Eight holes, four groups, five possible indications on the same plate. The interactive graphic shows which degrees of freedom each combination of SZ, CZ and the datums A, B and C locks.
This plate has 8 holes, of which two always form a group for functional reasons (A). The orientation of the 4 groups and of the holes they contain can be controlled through combinations of CZ and SZ.
The indications in the frame follow the order of the levels, from the largest collection to the smallest. On this plate there are two levels, the 4 groups and the 2 holes of a group. There can be more.
Further levels are possible. Suppose a part carries 4 groups, each group 2 subgroups and each subgroup 3 holes, then three arrows designate the levels and three indications stand in the frame:
Ø 0.8 SZ SZ CZ
A
Fictional example with three levels, not the plate from the drawing. The position is set to datum A only.
The position tolerance gives every hole a zone its axis has to lie in. How the eight zones stand relative to each other is decided by the modifiers. In this example the first indication in the indicator applies to the four groups relative to each other, the second to the two holes within a group.
Every zone applies on its own. The four groups may sit and rotate independently of each other, as long as each keeps to its own zone.
The zones form one unit with a fixed spacing and the same orientation. Translation and rotation are only possible together.
A sets the zones perpendicular to the upper face, B locks rotation in the plane of the drawing and holds the 22 mm, C holds the position in X.
Without a datum the pattern stays rigid in itself but free relative to the part. Only the datum ties the zones to faces, and every further datum takes one more degree of freedom. Whether the indication on your drawing matches the function is something we settle in a drawing review.
Pick an indication and watch what is still allowed to move. The locks at the dimensions snap shut as soon as the chosen indication holds that dimension.
Depending on the exact function of the plate, each of the cases shown can make sense.
Ø 0.3 SZ CZA
SZ CZ | A for example, when 4 individual parts are bolted to the plate and the hole spacing of 26 is what mainly governs the function.
Ø 0.4 CZ CZA
CZ CZ | A would make sense when something is fastened at all 8 holes at once while the hole pattern is still allowed to float relative to B and C.
Ø 0.6 SZ CZAB
SZ CZ |A|B is the one to use when the orientation among the parts and to datum B matters, but not the spacing between the 4 parts.
Ø 1.0 CZ CZAB
CZ CZ |A|B makes the spacing of the parts count as well.
Ø 1.5ABC
|A|B|C blocks all degrees of freedom of the holes absolutely through the 3 datums. This specification, widely used in practice, fixes the spacing and orientation of the 4 intended parts absolutely.
The five cases show how far a function can be broken down into single degrees of freedom, right down to the smallest fragment. That does not mean every part needs this depth. Many get by with a single indication. Breaking it down pays off where the function calls for it, or where an overly tight indication costs money in manufacturing.
Which combination fits the function is decided at the part, not by habit. In ISO GPS consulting we settle it together with your design, and in dimensional inspection the measuring program builds the same datum system the indication calls for.
