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Mechanical Design Reference

  • There was an engineer who had an exceptional gift for fixing all things mechanical. After serving his company loyally for over 30 years, he happily retired.

    Several years later the company contacted him regarding a seemingly impossible problem they were having with one of their multimillion dollar machines. They had tried everything and everyone else to get the machine to work but to no avail. In desperation, they called on the retired engineer who had solved so many of their problems in the past.

    The engineer reluctantly took the challenge. He spent a day studying the huge machine. At the end of the day, he marked a small "x" in chalk on a particular component of the machine and stated, "This is where your problem is".


    The part was replaced and the machine worked perfectly again.


    The company received a bill for $50,000 from the engineer for his service. They demanded an itemized accounting of his charges.


    The engineer responded briefly:





    One chalk mark                    $        1.00
    Knowing where to put it         $49,999.00

    Total                                    $50,000.00


    It was paid in full and the engineer retired again in peace.




    Here you will find mechanical reference data to help with your designs.





    Hardware -

    Screws and Fasteners








    Bolt Head markings







    Types of Washers











    Aircraft Hardware












    Taps and Dies








    Sheet metal









    Galvanic Scale

    Corroded end (anodic, or least noble)
    Protected end (cathodic, or most noble)

    Note: Groups of metals indicate they are closely similar in properties.

    Magnesium alloys
    Aluminum 2S
    Aluminum 17ST
    Steel or Iron
    Cast Iron
    Chromium-iron (active)
    18-8 Stainless (active)
    18-8-3 Stainless (active)
    Lead-tin solders
    Nickel (active)
    Inconel (active)
    Copper-nickel alloys
    Silver solder
    Nickel (passive)
    Inconel (passive)
    Chromium-iron (passive)
    18-8 Stainless (passive)
    18-8-3 Stainless (passive)

    Reference Source: Reference Data for Radio Engineers, Fourth Edition




    Geometric Dimensioning & Tolerancing (GD&T)


    Actual Size - Actual size is the measured size of the produced feature.


    Angularity - Angularity is the condition of a surface, axis, or center plane, which is at a specified angle (other than 0, 90, 180 or 270 deg.) from a datum plane or axis.



    Basic Dimension - A basic dimension is a theoretically exact value used to describe the exact size, profile, orientation or location of a feature. A basic dimension is always associated with a feature control frame or datum target.  Block tolerance does not apply and the applicable tolerance will be given to the feature control frame. Basic dimensions are enclosed by a rectangle.



    Bilateral Tolerance - A bilateral tolerance is a tolerance in which variation is permitted in both directions from a specified dimension (example +- .005).


    Center Plane - Center plane is the middle or median plane of feature.


    Circularity - See Roundness.


    Clearance Fit - A clearance fit is one having limits of size defined such that a clearance always results when mating parts are assembled.


    Concentricity - Concentricity describes a condition in which two or more features (cylinders, cones, spheres, etc.) In any combination have a common axis.  Measurement requirements for concentricity involves the complex task of mapping the referenced feature by way of opposed point measurements.  A through understanding of the measurement process  should be investigated before defining feature relationships using concentricity. 



    Coaxial - Coaxial describes a condition where two or more features have the same axis or centerline.


    Coordinate Dimension - (1) Either of two coordinates that locate a point on a plane and measured its distance from either of two intersecting straight-line axes along a line parallel to the other axis. (2) Any of three coordinates that locate that locate a point in space and measure its distance from any of three intersecting coordinate planes measured to that one of three straight-line axes that is the intersection of the other two planes.


    Coplanar - Coplanar describes a condition of two or more surfaces having all elements in the same plane.


    Cylindricity - Cylindricity describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis. 



    Datum - Datums are points, lines, planes, cylinders, axes, etc., a, from which the location, or geometric relationship of other part features may be established or related.


    Datum Axis - the datum axis is the theoretically exact centerline of the datum cylinder as established by the extremities or contacting points of the actual datum feature cylindrical surface, or the axis formed at the intersection of two datum planes.


