In order for the drawings to be dimensioned so that everyone can understand them, we need to follow standards that all people in the world must follow. Standards are created by these organizations:
A dimension is a numerical value expressed in appropriate units of measurement and used to define the size, location, orientation, form and other geometric characteristics of a part. The basic types of dimensioning are linear, aligned, angular, ordinate, radius/diameter, arc length.
Dimensions enable the designer of a product to express exact linear and angular distances. There are two types of dimensions used in blueprints: size and location. The first type is used to indicate the exact size of the product. The second gives exact locations of the holes, indentations, etc. on an object.
Dimensions are used to measure straight and angular distances. Straight distances are expressed in fractional or decimal form. Angular measures are expressed in degrees, minutes and seconds.
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Decimal dimensions are used on parts requiring accuracy. Mechanical manufacturing prints use decimal dimensioning entirely, even though the criticality of the dimensions may vary. There should be a note on the print stating the tolerances for the critical dimensions. Decimal tolerances commonly range from tenths (.10) to ten thousandths (.0001) of an inch. When the degree of accuracy is critical in a dimension, the tolerance becomes tighter. The number of decimal places that a dimension is carried will determine the decimal places a tolerance is carried. Ex: A dimension carried two decimal places (1.1O") would have a tolerance carried two places (±.01). If the dimension had been carried three places (1.100"), then the tolerance would be carried three places (±.001").
Fractional dimensions are used on parts which do not require tolerance of accuracy. In general, when an object is fractionally dimensioned it is implied that the overall tolerance is to be maintained plus or minus 1/64".
Angular dimensions are used to measure angles in parts. In mechanical manufacturing prints, angles are expressed in degrees (°), minutes ('), and seconds("). Angular tolerances can also be expressed in tenths (.10) or hundredths (.01) of degrees. Where degrees are indicated, the numerical value shall be followed by the symbol. Where decimal degrees less than one are specified, a zero shall precede the decimal value. Ex: 1°15" | 1°30'15" | 1.5° | 0°0'15"
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, width, 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). It's used to describe the approximate characteristics of the feature rather than the exact dimensions. 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:
Standard Linear dimensions. Dimension applies to the parts that are horizontal or vertical. Linear dimensions may not represent the true distance between beginning and ending dimension points because they do not take angle into account as aligned dimensions do.
Baseline dimensions are multiple dimensions measured from the same baseline. Ordinate dimensions measure the perpendicular distance from an origin point called the datum to an element. These dimensions prevent escalating errors by maintaining accurate offsets of the features from the datum. 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 dimensions are used to describe the theoretically exact size, profile, orientation or location of a feature or datum target. Those values can be recognized as “basic dimensions” because they are identified by the fact that they’re contained in a box drawn with a thin solid line. Basic dimensions identify tolerance information located in feature control frames that state geometric tolerances. Basic dimensions serve to orient and locate: 1. Tolerance zones and 2. Datum Targets.
Sheet Metal dimensions must shown overall dimensions, bend shape, and angles. Dimensioned features should be measurable during inspection from defined point to another. Dimensions should always be measured to the theoretical sharp and not a bend tangent. It is pointless to dimension flat patterns on prints if the part is formed and finished. The dimensions of the flat pattern will not be taken into consideration when manufacturing. The reason the flat pattern is insignificant to a manufacturer is because they have their own bend deductions based on the bend radius and angle, the material thickness and stiffness, the tooling used, and how much contact the tooling makes with the metal. On a side note, the flat pattern is definitely useful for estimating the fabrication of a part. It does serve a useful purpose for estimating, just not for manufacturing and/or inspection of the part.
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Assembly dimensions: When creating ortho drawings for assemblies, the only dimemsions needed are those that locate individual components with respect to each other. Constituent subassemblies should be treated as if they have already been constructed. So, dimensions pertinent to the smaller assemblies do not need to be included in the larger assembly drawings. Instead, dimensions between that subassembly and other assemblies/parts that it mates to need to be included. To improve ease of assembly, it is beneficial to provide locational dimensions between component features that actually mate to each other such:
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
Angle dimensioning: 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.
Arc and Circle dimensioning: 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 Taper dimensioning. Conical taper means the shaft smoothly tapers from one end to the other end, like a cone. Pro taper means the shaft is one constant diameter from the tip back maybe 12"-14" or so, and then begins to cone out toward the joint. The usual method of dimensioning a taper is to give the amount of taper in a note, such as TAPER .187 ON DIA. And then give the diameter at one end, plus the length, or give the diameter at both ends and omit the length.
Inside Surface Taper dimensioning. Tapered surfaces should be properly dimensioned to be machined accurately and verified with standard gages. A conical taper may be defined by a suitable combination of the following dimensions and tolerances, and a standard sized ball can be used to check an inside taper.
Slot dimensioning. By default slots are dimensioned to the centers of arcs/circles. ASME Y14.5M-1994 paragraph 1.8.10 provide three methods for the dimensioning of slots, with no stipulation regarding which is preferred for particular scenarios.
Feature Patterns dimensioning. ASME Y14.5-2009 provides a linear method to detail feature patterns, called repetitive features and dimensions. But the standard does not provide any tolerance rules for its prescribed scheme. Chain dimensioning accumulates tolerance as the pattern departs from the dimensioned start position. In some cases this is OK, but often this is unacceptable since the accumulation of tolerance can quickly lead to features that do not align to mating features on other components. The best way to avoid accumulation of tolerances is to use a methodology that does not rely on any form of direct dimensions. ASME Y14.5 actually suggests that GD&T should be used instead of direct dimensions to locate features. However, if GD&T; is not desired, the next best method is an ordinate dimension scheme.
Ordinate dimensioning is used when the X and the Y coordinates, from one location, are the only dimensions necessary. Usually the part has a uniform thickness, such as a flat plate with holes drilled into it. The dimensions to each feature, such as a hole, originate from one datum location.
Radial Patterns dimensioning. The proper method used in dimensioning holes on a bolt center diameter is provide the diameter of the bolt's circle and one angle to show the number of degrees between the center lines of the hole patterns spaced equally. An attached note specifies the number, size of the holes, and how they are drilled. When the hole patterns are not equally spaced on a bolt circle center diameter. A dimension for a bolt center, a note containing the number of holes and their size, and angle turned from a reference center line for locating the center lines of the other holes. Another method is to replace angular dimensions with linear dimensions.
Keyways and Keyseat dimensioning
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.
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
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.