8 Straightness of Features

8/11/2019 8 Straightness of Features

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THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY

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STRAIGHTNESS OF FEATURES OF SIZE


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APPLICATION

The same straightness symbol is used in the feature control symbolas for straightness of surface elements.

However, when not modified by MMC, the feature control symbol maybe directed either to the center line, or to extension lines from the

Diameter or thickness as shown.

STRAIGHTNESS OF FEATURES OF SIZE


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Positioning the feature control symbol between the views should beavoided.

Unless it is perfectly clear as to whether it refers to the lengthwiseor widthwise Straightness.

APPLICATION

If the cross section is circular thetolerance zone becomes circular and adiameter symbol then precedes thetolerance


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INTERPRETATION

A straightness tolerance applied to a center line means that allcenterlines between opposing line elements of the surface shalllie completely within tolerance zones having a width equal to thespecified tolerance


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MEASURING PRINCIPLE

The part to be measured ismounted using some suitable means ofsupport, such as between centers or onV blocks.

Two indicators are mounted

diametrically opposite one another,preferably on the same carriage andarranged to move parallel to thecenterline being measured as shown.

Indicators are placed at zero at

one end and differences in readingsbetween the two indicators are notedas the carriage is moved toward theother end.


7/18THE INSTITUTE FOR ENHANCEMENT OF TECHNOLOGY

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If a centerline is common to two ormore diameters of other features, astraightness tolerance directed to thecenterline applies to all features on thecommon centerline unless otherwisespecified

STRAIGHTNESS



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If the tolerance is intended to applyto only one thickness or diameter onthe common center line, a note may beadded beneath the feature control

symbol .

Alternatively the symbol may bedirected to extension lines from thesurfaces.

STRAIGHTNESS OF ONE FEATURE OF MULTIPLE DIAMETER PART



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DIRECTION OF APPLICATION OF STRAIGHTNESS

Straightness of a center line or medianplane applies only to center lines which run in

the direction of the line or line elements towhich the straightness tolerance is directed.

The width or diameter of the tolerance zonelies in the direction in which the arrowheadpoints.

In the rectangular part in Fig., the controlapplies only to longitudinal centerlinesbetween corresponding line elements of thetop and bottom surfaces.

If the cross section is circular or is a

regular polygon, with no means of determininga specific orientation for measuring purposes,the tolerance applies in all applicabledirections, as already explained.



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If there is a ready means ofidentifying the orientation, thetolerance applies only in thedirection indicated .



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If there may be some ambiguity anote should be added, such as THISDIRECTION ONLY .

If the part is circular and it isintended that the tolerance apply in all

directions a diameter symbol shouldprecede the tolerance value.



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If a tolerance is shown in two

directions it is measured in thesetwo directions and the tolerancezone is then a parallelopiped



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STRAIGHTNESS MAXIMUM MATERIAL CONDITION

It actually specifies a virtual size, equal to the maximum materiallimit of size plus or minus the specified geometrical tolerance

To signify that the maximummaterial principle the symbol

(M) is placed immediately afterthe tolerance value in thefeature control symbol

As there must always be a feature size dimension associatedwith such a tolerance it is useful and convenient to establish thisrelationship by directing the feature control symbol in line withthe dimension, or associating it with the dimension by a commonleader



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Tolerance of zero MMC, means that the virtual size limitcoincides with the maximum material limit.

If a feature is everywhere at its maximum material limit ofsize, no errors of straightness are permitted.




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A straightness tolerance modified by (M) means that the featureshould lie within a tolerance zone consisting of the line between

two parallel lines, in the same plane as the longitudinal section ofthe feature being evaluated.

These zone lines are separated by the special straightnesstolerance, plus the maximum material size of feature




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MEASURING PRINCIPLE

Gage consists of two straight and parallel gauging elementsbetween which the part must pass.

These gauging elements must be at least as along as thelength of the feature being gauged.

The gage must be maintained normal to the surface beingevaluated.



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It is very seldom used for round holes.

This is because such features are morereadily controlled by other geometrical

tolerances, such as cylindricity,perpendicularity, or position, all of whichcontrol straightness.

However straightness is very usefulcontrol for some internal features, such as

grooves and slots.

STRAIGHTNESS OF INTERNAL FEATURES


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If it is desired to ensure that the straightness error does notbecome too great when the part approaches the least material sizelimit, a maximum value may be added, as shown.

STRAIGHTNESS WITH A MAXIMUM VALUE

8 Straightness of Features

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