GD&T Flashcards
Benefits of GD&T
- Uniformity of design practice
- Less chance of misinterpretation
- Promotes interchangeability of parts
Different standards of GD&T
American National Standards Institute (ANSI); American Society for Mechanical Engineers (ASME); International Standards Organization (ISO)
In mass manufacturing, parts are required to be designed with some tolerance so they will:
fit any assembly
Each dimension has a:
tolerance
Dimensioning and tolerancing shall be
complete
Every dimension of a product shall be shown:
no more and no less
Dimensions are slected and arranged to suit the:
function and mating relationship of the part
Dimensions are slected and arranged to suit the:
function and mating relationship of the part
Dimensions should not be subject to more than one:
interpretation
Tolerances are used for:
lengths, widths, diameters and locations
What type of hole deviations are allowed for using just location tolerance for a through hole?
Several deviations from a perfect hole through the plate, all within the range of tolerance (the hole could be bent or at an angle that is not perpendicular)
Datum definition and what they can be
A datum is a reference locations for a dimension. A datum can be: a point, a centre line, or a plane. Note: these are virtual positions, not real positions.
Centrelines
derived for shafts and holes
Centrelines
derived for shafts and holes
Centreline locations for shafts
located half the measured diameter to the edge of the shaft
Centreline location for holes
the centreline is derived from the largest size pin that will fit into the hole (the ID)
Feature
a general term applied to a physical portion of a part (eg. a surface or a hole)
Datum feature
a feature of a part that contacts a datum (real, but datum is virtual)
Datum planes
If a part has an irregular surface, many different measurements are possible. Using a datum place (place irregular part on a flat plane) makes for a more accurate measruement.
Benefits of planning
Provides communication framework, helps allocate resources, provides benchmark for progress
Challenges of planning
realistic estimates of how long it will take and how long, how much effort/detail to put into planning
Gantt Chart
List of project activities, chronological time scale of activities; can include milestones (indicated by arrows); can include progress (indicated by bars)
Limitations of Gantt charts
Activity sequence (if one activity can start before another ends) is not obvious), no clear way of adjusting start-times to compensate for delays
CPM: activity
Any process; graphical representation is line with name (letter) above and duration below
CPM: events
graphically represented as a dot; shows which activities must be completed before the event can occur
Precedence
Graphically represented as all activities that come into an event; chart represented as ‘precedenc’ (eg. Acitvity : E | Precedence : D means D must happen before E)
How to draw chart before determining CP:
All events that have nothing preceding them begin at project start; all activities that don’t precede anything go to project end; all activities that preced etwo things, branch out after the event. Put duration under the line!!
Critical path
the path through which any delay (of any event) will delay the entire project). Normally the longest way way. Can be determined using the earliet time forward sweep and latest time reverse sweep.
Earliest time forward sweep/ latest time reverse sweep
Forward sweep: from each event, find the next single activity that takes the least amount of time. The latest time reverse sweep is the same (shortest activity = latest time). The CP is the path along which the earliest and latest times are the same for all events.
Critical Path and non-critical events.
Criticial path = least amount of time possible given no delays. Other events can be delayed without impacting project deadline.
CPM Steps
List all activities and expected durations, determine precedence, construct CPM diagram, determine earliest start time and latest start time, identify CPM
How to read sectional views:
Arrows indicate the direction of viewing, line (slicing plane) indicates where view is cut; line is dashed in the middle; put letters by arrows in normal view and lable the sectional view SECT A-A
Where to hatch
every object the cutting plane touches (not holes); hatch what touches the glass
uneven slicing planes
do not show corners of slicing plane
linetype for cross-sectioning
phantom linetype
hatching two adjacent pieces
different spacing, different directions
hatching three adjacent pieces
different directions
hatching lines
must be non-parallel to all edges; even spacing
revolved cutting planes
show holes and features at radial distance: the distance it would be at if it were revolved
cross-sectioning with ribs
omit hatching the rib (as if the plane were just before the ribs
hidden lines and cross-sectioning
hidden lines beyond cutting plane not usually shown unless needed for clarity
Setting tolerances
+/- 0.005 OR +0.3/-0.1 OR 5.7/5.6
Minimum clearance for mating parts
The smallest female part - the largest male part
Maximum clearance for mating parts
The largest female part - the smallest male part
Three standard fits between shafts and holes
clearance, transitional, interference
Clearance fit
the max diameter set by tolerance is always smaller than the min hole diameter (up to and including a zero clearance. Loose fit, free running fit, close running fit, sliding fit
Interference fit
The minimum shaft diameter is always larger than the maximum allowed hole diameter. They eliminate the need for threads, welds or other means of joining; often assembled by hydraulic press with lubricant and heat. Locational transition fit, medium drive fit, and force fit.
Transition fit
Can either be a clearance or interference fit for securely locating parts in an assembly while allowing disassembly.
How to denote screw threads
internal: leave a notch in inner circle; external: leave a notch in outer circle
fully defined
new definitions cannot be applied to a skecth without conflicting with those already existing
types of relations
horizontal, vertical, parallel, perpendicular, parallel, collinear, coincident, merge, intersection, concentric, tangent
Maximum material condition
MMC or M: shafts - at MMC when it is at the maximum diameter allowed by the tolerance, holes - at LMC when it is at minimum diameter allowed by tolerance
Least material condition
LMC or L: shafts- at min diameter, holes - at max diameter
What happens in terms of tolerance of the shaft as a hole departs from MMC
More tolerance is allowed
Features of size
a type of feature ( a cylindrical surface, or pair of two opposed parallel surfaces, etc.) associated with a size dimension. If you can use a calliper (big side) to measure it, it is a feature of size.
GD&T: Form
Straightness, flatness, circularity, cylindricity
GD&T: Orientation
Angularity, perpendicularity, parallelism
GD&T: Location
Position, concentricity, symmetry
Object line weight
heavy: 0.6mm
hidden line weight
medium : 0.3mm
center line weight
thin: 0.05mm
dimension and extennsion line weight
thin: 0.05mm
hatch line weight
thin: 0.05mm
phantom line weight
thin: 0.05mm
dimension line spacing
10mm from object and 6mm from eachother
crossing dim lines
never
