Flatness call-out

[Tizbass]

I have a question about a Flatness feature control frame on a part drawing.
The surface machined surface is approx 300MM x 700MM
Profile 0.75 X
Profile 0.5 - freestate
Flatness 0.1/ 75 x75
Max overlap 3MM
Min overlap 1MM

I'm good with most of it but Ive never seen the 75 x75 called out .....Does it mean I only measure flatness in section 75 x 75 (using the max/min overlap as my tolerance)and not the entire surface. This would seem to make sense otherwise the profile 0.5 freestate call out would be pointless. Am I right????

Thanks for any help, Randy

 

[pmarc]

Tizbass,

Flatness 0.1/75x75 means that every 75x75 square portion of the surface has to be inside tolerance zone, that is two parallel planes 0.1 apart.
I think profile in free state condition has nothing to do with the flatness, unless flatness is also specified in free state.

 

[MechNorth]

The 75 x 75 is a per-unit area requirement. It's in '94 in 6.4.2.1.1

It means that in any and every area of 75x75mm, the local flatness cannot exceed 0.1, with the workpiece restrained as specified. The min & max overlap is an interesting twist that I've not seen before; it means that there are discrete regions to be checked. Typically, the entire surface would be checked and any area where there is a sudden transition would be marked off & rechecked within a floating 75x75 area (i.e. a mylar cutout that you move around). The indicated overlap would actually allow a restrained flatness of 0.97 overall, which exceeds the freestate flatness. That isn't good.

As shown in the cited section, there's usually a composite feature control frame with an overall flatness callout on the top and a per-unit refinement underneath.

The profile 0.5 - freestate, without any datum reference, gives you freestate flatness on the feature. Not sure what the Profile 0.75X is about. A graphic would help.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

The profile is overall on the feature and is relative to at least one datum while the flatness cannot be over 0.1 mm in each 75X75 mm zone and is independent of the profile.

Flatness shown in this manner controls rapid change in the surface. If one had a flatness of 0.3, as an example, we could have it all in one area so the 75X75 zone controls rapid changes in the surface.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[Tizbass]

Yes it does help...Thanks. We were trying to hold a flatness of 0.1 over the entire surface which I don't think we needed to do.