straightness definition ?

[Madhu454]

thoughout the standard they have explained that , when straightness is applied to FOS say shaft , the straightness of median line is controlled.

but the definition of straightness uses the term AXIS . can anyone tell us why this diferance?

 

[pmarc]

Madhu454,
Look at attached picture. It is taken from Y14.5-2009. Although it aims to explain related and unrelated actual mating envelope concepts, I think it nicely shows the difference between axis and derived median line of cylindrical FOS.

http://files.engineering.com/getfil...b446-0a5f53a631df&file=fig_1-1_Y14.5_2009.JPG

Generally saying, derived median line is an imperfect "thing" and that is why its form can be controlled by straightness if needed.

Axis, which is an ideal straight element derived from unrelated actual mating envelope, cannot be controlled by straightness because one cannot control something that is already perfectly straight.

Your question reveals a shortcoming straightness definition in Y14.5M-1994. The term 'axis' should not be there. '09 edition eliminates this by stating that: "Straightness is a condition where an element of a surface, or derived median line, is a straight line."