Machining Fundamentals

Machining Fundamentals
Understanding Drawings
Chapter 3

Drawings
• Show the craft worker what to make and
identify the standards that must be followed
so the various parts will fit together
properly.
• The resulting parts will also be
interchangeable with similar components on
equipment already in service.

Drawings
• Range from a simple freehand sketch, to
detailed drawings for complex products.
• American National Standards Institute
(ANSI) |Developed symbols, lines, and
figures
• Symbols, lines, and figures on drawings are
known as the language of industry.

Dimensions
• A proper drawing includes all dimensions in
proper relation to one another.
• The dimensions are needed to produce the
part or object.
• Industries in the United States are in the
process of converting to the metric system
of measurement.

Fractional Dimensioning
• Drawings using fractional dimensioning
usually show objects that do not require a
high degree of precision
• Greater precision is indicated when
dimensions are given in decimal parts of an
inch.

Dual dimensioning
• A system that employs the US Conventional
system of fraction or decimal dimensions
and metric dimensions on the same
drawing.
• If the drawing is intended primarily for use
in the United States, the decimal inch will
appear above the metric dimension.

Metric Dimensioning
• All of the dimensions on the drawing are in
the SI Metric system, usually in millimeters.
• Until there is full conversion to metric in
the United States, a conversion chart will
appear on the drawing.

Information Included on
Drawings

Materials
• The general classification of materials to be
used in the manufacture of an object may be
indicated by the type of section line on the
drawing.
• Exact material specification is included in a
section of the title block
• May be found in the notes shown elsewhere
on the drawing.

Surface Finishes
• The quality of the surface finish (degree of
surface smoothness) is important in the
manufacture of products.
• The smoother the finish of a machined surface, the
more expensive it is to manufacture.
• Check mark and number are used to indicate
surface roughness in micro-inches or micro-meters

Surface Finishes
• A machinist compares surface finishes to
required specifications by using a surface
roughness comparison standard as a guide.
• If the surface finish is critical the finish is
measured electronically with a device called
a profilometer.

Tolerances
• The control of dimensioning to achieve
interchangeable manufacturing is known as
tolerancing.
• This controls the size of the features of a
part.
• A standard system of geometric
dimensioning and tolerancing has been
established.

Tolerances
• Are allowances, either oversize or
undersize, that are permitted when
machining or making a part.
• Fraction inch units the tolerance can be
assumed to be +/- 1/64 unless otherwise
indicated.
• The plus and minus tolerance is called a
bilateral tolerance.

Tolerances
• When the tolerance is only plus or only
minus it is called a unilateral dimension.

Quantity of Units
• Shown on the drawing is the number of
parts needed in each assembly.
• Included with the job information received
by the shop gives the total number of units
to be manufactured.
• This helps in ordering the correct amount of
material for the job.

Drawing Scale
• Drawings made other than actual size(1:1)
are called scale drawings.
• The scale is shown in a section of the title
block.
• A drawing made one-half size would have a
scale of 1:2
• A scale of 2:1 would mean that the drawing is
twice the size of the actual part.

Assembly or Subassembly
• Necessary to correctly fit the various parts
together.
• The term application is sometimes used in
place of the term next assembly.

Revisions
• Revisions indicate what changes were
made to the original drawing and when they
were made.

Names of the Object
• A portion of the title block provides this
information.
• It tell the machinist the correct name of the
piece.

Types of Prints

Blueprints
• Blueprint is often used to refer to all types
of prints.
• An actual blueprint has white lines on a
blue background
• The blueprint process is seldom used today
because of the time required to make a
print.

Diazo Process
• These are direct positive copies (dark lines
on a white background) of the original
drawing.
• Often referred to as white-prints or bluelines.

Xerographic (electrostatic)
Process
• This process makes a copy of the original
drawing
• The print can be enlarged or reduced in size
if necessary.
• Full color copies can be made on some
xerographic machines.

Microfilm Process
• The original drawing is reduced by
photographic means
• Finished negatives can be stored in roll
form or on cards
• To produce a working print, the microfilm
image is retrieved from files and enlarged
onto photographic paper.

Computer-Generated Prints
• Prints generated on a plotter from
information stored electronically in
computer memory.
• This same information can also be used to
control machine tools, using
CAM(computer aided manufacturing).

Types of Drawings used in the
Shop

Working Drawings
• Detail drawings. These consist of a drawing
of the part with dimensions, and other
information for making the part.
• Assembly drawings. These drawings show
where and how the parts, described on
detail drawings, fit into the completed
assembly.

Working drawings
• Subassembly drawings. Used to show the
assembly of a small portion of the
completed object.
• Exploded pictorial drawings. A drawing
with parts separated, put in proper
relationship.

Parts List
• Parts are identified by circled numbers and
are listed in a note.
• Some drawings will also include a parts list
or bill of materials listing all of the parts
used in the assembly.

Drawing Sizes
• Companies usually store their drawings in
the engineering department.
• Engineers prepare drawings on standardsize
sheets.
• This simplifies the stocking handling, and
storage of the completed drawings.

US Conventional Sheet Sizes
• A size = 8 1/2” X 11”
• B size = 11” X 17”
• C size = 17” X 22”
• D size = 22” X 34”
• E size = 34” X 44”

SI Metric Sheet Sizes
• A4 size = 210 X 297 mm
• A3 size = 297 X 420mm
• A2 size = 420 X 594 mm
• A1 size = 594 X 841 mm
• A0 size = 841 X 1189 mm

Geometric dimensioning and
Tolerancing
• A system that provides additional precision
compared to conventional dimensioning.
• It ensures that parts can be easily
interchanged.

