【转帖】inspection datum points

yang686526

inspection datum points
it has come to my attention that our tooling is being inspected using the same three points to establish the primary, secondary and tertiary datums.  i checked y14.5-1994, and it does not specify that datum points need to be unique.
i have not seen this on a drawing before, and was wondering if it is indeed allowed per the standard, or just our inspection departments own interpretation.  we are at the point of revisiting our company standards, and this is becoming a sticking point.  thanks in advance!
believe it if you need it or leave it if you dare.fff"> - robert hunter
am i missing the point or something?  if datum points are defined, they should be used.
"art without engineering is dreaming; engineering without art is calculating."
it's just that i am used to seeing six unique points to define the three datums.  the question is, is it acceptable (per y14.5) to use three points to define the three datums?
believe it if you need it or leave it if you dare.fff"> - robert hunter
i'm not seeing the issue either from the description.  can you go into more detail about your concern?
matt lorono
cad engineer/ecn analyst
silicon valley, ca
three (3) points define a plane such as a primary datum. the secondary is defined by 2 points (defferent point from the primary) while the tertiary is 1 point. one cannot define all three (3) datums with the same 3 point set up.
my background is in quality and measuring and i re  
thanks, dave.  that was my understanding also.  however, inspection claims that, as long as all of the points do not lie in a line, they can use all three to establish datum a, two of them to establish datum b (perpendicular to a) and the third for datum c (perpendicular to a and b).  i can't find a good argument against this, and am hoping for some logical reasoning as to why it is wrong (or is actually allowable).
believe it if you need it or leave it if you dare.fff"> - robert hunter
please forward a schematic of the datum points.
dave d.
picture a square block with two tooling balls on the top and one on the front face.  these are the three points being used to establish the datum structure.
believe it if you need it or leave it if you dare.fff"> - robert hunter
the 2 tooling balls on the top face will achieve an axis or straight line but not a plane. the 3rd ball could have been used or anti-rotation of the axis but you do not have an axis but a plane.
there is absolutely nothing here for the tertiary such as another ball at 1 end of the block.
i can only suggest that if you are following asme y14.5m-94. section 4.6.4 on page 73, it states:
"a primary datum plane is established by at least three target points or areas not on a straight line. see fig. 4-31. a secondary datum plane is usually established by two targets. a tertiary datum plane is usually established by one target. a combination of target points, lines, and areas may be used."
good luck!
dave d.
again, thanks for your input, dave.
  the argument is that they are using three points for datum a, two for b and one for c.  they just happen to be non-unique points, and the standard does not specify that they are to be unique.
  perhaps a square block was a poor example.  picture a mold shaped like a potatoe chip with three tooling balls placed around the edges.  the three are not in a line.  the three together establish the primary plane.  two of them establish the secondary, perpendicular to the primary, and one establishes the tertiary, perpendicular to the other two datum planes.
i apologize for repeating myself, but i am trying to ensure that i am expressing the situation accurately.
believe it if you need it or leave it if you dare.fff"> - robert hunter
a tool reference system can consist of 3 (min) or more tool balls.
an alignment (datums) can easily be created via 3 tools balls. it is an established & preferred process.
a 3d contour mold is a good example: with 3 t/b's i can align, then cms (laser, cmm, etc.) the tool surface, best fit if need be and re-identify the co-ordinates of the t/b's.
datum target points on a production part, forging or casting are different. these almost allways have to be 6 separate points that are used to establish datums.
y14.5 is more for production parts.