how to diemension a drawing

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How to dimension a newly created Part/Assembly

drawing

Yugandhar G

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Preface

Drawing is the language of engineers. Like any other language it has some rules to be followed. This session has been designed keeping the most common need felt by the new joiners and little experienced engineers in creation of new industrial drawing.

These session would give insight into the various drawings, dimensioning practices, GD&T representations and most commonly made mistakes by the associates.

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Contents

Various types of drawingsDrawings stages in product life cycleFundamental rules of dimensioningDatum selection and representations in drawingsVarious dimensioning schemesDo’s and Don'ts while creating a new drawingCommonly made mistakes in the drawings

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Various types of Drawings

Part drawings

As Molded Machined

Machining Forgings/ Casting

Sheet metal Plastics

As Cast / Forged Machined

Post bend Pre bend

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Importance of various drawings

• Part drawings :The purpose of these drawings to convey the manufacturing, measuring

dimension, testing requirements and to provide general notes like draft angles, fillet dimensions, general wall thickness etc.

• Assembly drawings :The purpose of this drawing is to show some important assembly

dimensions, overall dimension of assembly, assembly process requirements like insertion force, tightening torque requirements, test requirements, assembly sequence etc

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Importance of various drawings

• General arrangement(GA) / interface drawings :The purpose of this drawing is to convey mounting dimension with the

mating components, car assembly positions, testing requirements.Generally these drawings represent the top level assembly being supplied to its customer

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Examples of various drawings

Completely dimensioned part drawing

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Assembly drawings with important dimensions

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Assembly drawings with assembly machining dimensions

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Interfacing drawings with functional requirements, car co-ordinate position of important points

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Fundamental rules of dimensioning

• Dimensioning and tolerancing should clearly define the engineering intent

• No more dimensions than those necessary for complete definition shall be given.

• Dimensions shall be selected and arranged to suit the function and mating relation ship of a part and shall not be subject to more than one interpretation

• Importance dimension should have controlled tolerance, otherwise general tolerance would be used.

• Dimensions should be arranged to provide required information with optimum readability

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Drawings stages in product life cycle

Amount of detailing can vary according to the stage

• Concept : Few important details to convey the design concept forfurther evaluation

• Proto type : Moderate dimension to produce the parts for Design Validation purpose.

• Design release: Complete dimensioning of the part for regular production.

• After market / Service drawing: Few dimensions to identify the part from similar component. Basically overall length, mounting thread size etc

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Two main points before dimensioning

• Selection of origin (Datum) from which dimensions are shown.– Origin can be a Plane/Axis

• Type of dimensioning scheme– Ordinate / Chain / Direct– Combination of above

Both generally requires understanding of component geometry, interfacing components and its dimensioning scheme.

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Selection of Datum features

Datum features most often be:• Functional:Representative of seating features, mating features and

alignment features

• Accessible:Which is influenced by sufficient surface area and location

• RepeatableWhich is influence greatly by the feature’s precession of form

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Various dimensioning schemes

Chain Dimension: Any variation at one dimension would propagate till end.

Base line dimension: Tolerance higher than direct dimension Checking of part is easy.

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Various dimensioning schemes

Direct dimensioning: Used when least tolerance between two features required.

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Rectangular co-ordinate dimension.

•This scheme is used when there are series of hole in a plane

•Easy for inspection

Selection of origin and dimension scheme

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Rectangular co-ordinate dimension.

• This representation is used not to overload the drawing with dimensions.

• Improves readability

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Polar Dimensioning

•This is scheme is used when series of holes on a circular path at various angle

•Easy to visualize the hole PCD

•If rectangular co-ordinate scheme is used here, PCD need to be arrived.

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Starting of each pitch given from common reference

Direct dimensioning used locally to locate separate components.

This way tolerance accumulation at each set of holes can be avoided.

Combination of Ordinate and chain dimensioning

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Assembly exploded view

This hole act as a alignment feature. So dimensions are given

from here.Resting plane. So dimensions are given

from there


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This Plane act as a mating feature. So dimensions are

given from here.

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Origin taken from here, being it is bearing mating face

Direct dimensioning

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Selection of origin and dimension schemePre Bend drawing Post Bend drawing

Selected as datum, since it has long edges. It is preferred to select non

bending edges as datumSelected as datum,

since it has maximum resting area•Datum convention to be same in pre bend and post bend

•Principle view of prebend and post bend drawing to be of same orientation to aid easy understanding

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Selection of origin and dimension schemePre Bend drawing Post Bend drawing

Principle views of various components to be of same orientation to aid better

understanding. (Preferably as seen in assembly principle view)

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Detail view taken to avoid overloading of dimensions in principle view.

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Origin hole

In PCB plate components, origin to be hole center with ordinate dimension to be follow.

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Selection of origin and dimension schemeOrdinate

dimensioning from center, being

it is aligning feature

Direct dimensioning,

being these are related to

a single mating part

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Gear assembly sectional view shown for better understanding of slides to follow.

