DIMENSIONS IN MECHANICAL ENGINEERING DRAWINGS
In order for the drawings to be dimensioned so that evryone can understand them, we need to follow standards that all people in the world must follow. Standards are created by these organizations:
Dimensions are represented on a drawing using on of the two systems, unidirectional or aligned. Title block tolerance do not apply to basic dimensions. Basic dimesnions are used to define or position tolerance zones.
Types of Dimensions: There are two classifications of dimensions, size and location. Size dimensions define the height, widtyh, and depth of rectangular object. Size dimensions also define the diameter or radius of cylindrical or spherical objects. Location dimensions are used to determine where various features of the part are to be placed.
Reference Dimension: A reference dimension is used only for information purposes. It's either aduplication of a dimension that has been specified elsewhere on the drawing, or it's accumulated value of other dimensions. Reference dimensions areenclosed in parentheses. and never used for manufacturing or inspection purposes.
Nominal Dimension is dimensions half-way between the upper and lower limits of size (ex: .250 ±.005, the nominal dimension would be .2475 or .2525). Or for a toleranced dimension .25 ±.005 (ex:.25 would be a nominal dimension). Block tolerances do not apply to nominal dimensions. Nominal dimensions are never used for manufacturing or inspection purposes. Nominal dimensions must be identified either with the abbreviation NOM immediately following or under the dimension with a notte such as: UNTOLERANCES DIMENSIONS ARE NOMINAL.
Units of Measure:
TYPES OF DIMENSIONING
Standard Linear Dimensioning: Dimensioning from feature to feature is known as Chain Dimensioning. It is commonly used and easy to lay out. It does have possible consequences in the manufacturing of a part. Tolerances can accumulate, making the end product larger or smaller than expected. The accuracy of the final product is determined by the dimensions on the drawing. If all the dimensions originate from a common corner of the part, the object will be accurate. This is refered to as Datum Dimensioning. Datum insure the tolerance or errors in manufacturing do not accumulate.
Basic linear dimensions:
Sheet Metal dimensions:
Size dimensions: Size can be measured as Length, Width and Height, Diameter, Perimeter, Area, Volume or Mass. In GD&T the term feature-of-size (FOS) refers to any surface, or set of parallel surfaces associated with a size dimension. Example: A hole diameter (a cylindrical surface). Plate thickness (two opposed parallel surfaces). Actual size is a measured size. Basic size is the size from which the limits of size are derived by the application of allowances and tolerances
Chain Dimensioning: Dimensioning from feature to feature is known as Chain Dimensioning. It is commonly used and easy to lay out. It does have possible consequences in the manufacturing of a part. Tolerances can accumulate, making the end product larger or smaller than expected. In a chain of dimensions, dimension the width and height of the subject/part in the front view. The front view is preferred because it is supposed to provide the most information about an object’s geometry
Dimensioning Angles: Angled surface may be dimensioned using coordinate method to specify the two location distances of the angle. Angle surfaces may also be dimensioned using the angular method by specifying on location distance and the angle.
Dimensioning Arcs and Circles: Arcs and circles are dimensioned in views that show the arc or circle. Arcs are dimensioned with a leader to identify the radius; in some cases, a center mark is included. Circles should have a center mark and ar dimensioned with a leader to identify the diameter.
Dimensioning Curved Features and Arcs: The arrow can be inside for small arcs. Small arcs do not need center marks. Arrow can be outside. Large arcs use center marks. Use a capital "R" for dimensioning arcs. A center line is a dark line composed of alternate long and short dashes, and is used to represent the axes of symetrical parts or to denote centers. Do not create a gap when the centerline crosses the part outline. A centerline must end with a long dash.
Reference Dimensions: Designates more than one of the same feature. In this case, it is identifying there are two identical radius.
Chamfers: There are two options for external chamfer for 45 degree chamfer and basic dimension for angles other than 45 degrees and internal chamfers.
Fillets and Rounds: Large arcs use center marks. Small arcs do not need center marks. Arrow can be outside the arc. Use capital "R" for dimensioning the arcs.
Dimension a Countersink: Measurements that are nneded to create the desired countersink are Countersink Hole diameter (for screw head), Angle of countersink, Thru hole diameter (for screw head), Material thickness, Screw head height (industrial and aircraft).
Conical Tapers — Slot Dimensioning — Dimensioning Radial Patterns — Keyways and Keyseat...to be continous.
Reading a Hole Note:
Reading Thread Note: Axis of thread is coincident with the axis of its pitch cylinder or cone. The basic profile of a thread is the cylical outline, in an axial plane, of the permanently established boundary between the provinces of the external and internal threads. All deviations are with respect to this boundary. M#x# 6H is standard for internal metric thread. M#x# M4 is V-coil thread, repair tap, fit helicoil HSS Volkel Germany.
NOTES ON MECHANICAL DRAWING
Metric Title Block: The ASME Y14.5M-1994 standard requires the trailing zeros TO BE OMITTED in metric tolerance block. Thus, 0.240mm is written 0.24mm and then the decimal-place-specific standard dimensioning rubric becomes irrelevant. What's the proper way to specify title block tolerances in METRIC? Trailing zeros are trimmed for whole metric values, conforming to ANSI and ISO standards. Trailing zeros appear according to the ASME Y14.5M-1994 standard.
|RANGES IN NOMINAL LENGTHS IN MM||TOLERANCE CLASS|
|UP TO 10||0.02||0.2|
|10 TO 30||0.05||0.2|
|30 TO 100||0.1||0.4|
|100 TO 300||0.2||0.8|
Notes for Inseperable Assemblies and Weldment:
NOTE: UNLESS OTHERWISE SPECIFIED
MECHANICAL DRAWING AND TOLERANCES
Tolerance: A tolerance is the total amount by which a specified dimension may vary. It's the difference between the maximum and minimum limits. All dimensions except reference, nominal, maximum or minimum shall have a tolerance. Basic dimensions are used in conjunction with geometric tolerances. Title block tolerances do not apply to basic, reference, nominal,maximum,and minimum dimensions.
Coordinate Tolerancing is a method used to specify the allowable variations of size or location by apllying tolerances directly to the linear or angular dimensionss.