Manufacturing Engineering MECH 314

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Manufacturing Engineering - MECH 314

• Lecture 21

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Cylindricity Control
DATUM SYSTEMS

• March 5, 2007

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ANNOUNCEMENT
PROJECT AND LABS

• TEAMS MAY USE EITHER OF THE SCHEDULED LAB PERIODS
  TO GET HELP WITH THEIR DESIGN AND TOOLPATHS
  GENERATIONS
• i.e. you may use both the Wednesday 830-1030
  am, and the Friday 330-530 pm lab periods

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Form Geometric Characteristic Symbols

• There are four form control symbols
• straightness
• flatness
• circularity,
• cylindricity

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Cylindricity Application

• A cylindricity control is used on a drawing is to
  limit the surface conditions (out of round,
  taper, and straightness) of a shaft diameter.
• The cylindricity control limits the maximum
  allowable cylindricity error of the shaft
  diameter and the following statements apply
• The diameter must also be within its size
  tolerance.
• The cylindricity control does not override Rule
  1.
• The cylindricity control tolerance must be less
  than the total size tolerance.
• The cylindricity control does not affect the
  outer boundary of the FOS.

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Legal Specification Test for a Cylindricity
Control

• No datum references can be specified in the
  feature control frame.
• No modifiers can be specified in the feature
  control frame.
• The control must be applied to a cylindrical
  feature.
• The cylindricity tolerance value less than
  diameter tolerance?
• The tolerance value must be less than any other
  geometric control that limits the cylindricity of
  the feature.

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Inspecting Cylindricity

• When inspecting a part with a cylindricity
  specification, several separate parameters must
  be checked
• The size of the FOS,
• The Rule 1 boundary, and
• The cylindricity requirement.
• A cylindricity control can be inspected in a
  manner similar to the inspection for circularity.
  
• The difference is that for cylindricity, a spiral
  path (a helix) must be traced on the part
  surface.
• The surface points would then be compared to the
  two coaxial cylinders of the cylindricity
  tolerance zone.

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Form Control Summary

• A summarization of form control information is
  shown below

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DATUMS

• IMPLIED DATUMS
• An implied datum is an assumed plane, axis, or
  point from which a dimensional measurement is
  made.
• Implied datums are an old concept from coordinate
  tolerancing.

Implied Datums

• In this figure, the bottom and left sides of the
  block are considered implied datums.

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Shortcomings of Implied Datums

• Implied datums have two major shortcomings. They
  do not clearly communicate (to the drawing user)
• Which surfaces should contact the inspection
  equipment.

• In which sequence the part should be brought into
  contact with the inspection equipment.

Each sequence would produce different results for
the part measurements.
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Datum Features and Datums

• Datum features are part features and they exist
  on the part.
• Since a true geometric counterpart is
  theoretical, the datum is assumed to exist in,
  and be simulated by, the associated inspection
  (or processing) equipment.
• Datums are theoretical reference planes, points,
  or axis and are simulated by the inspection
  equipment.

• A datum feature simulator is the inspection
  equipment that includes the gage elements used to
  establish a simulated datum.
• For practical purposes, a simulated datum is
  considered a datum.

Planar Datums
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Datum Feature Symbol

• The symbol, called the datum feature symbol, used
  to specify a datum feature on a drawing is shown
  here
• The method of attaching this symbol to a part
  feature determines if it designates a planar
  datum or a FOS datum.
• The figure shows four ways of displaying the
  datum feature symbol to denote a planar datum.

Datum Feature Symbol
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Datum Reference Frame

• In order to restrict the six degrees of freedom
  on a part with planar datums, it takes the use of
  three datum planes.
• When three datum planes are used, they are
  considered to be a datum reference frame.
• A datum reference frame is a set of three
  mutually perpendicular datum planes.
• The datum reference frame provides direction as
  well as an origin for dimensional measurements.

Datum Reference Frame

• The planes of a datum reference frame have zero
  perpendicularity tolerance to each other by
  definition.

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The 3-2-l Rule

• Only applies on a part with all planar datums.
• The primary datum feature has at least three
  points of contact with its datum plane.
• The secondary datum feature has at least two
  points of contact with its datum plane.
• The tertiary datum feature has at least one point
  of contact with its datum plane.

• The three-point contact is the ideal condition.
• Form error on a planar datum feature can cause
  one point contact or "rocking" of a part on a
  primary datum plane.
• In these cases, the part must be stabilized on
  its primary datum plane.

Primary, Secondary, and Tertiary Datums
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Datum-Related Versus FOS Dimensions

• Only dimensions that are related to a datum
  reference frame through geometric tolerances
  should be measured in a datum reference frame.
• The hole locations are related to the datum
  reference frame, D primary, E secondary, and F
  tertiary.

• During inspection of the hole location
  dimensions, the part should be mounted in datum
  reference frame D-E-F
• but,
• The overall dimensions are not related to the
  datum reference frame

Datum-Related Versus Feature of Size Dimensions
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Inclined Datum Features

• On parts with datum features (surfaces) at angles
  other than 90, the datum reference frame will
  contain planes at the basic angle of the part
  surface
• The part shown here has a surface 60 from datum
  feature A this surface is designated as datum
  feature C.
• For this type of datum reference frame, the angle
  of the surface must be specified as basic, and
  the surface is controlled by a geometric
  tolerance.

Inclined Datum Features

• The inclined datum feature would have its datum
  feature simulator oriented at the basic angles
  shown on the drawing.

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Multiple Datum Reference Frames

• A part may have as many datum reference frames as
  needed to define its functional relationships.
• The datum reference frames may be at right angles
  or at angles other than 90.
• Also, a datum plane may be used in more than one
  datum reference frame.

datum reference frames

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Coplanar Datum Features

• Coplanar datum features are two or more datum
  features that are on the same plane.
• A single datum plane can be established from
  multiple surfaces.
• A datum feature symbol is attached to a profile
  control.
• The profile control limits the flatness and
  coplanarity of the surfaces.

Coplanar datum

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Datum Target Symbols

• A datum target application uses two types of
  symbols
• a datum target identification symbol, and
• symbols that denote which type of gage elements
  are to be used.

• The symbol is divided into two parts with a
  horizontal line.
• The bottom half denotes the datum letter and the
  target number associated with that datum
• the top half contains gage element size
  information when applicable.

DATUM TARGET EXAMPLE
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Datum Target Applications

• Note that flat-tipped gage pins are used to
  simulate the area datum targets specified on the
  drawing

• If the leader to the datum target is dashed (Al,
  A2, and A3), the gage pin contacts the back
  surface of the part in the view shown.

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Datum reference frame for irregular surfaces

• Gage elements that contact the points on the
  irregular surfaces are used to establish the
  datum reference frame.

• Note the use of basic dimensions to define the
  size and location of the datum targets.