Datumn Planes

[waqasmalik]

Hi all

I have a confusion regarding 3-2-1 concept of Datum Planes.Primary datum feature must have at least 3 points of contact with datum plane,2 in case of secondary and 3 in case of tertiary.Lets talk about primary datum feature,it must have at least 3 points of contact.It can be 4,5,6 whatever but more than 3 and flatness control given on primary datum feature is to ensure this at least 3 points concept.I have 2 questions

1) Why at least 3? Why not 4 points? I mean why 3 point?
2)what if my primary datum feature have 2 minor protrusions but under flatness tolerance.Obviously when part will be seated against the datum feature simulator then 2 points will be touching the datum feature simulator.so,will it be legal?

 

[pmarc]

waqasmalik,
Primary datum plane cannot be established by two points - this is purely a matter of geometry. Two points are sufficient to establish unique line in space, but that is all. There will still be infinite number of planes (candidate datums) passing through that line.

When a primary planar datum feature contacts its datum feature feature simulator literally in two points (for example two minor protrusions), the part will almost always be unstable. It will rock until a third point of contact is established. The issue is that in such a case it is quite difficult to predict the nature of the rocking, that is, it is hard to determine which point of surface will be the third one.

Flatness tolerance can be applied (but does not have to) to primary datum feature to reduce the number of possibilities, or in other words to limit the number of candidate primary datum planes.

 

[cjccmc]

"1) Why at least 3? Why not 4 points? I mean why 3 point?" Because 3 points define a plane. If there are 4 non-coplanar points there are several planes that can be defined.

For question 2), the peaks of the 2 protrusions would be 2 of the 3 points that define a tangent plane to the nominally flat surface, a plane can rotate about a "hinge line" between those 2 points until it contacts a third high point.

 

[waqasmalik]

Thanx. Now the concept which i have developed is that minimum of 3 points of contact is necessary to stop the part from rocking which will create problem in measurement. we will be sure that at least 3 points are in contact with datum feature simulator by trying to rock the part.If it rocks then there are less than 3 points of contact.Am i ok? Assume a flat plate with bottom face as primary datum feature.

 

[ewh]

As noted, it takes at least three points to define a plane.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[KENAT]

Milking stool v regular chair.

Milking stool should be stable pretty much how uneven the floor.

A regular chair however tends not to be unless it/the floor has some flex/give.

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