points to define a datum

[AliThePro]

I know that we need two point to define a line, so two points can be used to define a datum that is a line. I also know that we need three point to define a surface, therefore, three points can be used to define a surface that is a datum. But how about cylinders or spheres? Are three points still enough?

 

[ringman]

ali,

If you will look at Fig. 5-43 in the ASME Y14.4 std. You will see that the dia does not necessarily have to be for the full extent of the circumference. I believe this would relate to your situation in not having full circumferential surface. Therefore, I will still conclude that you can locate 2 diameters at a specified distance from one end of the cylinder and use the axis between them as the datum. Relatively simple.

 

[MechNorth]

Typically, you would use a collet (a multi-jaw chuck or other such device) to close down around the cylindrical datum feature. The centre axis of the collet simulates the datum axis of the cylindrical feature. If you don't want to use this method, you can take two groups of three points (prefer 7 & 11 pts to minimize lobing effects) to generate two circles, resulting in one common datum axis. Not a great way to do it, but useful if a taper is present. Personally, I'm always concerned about how well the generated axis represents the real one, but that's a decision for you. This is a fairly advanced topic as far inspection is concerned, so be careful using the two circles to generate the datum. Hope it helps.



Jim Sykes, P.Eng, GDTP-S
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