datum on the theoretic axis 2

If your part works in the assembly as you described "............to find the axis from 3 diameters" (which is highly unlikely) then use A-B-C as your primary common datum axis.
Describe how this part is assembled and maybe you can get better answers.

While it's true that showing a datum feature symbol on the center is incorrect, sometimes we can figure out the intent if we can see how datum A is used in a subsequent callout.
I suspect that they didn't want to make datum A from a combination of 3 centers (you could do that with hyphenated datums, as greenimi mentioned). But seeing the full GD&T could help the reader deduce the intended idea.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

Datum targets

Hi, no_marcin:

It is called "Multiple datum Features". You can use A-B-C as greenimi indicated. But I doubt you have such design intents in your application.

Below is what ASME Y14.5-2009 says about "Multiple datum features" (page 65 section 4.12).

MULTIPLE DATUM FEATURES
Where more than one datum feature is used to establish
a datum feature simulator for a single datum, the appropriate datum feature reference letters and associated
modifiers, separated by a dash, are entered in one compartment of the feature control frame. See para. 3.4.2 and
Fig. 4-22. Since the datum features have equal importance,
datum feature reference letters may be entered in any
order within this compartment. Where the intent is clear,
a datum feature reference letter may be used to define the
multiple surfaces as a single datum feature

Best regards,

Alex

Tick -- datum targets are best when you're using only a portion of the physical feature to create your combined datum.
If the target encompasses the full length, then we'd just keep it as a full letter and use the hyphen method.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

My solution ultimately would be to not allow my name to be associated with the drawing. No "drawn by". No "checked by". Most certainly NOT "approved by".

Hi,

Just an FYI, "Multiple datum features" in ASME Y14.5-2009 is renamed to "Common datum features" in Y14.9-2018.

7.12 COMMON DATUM FEATURES
When more than one datum feature is used to establish
a true geometric counterpart for a single datum, the
appropriate datum feature reference letters and associated
modifiers, separated by a dash, are entered in
one compartment of the feature control frame. See
para. 7.21(b) and Figure 7-27. Since the datum features
are equally important, datum feature reference letters
may be entered in any order within this compartment.
Where no material boundary modifier is applied, the
default RMB condition (Rule #2) applies. Where MMB
or LMB is the required boundary condition, the MMB
or LMB boundary modifier shall be applied following
each datum feature reference letter. Where the intent
is clear, a single datum feature reference letter may be
used to define the multiple surfaces as a single datum
feature. See paras. 6.3.2.1(g) and 7.12.1 and Figure 6-6.
Where applicable, each datum feature reference letter
shall be followed by a material boundary modifier.

p.s. I humbly accept the knowledgeable correction regarding use of datum targets in this case.