GD&T Flashcards
What does GD&T Stand for
Geometric Dimensioning & Tolerancing
What is Tolerancing
The total amount a specific Dimension is permitted to vary
Why do you need to tolerance?
Because every dimension cannot be perfect
What factors influence change in fit?
Temperature, humidity, atmosphere, dust, corrosion, wear, lubrication, etc
What are the two ways that a fit can be specified
Hole and Shaft basis
What are the 2 major fit systems?
ANSI and ISO
What are the 3 ISO general groups?
Clearance, Transition and interference fits
What are the 3 ANSI general groups
“RC”, locational and “FN” fits
“40H8” what does the first number mean
The basic size of the component
“40H8” What does the letter mean?
The letter indicates the class of fit (UPPER CASE FOR HOLES, lower case for shafts)
“40H8” what does the second number mean?
The second number indicates the international tolerance (IT) Grade
what kind of fit is “H11/c11”
a loose-running fit for wide commercial tolerances or allowances on external members
what kind of fit is “H9/d9”
free running fit not for use where accuracy is essential, but good for large temperature variation, high running speeds, or heavy journal pressures
what kind of fit is “H8/f7”
Close running fit for running on accurate machines and for accurate location at moderate speeds and journal pressures
what kind of fit is “H7/g6”
Sliding fit not intended to run freely, but to move and turn freely and locate accurately
what kind of fit is “H7/h6”
Locational clearance fit provides snug fit for locating stationary parts; but can be freely assembled and disassembled
what kind of fit is “H7/k6”
locational transition fit for accurate location, a compromise between clearance and interference
what kind of fit is “H7/n6”
locational transition fit for more accurate location where greater interference is permissible
what kind of fit is “H7/p6”
Locational interference fit for parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements
what kind of fit is “H7/s6”
Medium drive fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron
what kind of fit is “H7/u6”
Force fit suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical
What does a metric limit tolerance value specified on one line look like
12.50±0.25
what does a stacked inch limit tolerance value look like
1.062
1.000
what’s the specified dimension of : 76±0.4
76
what’s the lower limit of : 76±0.4
75.6 (76-0.4)
Whats the upper limit of: 76±0.4
76.04 (76+0.4)
Whats the tolerance of 76±0.4
0.8 (76.04-75.6=0.8)
What type of dimension appears on a drawing in parentheses?
A reference dimension
What does an inch dimension with an equal bilateral tolerance look like?
0.750±0.005
what does ISO stand for
International standards organization
what does ANSI stand for
The American National Standards Institute
Why use GD&T?
Proper use guarantees the form, fit and function of a product without the need to make assumptions or interpret requirements
What is a feature?
a feature is a general term that applies to a physical portion of a part. (an edge, surface, vertex, etc)
What is a feature of size (FOS)
one cylindrical or spherical surface; a set of 2 opposed elements or parallel surfaces that are associated with a size dimension
When is MMC most often used?
when FOS is used in an assembly
When is LMC most often used?
LMC is often specified to ensure a minimum wall thickness on a part
When is RFS used?
RFS is often applied to ensure symmetry, and is the default if MMC and LMC aren’t specified
What is basic dimensioning?
The basic dimension is the theoretical exact size that the feature of size should be
How are basic dimensions indicated on drawings?
Enclosed in rectangles
What are tolerance zones?
