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Flatness - Rectangular part using the three jack method

#1
I have always used the three screw method of flatness measurement. I encountered a rectangular part which using the three jack method displayed a flatness .105.

Upon attempting to reject this as it was grossly OOT, I received instruction to try a different method which seemed like a problem to me as I believe the three jack method to be tried, true, and definitive. The suggestion was to place the part on four jacks. One on each corner. Adjust until two opposing corners are at the same height. Then make the other set of corners the same height. So top left and bottom right are now the same height and I have zeroed my height gauge on them and the other two opposing corners are both .038 higher than the two 0 corners. Upon scanning with the height gauge I measured a flatness of .055. The parts are still OOT, but they are profoundly better as the surface now fits in a smaller zone. I don't know if I should accept this method. Is there a fundamental flaw that I am just not seeing? Any feed back would be appreciated.
 

Jim Wynne

Super Moderator
#2
Try setting the part on 1-2-3 blocks with the target surface facing the surface plate, then check the part with the height gage from underneath.
 
#3
That is just not how it is done. I have seen you reply similarly to an old thread on this site and what you are checking by doing that is parallelism. You have to use adjustable jack stands to remove any influence of the opposing surface's relationship with any point of contact. And you measure the surface face up. Not down. The three jack method is not in question here. I am only looking for insight to a fourth jack.
 

Jim Wynne

Super Moderator
#4
That is just not how it is done. I have seen you reply similarly to an old thread on this site and what you are checking by doing that is parallelism. You have to use adjustable jack stands to remove any influence of the opposing surface's relationship with any point of contact. And you measure the surface face up. Not down. The three jack method is not in question here. I am only looking for insight to a fourth jack.
The idea is to isolate the target plane, and the method I proposed will do that. I know the difference between flatness and a parallelism. There is no influence from the opposite surface. If you were to put the part on blocks with the target plane facing up and then draw the height gage across it, that would be checking parallelism.
 
#5
The blocks are parallel to the surface plate. If you rest the surface you are trying to measure on the blocks your surface will be then be subjected to the influence of its potential irregularities which also will not be able to be measured now because you have a 123 block in the way.
 

Jim Wynne

Super Moderator
#6
The blocks are parallel to the surface plate. If you rest the surface you are trying to measure on the blocks your surface will be then be subjected to the influence of its potential irregularities which also will not be able to be measured now because you have a 123 block in the way.
I understand the bit about the blocks being in the way of measuring the entire surface. I also understand about the blocks resting on the surface plate, but jacks have to rest on the plate as well. Never mind.
 

Proud Liberal

Quite Involved in Discussions
#10
Since flatness is defined as the smallest distance between 2 parallel planes that will enclose all points on a surface, be aware that anything short a a full 3-D vision scan of the surface will ALWAYS be an estimate of flatness and as such one can only claim that "the flatness is no worse than .XXX". If changing your setup or technique yields a lower value, use the lowest value you can obtain. But you can still only claim a "no worse than" result.
 
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