    Datum Feature - A datum feature is the actual component feature used to establish a datum.


    Datum Identification frame (Symbol) - The datum identification frame (symbol) contains the datum reference letter in a rectangular box, usually preceded and followed by a dash (any letter except I, O, or Q).


    Datum Line - A datum line is that which has length but no breadth or depth, such as, an intersection line of two planes, centerline or axis of holes or cylinders and/or reference line for functional tooling or gauging purposes.


    Datum Point - A datum point is that which has position, but no extent; such as, the apex of a pyramid or cone, center point of a sphere or reference point on a surface for functional tooling or gauging purposes.


    Datum Reference - A datum reference is a datum feature.


    Datum Reference Plane - is a set of three mutually perpendicular datum planes or axis established from the simulated datum  in contact with datum surfaces or features and used as a basis for dimensions for designs, manufacture, and inspection measurement.


    Datum Simulator - A datum simulator a surface of adequate precision oriented to the high points of a designated datum from which the simulated datum is established.  Examples: gage pin, block, surface of granite block.


    Diameter  Symbol - the diameter symbol, indicates a circular feature when used on the field of a drawing or indicates that the tolerance is diametrical when used in a feature control frame. 


    Datum Target - is a specified point, line, or area on a part that is used to establish the Datum Reference Plane for manufacturing and inspection operations. 


    Dimension - A dimension is a numerical value expressed in appropriate units of measure and indicated on a drawing.

    Feature - Features are specific component portions of a part and may include one or more surfaces, such, as holes, screw threads, profiles, faces or slots. Features may be individual or interrelated.


    Feature Of Size - One cylindrical or spherical surface, or a set of two plane parallel surfaces, each of which is associated with a size dimension.


    Feature Control frame (Symbol) - The feature control frame symbol is a rectangular box containing the geometric characteristics symbol, and the form, runout or location tolerance. If necessary, datum references and modifiers applicable to the feature or the datums are also contained in the box.


    Fit - Fit is a general term used to signify the range of tightness or looseness, which may result from the application of a specific combination of allowances and tolerance in the design of mating part features. Fits are four general types: interference, transition, line and clearance.

    Flatness - Flatness is the condition of a surface having all elements in one plane.



    Form Tolerance - A form tolerance states how far an actual surface of feature is permitted to vary from desired form the desired form implied by the drawing. Expressions of these tolerances refer to flatness, straightness, parallelism, perpendicularly, angularity, roundness, cylindricity, profile of a surface and profile of a line.


    Free State Variations - Free state variation is a term used to describe distortion of a part after removal of forces applied during manufacture.


    Geometric Characteristics - Geometric characteristics refer to the basic elements or building blocks, which form the language of geometric dimensioning and tolerancing. Generally, the term refers to all the symbols used in form, runout, and locational tolerancing.


    Geometric Tolerance - The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.


    Implied Datum - An implied datum is an unspecified datum whose influence on the application is implied by the dimensional arrangement on the drawing; example, Coordinate dimensioning is applied to part, the zero dimension is attached to a feature this feature is implied as a datum element; axis,  surface or plane.


    Interference Fit - An interference fit is one having limits of size so prescribed that an interference always results when mating parts are assembled.


    Least Material Condition - (LMC) - This term implies that condition of a part feature of size wherein it contains the least (minimum) amount of material, examples, largest hole size and smallest shaft size. It is opposite to maximum material condition (MMC).



    Limit Dimensions - In limit dimensioning only the maximum and minimum dimensions are specified. When used with dimension lines, the high limit is placed over the low limit. When used with a leader line or note, the low limit precedes the high limit.


    Limits Of Size - The specified maximum and minimum size of a given feature.


    Limits of Size Concept - The limits of size concept calls for perfect form at maximum material condition.  Also called the Taylor principle.


    Line to Line Fit - A line fit is one having limits of size so prescribed that surface contact or clearance may result when mating parts as assembled.


