Syntax question while dimensionning

I have a flat plate with 2 holes, one at each end. They are 2" c/c (then 1"
from the center line) and 0.5" from their end. What is the standard way to
dimension so thay it is relative to center? I dimension 2" c/c but for the
center thing should I draw a vertical centerline and dim1" (which is not
true from tolerance point of view) or just write a symetrical note? I want
the machinist to find the part center by "touching" both sides of the part,
set the zero to center and offset 1" at each side. How can I force that by
dimensioning?

--
Robin
Mech eng

SW2004+ SP1
AMD 1333 1024 ram
Spaceball 4000FLX
Hacked GeForce 3 to Quadro DCC

 

I think you're needing to look into Geometric Dimensioning &
Tolerancing. In the US the standard governing that is ASME/ANSI Y-14.5,
which is on a par with ISO standards. You'll actually find it in
section 5.13 and an illustration in figure 5-59, if you have the ability
to look it up. I can furnish you with an example if you email me
directly (remove/replace the capitalized MUNGE info in my email
address), but the bottom line is that you establish the length of the
bar as a datum by attaching a datum letter to the length dimension, then
you use positional tolerancing to reference that datum (as the secondary
or tertiary datum). The datum CAN be referenced at MMC (maximum
material condition) or LMC (least material condition) which allow a
bonus tolerance as the length departs towards the opposite end of the
size tolerance. But in your case you probably want it at RFS
(regardless of feature size) which gives no bonus tolerance at either
end of the size tolerance. For that you DON'T need to add a symbol to
the datum reference (the datum is simply presumed to apply at RFS).

Now, whether or not the machine shop will understand what you're
actually specifying is a different story. Probably the best thing is to
add a note to the drawing.

Mark 'Sporky' Stapleton
WaterMark Design, LLC
Charlotte, North Carolina, USA
(you can also email me as "" IF you substitute
"water" for its chemical symbol, which is "h2o")

 

For the simple answer, a note that says "PART IS SYMMETRICAL."

WT

 

Good point. In that case the note could say "HOLES ARE SYMMETRICAL ABOUT
THE CENTERLINE."

WT

 

But some time the part is not symetrical, just a few features.

 

I do not need GD&T, I just need to know if i should write on the dimension
sometihing like relative to center or something like that.

 

Well, you asked for "the standard way to dimension so that it is
relative to center" . . . and as far as I know there is no other
STANDARD way.