00:18:06.986 A:middle
A datum is a starting
point for a dimension.

00:18:07.476 --> 00:18:10.336 A:middle
Datum are theoretically
ideal location in space

00:18:10.856 --> 00:18:12.146 A:middle
such as a plane, centerlines.

00:18:13.356 --> 00:18:16.206 A:middle
Not every GDT feature
requires a datum.

00:18:17.156 --> 00:18:18.826 A:middle
The form controls don't.

00:18:19.886 --> 00:18:21.996 A:middle
Orientation and location do.

00:18:23.236 --> 00:18:27.146 A:middle
There are different ways of
defining a datum reference.

00:18:27.966 --> 00:18:31.626 A:middle
Now, we're going to
use these dashes before

00:18:31.626 --> 00:18:33.666 A:middle
and after the letter.

00:18:34.786 --> 00:18:35.946 A:middle
Here is an illustration

00:18:35.976 --> 00:18:38.106 A:middle
on how you're going
to do it on AutoCAD.

00:18:39.136 --> 00:18:44.136 A:middle
Again, simply attach it to
the -- to an extension line.

00:18:44.136 --> 00:18:47.726 A:middle
Let's understand
some of the controls.

00:18:47.726 --> 00:18:49.686 A:middle
Flatness, okay?

00:18:51.106 --> 00:18:52.276 A:middle
You see this symbol here?

00:18:52.276 --> 00:18:54.126 A:middle
Flat to within .25.

00:18:54.346 --> 00:19:00.796 A:middle
What that means is the surface,
okay, may be within this limit

00:19:01.076 --> 00:19:05.996 A:middle
of two parallel planes
or .25 apart.

00:19:07.296 --> 00:19:08.206 A:middle
Straightness.

00:19:08.726 --> 00:19:11.406 A:middle
Straightness within
.02 means that.

00:19:11.536 --> 00:19:15.646 A:middle
For instance it is the
-- yeah, this edge here.

00:19:16.996 --> 00:19:21.846 A:middle
Okay? What that means is as long
as any point in that edge is

00:19:21.846 --> 00:19:25.976 A:middle
within .02 tolerance
zone you're okay.

00:19:26.386 --> 00:19:32.606 A:middle
Here's an illustration of a
cylindrical tolerance zone.

00:19:32.736 --> 00:19:38.486 A:middle
So for instance here, for the
axis of the cylinder here,

00:19:38.916 --> 00:19:44.386 A:middle
cylindrical tolerance zone
of .04 means that as long

00:19:44.386 --> 00:19:51.226 A:middle
as any point in the axis of
the object is inside a cylinder

00:19:52.876 --> 00:19:59.426 A:middle
of the cylindrical tolerance
zone of .04, you're okay.

00:20:00.496 --> 00:20:02.286 A:middle
Circularity or roundness, okay?

00:20:02.736 --> 00:20:06.536 A:middle
Again, it's an indication
of how much you can deviate

00:20:06.536 --> 00:20:11.556 A:middle
from being a perfectly
circular cross section,

00:20:11.656 --> 00:20:12.906 A:middle
for instance, okay?

00:20:13.716 --> 00:20:15.316 A:middle
So here's your tolerance zone.

00:20:15.316 --> 00:20:18.306 A:middle
What that means is as long
as you are within that zone

00:20:18.306 --> 00:20:24.806 A:middle
of .25 wide, you're
okay for circularity.

00:20:25.886 --> 00:20:26.876 A:middle
Cylindricity.

00:20:27.196 --> 00:20:29.626 A:middle
Very similar, okay?

00:20:30.256 --> 00:20:34.366 A:middle
You're inside this
tolerance zone of .25

00:20:34.366 --> 00:20:39.116 A:middle
of your cylinder
anywhere along the length

00:20:40.016 --> 00:20:43.676 A:middle
of cylinder you'll be okay.

00:20:43.866 --> 00:20:44.276 A:middle
Parallelism.

00:20:44.276 --> 00:20:45.656 A:middle
So, here's the reference.

00:20:45.656 --> 00:20:47.416 A:middle
What that means is
this edge here

00:20:48.246 --> 00:20:49.996 A:middle
or this plane here
should be parallel

00:20:49.996 --> 00:20:51.236 A:middle
to the plane in the bottom.

00:20:51.596 --> 00:20:54.116 A:middle
This is your reference, 3.12.

00:20:54.166 --> 00:21:01.426 A:middle
So again, there's a tolerance
zone of .12 within any point

00:21:01.426 --> 00:21:06.036 A:middle
of your surface could
vary and still be parallel

00:21:06.116 --> 00:21:10.496 A:middle
to the datum plane
A. Concentricity.

00:21:11.186 --> 00:21:16.536 A:middle
It means having the same
center for two circles, okay?

00:21:16.536 --> 00:21:21.906 A:middle
And again, this is an example
of a cylindrical tolerance zone,

00:21:21.906 --> 00:21:26.296 A:middle
which means that the location
of the center can be anywhere.

00:21:26.986 --> 00:21:30.796 A:middle
The center of the circle
cylinder can be anywhere along

00:21:31.776 --> 00:21:35.126 A:middle
that cylindrical tolerance zone.