Bearing

Motor assembly

Motor mating plane

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Selection of origin and dimension schemeHousing – Machining drawing

Datum B, being this is bearing

seating diameter

Position tolerance given with respect to

Datum B.

These three holes are used to fix a retainer

plate to arrest Bearing.

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1

2

3


1) Dimension from center to end: To control end mating plane

2) Direct dimension form end to Bearing butting face: To control bearing position with respect to motor rotor

3) Direct dimension for Bearing width

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Selection of origin and dimension schemeHousing – Casting drawing

Some of the important dimension to ensure

sufficient clearance in assembly and to

ensure machining stock

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Mating face, so dimensions are given from here

Dimensions are given from center, since it is aligning feature

Cover – Machining drawing

Only machining details would be given.


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Selection of origin and dimension schemeOrigin is the machining plane in the

casting drawings also

Notes showing draft, radius, nominal

thickness, general tolerance

Cover – Casting drawing

Only casting details would be given.

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This hole act as a alignment feature. So dimensions are

given from here.

Here direct dimension

given to reduce the tolerance accumulation

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This defines the length of engagement of

mating component. These dimensions to be

given from end.

(not from center)

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Dimensioning of Taper parts

Unlike cylindrical parts, taper feature diameter varies along with

length. So fix a particular diameter called “GAUGE dia”,

corresponding length would have tolerance.

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Datum representation

If misaligned, it means to the tangent line, not to the axis

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If misaligned with

dimension, it means datum

to the top plane only.

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Reference dimensions

32.90 Ref

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Reference dimensions

• It is just a balance dimension in a set of dimension.• The tolerance on this dimension to be is arrived from its

driving dimension.• It is just given for ready reference ( to avoid calculations).• When the dimension is repeated in the drawing.• Generally reference dimension need not be used while

modeling.• Disadvantages of showing reference dimensions:Possibility of not updating when driving dimension changes.

This is the reason some of the reference dimensions will not match while conversion jobs

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Meaning of Basic dimension

• A dimension for which tolerance is specified elsewhere in the drawing, in this case it is specified in the hole positional tolerance. This also called True dimension.

• The Ø symbol in tolerance control frame to be used when feature is circular ie hole, arc and this circular tolerance zone is for the feature axis.

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Meaning of QA dimension

This is the index, based on which Quality personnel will

check the dimension. This would ensure they have not

missed any dimension

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Do's while creating a drawing

Selection of principle view:• As seen in the assembly orientation.• To have most important details to show.• Circular components to be detailed axis horizontal.

Selection of scale:• Drawing should not be dominated by too many dimensions

compared to part geometry.• When taken in actual print, geometry should be readable.

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• Different views for different purposes / details for complex parts

Have a separate to view to show related dimensions like Hole dimensions, placementSlot dimension and placement Undercut dimensionsGroove dimension etc

Place the reference dimension for a detail in its parent view only

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Sectional views:• When aligned with parent view:Follow the rule of I / III angel projection

• When not aligned with parent view for space reasons:Place in the proper side with respect to arrow from parent

view. Here also follow the angle of projection

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• Avoid crossing of leader lines• Use detail views to avoid cluster of dimensions• Always use upper case letters in drawing ( exceptions:

units etc)• Place the dimensions in most appropriate view• Use MS word to spell check if it is meaningful• Look for Ø, R symbols before circular dimension, since in

some software it does not come by default• Leader line to be slanted. ( Not horizontal or vertical)

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• Blank some components, tangent lines for clarity if possible

• Group all the related details to its view• Look for angle of projection when referring drawing from

other sources• Avoid repeated and redundant dimensions• Show center lines for a circular feature where it is being

dimensioned• Place the detail view hatch style as in parent view• Update the view to avoid unwanted datums, lines

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Do's while creating a drawingDo's while creating a drawing

• For non 45 degree chamfers show land, angle separately• Always keep mating component and its dimensioning

scheme in mind• Keep reference dimensions to the minimum• Follow the sequence in labeling the views (Excluding I, O,

Q, S)• For flexible parts like seals, O-rings, springs drawings

need to be created at free state• Do not keep trailing Zeros in Basic (True) dimensions

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If the dimension is over written (fake) or not to scale, underline the same.

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In multi sheet complex drawings show index in first sheet containing

• Datum planes and its definitions,

• Section and detail views reference and where it is shown

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For splines and gears, dimension over pins need

to be given

Dimension to be given in appropriate view like this.

Dimension not to be given here

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Separate views to show different zones

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Don'ts while creating a drawing

Find the errors in this drawing?

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Repeated

Avoid Redundantdimensions

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Section view with out hatching

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Commonly made mistakes in drawings

Lines missed for threads

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Ø missed

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Datum misaligned with dimension, which gives entirely different meaning

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Datum misaligned with dimension, which gives entirely different meaning

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Right

M5x25

Wrong

M5 x 25 –No space between

M 5x25 – No space between M and 5

M5X25 – use lower case “x”

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Queries?

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Thank You

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how to diemension a drawing

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