GD & T Characteristic - A-1 Straightness

A-1: Straightness : 

GD & T Characteristic - Straightness

What is Straightness?

Straightness is practically a One-Dimensional straight line that is supposed to have the length. 

Symbole - "  "

This characteristic is important for these objects which have their length (and its straightness) as one of the most important parameters 

Example: the spindle of a drilling machine, bed of planer machine, Cam-shaft of a multicylinder petrol engine, the screw of a screw jack, etc.

A short horizontal line (of a length twice that of the height of any alphabet or numerical figure written in the feature control frame), looking like a dash, is the symbol for this characteristic and can be seen at the chamber of the feature control frame. 

Types of Straightness -

1. Surface Straightness

2. Axial/Line straightness

1. Surface Straightness -

1. Surface Straightness

Whenever any functional Surface Element of an object like the inner face of a long slot, the sliding surface of a planer bed, the working face of a guide-rail, etc. is required to be reasonably straight then the concerned feature control frame indicating the permissible deviation at the straightness appears as shown. The mentioned feature control frame for the Straighmess of the shown feature indicates that the surface A (as good as a length) is allowed to deviate at its straightness, to 0.04 mm max. The surface roughness deviations are not to be accounted for within 0.04 mm. 

Such deviation at the straightness can be judged by aligning a "straight edge" with the component surface and observing the gap underneath the straight edge against some illumination. Even a feeler gauge also can be used at the gap for judging the amount of the deviation. Sometimes, the concerned surface of the object can be aligned with a surface plate or parallel block; and a feeler gauge inserted in the gap between the component and the surface plate can indicate the amount of deviation. For very long objects like planer machine beds. the use of Auto-collimator (an optical instrument) or the use of a Level Beam comparator can be much helpful.

2. Surface Straightness

2. Axial/Line straightness -

Sometimes, objects like shafts and spindles are required to be straight Then the corresponding Feature Control Frame is mentioned as shown. The expected deviation can now be in a circular zone. Accordingly. a zone descriptor symbol 'φ" is required at the second chamber of the feature control frame. Such deviation also can be checked by means of a surface plate, feeler gauge at different orientations of the component. 

Axial/Line Straightness

Straightness of a long surface, axis or an edge is functionally very important at the requirements of straight linear motion (shaping or planing machines), true rotational motions (machine spindles, gearbox shafts, lead screws); locking fitment (edges of a door panel, suitcase cover); true edge contacts (edge locators, V locators); etc. 

Relation With the other GD & T Symbole -

Surface -

Straightness is the 2-D version of Flatness as both are measured without datum. while the flatness measured the variation across the 2D plane and straightness only measured the variation of a straight line.

Axis - 

Axis straightness is also related to axis parallelism and axis perpendicularity. Both characteristics controlling a center axis with a cylindrical tolerance zone. 

Interview questions on Straightness-

1. Relative to datum - No.

Straightness is not related to the other feature of the component. That' why it is not required to provide datum reference for straightness tolerance. 

2. MMC Or LMC applicable - Yes, Only for Axis Straightness  

To Read More ... Flatness