They’re pictorial representation of the limits of size for a particular tolerance
What is the standardization of engineering drawing symbols controlled by
ASME Y14.5
What is this Symbol: Ø
Diameter
What is this Symbol: R
Radius
What is this Symbol: SR
Spherical Radius
What is this Symbol: SØ
Spherical Diameter
What is this Symbol: CR
Controlled Radius
What is this Symbol: X
Places or by
What is this Symbol: l_l
Counterbore
What is this Symbol: lSFl
Spotface
What is this Symbol: V
Countersink
What is this Symbol: a square
Square shape
What is this Symbol: ___
l
V
Depth or deep
What is this Symbol: <-O
Dimension Origin
What is this Symbol: ()
Reference
Characteristics of FORM
Never has a datum associated
Characteristics of PROFILE
SOMETIMES has a datum associated
Characteristics of LOCATION
ALWAYS has a datum associated
Characteristics of ORIENTATION
ALWAYS has a datum associated
Characteristics of RUNOUT
ALWAYS has a datum associated although datum may not be obvious
Material Condition: (M)
At maximum material condition
Material Condition: (L)
at least material condition
Material Condition: (P)
Projected tolerance zone
Material Condition: (F)
free state
Material Condition: (T)
tangent plane
Material Condition: Ø
Diameter
Material Condition: SØ
Spherical diameter
Material Condition: R
Radius
Material Condition: SR
spherical radius
Material Condition: CR
Controlled Radius
Material Condition: ()
Reference
Material Condition: {ST}
Statistical Tolerance
Material Condition: <–>
Between
What is a datum
a datum is a reference from which measurements are made (assumed to be perfect)
What is a datum feature?
An actual feature that is used to establish a datum, like a face, bore or shaft
What are datum symbols?
Symbols that are used to identify datum and in situations where more than 1 datum is used, their order of precedence is shown in the feature control frame
What is a datum centre plane?
A plane that splits a symmetrical feature such as a slot or tab (needs a leader)
What are the 6 degrees of freedom
The two rotational directions around the each of the xyz axis
What is the symbol for coplanar surface datums?
L-M in the control frame
What does A mean in the feature control frame?
Primary datum reference
What does B mean in the feature control frame?
Secondary Datum reference
What do letters connected by ‘-‘ in the feature control frame mean?
‘A-B’ multiple datum reference
When do coaxial features exist?
When more than 1 features share a common axis
for an external datum feature, how is the datum centre plane created?
Its established by 2 parallel planes at the minimum separation
for an internal datum feature, how is the datum centre plane created?
its established by 2 parallel planes at maximum separation
A parts tolerance should be as liberal as possible to be?
Consistent with the parts function
The limits of the size of a feature controls?
the amount of variation in both size and form
What is the “envelope rule”?
if the feature is at MMC its geometric form must be perfect
What does RFS mean
Regardless of feature size
What does RMB mean
Regardless of material boundary
What features are applied only on a RFS basis
Circularity, cylindricity, profile, circular runout, total runout, concentricity & symmetry
What are the exceptions to rule #1 Extreme form variations
*parts that have stock sizes: sheet metal, gaskets, tubing, etc
*a feature of size of a non-rigid part (rubber)
What can override rule #1 Extreme form variations
*apply a straightness control to the FOS
*By adding a special note to the FOS
*By means of the “independency” symbol (I)
What does FOS mean?
Features of Size
What is the symbol for RFS?
It doesn’t have one, its assumed when no other condition is specified
What is Rule #2 “all applicable geometric tolerances”
RFS applies, unless MMC or LMC is specified
What is Bonus Tolerance?
It allows the tolerance available to vary with the actual size of the part
When is Bonus Tolerance permissible
When a MMC or LMC modifier is used in the tolerance portion of the feature control frame
What is a bonus tolerance derived from?
from the FOS tolerance
What is flatness
a form of tolerance and it controls the whole surface by means of 2 planes specifying the tolerance
Flatness Independency
defining size tolerance as independent of form tolerance
What is circularity?
the control of “out-of-roundness”
How is circularity measured?
it is measured at any/each cross section perpendicular to the axis
Does circularity reference a datum?
No, it controls form
How large can the circularity tolerance be?
It must be less than a tolerance of size
How is circularity show in the control frame?
a circle followed by a dimension
Where does Free State Variation exist?
Where a part distorts when manufacturing forces are removed
How is parallelism gauged?
Its gauged relative to a datum or another feature (Face A is parallel to face B)
Parallelism geometric tolerance is defined by?
Parallel planes or cylindrical zones that are parallel to a datum plane
Theoretically what is a tangent plane?
Its an exact plane that is considered to “lie” on the high points of the feature surface
How can perpendicularity be specified?
Its always specified RFS and a datum is required
