Principles of Engineering Drawing
Thayer Machine Shop
Drawing and Tolerancing
This Tutorial reviews the following principles:
Drawing: How to interpret and create engineering drawings Dimensioning: How to communicate dimensions properly Tolerancing: How to use geometric and dimensional tolerances to specify how much variation is acceptable during manufacture
Why Engineering Drawings?
• Engineering drawing is a formal and precise way of communicating information about the shape, size, features and precision of physical objects. • Drawing is the universal language of engineering. • Engineering drawing could be a complete course in itself, but we only have 80 minutes so... This is only going to cover the very basics.
• Just like written language has standards, the “grammar” of technical drawing is defined by... the ANSI
Y14.5 or the ISO standard
• The ANSI standards must be understood to read a drawing. • Lets look at a sample drawing... 4
PART NAME: Left Mtg. Bracket MATERIAL: AL6601-T6 NOTES: Units in Inches DRAWN BY: BC TOL: REV: QTY: 1
Deburr all edges
5 DATE: 6-1-97
Basic Information Included in a Drawing • Projected Views: Show as many sides as needed for completeness. • Cross Sections: A view that is good for showing interior features. • Table: Lower right corner, with material information, part name, designer etc. and finally
• DIMENSIONS!!!: These are the most important and
most complicated part of the drawing. There is more to it than just the numerical values!
Which is better?
0.250” + .003” 0.750” + .003” 1.000” + .003” 0.250” + .003”
A Dimensioning Example,
showing that placement should match intent
These drawings show bolts holes for mounting a flange onto a plate. When mounting the flange, the position of the holes with respect to each other is very important, or else the flange (or part) won’t fit. It makes sense to dimension the distance between the holes, instead of the distances to the edge. 0.250” + .003” 0.750” + .003” 1.000” + .003” 0.250” + .003”
Dimension placement matches intent
Dimension placement does NOT match intent
(every part needs some)
There are two types of tolerances: Dimensional Tolerances and Geometric Tolerances
What’s the difference?
• Dimensional tolerances control _______________.
• Geometric tolerances control __________ & __________.
Geometric tolerances affect dimensional tolerances!
Types of Dimensional Tolerances
1.375 + .004
Plus & Minus Tolerancing
Both methods are acceptable.
The distance between X and Y is a critical dimension. The total variation in the distance between X and Y depends on how the drawing is dimensioned.
How much tolerance is specified on the distance between X and Y in each example? (a) + (b) + (c) +
Geometric Tolerancing is used to specify the shape of features. Things like: •Straightness •Flatness •Circularity •Cylindricity •Angularity •Profiles •Perpendicularity •Parallelism •Concentricity •And More...
Geometric Tolerances are shown on a drawing with a feature control frame.
The Feature Control Frame
This feature control frame is read as: “The specified feature must lie perpendicular within a tolerance zone of 0.05 diameter at the maximum material condition, with respect to datum axis C. In other words, this places a limit on the amount of variation in perpendicularity between the feature axis and the datum axis. In a drawing, this feature control frame would accompany dimensional tolerances that control the feature size and position. 14
Geometric Characteristic Symbols
A feature control frame gives information about geometric 15 tolerances on the feature.
Straightness at MMC
Demo Sketch your observations:
2 Rods: .375” diameter...
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