Dimensioning Skills

UNIT VI: Basic Dimensioning Skills

Competency: 006.00
Demonstrate basic dimensioning techniques.

Objective: 006.01
Identify the accepted standards for mechanical dimensioning practices.

Introduction: The purpose of this unit is to introduce students to basic dimensioning techniques related to technical drawing. The previous units were mainly focused on describing the shape of objects. This unit will focus on describing the size and location of features. Standards for dimensioning technique, dimension placement, and general rules for dimensioning will be covered per Dimensioning and Tolerancing, ASME Y14.5M-1994. Standards for architectural dimensioning are to be covered in Architectural Drafting II.

Although most of this unit focuses on dimensioning technique (how dimensions should look on a drawing), students should be introduced to methods for determining the critical dimensions for a part. Since most objects exist within the context of an assembly, it is recommended that teachers make the students aware of how the part functions within the assembly before dimensioning the drawing.

R1(192-220):R2(230-244)

Identify the following standard practices for dimensioning:

• Dimension lines:
• Shows the beginning and the end of the measurement.
• Terminated by arrowheads.
• Thin line weight.
• Should be broken to allow for the numbers to be inserted.
• Must be a minimum of .375" or 10mm away from the object.
• Must be a minimum of .250" or 6mm away from parallel dimension lines.
• Extension lines:
• Extend the edge of the object.
• Thin line weight.
• There should be a visible gap (.0625" or 1mm) between the object and the start of the extension line.
• Extension lines should extend about .125" or 2.5mm beyond the last dimension line.
• Leader lines:
• Are drawn from a note or dimension to place where the note applies.
• Are drawn at an angle (usually 30°, 45°, or 60°).
• Should have a short (.125" or 3mm) shoulder that if extended, would intersect the note at mid-height.
• May end with an arrowhead or dot.
• If extended, the leader would pass through the center of arcs or circles (radial).
• Leaders should not cross over or through other leaders or dimension lines.
• Avoid making leaders parallel or perpendicular to visible edges.
• Should extend from the first word or the last word of the note.
• Arrowheads:
• Can be solid filled or open.
• Should be approximately .125" or 3mm long.
• Should be approximately 2.5 to 3 times as long as wide.
• Dimensioning placement for reading.

Unidirectional - recognized as standard for engineering drawing per ASME Y14.5M-1994.

• Aligned - preferred practice in architectural drawing.

UNIT VI: Basic Dimensioning Skills

Competency: 006.00
Demonstrate basic dimensioning techniques.

Objective: 006.02
Explain the procedures for dimensioning mechanical drawings.

R1(192-220):R2(230-244)

• Procedures for using decimal and metric measurement.
• Decimal inches:
• Decimals are the ANSI standard.
• Decimals are easier to add, subtract, multiply and divide than fractions.
• Preferably, decimals should be rounded to two decimal places. (Unless more precision is required.)
• Omit zero before the decimal point for values of less than one.
• Display trailing zeros equal to the drawing's precision.
• Fractional inches:
• Used where close tolerances are not important.
• The horizontal fraction bar is preferred.
• Omit the inch mark when dimensions are all in inches.
• Metric:
• Where linear measurement are less than 10,000 millimeters, the millimeter is the standard unit of measure.
• The abbreviation for millimeters (mm) is usually omitted when all dimensions are in millimeters.
• The period is used as a decimal point only in English speaking countries, others use a comma.
• If the value is less than one millimeter, a zero should precede the decimal point.
• Omit trailing zeros.
• Be able to list, explain, and give examples of the following general rules of dimension placement.
• The number one rule of dimensioning is that of clarity.
• Place dimensions where the shape is best shown.
• Shortest dimensions placed closest to the object.
• Group and align dimensions when possible.
• Avoid duplicate and/or unnecessary dimensions.
• Do not place a dimension to coincide with a line of a drawing.
• Try to avoid placing dimensions inside a view.
• Avoid crowding dimensions.
• Avoid dimensioning to hidden features.
• Place dimensions between the views to which they relate.
• Lines should be thin and contrast noticeably with visible lines.
• Dimensions should be included that describe both size and location of features.
• The diameter of cylinders is dimensioned in the rectangular view. The diameter of machined holes is dimensioned in the circular view.

• Identify , explain, and be able to illustrate the correct placement of dimensions on the following circular features:
• Arcs
• dimension by using the radius
• The letter "R" should precede the arc size.
• Circles
• dimension by using the diameter
• The diameter symbol should precede the circle size.
• Use a "X" when describing the number or quantity of circles.
• Correct placement of dimensions on angular features where the angles are expressed in degrees.
• A point-to-point dimension consisting of "chains" of dimensions placed end to end.
• one dimension is omitted
• adequate for simple parts
• ANSI establishes the standard or "correct" rules regarding dimension placement when creating technical drawings.

UNIT VI: Basic Dimensioning Skills

Competency: 006.00
Demonstrate basic dimensioning techniques.

Objective: 006.03
Construct dimensions on an engineering drawing.

Requirements: Each student is required to apply basic dimensioning techniques to a drawing.
1.   Using the drafting equipment provided, give all of the dimensions and notes required to describe the object’s size and features.
2.   The drawing is produced at a scale of 1:1 (full size).
3.   Use decimal inches for all measurements. Give dimensions to two (2) decimal places.
4.   Use accepted drafting standards for lines and freehand lettering.
5.   Letter your name, problem number (006.03.001), scale, and date in the title block.
6.   Time Limit = 60 minutes.


Assessment: The dimensioned drawing should be evaluated based on the following criteria:
Dimensioning concepts and techniques                                           50 points
Accuracy                                                                                          25 points
Line weight, technique and neatness                                               20 points
Lettering                                                                                             5 points

Rubric for BASIC DIMENSIONING SKILLS – Board drawing
Construct dimensions on an engineering drawing - 006.03

Dimensioning concepts and techniques

Accuracy

Line weight/technique/neatness

Lettering

                                                                                                   Total Score                          

Rubric for BASIC DIMENSIONING SKILLS using CAD
Apply simple dimensioning to an engineering drawing - 006.03

Dimensioning concepts and techniques

Accuracy

Dimension, Extension, Center, and Leader Lines

Dimensioning Text and Notes

                                                                                                   Total Score                          

AUTHENTIC ASSESSMENT: Product Development

Basic Dimensioning Skills Project Assessment

• Each team member will add basic dimensions to multiview drawing of individual design concept.
• The multiview drawing original should be preserved and dimensions added to a copy if drawn using instruments rather than CAD.
• Evaluate drawings based on Basic Dimensioning Skills Performance Objective 006.02
• Teams will generate materials lists and cost estimates.
• Begin prototype construction.
• Class periods may be used if time permits.
• Prototype construction may be an out-of-class assignment.
•  Objective 007.05