Flatness of a derived median plane per unit length.

[drstrole]

I am looking for input on how to specify flatness per unit length for longer pieces of shaft, 300+ mm in length. Pre 2009 we used straightness of a median plane and specified it per 30mm. Using the 2009 standard, we would use flatness and thus my question. Do I specify the length per inch (option 1) or as an area (option 2) & is the area intuitive? Since flatness is a measurement of area, I tend to lean towards option 2, but it seems less than intuitive. I do not see any support in Y14.5 or any other text that I currently have. Alternatively, is there another correct method that I am missing?

Drstrole
GDTP - Senior Level

 

[Belanger]

For your situation, the standard doesn't really say. Even for surface flatness, I've often seen prints where there was only one number after the slash. But paragraph 5.4.2.2 does say that "the size of the unit area...is specified," so I see that as requiring a 2D dimension(s).

Seems to me, if that is ironed out, then the same concept applies to FOS flatness.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[fsincox]

I was not under the impression you could not use straightness as before, I would use flatness for square parts but for a round shaft, I would still use straightness.
Frank

 

[MechNorth]

'09 added flatness of a derived median plane because that's what '94s straightness of a derived median plane actually meant; it was a correction of sorts to the standard.

Per J-P, I see no reason that extension of principles wouldn't get you to per-unit-area on a derived median plane. As for per unit length or per unit area, it's a surface rather than a line element control, so per unit area is appropriate.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Let me guess, shaft does not imply a round cross section to some people?
Frank

 

[CheckerHater]

Let me guess, shaft does not imply a round cross section to some people?

Lack of diameter symbol doesn't help either.

 

[fsincox]

Good point, I should have noticed that too, My bad.
thanks CH

 

[CheckerHater]

No, seriously.
"Diameter" symbol was introduced in 1973, "square" in 1982.
What other kind of shaft can you think of?

 

[MechNorth]

I've seen triangular, pentagonal, hexagonal, octagonal and other shapes of shafts. Function and bar stock.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerHater]

I wouldn't want to derive median plane from triangular shaft early in the morning.

 

[fsincox]

I would most likely use axial straightness on all of those, execpt squares and rectangles, unless I was also specifically concerned with controlling twist.
Frank