interpretation geometric tolerances

[volaff]

Bye to all,
I have a doubt about the geometric tolerances I saw on a catalog.

I can't understand the circled tolerance as I have a doubt about reference to.

in my interpretation it seems to be the support surface of the dotted part, but I'm not sure.
In case my interpretation was correct, I just can't understand the tolerant tolerance.

thanks to anyone who will read

 

[MassiVonWeizen]

tolerance is symmetry
the reference to is the diameter

means that compared to the ø20 size the holeø8 must be symmetric with a 0.1 error

 

[volaff]

I'm a little confused.
Was it not better to give a symmetry than the axis of the ø20 hole than the diameter?

 

[MassiVonWeizen]

the axis is an ambiguous reference because it can refer to the diameter ø44, to the m30 and to the ø45; not being a tangible element can be extrapolated from any diameter present.
If you use the diameter axis ø45, which is not functional and has a very great tolerance, you may not be sure that it is the same as the diameter ø20 which is the important one for the operation of the plug.

 

[volaff]

Thank you very much all clear now.
but, in fact, from the practical point of view, how does the symmetry of a hole translate to another?
I have nothing practical in mind.
thank you anyway!

 

[MassiVonWeizen]

for me it should be understood as an image

 

[volaff]

Thank you very much.
Perhaps I am accustomed to thinking about symmetry always with respect to one axis and not of a diameter compared to another.
Now it's all clearer.
Thank you so much!

 

[Mito125]

for me it should be understood as an image

with this image I have finally understood, thank you