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Dimensioning and TolerancingPowerPoint Presentation

Dimensioning and Tolerancing

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Presentation Transcript

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

- Shaft in hole fits are very common
- Both shaft and hole diameter vary
- Dimensions and tolerances must be specified to guarantee the desired 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

- 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

- Used to accurately locate parts during assembly
- Tradeoff between ease of assembly/disassembly and accuracy of location
- Example: locating dowels or pins

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

- Often parts are assembled with multiple shafts mating with multiple holes
- Examples?

Objectives

- Close fit with no play and good locational accuracy
- High probability of parts fitting despite variations in dimensions

Types of Variations

- Size tolerances on hole diameters
- Size tolerances on shaft diameters
- Positional tolerances on hole locations

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

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

- 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)

- 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

- 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

- Form controls
- compare feature to ideal geometry

- Orientation controls
- compare orientation of features to datums

- Location controls
- compare location to datums

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

- 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

- 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

- 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

- 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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