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Dimensioning and Tolerancing. Tolerancing. No manufacturing process can produce parts with exact dimensions Allowable variations or tolerances must be specified by the designer, with two objectives: ensure fit and function minimize manufacturing cost. Example - shaft in hole fit.

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tolerancing
Tolerancing
  • No manufacturing process can produce parts with exact dimensions
  • Allowable variations or tolerances must be specified by the designer, with two objectives:
    • ensure fit and function
    • minimize manufacturing cost
example shaft in hole fit
Example - shaft in hole fit
  • Shaft in hole fits are very common
  • Both shaft and hole diameter vary
  • Dimensions and tolerances must be specified to guarantee the desired fit
shaft in hole

Tolerance

Minimum clearance

Shaft in Hole
types of fit
Types of Fit
  • Clearance fit
    • largest shaft diameter is smaller than smallest hole diameter
    • there is always clearance
  • Interference
    • smallest shaft diameter is larger than largest hole diameter
    • there is always interference
  • Transition
    • there could be either interference or clearance
clearance fits
Clearance Fits
  • Loose running
    • lots of play, where accuracy is not important
  • Free running
    • less play, good for moving parts
  • Close running
    • close fit for moving parts, high accuracy required
transition fits
Transition Fits
  • Used to accurately locate parts during assembly
  • Tradeoff between ease of assembly/disassembly and accuracy of location
  • Example: locating dowels or pins
interference fits
Interference Fits
  • Used for force or press fits
  • Results in permanent assembly without need for fasteners or other joining operations
  • High locational accuracy
multiple holes and shafts
Multiple holes and shafts
  • Often parts are assembled with multiple shafts mating with multiple holes
  • Examples?
objectives
Objectives
  • Close fit with no play and good locational accuracy
  • High probability of parts fitting despite variations in dimensions
types of variations
Types of Variations
  • Size tolerances on hole diameters
  • Size tolerances on shaft diameters
  • Positional tolerances on hole locations
must fit in worst case
Must Fit in Worst Case
  • Holes at minimum size
  • Shafts (e.g. bolts) at maximum size
  • Holes at minimum distance on one part
  • Holes at maximum distance on the other part
worst case

Maximum

Smallest hole

Minimum

Largest shaft

Worst Case
geometric dimensioning and tolerancing gd t
Geometric Dimensioning and Tolerancing (GD&T)
  • GD&T symbols specify additional tolerancing information for 3D geometry
  • Used in addition to standard +/- dimensioning
  • Used properly, GD&T can allow looser tolerances to minimize manufacturing cost
datums
Datums
  • A datum is a plane, centerline or point used as a reference starting point for dimensions
  • Often flat faces of a part or centerlines of holes are used as datums
  • There can be several datums, labeled A, B, C, etc.
maximum material condition mmc
Maximum Material Condition (MMC)
  • MMC is the condition where a feature has the maximum volume or material
  • For a hole, it is the smallest size
  • For a shaft, it is the largest size
departure from mmc
Departure from MMC
  • As a feature departs from MMC, it moves away from the worst case
  • This may permit tolerances on other dimensions to be increased while still achieving fit
  • For example, if the holes in the previous example are larger than the minimum, the tolerance on the spacing can be increased and the parts will still fit
geometric controls
Geometric Controls
  • Form controls
    • compare feature to ideal geometry
  • Orientation controls
    • compare orientation of features to datums
  • Location controls
    • compare location to datums
form controls
Form Controls
  • Straightness
    • compares a line or axis to a perfectly straight line
  • Circularity
    • compares a circular cross section to a perfect circle
  • Flatness
    • compares a flat surface to a perfect plane
  • Cylindricity
    • compares a cylindrical feature to a perfect cylinder
    • includes axis straightness, circularity and taper
orientation controls
Orientation Controls
  • Parallelism
    • a line or surface must be parallel to a datum
  • Perpendicularity
    • a line or surface must be perpendicular to a datum
  • Angularity
    • a line or surface must be at an angle to a datum
  • Line profile and Surface profile
    • line and surface profiles compare features to ideal profiles
location controls
Location Controls
  • Concentricity
    • controls deviation of concentric axes of cylindrical elements
  • Runout
    • measures “wobble” in surface of cylindrical feature as it is rotated about an axis
  • Position
    • Locates features relative to datums
    • allows larger “bonus” tolerances as features depart from MMC
feature control frames
Feature Control Frames
  • GD&T controls are added to drawings as feature control frames, using standard symbols
  • In I-DEAS, they are defined by completing a form
tolerancing in i deas
Tolerancing in I-DEAS
  • Tolerances can be applied to constraints
  • I-DEAS tolerance analysis uses variational geometry to analyze the effect of tolerance stack-up
  • GD&T symbols can be applied as annotations in modelling mode
  • Annotations are automatically included in drawings
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