Flatness Call Out confusing me - What they are wanting and how to check it?

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P

Phil Fields

#22
Missilegypsy,
David's procedure was correct for measuring flatness due to the 1st process of leveling the part by finding the 3 high points (this is time consuming). Once the 3 high points are found and equalized, it is fairly easy to scan the part and find the low point, thus the flatness.

Phil
 

Jim Wynne

Staff member
Admin
#23
This is the same argument I have had many times before, but what all of you are measuring is Paralellism not Flatness. Flatness is measured from the underside of the piece on the 3 points from the surface plate up to the part and swept across the bottom of the part with a dial indicator attached to the height gauge.
It would be a parallelism measurement if the part were lying directly on the surface plate, or if it were resting on blocks. When the part is resting on the tips of the jacks and when the indicator is universally zeroed over those tips, the plane has been isolated. Look at it this way: Say the indicator reads zero over each of the jacks, but reads -.005" over the center of the part, away from the zero areas. Where is that .005" coming from? Not from the surface plate or the jacks, which would be parallelism.
 
2

20110525R

#24
Missilegypsy,
David's procedure was correct for measuring flatness due to the 1st process of leveling the part by finding the 3 high points (this is time consuming). Once the 3 high points are found and equalized, it is fairly easy to scan the part and find the low point, thus the flatness.

Phil
Sorry, but you are also incorrect, that is Paralellism.
 
Q

qualitytrec

#26
I have always understood flatness to be a best fit feature unless tied back to datums (which I have not seen). As such, it is important to adjust the part until you have the least amount of difference in your sweep, this can be done in either orientation as long as jacks are used. the important thing is to establish your zero at the low point (if using the top surface, which is easier in my experience) after balancing the part to the least variance.

// would be measured if you balanced the bottom of the piece to the least variance then measured the top (or vicea versa). If you are balanced on the surface you are evaluating then it would be flatness.

A lazer scan would be the easiest way if it is available but if you need to use the indicator technique then this is the best way i have found.

Hope this helps.

Mark
 
T

True Position

#27
I have always understood flatness to be a best fit feature unless tied back to datums (which I have not seen). As such, it is important to adjust the part until you have the least amount of difference in your sweep, this can be done in either orientation as long as jacks are used. the important thing is to establish your zero at the low point (if using the top surface, which is easier in my experience) after balancing the part to the least variance.

// would be measured if you balanced the bottom of the piece to the least variance then measured the top (or vicea versa). If you are balanced on the surface you are evaluating then it would be flatness.

A lazer scan would be the easiest way if it is available but if you need to use the indicator technique then this is the best way i have found.

Hope this helps.

Mark
This is why the method is so time consuming. You setup the part, find the indicator TIR, adjust the jacks, check TIR, if better, this is the new flatness. Continue improving until you find the minimal TIR.
 
2

20110525R

#28
I have always understood flatness to be a best fit feature unless tied back to datums (which I have not seen). As such, it is important to adjust the part until you have the least amount of difference in your sweep, this can be done in either orientation as long as jacks are used. the important thing is to establish your zero at the low point (if using the top surface, which is easier in my experience) after balancing the part to the least variance.

// would be measured if you balanced the bottom of the piece to the least variance then measured the top (or vicea versa). If you are balanced on the surface you are evaluating then it would be flatness.

A lazer scan would be the easiest way if it is available but if you need to use the indicator technique then this is the best way i have found.

Hope this helps.

Mark
That is correct Mark, you hit the nail on the head when you said "If you are balanced on the surface you are evaluating then it would be flatness.". That is the right statement.....
 
H

Hodgepodge

#29
Here is how to check flatness without at CMM. This method has been used for years and years.
I've never seen this method before. It seems like a royal pain. A faster method is to use blocks or jacks (sometimes called "points") that are high enough to sweep an indicator under. This method isn't without its faults, but it is easy enough to establish the proper plane.
 

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Stijloor

Staff member
Super Moderator
#30
Friends,

Now I know why I am so damn tired after teaching my 3-day GD&T course. :D

I always ask my students to be open-minded and accept the official ASME Y 14.5 definitions of geometric concepts. As witnessed here, I still have a lot of work to do!

Stijloor.
 
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