Flatness measured to locations of a plane

[Jeff Chang]

Is any specification that defined flatness measured locations for a plane. How many measured points are suitable or it can reduce gap btw supplier and my measurement.
To define same locations may be helping a little be but I am worring about leak significant locations that determine flatness pass or fail.

 

[Jim Wynne]

Re: Flatness measured locations

A flatness requirement applies to the entire surface indicated on the drawing. If you have a relatively large surface but don't want the flatness callout to apply to all of it, you can highlight the area of interest, and apply dimensions that define the limits of the area.

 

[Jeff Chang]

Re: Flatness measured locations

Thanks Jim,
Since the drawing did not defined which area of surface that are more important, the drawing call out flatness should apply to all of it.
I would like to ask them folow my measurement.
For example: a welding frame 170'*78' made by 5' rectangle tube as double cross.
First, to visual check frame each straight tube with 40 inch steel ruler. and mark all lower and highest location.
Second, measure those location by laser arm distance btw point to point 10 inch max.
Finally to check any area out of spec locations, add measured point near orignal locations for result confirmation.

 

[Jim Wynne]

Re: Flatness measured locations

Thanks Jim,
Since the drawing did not defined which area of surface that are more important, the drawing call out flatness should apply to all of it.
I would like to ask them folow my measurement.
For example: a welding frame 170'*78' made by 5' rectangle tube as double cross.
First, to visual check frame each straight tube with 40 inch steel ruler. and mark all lower and highest location.
Second, measure those location by laser arm distance btw point to point 10 inch max.
Finally to check any area out of spec locations, add measured point near orignal locations for result confirmation.

I'm not sure what your size notations mean. In the US at least, you're describing something that's 170 feet x 78 feet, but I'm pretty sure that's not what you mean. In any event, you need to make sure that if flatness is called out in accordance with standard GD&T definitions that you're actually measuring flatness and not something else such as parallelism. It's a good thing to come to an agreement with your supplier on measurement methods, but without being able to see the drawing and the weldment itself it's hard to give advice. The first thing you need to determine is whether the welding fixture and process is capable of doing what you need.