roshaliza hamidon (dpt 312 08/09) chapter 3 measurement and tolerances dpt 312 metrology

ROSHALIZA HAMIDON (DPT 312 08/09)

CHAPTER 3MEASUREMENT AND TOLERANCES

DPT 312METROLOGY


Introduction

DEFINITION

1. Engineering designWhat is to be manufactured?

2. ProductWhat has been manufactured?

3. Part inspectionHow we compare the product with the engineering design? Ensure the most

economical and effective production

of the parts

Geometric dimensioning and

tolerancing


Tolerance

o The total amount that a specific dimension is permitted to vary

o In dimensional metrology, tolerances are applied to both position (where) and size (how big) dimensions (see figure 3.1)

o Both types of dimensions must have tolerances for economical manufacture


A

B

D

C

E

Figure 3.1 Tolerances apply to both dimensions of size (A,B, E) location and dimensions of location ( C and D)

A

B

D

C

E

A

B

D

C

E

A

B

D

C

E


Geometric dimensioning and tolerancing (GD&T)

GD&T is a means of dimensioning and tolerancing a drawing with respect to the actual function or relationship of part features that can be most economically produced

It is a language of symbols used on mechanical drawings to efficiently, and accurately communicate geometry requirements for features on parts and assemblies.

This type of dimensioning and tolerancing should be used when:

i. Features are critical to be functionality or interchange ability of the part

ii. Datum references are desirable to ensure consistency between design, manufacturing and inspection

iii. Computerization techniques in design and manufacturing are being used or are desirable

iv. Standard interpretation or tolerance is not already implied


Key terms in GD&T

Feature – general term applied to a physical portion of a part such as a surface, hole or slot

Datum- a theoretically exact plane, point axis from which a dimension is measured

Datum feature – part feature that contacts a datum (use as the origin for measurement)

Datum reference plane- a set of three mutually perpendicular datum planes (see figure 3.2)

Feature of size- one cylindrical or spherical surface, or a set of two opposed parallel surface associated with a size dimensioni. internal – the diameter of a hole or the width of a slotii. External – the width or length of a block or a shaft diameter


Figure 3.2: Datum Reference Plane


SYMBOL AND MODIFIERS

The language of geometric tolerancing is a set of symbols.

This symbols are divided into five types of dimensioning control

1. Form 2. Profile 3. Orientation 4. Location5. Runout

* Geometric controls of form never use a datum reference. *Form control (straightness, flatness, circularity or cylindricity) are always relative to themselves and not other features.*some geometric controls (orietation, location, or runout) must have a datum reference


1. Form tolerance

State how far an actual surface or feature is permitted to vary from the desired form implied by the drawing


2. Profile tolerance

States how far an actual surface or feature is permitted to vary from the desired form on the drawing and/ or vary relative to a datum


3. Orientation tolerance

States how far an actual surface or feature is permitted to vary relative to a datum


4. Location tolerance

States how far an actual size feature is permitted to vary from the perfect location implied by the drawing as related to a datum or other feature


5. Runout feature

States how far an actual surface or feature is permitted to vary from the desired form implied by the drawing during full 360 degree rotation of the part on a datum axis


Material condition

1. Maximum material condition (MMC) The condition in which a feature of size

contains the materials within its stated tolerance limit

2. Least material condition (LMC) LMC is the condition in which a feature of size

contains the least amount of material within its permissible limits

3. Regardless of Feature Size (RFS) Indicates a geometric tolerance that applies

at any increment of size of the feature within its permissible limits.


APPLICATION OF GEOMETRIC TOLERANCING

Form tolerance Straightness Flatness Flatness Circularity (Roundness) Cylindricity

Profile tolerance Profile of line Profile of surface

Orientation tolerances Angularity Perpendicularity Parallelism

Location tolerance Position Concentricity Symmetry

Runout tolerances Circular runout Total runout


1. Form Tolerance

State how far an actual surface or feature is permitted to vary from the desired for

Straightness , flatness, circularity, and cylindricity are most frequently applied to single features or portions of a feature.


The condition where one line element of an axis, or surface is in straight line. (see figure 3.3)

Straightness tolerances can be applied to an axis or to a surface

i. Straightness


Figure 3.3: Straightness tolerance


Figure 3.4: Straightness tolerance applied to an axis

o When applied to an axis, straightness is specified in the view where the axis is in straight line

oThe tolerance zone is a space between two parallel straight lineso MMC of LMC


Figure 3.5: Straightness tolerance applied to a surface

o When a surface is to be controlled, the feature control frame is attached to the surface with leader or extension lineoRFS


ii. Flatness

The condition of surface having all elements in one plane

When flatness is specified, the feature control frame is attached directly to the surface or to an extension line of the surface

The flatness tolerance zone is defined by two parallel planes

All points of the surface must be within the limits of the tolerance zone defined by these two planes. (see figure 3.6)

The smaller the tolerance zone, the flatter the surface

Always applied RFS, no feature modifier such as MMC or LMC are allowed


Flatness (cont’)

Figure 3.6: Flatness tolerance


iii. Circularity (roundness)

Circularity is identified with any given cross section taken perpendicular to the axis of a cylinder or cone or through the center of sphere

The tolerance is bounded by two concentric circles

Each circular element of the surface must be contained within these concentric circles

The circularity tolerance must be less than the size tolerance, except for parts subject to free state variation

For RFS


Figure 3.7: Circularity tolerance


iv. Cylindricity

Defined as the condition of a surface of revolution in which all points of the surface are equidistant from a common axis