Definitions

Geometric Characteristic
Symbols
• They are employed to provide clarity and
precision in communicating design
specifications
• These symbols are standardized by the
American society of Mechanical Engineers
(ASME).

Geometric Tolerance
• A general term that refers to tolerances
which control form, profile, orientation,
location, and run-out.

Basic Dimension
• A numerical value denoting the exact size,
profile, orientation, or location of a feature.

True Position
• Of a feature is its theoretically exact
location as established by basic dimensions.

Reference Dimension
• Is a dimension provided for information
only.
• It is not used for production or inspection
purposes.

Datum
• Is an exact point, axis, or plane.
• It is the origin from which the location or
geometric characteristic of features of a part
is established.
• It is identified by a solid triangle with an
identifying letter.

Feature
• Is a general term applied to a physical
portion of a part, such as a surface, pin,
hole, or slot.

Datum Feature
• Is the actual feature of a part used to
establish a datum.

Maximum Material Condition
(MMC)
• Is the condition in which the size of a
feature contains the maximum amount of
material within the stated limits of size.
• Examples include a minimum hole diameter
and maximum shaft diameter, both of which
result in the greatest possible amount of
material being used
• MMC is indicated by an M within a circle.

Least Material Condition (LMC)
• The condition in which the size of a feature
contains the least amount of material within
the stated tolerance limits
• Examples include a maximum hole
diameter and minimum shaft diameter.
• LMC is indicated by an L within a circle.

Regardless of Feature Size (RFS)
• Specifies that the size of a feature tolerance
must not be exceeded.
• RFS is assumed for all geometric tolerances
unless otherwise specified.

Limits of size
• The maximum and minimum sizes of a
feature.

Actual size
• The measured size of a part after it is
manufactured.

Application of Geometric
Dimensioning and Tolerancing
• Datum identification symbol. A datum
identifying symbol consists of a square
frame that contains the datum reference
letter.
• All letters but I,O, and Q may be used.
• A rectangular frame with the datum
reference letter preceded and followed by a
dash may be found on older drawings.

Application of Geometric
Dimensioning and Tolerancing
• Feature control frame. A feature control
frame is used when a location or form
tolerance is related to a datum.
• It contains the geometric symbol, allowable
tolerance, and the datum reference letter.
• It is connected to an extension line of the
feature, a leader running to the feature, or
below a leader-directed note of the feature.

Form Geometric Tolerances
• Control flatness, straightness, circularity
(roundness) and cylindricity.

Flatness
• Is a measure of the variation of a surface
perpendicular to its plane.
• The flatness geometric tolerance specifies
the two parallel planes within which all
points of a surface must lie.

Straightness
• Describes how closely the surface of an
object is to a line.
• A straightness geometric tolerance
establishes a tolerance zone of uniform
width along a line.

Circularity
• Is characterized by any given cross section
taken perpendicular to the axis of a cylinder
or a cone, or through the common center of
a sphere.
• A circularity geometric tolerance specifies a
tolerance zone bounded by two concentric
circles, indicated on a plane perpendicular
to the axis of a cylinder or a cone.

Cylindricity
• Represents a surface in which all points are
an equal distance from a common center.
• The cylindricity geometric tolerance
establishes a tolerance zone that controls the
diameter of a cylinder throughout its entire
length.

Profile Geometric Tolerances
• Controls the outline or contour of an object
and can be represented by an external view
or by a cross section through the object.
• It is a boundary along the true profile in
which elements of the surface must be
contained.

Profile Line Geometric Tolerance
• A two-dimensional (cross-sectional)
tolerance zone extending along the length of
the element.
• It is located using basic dimensions.

Profile Surface Geometric
Tolerance
• Is three dimensional and extend along the
length and width of the surface.
• For proper orientation of the profile, a
datum reference is usually required.

Orientation Geometric
Tolerances
• Control the degree of parallelism,
perpendicularity, or angularity of a feature
with respect to one or more datum's.

Angularity
• Is concerned with the position of a surface
or axis at a specified angle to a datum plane
or axis.
• The specified angle must be other than 90
degrees.

Angularity Geometric Tolerance
• Establishes a tolerance zone defined by two
parallel lines, planes, or a cylindrical zone
at a specified basic angle other than 90
degrees.

Perpendicularity Geometric
Tolerance
• Specifies a tolerance zone at right angles to
a given datum or axis.
• It is described by two parallel lines, planes,
or a cylindrical tolerance zone.

Parallelism
• Describes how close all elements of a line
or surface are to being parallel to a given
datum plane or axis.

Parallelism Geometric Tolerance
• Is a tolerance zone defined by two lines
parallel to a datum within which the
elements of a surface or axis must lie.

Location Geometric Tolerances
• Are employed to establish the location of
features and datum's
• The define the zone within which the
center, axis, or center plane of a feature may
vary from the true position.
• Also know as positional tolerances

Concentricity
• Defines the relationship between the axes of
two or more of an objects cylindrical
features.

Concentricity Geometric
Tolerance
• Is express as a cylindrical tolerance zone.
• The axis or center point of this zone
coincides with a datum axis.

Symmetry
• Indicate equal or balanced proportions on
either side of a central plane or datum.

Symmetry Geometric Tolerance
• Is a zone within which a symmetrical
surfaces align with the datum of a center
plane or axis.

Run-out Geometric Tolerances
• Total run-out controls circularity,
straightness, angularity, and cylindrically of
a part when applied to surfaces rotated
around a datum axis.
• Circular run-out is applied to features
independently and controls circularity of a
single circular cross section

Summary
• Geometric dimensioning and tolerancing is
far more involved that described in this
presentation. If you wish more information
on this you may study a copy of ASME
Y14.5M-1994, or enroll in a class on
geometric dimensioning and tolerancing at
the college of your choice.

Questions