    Location Tolerance - A tolerance states how far an actual feature may vary from the perfect location implied by the drawing as related to datums or other features. Expressions of these tolerances refer to the category of geometric characteristics containing position, concentricity, and symmetry.


    Maximum Material Condition - (MMC) Maximum material condition is that condition of a part feature wherein it contains the maximum amount of material within the stated limits of size. That is: minimum hole size and maximum shaft size.



    Modifier - A modifier is the term used to describe the application of geometric principles. The principles include MMC, LMC, RFS, projected tolerance zone and diameter.


    Nominal Size - The nominal size is that stated designation which is used for the purpose of general identification, examples: 1.400, .050 .


    Parallelism - Parallelism is the condition of a surface, line, or axis, which is equidistant at all, points from a datum plane or axis. 



    Perpendicularity - Perpendicularity is the condition of a surface, axis, or line, which is 90 deg. From a datum plane or a datum axis.



    Position Tolerance - Position tolerance (formerly called true position tolerance) defines a zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position.



    Principle Of Independency - This principle sets no limits to the number of errors of form possessed by individual features of a work piece. International designer, except the USA, use this principle.


    Profile Of A Line - Profile of a line is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, along a line element of a feature.



    Profile Of A Surface - Profile of a surface is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, on a surface.



    Projected Tolerance Zone - A projected tolerance zone applies to a hole in which a pin, stud, screw, etc., is to be inserted. It controls the perpendicularity of the hole to the extent of the projection from the hole and as it relates to the mating part clearance. The projected tolerance zone extends above the surface of the part to the functional length of the pin, stud, and screw relative to its assembly with the mating part.



    Reference Dimension - A dimension, usually without tolerance, used for information purposes only. It does not govern production or inspection operations. A reference dimension is a repeat of a dimension or is derived from other values on the drawing or related drawings. Symbology: (.250)


    Regardless Of Feature Size - (RFS) - This is the condition where the tolerance of form, runout or location must be met irrespective of where the feature lies within its size tolerance.  Optional for Position.



    Roundness - Roundness describes the condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected by any plane (1) perpendicular to a common axis (cylinder, cone), or (2) passing through a common center (sphere) are equidistant from the center.



    Runout - Runout is the composite deviation from the desired form of a part surface of revolution through on full rotation (360 deg) of the part on a datum axis.



    Runout Tolerance - Runout tolerance states how far an actual surface of feature is permitted to deviate from the desired form implied by the drawing during one full rotation of the part on a datum axis. There are two types of runout: circular runout and total runout.


    Size Tolerance - A size tolerance states how far individual features may vary from the desired size. Size tolerances are specified with ether unilateral, bilateral or limit tolerance methods.


    Specified Datum - A specified datum is a surface or feature identified with a datum identification symbol of note.

    Squareness - See Perpendicularity.


    Straightness - Straightness describes a condition where an element of a surface or an axis is a straight line.



    Symmetry - Symmetry is a condition in which a feature (or features) is symmetrically disposed about the center plane of a datum feature. 


    Tolerance - A tolerance is the total amount by which a specific dimension may vary; thus, the tolerance is the difference between the maximum and minimum limits.


    Transition Fit - A transition fit is one having limits of size so prescribed that either a clearance or an interference mat result when mating parts as assembled.


    True Position - True position is the theoretically exact location of a feature established by basic dimensions, also referred to as "Position or positional tolerance"



    Total Runout - Total runout is the simultaneous composite control of all elements of a surface at all circular and profile measuring positions as the part is rotated through 360.



    Unilateral Tolerance - A unilateral tolerance is a tolerance in which variation is permitted only in one direction from the specified dimension, example, 1.400  +.000/ -.006.


    Virtual Condition (Size) - The boundary generated by the collective effects of MMC, size limit of a feature and any associated geometric tolerance, virtual condition must be considered in determining the fit between mating parts. The term "virtual condition" is preferred over "virtual size."


    More to come...







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