Difference Between Flatness & Straightness

With regard to surface shapes, flatness = 1/bumpiness; with regard to linear shapes, straightness = 1/curviness. My apologies for such esoteric theory at an early stage.

Straightness is the 2-dimensional analog of flatness, which is 3-dimensional.

That would be the way I interpret it.

Flatness is the degree of variation in surface height with regard to a reference 2D plane.

Straightness is generally the linear variation compared to a reference 1D strait line.

No

Straightness is usually put on surfaces that aren't flat. It is used to control the straightness of a surface or an axis.it specifies how much the surface or axis is allowed to vary from the perfect straight line mentioned. Straightness tolerancing is usually applied on cylindrical features (shafts,bars etc.) to control bowing and other distortions. Of course it may also be applied to flat surfaces, when it would mean that each longitudinal element of the surface must lie within the tolerance zone specified, in the direction denoted. The straightess tolerance is some time put on the axis too (hole or shaft) but there the preferable would be the cylindricity.

Flatness anyway had to be on a flat surface.

Flatness is a 3D form control of the surface. Surface straightness is a 2D form control of the surface only applying to the view in which the straightness control is situated. For most 3D surfaces, the design requirement is usually that the entire surface be controlled, so flatness would be the tool of choice in controlling the variation.

http://www.playtex.com/

Straightness is related to a line. Flatness is related to a plane.

Straightness and Flatness View as html

GA

To try to put it more accessible language;

Straightness is deviation from a (perfect) axis or edge.

Flatness is deviation from a (perfect) plane

E.g. the letter 'S' can be flat, but is not straight.

A 'straight road' is not flat.

Similarly, a tube or shaft, can be straight, but is not flat.

They can appear to occur together, but are applying to quite different aspects.

e.g. a 'D' shaped extrusion may have both flatness and straightness tolerances, as either could be 'off' independently of the other.

I.e. the terms are not interchangeable.

When I was a teenager I had a girlfriend who straight away told me she was flat. Has things changed since then? ....being a long time now maybe, new technology....

How do we check for flatness and straightness?

Surface plate, dial indicator, laser, CMM, any number of ways.

Dunno. It can't be seen from here. Posting pictures might help.

I take my hat of to you - trying to get a question clarified after 13 years !

This place is getting surreal. I just noticed an update to a question about chess that vaporized years ago. Not saying who, but it was posted by a stellar avian .

Shhh....

Dear Friend,

STRAIGHTNESS refers/applies to a LINE.

FLATNESS applies to a surface with out undulations.

DHAYANANANDHAN.S

You see a woman with no "boobs" - this is Flat and if she is not crouched then - this is straight. This then defines both and you you cannot use them together as same.

For simple explanation let us take two coke bottle...Put these two coke bottle in straight.i.e (2 coke bottle should lie in same axis-collinear)... Deviation in the axis of two bottle is known as straightness error....In a same way if u could see the bottle surface it is not flat...i.e(bottle seems to be some wavy nature(viewing plane))...