The tolerance zone for cylindricity is bound by two concentric cylinder

All surface elements must lie within these concentric circles

The tolerance applies simultaneously to both the circular and the longitudinal elements of the surface

For RFS


Figure 3.8: Cylindricity tolerance


2. Profile tolerance

Allows us to control the form or shape of a surface

A profile is the outline of an object represented by a cross section through the part or by and end view of the part

Basic dimensions are generally used to define a profile

Profile tolerance can be applied as either a profile of line, or profile or a surface


i. Profile of line

Used where parts have a change in the cross section through the length

The tolerance zone is two dimensional, extending along the length of the applicable feature

See figure 3.9


Figure 3.9: Profile of a line with all around

symbol


ii. Profile of a surface

Use to control the entire surface of single entity

The tolerance zone is three dimensional, extending along the total length and width or circumference of the part or feature


Figure 3.10 Application of profile of a surface


3. Orientation tolerance

Orientation tolerance control features in relation to one another; therefore a datum reference is required

Orientation controls can be applied to the surface or the axis of the part feature.

Angularity, parallelism, and perpendicularity are orientation tolerances


i. Angularity

Angularity is the condition of a surface, center plane, or axis at a specific angle from a datum plane or axis.

The tolerance zone is established by two parallel planes or a cylindrical zone at any specified angle other than 90%, to a datum plane or an axis

Where angularity is applied to a surface, the feature control frame is connected to the surface by a leader

See figure 3.11 The angle is specified by a basic angle from the

datum plane For RFS


Angularity tolerance applied to a surface

Angularity tolerance applied to an axis

Figure 3.11


ii. Perpendicularity

Perpendicularity is the condition when a surface, center plane or axis is at exactly 90 degree to a datum

The tolerance zone for perpendicularity is established by two parallel planes or cylindrical zones that are 90 degree to a specified datum plane or axis

Perpendicularity can be applied to a surface or an axis When applied to a surface, the shape of the tolerance

zone is parallel planes that are at a 90 degree angle to a datum plane.

See figure 3.12 The tolerance value identifies the size of the tolerance

zone All elements of the surface must lie within this

tolerance zone For RFS


Perpendicularity tolerance applied to

surface

Perpendicularity tolerance applied to an

axis

Figure 3.12


iii. Parallelism

Parallelism is the condition where a surface, center plane or axis is exactly parallel to a datum.

Parallelism may be applied to a surface, resulting in a tolerance zone of two parallel planes, or applied to an axis resulting in a cylindrical tolerance zone.

When surface is controlled parallel to a datum, all elements of that surface must lie within two parallel planes, parallel to the datum

Parallelism may be applied to the axis of two or more features where a parallel relationship is required.

See figure 3.13


Parallelism tolerance applied to a surface Parallelism tolerance

applied to an axis

Figure 3.13


Location tolerance

Location tolerance are used to locate or position features from datums

Location tolerances include position, concentricity and symmetry

Position tolerancing provides the maximum benefit of GD&T, allowing increase in tolerance of the feature by 57% and reducing scrap

Location tolerances can be used to control position, symmetry and coaxiality


i. Position

Position tolerance defines a condition where the center, axis or center plane of a feature of size is allowed to vary from true position

True position is the theoretically exact location of a feature.

The location of each feature is given by basic dimension and the location tolerance is indicated by the position symbol, a tolerance value, applicable modifiers and datum references


Figure 3.14: Position tolerance


ii. Concentricity

Concentricity is used to control the relationship between the axes of two or more cylindrical features.

When the axes of each cylinder fall on the same centerline, they are concentric. (see figure 3.15)

The tolerance zone for concentricity is a cylindrical tolerance zone whose axis coincide with the axis of the datum feature

Concentricity is a relative measurement, so it requires a datum specification

The tolerance can only be applied on an RFS basis


Figure 3.15: Concentricity tolerance. Concentricity is defined as the condition where all median points of diametrically opposed elements of a cylinder are

congruent with the axis of a datum feature


iii. Symmetry

Symmetry is a positional tolerance where the median points of all opposed elements of two or more feature are congruent with the axis or center plane of a datum feature

Concentricity and symmetry are similar concepts The difference is that they are applied to different

geometric configurations- concentricity applies to cylindrical features while symmetry is applied to planar features

The tolerance zone is centered about the center plane of the datum

Symmetry is always used with a datum reference and applied on an RFS basis.


Figure 3.16: Symmetry tolerance


Runout tolerance

Runout tolerance are a combination of tolerances used to control the relationship of one or more features of a part to a datum axis.

The features can be surfaces perpendicular or surfaces around the datum axis

There are two types of runout control: 1. Circular runout2. Total runout


Circular runout

Circular runout controls circularity and coaxiality (the condition where two or more features share a common axis)

The tolerance is measured by full indicator movement of a dial indicator placed at several locations while the part is rotated 360 degree.

Circular runout is measured as a single circular element at each measured location


Figure 3.17: Circular runout


Total runout

Total runout is used to provide total composite control of all surface elements. (see figure 3.18)

The total tolerance is applied to both circular elements and the profile

When applied to the surface around and at right angle to a datum axis, total run out may be used to control a combination of circularity, straightness, angularity, taper and profile.


Figure 3.18: Total runout


References

Dotson C.l., Fundamentals of Dimensional Metrology, Thomson Delmar Learning, 2006

Meadows J.D., Measurement of Geometric Tolerance in Manufacturing, Marcel Dekker, 1998

Griffith G.K, Geometric Dimensioning and Tolerancing, Prentice Hall, 2002.

roshaliza hamidon (dpt 312 08/09) chapter 3 measurement and tolerances dpt 312 metrology

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